Remedy Evaluation Work Plan Units 1 & 2 Stage I and II

Prepared for

Talen Montana, LLC

P.O. Box 23 Colstrip, Montana 59323

Remedy Evaluation Work Plan

Units 1 & 2 Stage I and II Evaporation

Ponds Site

Colstrip Steam Electric Station, Colstrip, Montana

Prepared by

10211 Wincopin Circle, 4th Floor Columbia, Maryland 21044

Project Number MR1149D

July 2016

i 7/29/2016 MR1149D/Remedy Evaluation Work Plan - SOEP STEP

TABLE OF CONTENTS

ABBREVIATIONS AND ACRONYMS…………………………………………….....iv

1. INTRODUCTION ................................................................................................... 1

1.1 Purpose ........................................................................................................... 1

1.2 Work Plan Organization ................................................................................. 1

2. BACKGROUND ..................................................................................................... 3

2.1 Facility Description ........................................................................................ 3

2.1.1 Construction/Operations .................................................................... 3

2.1.2 Prior Releases and Response Actions ................................................ 5

2.2 Physical Setting .............................................................................................. 7

2.3 Geology .......................................................................................................... 8

2.4 Hydrogeology ................................................................................................. 8

2.4.1 Hydrostratigraphic Units .................................................................... 8

2.4.2 Groundwater Flow ............................................................................. 9

2.4.3 Numerical Groundwater Flow Model .............................................. 10

2.5 Process Water Indicator Parameters ............................................................. 11

3. CONCEPTUAL SITE MODEL ............................................................................ 12

3.1 Source Area(s) and Constituents of Interest ................................................. 12

3.2 Release Mechanism ...................................................................................... 14

3.3 Transport Pathways ...................................................................................... 14

3.4 Environmental Fate ...................................................................................... 15

3.5 Potential Exposure Point(s) .......................................................................... 15

3.6 Potential Receptors/Health Risks ................................................................. 15

4. REMEDIAL ACTION OBJECTIVES .................................................................. 16

4.1 Objectives and Implementation of the AOC ................................................ 16

4.2 Compliance Evaluation ................................................................................ 17

4.3 Source Control Upgrade Remedial Action Objectives ................................. 18

4.4 Migration Management Upgrade Remedial Action Objectives ................... 18

4.5 Institutional Controls .................................................................................... 18

5. PRELIMINARY SCREENING OF REMEDIAL ACTION TECHNOLOGIES . 19

6. ASSEMBLING REMEDIAL ACTION ALTERNATIVES ................................. 22

7. EVALUATING REMEDIAL ACTION ALTERNATIVES ................................ 24

7.1 General Standards ......................................................................................... 24

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7.1.1 Protection of Human Health and the Environment .......................... 24

7.1.2 Attainment of Cleanup Criteria ........................................................ 25

7.1.3 Source Control ................................................................................. 25

7.1.4 Compliance with Standards for Management of Wastes ................. 25

7.2 Detailed Evaluation Factors ......................................................................... 25

7.2.1 Performance ..................................................................................... 26

7.2.2 Reliability ......................................................................................... 27

7.2.3 Ease of Implementation ................................................................... 27

7.2.4 Potential Impacts .............................................................................. 28

7.2.5 Time to Start/Complete .................................................................... 28

7.2.6 Cost .................................................................................................. 29

7.2.7 Permits and Approvals ..................................................................... 29

7.2.8 Community Concerns ...................................................................... 30

8. PREFERRED REMEDIAL ACTION ALTERNATIVE ...................................... 31

9. NEED FOR ADDITIONAL DATA OR TREATABILITY STUDIES ................ 32

10. SCHEDULE AND DELIVERABLES .................................................................. 33

11. REFERENCES ...................................................................................................... 34

LIST OF TABLES

Table 2-1 List of Process Ponds – SOEP/STEP Site

Table 2-2 List of Geologic Units – SOEP/STEP Site

Table 2-3 Preliminary List of Constituents of Interest and Indicator Parameters – SOEP/STEP SiteTE

Table 5-1 Preliminary Screening of Remediation Technologies – SOEP/STEP

Table 6-1 Preliminary Assembly of Remedial Action Alternatives – SOEP/STEP

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LIST OF FIGURES

Figure 1-1 Administrative Order on Consent Site Areas

Figure 2-1 SOEP/STEP Process Ponds and Capture Wells

Figure 2-3 Hydrologic Regime East Fork Armells Creek

Figure 3-1 SOEP/STEP Site Conceptual Site Model

LIST OF APPENDICES

Appendix A SOEP/STEP Site Geologic Cross Sections

Appendix B SOEP/STEP Figures Showing Interpreted Extent of Indicator Parameters

Appendix C SOEP/STEP Figures Showing Uncaptured Areas

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ABBREVIATIONS AND ACRONYMS

amsl above mean sea level AOC administrative order on consent BSL baseline screening level CAA corrective action alternative CAO corrective action objective CCR coal combustion residuals CCRA cleanup criteria and risk assessment COI constituent of interest CQA construction quality assurance Creek East Fork Armells Creek CSES Colstrip Steam Electric Station CSM conceptual site model DEQ-7 Montana numerical water quality standards, Circular 7 EHP evaporation holding pond gpm gallons per minute HDPE high-density polyethylene MCL maximum contaminant level MDEQ Montana Department of Environmental Quality MW megawatt POC point of compliance Talen Talen Montana, LLC RAA remedial action alternative RAO remedial action objective RFP/RPP reinforced polypropylene SC specific conductance SCEM site conceptual exposure model SOEP Stage I Evaporation Pond STEP Stage II Evaporation Pond TDS total dissolved solids USEPA U.S. Environmental Protection Agency VSEP vibratory shear-enhanced process

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1. INTRODUCTION

1.1 Purpose

This Remedy Evaluation Work Plan (work plan) has been prepared by Geosyntec Consultants, Inc. (Geosyntec) on behalf of Talen Montana, LLC (Talen), the successor of PPL Montana, LLC, pursuant to Article VI.C of the Administrative Order on Consent (AOC) Regarding Impacts Related to Wastewater Facilities Comprising the Closed-Loop System at the Colstrip Steam Electric Station (CSES) located in Colstrip, Montana (MDEQ, 2012). It addresses the constituents of interest (COIs) in groundwater at the Units 1 & 2 Stage I and II Evaporation Ponds Site (a.k.a., the SOEP/STEP). A map of the Colstrip area, including the SOEP/STEP site, is shown on Figure 1-1.

The AOC for the CSES comprises three areas: i) the Plant Site; ii) the SOEP/STEP Site; and iii) Units 3 & 4 Effluent Holding Pond Site (Units 3 & 4 EHP). A fourth area identified in the AOC, process water pipeline spills, is addressed as part of each of these three areas. The Plant Site and the SOEP/STEP Site are largely located within the East Fork Armells Creek drainage basin. The Units 3 & 4 EHP Site is located in the Cow Creek drainage basin. The Plant Site and Units 3 & 4 EHP Site are addressed in separate work plans.

Several remedial actions have already been completed and/or are ongoing at the SOEP/STEP Site. Those include, but have not been limited to: i) the upgrade or closure of certain process ponds (also called cells and clearwells) (Geosyntec, 2015); ii) the installation and operation of a groundwater capture system; and iii) cleanup response actions for pipeline spills. A significant component of the remedy evaluation will include an evaluation of the performance of the current process cell liner systems and the current groundwater capture system, with a focus on their adequacy and the need for enhancements and/or replacement by other technologies.

A multiple lines of evidence approach, including an evaluation of mass flux and fate and transport modeling, will be used in the remedy evaluation for the SOEP/STEP site. The evaluation techniques will be presented in the remedy evaluation report for the SOEP/STEP Site along with performance metrics and an explanation of their intended use.

1.2 Work Plan Organization

This work plan describes the approach for identifying and evaluating potential remedial actions for releases from the process water/scrubber systems at the SOEP/STEP Site. Background information is provided in Section 2 and the Conceptual Site Model (CSM) is presented in Section 3. The Remedial Action Objectives (RAOs) are defined in Section 4. The rationale that will be used to identify and screen remedial action technologies for the SOEP/STEP Site is described in Section 5 and a preliminary screening (subject to revision during implementation of this work plan) is included. Section 6 describes how those remedial action technologies that are retained will be assembled into Remedial Action Alternatives (RAA) to achieve the RAOs. A preliminary assembly of alternatives (subject to revision during implementation of this work plan) is provided. Section 7 describes the planned approach for detailed evaluation of the


assembled remedial alternatives. Section 8 describes how the preferred remedial action alternative will be identified. Section 9 describes the planned approach for identifying additional data gaps (if any) including the potential need for treatability studies. The schedule for completion of the work and contents of the Remedy Evaluation Report are discussed in Section 10. References are listed in Section 11. Supporting information is provided in tables, figures, and appendices that follow the text.


2. BACKGROUND

2.1 Facility Description

2.1.1 Construction/Operations

The CSES consists of four operating coal-fired electric generating units. Units 1 and 2 are 333 megawatts (MW) each and have been operating since 1975. Units 3 and 4 are 805 MW each and have been operating since 1983 and 1986, respectively. The CSES is co-owned by Talen; PacifiCorp; Puget Sound Energy, Inc.; Portland General Electric Company; Avista Corporation; and NorthWestern Corporation. Talen is also the operator of the CSES (Hydrometrics, 2016).

The CSES uses a closed-loop process water/scrubber system. Castle Rock Lake serves as a reservoir for the City of Colstrip and CSES. Water is piped from the Yellowstone River to Castle Rock Lake (a.k.a., the Surge Pond) via a 29-mile long pipeline. From Castle Rock Lake, water is piped to holding tanks at the Plant Site for use in the boilers, cooling towers, and scrubber systems for Units 1 through 4. Slurries of fly ash are transported to disposal ponds at SOEP/STEP and the 3&4 EHP areas. Bottom ash is slurried to holding ponds on the Plant Site, dewatered, and then transported to the 3&4 EHP area for disposal. Previously, the SOEP/STEP ponds were used for fly ash settling/decanting and evaporation of excess water. However, a paste plant was constructed in 2008 that dewaters scrubber slurry prior to fly ash (paste) disposal in the ponds. Clear water has always been recirculated and is re-used at the Plant Site for various purposes including scrubbing (Hydrometrics, 2016).

There are three general areas where process water is stored in ponds at the CSES:

1. The Plant Site contains Units 1 through 4 and several associated process ponds;

2. The SOEP/STEP Site is used for disposal of scrubber slurry (fly ash) from Units 1 and 2 and is located approximately 2 miles northwest of the Plant Site; and

3. The Units 3 & 4 EHP Site is used for disposal of scrubber slurry (fly ash) from Units 3 and 4 and bottom ash from all four generating units and is located approximately 2.5 miles southeast of the Plant Site.

The Plant Site and Units 3&4 EHP are addressed in separate work plans. The SOEP and STEP were constructed behind earthen dams built across a tributary drainage to East Fork Armells Creek. Fly ash scrubber slurry from Units 1 and 2 is transported via a pipeline to the SOEP/STEP. Clear water from the process ponds is then piped back to the Plant Site for re-use. The pond system at the SOEP/STEP Site has been operating since 1975 (Hydrometrics, 2016). A paste plant was constructed on the south side of STEP in 2008 to remove water from the fly ash scrubber slurry and began operating in 2009. The historic and current process ponds at the SOEP/STEP Site, including the current and future status of each pond, are summarized in Table 2-1. The locations of the historic and current process ponds at the SOEP/STEP Site are shown on Figure 2-1.


The SOEP received fly ash slurry from the Plant Site Units 1 & 2 A/B Pond between 1975 and 1997. The SOEP was constructed with the approximately 70-foot high Stage I Dam and a partial liner consisting of natural clay. With the partial clay liner, the seepage rate was estimated by the Bechtel design to be between 85 and 115 gallons per minute (gpm) assuming a full pond, saturated conditions, limited fractures in bedrock, and steady state conditions. The Stage I Main Dam was constructed with chimney drains, a blanket drain, and a toe drain. Water from those drains was routed to a sump that returned seepage water to the SOEP (Hydrometrics, 2016). The SOEP was full in 1997 and completely reclaimed in 2002. An engineered evapotranspiration cap has been constructed over the SOEP to reduce infiltration and leachate generation. Groundwater capture systems, which are generally located north and south of SOEP, are used to intercept seepage to groundwater from the SOEP area.

The STEP was constructed in 1992 directly down the drainage (east) from the SOEP (Figure

2-1). The STEP was constructed via the Stage II Main Dam that is approximately 88 feet high. The Stage II Main Dam was constructed with a central core grout curtain that extends horizontally along the entire length of the dam, and vertically through the alluvium where it is keyed into the underlying siltstone. The Stage II Main Dam was also constructed with chimney drains and toe drains that discharge to a sump that conveys water to Cell E (Hydrometrics, 2016).

The STEP currently consists of five cells (A, B [New Clearwell], D, E, and Old Clearwell). The Stage II Main Dam forms the east sides of Cell E, the Old Clearwell, and Cell D. The STEP cells are lined with either high-density polyethylene (HDPE) or reinforced polypropylene (RFP or RPP). The cumulative seepage from the STEP cells is estimated at approximately 21.5 gpm (Hydrometrics, 2016). The current use of the STEP cells is as follows:

1. Cell A, commissioned in 1992, is full and no longer receives fly ash scrubber slurry/paste, captured groundwater, or process water;

2. Cell E, commissioned in 2003, is currently the only cell receiving paste from the paste plant. Cell E also receives water from the STEP Main Dam Sump;

3. Cell B was constructed in 2006 to provide additional volume to store process water. In 2011, Cell B was equipped with pumps and converted to the New Clearwell. The New Clearwell receives clear water from the paste plant and returns it to the Plant Site for re-use in the scrubbers. The New Clearwell also receives captured groundwater. The New Clearwell was constructed with an underdrain collection system – no water was not detected in the underdrain in 2015;

4. The Old Clearwell, commissioned in 1992, is now used to store clear water from Cell E, but has also received some flyash in the past. The Old Clearwell is planned to be filled with paste; and

5. Cell D was constructed in 2011 to provide additional volume to store clear water and paste as needed. Cell D was constructed with an underdrain collection system – no water was detected in the underdrain in 2015.


In addition to the five existing cells listed above, a future Cell (Cell C) is planned for construction north of Cell E, when needed.

2.1.2 Prior Releases and Response Actions

The closed-loop process water/scrubber system was designed to minimize impacts to water resources; however, process pond seepage, pipeline spills and incidental tears in pond liners have resulted in releases of process water to groundwater and surface water in various portions of the CSES. Summaries of past releases and investigations can be found in the Units 1 & 2 SOEP/STEP Evaporation Ponds Site Report (Hydrometrics, 2016).

Talen has implemented several completed and ongoing remedial actions at the SOEP/STEP Site in response to changes in groundwater quality and/or in response to past spills or releases. Those remedial actions have included, but are not limited to: i) operational (groundwater and surface water) monitoring; ii) groundwater capture and re-use; iii) operational changes (pond upgrade/closure and paste production); and iv) additional best management practices (BMPs) for process water management. Captured groundwater at the SOEP/STEP site is pumped to the STEP. The paste plant removes approximately 90% of the free available water in the fly ash scrubber slurry (Geosyntec, 2015). Captured groundwater and clear water from the paste plant are piped to the Plant Site, treated using a vibratory shear-enhanced process (VSEP), and then re-used at the Plant Site (Hydrometrics, 2016).

Process water in the ponds has some of the chemical characteristics of the ash with which it has had contact. To track those chemical characteristics, grab samples for water quality testing are collected near the surface of active process ponds at a minimum frequency of once every three years. Water seeping from the ponds migrates downward and may eventually reach and mix with groundwater if not contained by the pond liners, underdrains, and main dam seepage control structures (chimney drains, toe drains, valley drain, main dam sump). Groundwater beneath the ponds has some of the chemical characteristics of the local soil, coal, and rock through which it has flowed. Some COIs in process water also occur naturally in local strata at, or above, background concentrations. Several indicator parameters have been used at the SOEP/STEP Site to differentiate between groundwater that has been impacted by process water and the native groundwater quality. Indicator parameters include specific conductance (SC), dissolved boron, chloride, sulfate, and the ratio of calcium to magnesium. Those indicator parameters, together with the respective background concentrations (Background Screening Levels [BSLs], Neptune, 2016), are useful in evaluating the potential presence of process water COIs in groundwater and have been accepted as an evaluation method for water quality evaluation by Talen and the Montana Department of Environmental Quality (MDEQ) (Hydrometrics, 2016).

Routine groundwater quality monitoring is conducted in and around the SOEP/STEP Site via a network of monitoring wells and capture wells. The MDEQ-approved monitoring program is outlined in the Water Resources Monitoring Plan (Talen, 2015). Biannual groundwater monitoring is conducted to document seasonal variability. For example, in 2015, 214 wells


were sampled during the first half of the year and 168 wells were sampled during the second half of the year.

Synoptic surface water monitoring is conducted at multiple stations along East Fork Armells Creek. Synoptic surface water/groundwater monitoring was conducted in March and October 2015. Surface water and streambed soil/sediment was sampled at twelve locations, and groundwater elevations were measured at nineteen locations immediately adjacent to East Fork Armells Creek.

Where indicator parameters in groundwater have been interpreted to indicate the presence of process water COIs at the SOEP/STEP Site, Talen operates and maintains a groundwater capture system consisting of sixty wells. The majority of groundwater capture wells are located on the east side of the STEP Main Dam (see Figure 2-1).

Groundwater capture wells are routinely monitored for flow, hours pumps operated, water level, and SC. Monitoring is conducted to: i) measure water levels and flow rates; ii) make adjustments to drawdown; iii) estimate capture volumes; and iii) to identify problems. One problem is scale build-up inside some of the conveyance piping, which increases water pressure and renders flow meters inoperable. Flow rates are measured through sample ports at the wellheads under atmospheric pressure, and are higher than the actual flow within the pipelines which operate under greater than atmospheric pressure. To compensate, the flow rates measured at the wellheads are reduced by 25 percent beginning with the SOEP/STEP Site Report (Hydrometrics, 2016). Modeling of future groundwater capture systems as part of the remedy evaluation will continue to include the adjustment of 0.75 times the measured flow rate and will also include sensitivity analyses of flow rate measurements. The 0.75 adjustment potentially results in a conservative assessment (i.e., potentially under-predicts actual capture). In 2015, the average total adjusted flow rate for the groundwater capture system at the SOEP/STEP site was 183 gpm, and an additional 4 gpm was collected from the STEP Main Dam Sump (Hydrometrics, 2016).

Additional mitigation measures that have been completed in the SOEP/STEP site (Hydrometrics, 2016) include the following:

Installation of an engineered vegetated cap during reclamation of the SOEP in 2002 to limit infiltration and provide evapotranspiration of precipitation;

Installation (in 2006) of RPP double-lined cells for the New Clearwell (Cell B) and Cell D with underdrain collection systems between the primary and secondary liners, and beneath the secondary liners. Cell A, Cell E and the Old Clearwell were single-lined with HDPE in 1992;

Utilization of paste technology (beginning in 2009), forced evaporation and water management practices (re-use) to reduce the volume of water (and hydraulic head) in the ponds;


Supplying municipal water to residences and businesses east of the STEP Main Dam, and the planned sealing of private wells without well logs or with questionable annular seals (PW-704 was sealed and replaced in 2012); and

Installation of new scrubber slurry and clearwater return pipelines to replace the original pipeline.

The adequacy of these systems will be investigated as part of the Remedy Evaluation.

2.2 Physical Setting

The SOEP/STEP Site is located north of the City of Colstrip, which lies within Rosebud County in south central Montana. The location of SOEP/STEP Site is depicted on Figure 1-1. The nearest surface water features are East Fork Armells Creek to the east and Castle Rock Lake to the south. Other nearby surface water features include treated sewage lagoons and a golf course storage pond using water from the Colstrip Sewage Treatment Plant. Farther north is Stocker Creek, which flows intermittently to the east and into East Fork Armells Creek. The average annual precipitation in the Colstrip area over the last 40 years, which roughly coincides with the operating life of the SOEP/STEP Site, is approximately 15.2 inches (NewFields, 2016).

Regionally, East Fork Armells Creek is an intermittent stream, but it generally flows continuously through the City of Colstrip and along the eastern edge of the SOEP/STEP Site. Overall, East Fork Armells Creek is a gaining (increasing in flow) stream along this reach, but it does have localized losing (decreasing in flow) reaches (Hydrometrics, 2016). Figure 2-3

shows the reaches that are gaining and losing and hydrographs for each monitoring station. The gaining and losing reaches were determined based on the period-of-record harmonic mean discharge values at monitoring stations. The stream discharge measurements shown in the hydrographs on Figure 2-3 were collected during spring synoptic runs.

The CSES is a zero discharge facility, which means there is no direct process water discharge from the process ponds to surface water. The ground surface topography slopes downward from the SOEP/STEP Site toward East Fork Armells Creek. Groundwater from most of the SOEP/STEP Site flows in the direction of East Fork Armells Creek, although capture wells at the SOEP/STEP Site limit the migration of groundwater (Hydrometrics, 2016). Surface water in East Fork Armells Creek varies with respect to depth and flow rate throughout the year. East Fork Armells Creek adjacent to the SOEP/STEP Site is generally shallow and slow moving (Ford Canty & Associates, 2015).

Castle Rock Lake is located south of the SOEP/STEP Site and west of East Fork Armells Creek. Castle Rock Lake was constructed via a main dam across a tributary drainage to East Fork Armells Creek. Castle Rock Lake was constructed with natural clay soils. Water for Castle Rock Lake is supplied by the Yellowstone River via a 29-mile long pipeline originating at the Nicols pump station west of Forsyth, Montana. Water in Castle Rock Lake is either routed to the City of Colstrip Water Treatment Plant and distributed to residents and businesses, or piped to the Plant Site as “raw” water. Water levels in Castle Rock Lake are regulated between an


elevation of about 3,280 feet above mean sea level (amsl) in the summer and 3,284 feet amsl in the winter months. The quality of the water is generally good and reflects that of the Yellowstone River (Hydrometrics, 2016).

2.3 Geology

The uppermost bedrock formation in the SOEP/STEP Site area is the Tongue River Member of the Fort Union Formation. Overall, the Fort Union Formation in the Colstrip area is approximately 650 feet thick. The Tongue River Member, which is exposed at the ground surface in some areas of Colstrip, has a maximum thickness of about 350 feet. The Tongue River Member is comprised of claystone, siltstone, fine-grained sandstone, and coal. The two main near-surface coal seams in the Colstrip area are the Rosebud Coal and McKay Coal; a third seam called the Robinson Coal is found at depth. Only the Rosebud Coal is mined in the Colstrip area because the McKay Coal may cause boiler scaling (NewFields, 2016). Strata beneath the McKay Coal are referred to as the Sub-McKay. Clinker is present in the SOEP/STEP Site area. Clinker is thermally altered and collapsed overburden rock that forms when a coal seam burns in situ (Hydrometrics, 2016).

Where the bedrock is not exposed, it is overlain by unconsolidated deposits and anthropogenic materials (i.e., fill and mining spoils). The unconsolidated deposits include fine-grained and coarse-grained alluvium and associated colluvium along and beneath East Fork Armells Creek, fill, and mine spoils (Hydrometrics, 2016).

The thicknesses and extents of the geologic units vary across the SOEP/STEP Site area, andindividual depositional units may be discontinuous (Hydrometrics, 2016). Descriptions of the main geologic units in the SOEP/STEP Site area are provided in Table 2-2.

2.4 Hydrogeology

2.4.1 Hydrostratigraphic Units

The primary water-bearing units at the SOEP/STEP Site are: (i) alluvium deposits associated with East Fork Armells Creek and its tributary drainage bottoms beneath the SOEP/STEP Site ponds and Castle Rock Lake; (ii) McKay Coal; and (iii) Sub-McKay siltstones and sandstones (NewFields, 2016). Of these units, the most permeable layers are basal (alluvial) gravels along East Fork Armells Creek, cleated McKay Coal, and fractured Sub-McKay sandstones facies. Groundwater also flows through fractures in the interburden (where saturated) and the Sub-McKay siltstones, claystones, and shales. Due to erosion, the Rosebud coal/clinker and interburden are absent in most areas of the SOEP/STEP Site except for a few locations where the Rosebud coal/clinker forms a cap rock on hilltops and ridges (NewFields, 2016). Where present, clinker has a higher hydraulic conductivity compared to the other bedrock units. However, groundwater is rarely found in clinker due to its occurrence as a cap rock and its ability to transmit groundwater to underlying units. Groundwater is also present in the spoils west (upgradient) of the SOEP/STEP site (Hydrometrics, 2016).


Hydrogeologic cross sections of the SOEP/STEP are shown in Appendix A (NewFields, 2016). Note that hydrostratigrahic units at the SOEP/STEP Site are variable, and cross sections along other transects may show significant differences.

2.4.2 Groundwater Flow

Groundwater flow patterns within the hydrostratigraphic units at the SOEP/STEP Site are a function of: (i) geology; (ii) areas of recharge and discharge; (iii) ground surface topography; (iv) process ponds and dams; and (v) groundwater capture. Recharge to the groundwater system is through infiltration of precipitation, seepage from process ponds and Castle Rock Lake, and residential lawn and garden watering. The recharge rate is a function of ground cover, geology, process pond liner systems, and Castle Rock Lake bottom sediments. The background recharge is estimated to be 0.22 to 0.44 inches/year, which is approximately 1.5 percent of average annual precipitation (NewFields, 2016).

Hydraulic conductivities of hydrostratigraphic units measured at the SOEP/STEP Site are highly variable. Coarse (basal) alluvium and localized areas of coarse fragmented spoils (typically near the base of old mine cuts) have the highest hydraulic conductivity while fine alluvium, spoils and bedrock units typically have lower values. The following table (NewFields, 2016, Table 2-1) summarizes hydraulic properties and statistics for select hydrostratigraphic units that been tested. These properties and statistics are based on hydraulic tests conducted at over 90 wells located at the Plant Site, the SOEP/STEP Site, and the City of Colstrip

Summary of Aquifer Properties by Hydrostratigraphic Unit

Hydro Stratigraphic

Unit

Geometric Mean

Transmissivity (feet2/day)

Geometric Mean

Hydraulic Conductivity

(feet/day)

Minimum Hydraulic

Conductivity (feet/day)

Maximum Hydraulic

Conductivity (feet/day)

Geometric Mean

Saturated Thickness

(feet)

Geometric Mean

Storativity

Alluvium 225 18.3 0.15 355 12 0.0003

Rosebud Coal 149 12.5 0.9 65 12 N/A

Interburden 13 1.1 0.02 39 13 N/A

McKay Coal 26 2.3 0.06 9.3 10 N/A

Sub-McKay 41.5 2.5 0.03 242 14.1 0.0008

Note: N/A – not applicable

Shallow groundwater first occurs in spoils to the west (upgradient) of the SOEP/STEP Site. Where the Rosebud Coal is missing (burned) southeast of the SOEP and immediately west of the Castle Rock Lake, first groundwater is either found in the Rosebud/McKay interburden or McKay Coal. Beneath the SOEP/STEP Site ponds, shallow groundwater is found in alluvium.


Shallow bedrock and alluvial groundwater elevations are highest west of Castle Rock Lake and southeast of the SOEP. Groundwater elevations west of Castle Rock Lake indicate flow towards the SOEP (Hydrometrics, 2016).

Alluvial groundwater flows eastward under the SOEP and STEP, but is interrupted by the Stage I and Stage II Dams (Figure 2-1). Alluvial groundwater directly east of the Stage II Dam flows eastward until it reaches East Fork Armells Creek alluvium (unless interrupted by the capture system). Groundwater in the East Fork Armells Creek alluvium then flows northward (Hydrometrics, 2016).

Downward vertical gradients are observed at the SOEP/STEP Site. Reductions in hydraulic head caused by capture system pumping may potentially reduce the downward gradients (Hydrometrics, 2016).

Groundwater in the Sub-McKay generally flows to the northeast under a regional gradient from the Bighorn Mountains approximately 75 miles to the south toward the Yellowstone River located about 30 miles to the north. The Yellowstone River and lower reach of Rosebud Creek are thought to be the regional discharge points for this deep regional flow system. Groundwater flow from the upper sub-McKay bedrock flows to the alluvium where it sub-crops in East Fork Armells Creek alluvium.

2.4.3 Numerical Groundwater Flow Model

A calibrated numerical groundwater model was prepared for the SOEP/STEP Site (NewFields, 2016) to simulate groundwater flow and advective transport under a variety of hydrologic conditions. The model is able to distinguish flow within and between spoils, fine and coarse alluvium, overburden, Rosebud Coal, interburden, McKay Coal, and shallow/deep Sub-McKay (NewFields, 2016).

The calibrated model may be used as an interpretive and predictive tool for evaluating: (i) groundwater flow; (ii) capture system effectiveness; and (iii) effects of contemplated remedial alternatives. The model can be used to predict the effects of modifying current pumping rates, turning capture wells on or off, or adding capture wells, to modify capture. The model can also be used to predict the effects of planned pond upgrades or closures as source control measures, and to evaluate fate and transport. As these types of model runs are predictive in nature, no changes will be made to the model calibration.

Inherent in any model is a degree of uncertainty. Results of capture zone analyses, together with groundwater monitoring data, provide an understanding of the efficacy of the capture system at the present time. However, capture analyses do not take into account operational down time of capture wells for maintenance/repairs, or variability in flow from unusual seasonal climatic patterns. In addition, the flow rates from the groundwater capture wells have some uncertainty as do the seepage rates from the process pond liner systems (NewFields, 2016). The model simulates groundwater flow as a porous media whereas it is actually a combination of porous media flow and fracture flow for the bedrock units.


Groundwater capture wells have been installed in areas of the SOEP/STEP Site with identified process water impacts. A majority of the groundwater capture wells are located on the east side of the Stage II Dam. Overall, groundwater quality in remedial areas of the SOEP and STEP has shown an improvement (Hydrometrics, 2016). Other areas where the calibrated flow model indicates impacted groundwater may be leaving the SOEP/STEP Site or where groundwater capture may not be effective (NewFields, 2016, Section 8.0) will be considered in the remedy evaluation. Figures from the calibrated flow model showing uncaptured areas (NewFields, 2016) are included in Appendix C.

2.5 Process Water Indicator Parameters

Article IV.B of the AOC states that the COIs include, but are not limited to, sulfate, boron, selenium, potassium, sodium, magnesium, total dissolved solids (TDS), and salinity as measured by specific conductance (SC) (Table 2-3). TDS and SC are essentially two measures of the same underlying property. Of these, several indicator parameters are used along with the BSLs to evaluate the potential presence of process water COIs in groundwater at the SOEP/STEP Site (Hydrometrics, 2016). These include TDS, SC, dissolved boron, chloride, sulfate, and the ratio of calcium to magnesium. Groundwater affected by process water contains TDS, sulfate, chloride and boron concentrations that are elevated relative to background or unaffected reference areas concentrations. In most cases, the water also exhibits low ratios of calcium to magnesium. Chloride is given less weight compared to the other indicator parameters because the chloride BSL appears to be erroneously low.


3. CONCEPTUAL SITE MODEL

This section describes the current Conceptual Site Model (CSM) that provides the foundation for the remedy evaluation at the SOEP/STEP Site. The initial hydrogeologic CSM was developed by Maxim (2004) in preparation for the development of a numerical groundwater flow model. The hydrogeologic CSM for the SOEP/STEP Site was updated by Maxim (2005), Geomatrix (2007) and Newfields (2016).

For purposes of the remedy evaluation, the CSM includes the following components:

Source Area(s) and COIs; Release Mechanisms; Transport Pathways; Environmental Fate along Pathways; Potential Exposure Point(s); and Potential Receptor and Associated Potential Health Risk.

The CSM for the SOEP/STEP Site is described below and summarized on Figure 3-1.

3.1 Source Area(s) and Constituents of Interest

The main source areas are the process water ponds at the SOEP/STEP Site and associated pipelines. As described on Table 2-1, the SOEP has been reclaimed, and Cell A (STEP) is full and no longer receives flyash scrubber slurry. In some cases, there might be salts in the unsaturated zone soils (e.g., below the pond liners or where there have previously been flyash pipeline failures) that act as secondary source areas. Historical releases outside the footprint of the ponds, mainly due to accidental releases from the process water pipelines at the SOEP/STEP Site, are documented in in “Table 3-1, Summary of Releases of Actions Taken, Units 1 & 2 SOEP/STEP Evaporation Ponds” (Hydrometrics, 2016).

Implementation of the Master Plan (Geosyntec, 2015) in response to the Federal CCR Rule is occurring in a phased approach over several years. As a comparative metric for possible source control measures for the remedy evaluation, Geosyntec will use mass flux estimates before and after planned cell modifications to predict groundwater quality improvements arising from planned cell upgrades or closures, or changes in process water chemical profiles.

The baseline mass flux for the COIs from each pond or cell to the groundwater will be calculated by multiplying the concentrations of COIs in the process ponds (Table 2-3) by the seepage estimates for the ponds (Tables 2-6A through 2-6B) presented in the SOEP/STEP Site Report (Hydrometrics, March 2016). If a pond is currently planned to be upgraded or closed, a new seepage rate will be estimated. For a pond that has a new chemical profile or a new seepage rate, those data will be used to calculate the new (future) mass flux for that pond to groundwater. The new mass flux will be compared to the baseline mass flux for each pond to evaluate if there is a change (i.e. a decrease) in mass flux to groundwater. The new mass flux at a given pond will be evaluated in terms of its effectiveness as a source control measure in the remedy


evaluation. Fate and transport modeling might be used, potentially along with other tools and lines of evidence, to evaluate whether additional source control measures, beyond those envisioned in the Master Plan, are necessary to achieve Cleanup Criteria.

If some of the process ponds or cells contribute the greatest load (i.e., mass flux) of indicator parameters to the shallow groundwater system, after accounting for uncertainty in the estimated mass flux, then they should receive a greater degree of source control compared to other ponds. Those COIs that are present at higher concentrations in groundwater and highly mobile in water should serve as good indicator parameters for evaluating mass flux. Limitations or assumptions that are used in the evaluation of mass flux will be documented in the Remedy Evaluation Report. The load for an individual cell depends upon both the seepage rate of the contained process water and the concentration of its dissolved solutes. For example, the dissolved solute flux to groundwater from a cell with a low seepage rate but very high solute concentrations might be more significant than that from a storm water detention pond with a higher seepage rate but much lower solute concentrations. The effect of implementation of the Master Plan (Geosyntec, 2015) on the cells and preferred remedy will be described in the Remedy Evaluation Report.

Since mass flux will be used to evaluate the source control effects of cell upgrades or closures, or changes in chemical profiles, it will also prove useful to evaluate the source control effects in the groundwater flow regime. The calibrated groundwater flow model and the COI concentrations in groundwater will be used to calculate the mass flux of COIs through a vertical plane downgradient from the ponds in the groundwater flow regime. The background mass flux will then be calculated using the MDEQ-approved 2015 BSLs. The difference will be the reduction that is needed to attain the MDEQ-approved 2015 BSLs. The reduction in mass flux needed in groundwater will then be compared to the reduction in mass loading achieved from the planned cell upgrades/closures. The difference between the reduction in mass flux needed in groundwater and the reduction in mass loading achieved from the cells will then be used to predict the locations where compliance with Cleanup Criteria may be achieved, and the locations where additional remediation may be needed. As part of a multiple lines of evidence approach, changes in center of mass over time, or other techniques, may also prove useful to visualize source control effects, groundwater capture, or natural attenuation in certain areas of the SOEP/STEP Site.

The COIs are defined in Article IV.F. of the AOC as “…those parameters found in soil, groundwater or surface water that (1) result from Site operations and the wastewater facilities and (2) exceed background or unaffected reference area concentrations.” Article IV.B of the AOC states that COIs (‘regulated substances’) include, but are not limited to, sulfate, boron, selenium, potassium, sodium, magnesium, TDS, and salinity as measured by SC (Table 2-3). It should be noted that COIs will also be addressed in the risk assessment currently being prepared by Ford Canty & Associates for the SOEP/STEP Site. It is anticipated that the risk assessment will narrow the COIs to a list of constituents of potential concern (COPCs) strictly from a risk assessment perspective. The use of the term COIs in this work plan should not be confused with the term COPCs that will be used in the risk assessment.


3.2 Release Mechanism

For the process ponds, the primary release mechanism is potential seepage through the engineered pond liner system and the dam sumps by solutes that are already in a dissolved state in the process water contained in the pond. This is the main release mechanism to be addressed by source control technologies in the remedial alternatives. It is possible that there are salts of these solutes in the soils beneath the ponds (Hydrometrics, 2016). In such cases, dissolution of those salts would be an additional release mechanism. Potential release by volatilization or entrainment of process water droplets into ambient air from the pond water surface is considered to be insignificant and is therefore not included as a release mechanism to be addressed by the remedy.

For the accidental releases from the process water pipelines, loss of containment is the relevant release mechanism. This release mechanism has been, and continues to be, addressed by system repairs, operator training, and operation and maintenance procedures.

3.3 Transport Pathways

For releases from the ponds, migration by groundwater advection is the main transport pathway. Several indicator parameters have been established for the SOEP/STEP Site. They include SC, boron, sulfate, chloride, and calcium-to-magnesium ratio. The indicator parameters are highly mobile in groundwater and have a higher concentration in impounded process water versus background (i.e., BSLs) and therefore are more useful than relatively immobile constituents for evaluating the extent of process water migration. The figures provided in Appendix B are from the Site Report and show the interpreted extent of indicator parameters above the MDEQ-approved BSLs for the SOEP/STEP Site (Neptune, 2015). For the SOEP/STEP Site, groundwater transport pathways are mostly toward East Fork Armells Creek in the shallow groundwater system and to the northeast in the deeper (Sub-McKay) groundwater system. The existing groundwater monitoring data indicate that the COIs have migrated farther (laterally) from the ponds in the shallow groundwater zones compared to the deeper zones. These transport pathways are mitigated by the existing groundwater capture systems. This remedy evaluation will address whether the existing capture systems adequately contain affected groundwater.

Transport pathways for accidental releases from the pipelines are via overland flow (controlled by ground surface topography) toward East Fork Armells Creek, and via infiltration to groundwater. If there are COIs remaining in the affected soils following emergency response actions, then leaching by infiltration of precipitation is another possible transport pathway. This is also potentially applicable to salts in soils of the unsaturated zone beneath the ponds (if any).

For both the ponds and accidental pipeline releases, if COIs enter East Fork Armells Creek, they are transported by surface water in the downstream direction (to the north). If COIs enter the alluvium associated with East Fork Armells Creek but do not reach the creek, they are transported downgradient (to the north) by groundwater advection.


3.4 Environmental Fate

Some of the COIs are highly mobile in groundwater and surface water and hence undergo little attenuation other than dilution and dispersion. Some adsorption to aquifer solids is possible for some constituents. It is possible that some mineral precipitation occurs (e.g. gypsum) when the magnesium sulfate-rich pond water mixes with more calcium-rich groundwater. Gypsum precipitation might also be facilitated by calcium-magnesium ion exchange reactions with clays in the aquifer solids. No volatilization, decay, or biodegradation of these constituents is expected. Detailed discussion on the environmental fate of COIs will be provided in the Remedy Evaluation Report, including: (i) the results of fate and transport modeling to be performed using the calibrated groundwater flow model; and (ii) an evaluation of potential natural attenuation processes including adsorption, chemical reactions and biodegradation.

3.5 Potential Exposure Point(s)

Potential exposure points include groundwater wells and East Fork Armells Creek. Talen has decommissioned a number of private water supply wells and provided alternative water supplies to those residents.

The reach of the East Fork Armells Creek downstream of the SOEP/STEP Site may have received loading of COIs via groundwater seepage in the past. The groundwater capture system and source control upgrades are intended to mitigate future migration to these potential exposure points. The Remedy Evaluation will consider whether the existing system adequately captures current seepage from the ponds, thus mitigating these potential exposure points.

3.6 Potential Receptors/Health Risks

A Site Conceptual Exposure Model (SCEM) will be prepared as the first step in the Exposure Assessment conducted as part of the Cleanup Criteria and Risk Assessment (CCRA) by Ford Canty & Associates, and could list the following as potential receptors: residents, outdoor workers, trespassers, trench workers, and terrestrial and aquatic ecological receptors. The identification of chemicals of potential concern and consequent evaluation of health and ecological risks will be completed during the CCRA following guidance from the U.S. Environmental Protection Agency (USEPA), AOC and MDEQ.


4. REMEDIAL ACTION OBJECTIVES

This section presents the Remedial Action Objectives (RAOs) that will be used to screen remedial action technologies and evaluate remedial action alternatives for the SOEP/STEP Site.

4.1 Objectives and Implementation of the AOC

The AOC embraced a risk-based remediation approach for the process water systems at the CSES. The AOC states “…the Department and Talen have concluded that a comprehensive, risk-based approach incorporating all tools and requirements applicable under Montana’s generally applicable environmental laws, including adaptive management practices available thereunder, is needed to address groundwater contamination and seepage.”

The AOC includes requirements for a Cleanup Criteria and Risk Assessment Report (CCRA) to be completed prior to, or in parallel with, the Remedy Evaluation Report. The CCRA will establish risk-based cleanup criteria for the remedy under the AOC and will address COIs that include “parameters found in soil, ground water or surface water that (1) result from Site operations and the process water facilities and (2) exceed background or unaffected reference areas concentrations.”

The AOC indicates that the applicable cleanup criteria for COIs in groundwater and surface water (except for the evaluation of ecological receptors) are the most current version of Circular DEQ-7 (Montana Numeric Water Quality Standards), the USEPA MCLs, or the risk-based screening levels contained in the most current version of Montana Risk-Based Guidance for Petroleum Releases, whichever is more stringent. For constituents where there is not a value given in the above sources, the cleanup standard is the tap water screening level contained in the most current version of the USEPA Regional Screening Levels for Chemical Constituents at Superfund Sites. If the above criteria are not adequate to protect ecological receptors, then a cleanup value that results in an acceptable ecological risk, determined using the most current versions of standard USEPA ecological risk assessment guidance, is the cleanup standard. The Cleanup Criteria may not be more stringent than the background or unaffected reference area concentrations.

For COIs in soil, the cleanup value is the most stringent of a cumulative human health risk of 1E-05 for carcinogens or a cumulative hazard index of one for non-carcinogenic COIs. If the above value is not protective of potential ecological receptors, then the cleanup level is a value that results in acceptable ecological risk as determined using the most current version of standard USEPA ecological risk assessment guidance, or the risk-based screening level contained in the most current version of Montana’s Risk-Based Guidance for Petroleum Releases. The Cleanup Criteria may not be more stringent than the background or unaffected reference area concentrations.


While the AOC does not specify the point of compliance (POC) for groundwater remediation, Paragraph 1.I of the AOC (MDEQ, 2012) indicates that [Talen] must ‘contain any impacts on [Talen] land’. For surface impoundments that are subject to the USEPA’s Federal CCR Rule1, the POC will be the edge of the impoundment when that POC becomes effective. The SOEP and Cell A were full and stopped receiving CCR prior to the effective date, and thus are not subject to the Federal CCR Rule. Therefore, a consistent POC among all the cells at the SOEP/STEP Site is not necessarily appropriate. Geosyntec will initiate the remedy evaluation for the SOEP and Cell A assuming the initial POC is the same as that for non-hazardous solid waste facilities, i.e. up to 150 meters downgradient from the cell boundary (but on Talen property and not beyond the property boundary or the nearest receptor.

For COIs that are also regulated by the Federal CCR Rule, and have been released from cells that are subject to the Federal CCR Rule groundwater monitoring and corrective action requirements, the POC will be the downgradient edge of the cell, but that POC will not be effective until October 2017. The New Clearwell (Cell B), the proposed Cell C, Cell D, Cell E, and the Old Clearwell are subject to the Federal CCR Rule.

4.2 Compliance Evaluation

The CCRA will be completed in parallel with the Remedy Evaluation. For purposes of this work plan, it is assumed that cleanup levels will need to be established for several parameters, likely to include boron, sulfate, TDS, and selenium. It is also assumed that the cleanup level for boron, sulfate, and TDS will be based upon DEQ-approved 2015 BSLs because they are greater than the values identified in the AOC, or no values are available. The selenium cleanup levels will be based upon the MCL because the DEQ-approved 2015 BSLs for this metalloid are below the MCL. It is also assumed that there are currently no complete exposure pathways to groundwater for potential human receptors, and that little if any impact to East Fork Armells Creek is occurring. The CCRA will address potential future groundwater exposure, and may result in Cleanup Criteria above the DEQ-approved 2015 BSLs at specific locations. The POC for attaining the Cleanup Criteria in groundwater for non-CCR Rule cells is based upon the Federal RCRA Subtitle D program and is set no more than 150 meters downgradient from the edge of each cell, but cannot extend beyond the downgradient property boundary and the nearest receptor. For COIs that are also regulated by the Federal CCR Rule, and have been released from cells that are subject to the Federal CCR Rule groundwater monitoring and corrective action requirements, the POC will be the downgradient edge of the cell footprint, but that POC will not be effective until October 2017.

1 “Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals from Electric Utilities; Final Rule.” 40 CFR Parts 257 and 261. April 17, 2015.


4.3 Source Control Upgrade Remedial Action Objectives

The RAOs for Source Control Components of the overall remedy are to:

“control future release of COIs to the groundwater to the extent necessary to achieve the cleanup levels at the downgradient point of compliance in a reasonable period of time.”

During implementation of this work plan, calculations of the expected timeframes to achieve the cleanup goals at the POC will be provided along with a justification as to whether or not they are reasonable. Depending upon the existing mass loading of solutes to the groundwater system from various ponds, more aggressive source control actions may be necessary at some source areas compared to others.

4.4 Migration Management Upgrade Remedial Action Objectives

The RAOs for Migration Management Components of the overall remedy are to:

“Prevent potential current and future exposure of human and ecological receptors to COIs at concentrations greater than cleanup criteria in groundwater beyond the point of compliance, and in surface water in East Fork Armells Creek, and to restore water quality to Cleanup Criteria or background, whichever is greater, in a reasonable period of time.”

4.5 Institutional Controls

Existing or new institutional controls might contribute to controlling potential exposure to COIs in the short term until such time that the remedy has achieved the cleanup goals. The RAOs for institutional controls, if included as a remedy component, would be to alert potential receptors to the presence of COIs and to reduce or eliminate potential exposure. An implementation plan for institutional controls, if included as a part of the preferred remedy, will be included in the Remedy Evaluation Report.


5. PRELIMINARY SCREENING OF REMEDIAL ACTION TECHNOLOGIES

This section describes the approach for initial identification and screening of remedial technologies. In this evaluation, a “technology” might include a:

Component, such as a capture well or a process water pond or cell liner upgrade; Activity, such as groundwater flow and transport modeling; or

Institutional control, such as a property deed restriction to control or eliminate a potential exposure pathway.

Potential technologies will be identified based on their applicability to the physical setting and subsurface conditions described in Section 2.0, and the ability of the technologies to address the RAOs described in Section 4.0. Technologies will be eliminated from consideration if their use is reasonably precluded by Site and/or COI characteristics.

Technologies will be screened using the following criteria:

Effectiveness; Implementability; and Qualitative cost.

These three screening criteria effectively combine the remedy evaluation criteria discussed in Article VI.C. in the AOC. The objective of the screening is to identify and retain technologies that alone or in combination provide a workable number of alternatives for detailed evaluation.

Each criterion will be scored as above average, average, or below average to facilitate comparison between technologies and decision making regarding assembly of remedial action alternatives. In some cases, the result of such scoring would be highly dependent on the specific COI and the application and design of the technology, and these will be marked accordingly. In other cases, the criteria may not apply, and these will be so marked. Note that some technologies may perform better when combined in a remedial alternative than as the sole technology.

Effectiveness will be assessed by considering the ability of the technology to achieve the RAOs. As previously stated, several remedial technologies have already been implemented and/or are ongoing at the SOEP/STEP. For technologies that have already been implemented at the SOEP/STEP, their effectiveness to date and their potential for enhancement will be evaluated.

In regard to implementability, ratings will be applied based on availability, maintenance needs, and ease of implementation (e.g. site infrastructure obstructions). Above average ratings will be applied where there are many vendors/suppliers, the technology is relatively low maintenance, and would be easily implemented. Conversely, below average ratings will be applied where the technology has few vendors/suppliers, the technology has high maintenance


and would be difficult to implement. Average ratings will be applied for those technologies where these factors fall at a relative midpoint.

For qualitative cost, consideration will be given to both capital and operation and maintenance costs. In this manner, total costs will be compared as either lower or higher than the current average site remediation costs.

A preliminary screening of technologies has been completed for this work plan and the results are illustrated in Table 5-1. This preliminary screening is subject to revision during implementation of this work plan. In Table 5-1 and in the following section, the technologies are grouped to facilitate assembling the technologies into remedial action alternatives (RAAs). Technologies are grouped together by:

Source control; Groundwater migration management; and Institutional controls.

Potential source control technologies include:

1. Install or upgrade liners in the process ponds; 2. Dewatering ash prior to placement in ponds using paste technology or dry stacking; 3. Cap/close process ponds; 4. Slurry wall/grout curtain/sheet pile; 5. Sumps/interceptor trenches; 6. Horizontal capture wells; 7. Perimeter (vertical) capture wells; and

8. Excavation of source material.

Groundwater migration management technologies include:

1. Capture wells/trenches; 2. Recharge barriers (gradient control); 3. Permeable reactive barriers; 4. Natural attenuation; and 5. Point of use monitoring/treatment.

Institutional control technologies that restrict potential groundwater exposure include:

1. Town ordinances; 2. Deed restrictions; 3. Easements;


4. Reservations; 5. Covenants; 6. Controlled groundwater areas; and 7. Zoning.

These are a mix of proprietary controls (pertaining to controls tied to specific land and ownership) and governmental controls (pertaining to controls on land use in areas under jurisdiction of an empowered governmental body).

Table 5-1 is marked as appropriate where a factor is inapplicable, or if consideration of a particular factor would be highly dependent on the specific COI and the application/design of the technology.

The screening shows that most of the listed technologies are feasible, with a degree of variability in effectiveness, implementability, and cost. During implementation of this work plan, other technologies might be identified and considered that are not included in this preliminary screening. The actual technologies might vary somewhat.


6. ASSEMBLING REMEDIAL ACTION ALTERNATIVES

Remedial action alternatives will be assembled from the technologies listed in Table 5-1 along with other promising technologies identified during implementation of this work plan, based on professional judgment. A remedial action alternative may be comprised of either an individual technology or remedial approach, or a combination of multiple technologies or remedial approaches. No further action (i.e. continuing the existing remedial systems as currently operated) will be included in the evaluation as a baseline for comparison. A range of ‘aggressiveness’ of the alternatives will be included such as No Further Action, Source Control upgrades only, Source Control upgrades plus additional distal groundwater plume management.

The assembly of remedial action alternatives will depend on multiple factors such as:

Nature and extent of COIs – Some technologies (or remedial approaches) may be effective for some COIs but not for others. If multiple COIs are identified, then it is possible that more than one type of remedial technology (or approach) may be required.

Targeted media – The technologies (or remedial approaches) to control migration of COIs may vary depending whether the COIs are observed in soil, surface water, or groundwater. For groundwater, the technologies (or remedial approaches) for source control may differ from those for migration management or for institutional controls.

RAOs - One technology (or remedial approach) may be sufficient for meeting some or all of the RAOs but if not, then use of multiple technologies (or remedial approaches) may be required either spatially, temporally, or both. Each assembled alternative will need to meet the general standards of the AOC: i) protect human health and the environment, ii) attain Cleanup Critiera, iii) control the source(s) of release so as to reduce or eliminate further release of COIs, and iv) comply with standards for management of wastes. Remedial actions that meet these general standards are consistent with the requirements of the AOC.

Performance and regulatory standards – Depending on the COIs, one or multiple technologies (or remedial approaches) may be required to meet the performance and regulatory standards. Other metrics (e.g., RAOs, time) besides regulatory standards (i.e., Cleanup Criteria, POC) will be considered as performance standards for remedy evaluation.

The assembled remedial action alternatives will be documented in tabular format in the Remedy Evaluation Report. There will likely be several alternatives that include groundwater capture since that technology is already in place and functioning at the site. Various configurations of groundwater capture may be included as distinct alternatives, such as addition of capture wells to the existing capture well field in appropriate locations to improve COI capture, as well as reconfiguration of the existing capture well field (potentially including decommissioning of


select wells) to improve system efficiency while meeting RAOs. Alternatives that rely more on source control upgrades along with institutional controls and/or alternative water supplies for distal plume areas (rather than capture wells) will also be included. Where appropriate, innovative in situ technologies such as permeable reactive barriers may be included in certain areas such as shallow alluvium, along with capture wells for deeper fractured bedrock areas.

A preliminary assembly of remedial alternatives is included in Table 6-1. This table was developed based upon the considerations described above and are intended to convey the general approach to be used for assembling a long list of potential remedial alternatives that will be screened using the criteria described in Section 7 below. During implementation of this work plan, other alternatives might be assembled and considered that are not included in this preliminary list. Also, different alternatives might be considered for different portions of the SOEP/STEP Site based upon varying site conditions.


7. EVALUATING REMEDIAL ACTION ALTERNATIVES

This section describes how the assembled RAAs will be evaluated with respect to the applicable requirements in the AOC. First, the assembled RAAs will be evaluated based on general standards (described in Section 7.1 below) to demonstrate that the assembled alternatives meet basic requirements such as protecting public health and the environment, controlling human and ecological exposure to hazardous substances, and achieving compliance with applicable state and federal regulations.

Next, the assembled remedial action alternatives will be evaluated in more detail considering their ability to achieve the RAOs presented in Section 4.0 of this work plan in the context of the Site and COI characteristics. Site and COI characteristics affect areas and depths of installation, material volumes, methods and sequence of construction, flow and reaction rates, sampling and analysis, effort to optimize, energy use, waste management and other relevant issues that require consideration for the alternative to be effective. Tables listing the total areas and volumes of media impacted by COIs exceeding Cleanup Criteria, and table(s) presenting a written summary of the evaluation of each remedial alternative will be included in the Remedy Evaluation Report.

These evaluations will be used to identify the preferred remedial action alternatives for the Site.

7.1 General Standards

The Remedy Evaluation will include a demonstration of how each assembled remedial action alternative will:

Protect human health and the environment;

Attain cleanup criteria;

Control the source(s) of release so as to reduce or eliminate further release of COIs; and

Comply with standards for management of wastes.

Remedial actions that meet these standards are generally consistent with the AOC.

7.1.1 Protection of Human Health and the Environment

Each alternative will be evaluated in terms of demonstrable mitigation of potential risk to public health, safety or welfare and the environment from the Site. The evaluation will include an estimation of the relative degree to which each alternative will eliminate, reduce or control potential risk. For example, in the case of source control actions, such as the upgrade of a liner, this demonstration will include construction quality assurance (CQA) documentation of liner construction. For groundwater migration management, this will include an evaluation of capture zone for hydraulic control systems and identification of potential data gaps.


7.1.2 Attainment of Cleanup Criteria

The manner and rate in which each alternative attains cleanup criteria (listed in Section 4.0 of this work plan) will be described. The evaluation will include the monitoring network required for each alternative. It will also evaluate whether installation of additional capture wells might accelerate the overall cleanup timeframe in a cost-effective manner.

It should be noted that Site-Specific Cleanup Criteria and a list of constituents of COPCs will be provided in the CCRA, which is anticipated completed by Ford Canty & Associates in parallel with the Remedy Evaluation. The Remedy Evaluation will be based on the MDEQ-approved BSLs, the Circular DEQ-7 values, and the draft cleanup criteria report values. Per the AOC, the MDEQ-approved BSLs serve as the Cleanup Criteria for certain COIs where the BSLs are greater than the DEQ-7 values and the risk-based concentrations.

7.1.3 Source Control

The evaluation will include an estimation of the relative degree to which each alternative will reduce or eliminate, to the extent feasible, further releases of COIs. In the case of an engineered cap or cover system, this evaluation may include CQA documentation of cap construction. In the case of groundwater capture, this evaluation may include potentiometric surface maps and evaluation of improvement in water quality at areas downgradient of the capture well capture zone, as well as the results of numerical groundwater flow modeling.

7.1.4 Compliance with Standards for Management of Wastes

The evaluation will demonstrate that the alternative is capable of complying with standards for management of wastes, which is that wastes shall be managed in a manner that complies with applicable regulatory requirements. This will be done by identifying the compliance requirements for the RAAs and demonstrating how each alternative would meet these requirements.

7.2 Detailed Evaluation Factors

The Remedy Evaluation will include a detailed comparative evaluation of each assembled RAA based on:

Performance; Reliability; Ease of implementation; Potential impacts; Time to start/complete; Cost; Permits and approvals; and Community concerns.


Consideration of these factors in selection of remedial actions is consistent with the requirements of the AOC.

7.2.1 Performance

The performance of each assembled remedial action alternative will be evaluated in terms of long and short-term effectiveness and protectiveness. The performance of an assembled remedial action alternative is defined by its ability to eliminate or substantially reduce the inherent potential for the wastes in the contaminated media to cause future environmental releases or other risks to human health and the environment. Changes to ponds at a result of the implementation of the Master Plan will be incorporated into the evaluation of each remedial action alternative. The evaluation will be based on consideration of the following:

i) Ability to achieve the RAOs within a reasonable period of time under the specific circumstances for the SOEP/STEP Site;

ii) Magnitude of reduction of existing risks and magnitude of residual risks in terms of likelihood of further releases;

iii) Type and degree of long-term management required, including monitoring, operation and maintenance;

iv) Short-term risks that might be posed to workers, the community or the environment during implementation, including potential threats to human health and the environment associated with construction, excavation/extraction, transportation and disposal;

v) Estimated time until RAOs, performance standards, and regulatory standards are achieved;

vi) Long-term reliability of the engineering and institutional controls; and vii) Potential need for replacement of the remedy.

Alternatives that include groundwater capture will require consideration of site specific conditions such as the uncertainty of pumping rates from the capture wells due to scaling, uncertainty in the amount of time that capture wells are off line for maintenance and repairs, and uncertainty of seepage rates from the process ponds on the reliability of the groundwater model calibration. The evaluation of groundwater capture alternatives will be based on the groundwater conceptual model developed by NewFields in the Site Report (Hydrometrics, 2016) It is also recognized that the performance of some alternatives such as the groundwater capture systems may not have been fully optimized at the SOEP/STEP Site and may be evaluated based on the USEPA guidance on Methods for Monitoring Pump-and-Treat Performance and A Systematic Approach for Evaluation of Capture Zones at Pump and Treat Systems (USEPA, 1994; 2008). As previously stated, a significant component of the remedy evaluation will include an evaluation of the current groundwater capture systems with a focus on enhancement, if appropriate. This will be done largely using the groundwater model developed by NewFields


in the Site Report. If deemed appropriate, a revised groundwater capture system may be considered as the preferred remedial technology, either alone or coupled with another remedial technology. 7.2.2 Reliability

The reliability criterion refers to an assembled remedial action alternative’s certainty of success in both the short- and long-term, as well as under changing environmental conditions. Inherent in its success is the ability of the assembled RAA to control the source. The evaluation of reliability will consider the following factors:

How frequently and successfully a particular RAA has been implemented at other sites with similar geology, hydrogeology and COIs conditions;

Potential impacts, if any, on immediate receptor(s) in case a technology in a RAA fails; How the technologies in the RAA will respond to environmental changes such as storm

events, earthquakes, groundwater flow changes from changes in off-site land use, etc.; The extent of long-term operation and management required after implementation; The extent to which the remedial action technology will control the source and reduce

further releases; and The extent to which remedial technologies may be used and the extent to which

treatment is reversible.

The assembled RAAs will be evaluated in terms of their overall projected useful life as well as their effectiveness in source control. A RAA that reliably meets RAOs and controls the source(s) with little long-term management may be preferred.

7.2.3 Ease of Implementation

The Remedy Evaluation will include an estimation of the relative ease or difficulty of implementing the assembled RAA. This criterion refers to the physical, technical, and administrative ease of implementing a remedial action alternative. It includes a consideration for time and resources needed to obtain necessary approvals, a consideration of how readily the remedial components are available, and a consideration of how much disturbance/interruption the remedial action alternative will cause at the site and on the surroundings. This evaluation will be based on consideration of the following:

Administrative activities required for implementation such as permits, approvals, access agreements, etc. and the time requirement for these activities;

The availability of components, equipment, and technical services, and disposal facilities, if needed;

The time and resources for construction and startup, and the ease of operation and maintenance;


Suitability of the spatial footprint of the remedial action alternative. In some cases, the use of a certain technology may be precluded by the site-specific terrain, buildings, roads, and underground and overhead utility lines;

Degree of difficulty associated with constructing the technology; Expected operational reliability; Need to coordinate with and obtain necessary approvals and permits from other

agencies; Availability of necessary equipment and specialists; The disturbances or concerns to the site and surrounding areas/communities; and Available capacity and location of needed treatment, storage and disposal services.

An assembled RAA that is the relatively easy to implement and sustain in order to meet RAOs and performance/regulatory standards may be preferred.

7.2.4 Potential Impacts

The potential impacts of an assembled RAA refer to unintended effects that may occur during implementation. The factors to be considered in evaluation of potential impacts of an assembled remedial action alternative include the following:

Exposure to hazardous substances; Safety measures associated with remedial action alternative construction and operation; Risk of cross-media contamination; and Risk of exposure to residual levels in environmental media in case it may not be

possible to remove all of the COIs even with the best available technologies, or exposure to treatment residuals.

Potential impacts will also be evaluated as part of the reliability criterion. An assembled RAA that has fewer potential negative impacts on the site and its surroundings while meeting RAOs and performance/regulatory standards may be preferred.

7.2.5 Time to Start/Complete

The time to start refers to the time, after MDEQ approval of the remedy, that will be required to: (1) conduct sampling to fill data gaps if required; (2) design, construct, run and analyze required treatability studies either at the benchtop or pilot scale; (3) design and construct either source control or mitigation management technologies; and (4) implement institutional controls, if applicable. The time to complete refers to the time that will be required to meet RAOs and performance/regulatory standards. All calculations of the estimated time required for each alternative to achieve Cleanup Criteria and RAOs will be documented in the Remedy Evaluation Report. The time to start and complete a remedial action alternative will be


evaluated as part of the ease-of-implementation criterion and also by considering the following factors:

Time required to obtain the necessary permits and approvals for RAA implementation; Ease of construction and availability of materials, resources, and technical services; and Processes such as natural attenuation of COIs that may affect estimated cleanup times.

Some RAAs may require more time to start but may achieve RAOs sooner than other RAOs. An assembled RAA that has shorter start and completion times or that provides a good balance between the start and completion times may be preferred.

7.2.6 Cost

The cost of an assembled RAA is an important distinguishing consideration, particularly when multiple RAAs provide the same level of performance and reliability, and offer equivalent protection of human health and environment. An estimate of the cost of each RAA will be developed, including both capital and operation and maintenance costs over an estimated timeframe required to achieve Cleanup Criteria. The cost of each alternative will be expressed in current US dollars. The costs may vary amongst RAAs depending on the technology(ies) included in the RAAs. The cost is not a consideration if only one RAA is evaluated or in the case where one RAA is the only one that is effective and protective. Evaluation of RAA cost will include the following components:

Cost of design, permitting and site preparation; Cost of materials, construction, and engineering; Cost operation and maintenance, and disposal of treatment residuals; Cost of sampling, monitoring, and reporting; Cost of system decommissioning; and Other miscellaneous costs such as project management, health and safety, training, etc.

An RAA that meets the RAOs and performance/regulatory standards with a lower overall cost of implementation and operation may be preferred.

7.2.7 Permits and Approvals

The anticipated permits and approvals, and applicable statutes and regulations, will be listed in the Remedy Evaluation Report and classified as federal, state or local. As discussed above in the ease-of-implementation and time-to-start criteria, the RAAs will be evaluated for the ease of obtaining necessary approvals and permits for implementation and operation. The criterion will consider the time and resources required to obtain necessary approvals and permits, and how the RAAs will comply with the listed statues and regulations. A RAA that meets the RAOs and performance/regulatory standards with fewer requirements for obtaining approvals and permits may be preferred.


7.2.8 Community Concerns

The evaluation will include an estimation of the relative degree to which community concerns are addressed by each RAA. An initial scoping meeting with MDEQ may provide some insight on community concerns. As provided in the AOC, MDEQ will solicit and consider public comment on the Remedy Evaluation Report. For a given RAA, the community concerns may stem from the following:

Risk of exposure to hazardous substances during construction and/or operation; Disturbances during implementation and operation such as noise, dust, odor, increased

traffic, etc; Risk of exposure to residual levels of COIs in case it may not be possible to remove all

of the COIs even with the best available technologies; Potential side effects to other environmental media; and Aesthetics and other socio-economic concerns.

In addition to the above factors, the evaluation of remedial action alternatives will consider community acceptance and opportunities for ongoing community participation. A RAA that meets the RAOs and performance/regulatory standards and is capable of achieving community acceptance may be preferred.


8. PREFERRED REMEDIAL ACTION ALTERNATIVE

The preferred RAA will be identified based on the results of the evaluation described in Section 7.0 of this work plan. A table or tables will be prepared summarizing: i) the advantages and disadvantages of each alternative; ii) how each alternative satisfies the RAOs, including cleanup criteria in Article IV. G. of the AOC; iii) rationale for the identification of a preferred remedy; and iv) identification of sampling or treatability studies that are needed, if any. The table or tables will include a written discussion of each alternative including the No Further Action alternative. Sampling or treatability studies recommended would be discussed in the Remedy Evaluation Report and details (and laboratory selection) of the treatability studies should be specified by the designer at the remedial design stage. Treatability studies for unproven/innovative technologies may be warranted at the remedy evaluation stage and a separate work plan would be prepared for MDEQ review in that case.

This information along with a schedule for submission of a Remedial Design/Remediation Action Work Plan will be compiled and submitted in a Remedy Evaluation Report to document this effort.


9. NEED FOR ADDITIONAL DATA OR TREATABILITY STUDIES

The Remedy Evaluation process will include identification of data gaps to be addressed either during remedy evaluation or during remedy design.

The Remedy Evaluation Report will also include, as necessary, identification of information regarding the need for treatability studies to support the design of the preferred remedial measure alternative. Sampling or treatability studies recommended prior to submittal of the Remedy Evaluation Report will be presented in the form of work plans for DEQ to review.


10. SCHEDULE AND DELIVERABLES

A preliminary schedule for completion of the work described herein and delivery of the Remedy Evaluation Report to MDEQ is December 2016, or three months after MDEQ approval of any predecessor report. The schedule for the implementation of this work plan assumes that no additional site characterization or treatability studies are needed prior to the remedial design stage.

A Remedy Evaluation Report will be prepared. The report will document the screening of technologies, assembling of RAAs, and the evaluation of RAAs, leading to the identification of a preferred alternative. The report will include the following:

1. An introduction describing the purpose and organization of the report, including reference to the extensive list of interim actions listed in the SOEP/STEP Site Report (Hydrometrics, 2016) and an identification/description of significant interim actions that may have occurred in the past 5 to 10 years and how they have addressed the RAOs.

2. Presentation and discussion of results of data that support the rationale for the remedy selection, such as groundwater modeling results, mass flux estimates and cleanup timeframe estimates (including data and estimates used) that may have been developed for the Site Report, CCRA or for the remedy evaluation itself.

3. A summary of changes to the work described in this work plan, if any.

4. A presentation and discussion of the results of the detailed alternatives analysis.

5. Figure(s) illustrating the anticipated location of elements of the preferred alternative based on the information that is available at the time of the Remedy Evaluation Report. The anticipated location may be subject to change as new information may become available from sampling or treatability studies, or during remedial design.

6. A discussion of estimated time to meet the RAOs.

Supporting information such as engineering calculations, cost estimate backup, data collected during the implementation of the Work Plan to fill data gaps, and applicable reports concerning treatability studies for potential remedy technologies evaluation will be provided in appendices.


11. REFERENCES

Geomatrix, 2007. Conceptual Model Update Report, Stage I and II Evaporation Ponds and Plant Site Areas, Colstrip Steam Electric Station, Colstrip, Montana. Geomatrix Consultants, Inc., Missoula, Montana. July 2007.

Geosyntec, 2015. Master Plan Summary Report, Colstrip Steam Electric Station. Geosyntec Consultants, Inc., Columbia, Maryland. June 2015.

Hydrometrics, 2016. Talen Montana, LLC, Colstrip Steam Electric Station, Administrative Order on Consent, Units 1 & 2 Stage I and II Evaporation Ponds Site Report. Hydrometrics, Inc., Billings, Montana. March 2016.

Maxim, 2004. Preliminary Conceptual Hydrogeologic Model, State I and II Evaporation Ponds and Plant Site Areas, Colstrip Steam Electrical Station, Colstrip Montana. Maxim Technologies, July 2004.

Maxim, 2005. Technical memorandum, groundwater modeling of the Stage II Evaporation Pond Area, Colstrip, Montana, March 2005. MDEQ, 2012. Administrative Order on Consent Regarding Impacts Related to Wastewater Facilities Comprising the Closed-Loop System at Colstrip Steam Electric Station, Colstrip Montana. Montana Department of Environmental Quality, July 2012.

Neptune and Company, Inc., 2016. Final Report on Updated Background Screening Levels, Plant Site, 1&2 SOEP and STEP, and 3&4 EHP, Colstrip Steam Electric Power Station, Colstrip, Montana, 22 January 2016.

NewFields, 2016. Stage I & II Evaporation Ponds Area Groundwater Conceptual Model and Expanded Numerical Model, Colstrip Steam Electrical Station, Colstrip, Montana. NewFields, March 2016.


TABLES

Geosyntec Consultants

Table 2-1_SOEP STEP Process Ponds

Hydrometrics, Inc. Revised 7/29/2016Table 2-1

Page 1 of 1

Waste Water FacilityTotal Capacity

(acre-feet)/ft3 (1)Surface Area (acres)/ft2 (2)

Pond WaterElevation

(April 2015)(ft amsl)

Years in-service Current Status Future Status Lining Pond Function /Comments

Stage I Evaporation Pond(SOEP)

(2,350)/102,366,000

(114)/4,965,840 NA 1975-

1997 Closed NA Partial liner of compacted clay

Received flyash transported with scrubber slurry from Units 1&2 for final disposal. This pond was full in 1997 and the reclamation program for this pond was completed in 2002. There has been limited grazing on this reclamation since 2003. In 1995, a groundwater collection system was installed west of this pond. In 1999 and 2001, this west groundwater collection system was expanded. In 2000, a groundwater collection system was installed south of this pond. In 2006, wells were installed in the pond boundary to evaluate dewatering of the scrubber material.

Stage II Evaporation Pond(STEP)

Inclusive of all active cells described below

(4,370)/190,357,200

(159.3)/6,939,108 See Below 1992-

present See Below See Below HDPE or RFP

Receives flyash transported with scrubber slurry from Units 1&2 for final disposal. Started receiving slurry in 1994. Clearwater is collected in the Clearwell and returned to the scrubbers for re-use. In 1999, a groundwater collection system was installed east of this pond. This area's groundwater collection system has been expanded in recent years.

- Cell A NA (42.1)/1,833,876 3,269 1992-

2015 Not operating To be closed in 2019 HDPECell A was lined during initial construction. This cell is essentially full and no longer recieves flyash scrubber slurry.

- Cell B (New Clearwell)

(281.4)/12,257,784

(15.3)/666,468 3,265 2006-

present Operating

Verify that groundwater quality is not impacted by Cell B, and if verified, operate with existing

liner.

Double-lined RPP withunderdrain collection

systems.

Constructed in 2006, Cell B is double-lined with a 45 mil RPP primary liner, a 36 mil RPP secondary liner, a collection system between the primary and secondary liners, and another collection system beneath the secondary liner. This cell was operated as Cell B from 2006 to 2011 to provide additional volume to store process water. It was put into service as the New Clearwell in 2011. Receives clear water from paste plant and returns to scrubbers for reuse.

- Cell C NA (17)/740,520

This cell has not been constructed but will be lined, if needed.

- Cell D (623.6)/27,164,016

(24.7)/ 1,075,932 3,262 2011-

present Operating

Verify that groundwater quality is not impacted by Cell D and,

if verified, operate with existing liner.

Double-lined RPP withleachate collection.

Receives clear water or paste from the STEP systems as needed.

- Cell E (743)/32,365,080

(46.8)/ 2,038,608 3,268 1992-

present OperatingContinue to use for paste

disposal until 2018; close in 2020

HDPE

This currently active cell recieves paste from the paste plant. In 1998, Cell E weirbox outlet developed a leak that was repaired. In 2000, the Cell E weirbox outlet developed a leak that was repaired. In 2006, a small hole in the liner on the north side of Cell E was found just under the water level. The water that leaked was recovered on the north side of the E/C dike. The hole was repaired.

- Old Clearwell (437)/19,035,720

(10)/435,600 3,266 1992-

present Operating Dewater in 2022 HDPE

The Old Clearwell was lined during its initial construction. The Old Clearwell receives clear water from the settling portion of Cells A and E, and will eventually be filled with paste. The Old Clearwell was struck by a barge in October 2007; and some seepage was observed beneath the liner. A capture system was installed and repairs to the liner were completed in June 2008.

Notes:

HDPE - high density polyethyleneRFP or RPP - reinforced polypropylene Source: Geosyntec (2015); Hydrometrics (2016)

NA - not applicable. Facility is no longer in service or facility is not parameterized by the dimension listed.

CCR - Coal Combustion Residual

(1) Design Capacity - To convert acre feet to cubic feet multiply by 43,560 square feet(2) Area of pond footprint - top dimensions - multiply acres by 43,560 to get square feet

TABLE 2-1 LIST OF PROCESS PONDS

Colstrip Steam Electric StationColstrip , Montana

Proposed/ Permitted (not constructed)

SOEP/STEP SITE

Geosyntec Consultants TABLE 2-2

LIST OF GEOLOGIC UNITS SOEP/STEP SITE

Colstrip Steam Electric Station

Colstrip, Montana

1 MR1149D\Table 2-2 List of Geologic Units

UNIT DESCRIPTION APPROXIMATE

THICKNESS DISTRIBUTION OR EXTENT

Coarse Alluvium

Fluvial deposits of gravel and sand generally in a fining-upward sequence. Basal gravels typically overlie bedrock and grade upwards into poorly sorted sand and silt.

Up to 40 feet total.

Present in drainage valley bottoms beneath SOEP/STEP ponds, and along and beneath East Fork Armells Creek; less extensive along Stocker Creek. Absent elsewhere.

Fine Alluvium Fluvial deposits of silty clay to clayey silt, generally gradational with colluvium.

Colluvium

Erosional and gravitational deposits of silt and silty clay, with coarser deposits present locally.

Variable. Beneath ridge tops and slopes, lines margins of East Fork Armells Creek; absent elsewhere.

Clinker

Highly vesicular and fractured rock composed of thermally altered shale and collapsed overburden resulting from historic burning of underlying coal seams.

Up to 90 feet, where present.

Along most of the higher ridgetops, and in the southern portion of SOEP/STEP Site area.

Fill Intermixed sandy silt and clay. Variable, where present. Used to construct berms and dams for process water ponds, and to reclaim the SOEP area.




Colstrip, Montana




Spoils (former Overburden)

Silt, clay, sandstone and coal fragments used to backfill areas where Rosebud Coal was mined.

0 feet. Absent in the SOEP/STEP Site Area (present west of the SOEP).

Rosebud Coal

Cleated (fractured) coal. Only coal seam that is mined. Historical burning in situ has formed Clinker in some areas.

20 to 25 feet, where present.

Absent north of the SOEP/STEP ponds due to the dip and/or fault that may exist in the drainage valley bottom. Absent in the East Fork Armells Creek and Stocker Creek valleys due to erosion.

Interburden

Siltstone and claystone with some minor fine sandstone; may be fractured. Separates Rosebud Coal and underlying McKay Coal.

10 to 20 feet, with a maximum of 50 feet.

Present south of the SOEP/STEP ponds; likely absent north of the SOEP/STEP ponds due to the dip and/or fault that may exist in the drainage valley bottom. Absent in the East Fork Armells Creek and Stocker Creek valleys due to erosion.




Colstrip, Montana




McKay Coal Cleated coal. Not mined due to poor boiler quality.

7 to 15 feet, but mostly 7 to 9 feet.

Present south of the SOEP/STEP ponds; likely absent north of the SOEP/STEP ponds due to the dip and/or fault that may exist in the drainage valley bottom. Absent in the East Fork Armells Creek valley due to erosion.

Sub-McKay

Interbedded claystone, siltstone, fine sandstone and thin coal seams (including Robinson Coal) of the Tongue River Member beneath the McKay Coal. Units may be fractured.

Up to 350 feet.

The Sub-McKay is continuous across the SOEP/STEP Site area, although individual units may pinch out laterally and become relatively discontinuous. The Sub-McKay is divided into shallow and deep zones based on differing groundwater flow directions only; lithologically, the shallow and deep zones are similar.

Sources: Hydrometrics (2016); NewFields (2016)


1 MR1149D\Table 2-3 COIs

TABLE 2-3 PRELIMINARY LIST OF CONSTITUENTS OF INTEREST

AND INDICATOR PARAMETERS SOEP/STEP SITE


Colstrip, Montana

1 Listed in Article IV.B of the AOC as 'regulated substances'. Constituents of Interest may be added or eliminated based on the risk assessment that will be conducted in parallel with the remedy evaluation. 2 Used to indicate process water impacts to groundwater. 3 The chloride baseline screening level (BSL) included in Neptune (2016) and approved by Montana DEQ appears to be erroneously low and hence is given less weight compared to the other indicator parameters.

Element / Compound Constituent of Interest1

Indicator Parameter2

Boron X X

Calcium X

Chloride3 X

Magnesium X X

Potassium X Salinity (as Specific Conductance) X X

Selenium X

Sodium X

Sulfate X X Total dissolved solids (TDS) X X

TABLE 5 -1 PRELIMINARY SCREENING OF REMEDIATION TECHNOLOGIES

SOEP/STEP SITE

Colstrip Steam Electric StationColstrip, Montana


Evaluate Optionsfor Enhancing the Performanceof the Existing Capture System

Enhance the existing near-source capture system by modifying the well field near a source area (pond or cell) by replacing existing vertical wells, and/or adjusting pumping rates.

Average

Effective for hydraulically controlling plume migration. Captured groundwater would continue to be pumped to one or more process ponds and then reused at the Plant Site. Continued reduction in COI mass would be achieved in groundwater.

Easy

Optimization analysis of existing capture system would be needed. Volume of groundwater being captured may increase.

Lower than Average

May need to re-configure conveyance piping for captured groundwater. No additional operating and maintenance costs would be anticipated.

Retained, groundwater capture system is already in-place

Expand Existing Capture System with Additional Capture Wells

Expand the existing near-source capture system by installing additional horizontal/vertical wells beneath/near a source area (pond or cell).

Above Average

Effective for hydraulically controlling plume migration, especially in certain areas where additional capture is needed and cannot be achieved by adjusting pumping rates on existing wells. Captured groundwater would continue to be pumped to one or more process ponds and then reused at Plant Site. Continued reduction in COI mass would be achieved in groundwater.

Average

Optimization analysis of existing capture system would be needed prior to installation of new horizontal/vertical wells. Volume of groundwater being captured may increase.

Highly dependent on location, existing infrastructure and depth.

Existing ponds, dams, roads, buildings and utilities would affect new well locations and installation. Costs usually increase with the depth/number of new wells. May need to re-configure conveyance piping for captured groundwater.

Retained, expand groundwater capture system that is already in-place

Sumps/Interceptor Trenches

Pump groundwater from an existing and/or new sump or interceptor trench near a source area (pond or cell).

Average

Effective for hydraulically limiting mass flux seeping from a process pond to groundwater. Captured groundwater would continue to be pumped to one or more process ponds and then reused at Plant Site. Reduction in COI mass would be achieved in groundwater.


Installation of new sump(s) or interceptor trench(es) might be required. Difficult to implement near or beneath existing ponds, dams, roads, buildings and utilities.


Existing ponds, dams, roads, buildings and utilities would affect new sump-trench locations and installation. Costs would vary with the depth/number of sumps/interceptor trenches, and the length of operation.

Retained, technology has been implemented at certain locations:• chimney drains and toe drains for Stage I and Stage II Main Dams, and• underdrains for New Clearwell (Cell B) and Cell D

Install/UpgradePond/Cell Liners

(All existing process ponds currently have some kind of bottom liner, see

Table 2-1)

Install a geosynthetic clay liner (GCL) in the bottom of a pond or cell.

Above Average for some cells.

Effective for reducing future mass flux of COIs from some process ponds; however, this technology alone would not be expected to remediate existing groundwater impacts.

Average for a new process pond. Highly dependent on amount of ash in an existing process pond.

Installation of a new bottom liner in an existing disposal pond would require dewatering and removing the ash.

Average for a new process pond. Highly dependent on amount of ash in an existing disposal pond.

Retained, potentially applicable to Old Clearwell

Paste Technology/Solidification

(flyash disposal ponds only)

Add polymer to flyash scrubber slurry to form a paste. Removes up to 90% of free available water in the scrubber slurry prior to placement, and thus reduces hydraulic head on the pond liner. Paste solidifies (hardens) to some degree, but can still be graded.

Above Average

Effective for eliminating water storage and thus reducing seepage losses and loading of COIs from flyash disposal ponds to groundwater. However, this technology alone would not be expected to remove existing groundwater impacts, and would not be expected to achieve RAOs in the source area within a reasonable timeframe.

Average

Proven technology as polymer feed systems are readily available and paste plant is already in-place and operating. Positive displacement pumps would continue to move paste via pipes to desired disposal location. Removed water would continue to be returned the Plant Site for re-use in the scrubbers.

Higher Than Average

Operating cost is high, considering the cost of polymer, continuous maintenance, paste transport, energy expenses, labor, consumables and equipment maintenance.

Retained, technology is already being implemented

StatusCategory Technology Description Effectiveness Implementability Cost

Source Control(containment, treatment,

removal)

MR1149D\Table 5_1_Technology Screening-SOEP STEP Page 1 of 5


SOEP/STEP SITE




Close/CapPonds/Cells

Close a pond or cell by either dewatering and removing the ash, or covering the ash in-place with an engineered cap or liner.

Above Average

Effective for preventing infiltration of precipitation into the ash, and thus effective for reducing the mass flux of COIs through pond liner. However, this technology alone would not be expected to remediate source areas.

Average

Requires dewatering the pond to fill with ash or paste, and then grading the ash or paste prior to covering it with an engineered cap. For flyash/paste disposal ponds, an engineeered liner may be installed on top of the existing ash or paste prior to dry stacking (see below).

Average

Engineered caps or liners typically have average costs to design and construct, and low costs to maintain.

1Retained, the closure/capping of certain ponds is already being implemented as part of the Master Plan1

Dry Stacking


Reconfigure paste plant to produce a dry flyash that is placed (stacked) on top of lined flyash/paste disposal ponds.

Above Average

Effective for eliminating water storage and thus reducing seepage losses and loading of COIs from flyash disposal ponds to groundwater. However, this technology alone would not be expected to remediate source areas.

Average

Requires reconfiguring paste plant and lining flyash/paste disposal ponds. The dry flyash cake is no longer pumpable and needs to be transported by truck/conveyor to desired disposal location. Requires dust control.

Higher Than Average

High capital and operating costs, and conversion of air pollution control devices.

Retained, dry stacking will be implemented when the existing ponds are full as part of the Master Plan1

Slurry Wall/Grout Curtain

Surround source area (pond or cell) with a physical barrier typically constructed of low permeability soil-bentonite and/or Portland cement. Slurry walls are typically installed at depths of less than 50 feet. Grout curtains are generally used at slightly shallower depths (30-40 feet maximum).

Highly dependent on location, existing infrastructure, depth, and permeability of underlying soil/rock.

Effective for containing impacted groundwater within the source area and/or diverting unimpacted groundwater flow around the source area.

Highly dependent on existing infrastructure around the proposed location for the slurry wall/grout curtain, and the proposed depth of the slurry wall/grout curtain. More difficult to implement in bedrock than in unconsolidated deposits (e.g., alluvium).


Existing ponds, dams, roads, buildings and utilities would affect the installation method and related cost. Costs would vary with depth and length of physical barrier to be installed and presence of bedrock.

Retained, cutoff walls have already been implemented in the Stage I and Stage II Main Dams

Source Removal

(excavation)

Excavate coal combustion solids and impacted soils and dispose in a new location.

Above Average

Effective for eliminating some sources of COIs from potentially impacting groundwater; however, this technology alone would not be expected to remediate the existing groundwater impacts.

Highly dependent on location, existing infrastructure, and area/depth of the proposed excavation.

Difficult to implement near or beneath existing ponds, dams, roads, buildings and utilities. Requires storm water and dust control, and requires suitable backfill and restoration. May require dewatering if groundwater is encountered.

Highly dependent on location, existing infrastructure, and area/depth of the proposed excavation.

Limited excavations are cost effective. The cost increases as areas and depths increase, and where existing infrastructure requires replacement or relocation.

Retained, excavations have been implemented in limited areas (e.g., pipeline spills)

Monitored Natural Attenuation

(MNA)

Collect additional groundwater data to evaluate the ability of natural processes to reduce COI concentrations to acceptable levels over time. Characterize aquifer solids to identify attenuation mechanisms and evaluate their capacity, longevity and reversability. Use modeling tools to predict the fate and transport of COIs. Continue groundwater monitoring to demonstrate MNA is occurring.

Highly dependent on COIs, aquifer characteristics and groundwater characteristics.

The effectiveness of MNA on organic compounds is proven at many sites, but its effectiveness on inorganic compounds (soluble cations and anions) is not as well understood. Aquifer/groundwater characteristics such as source strength and persistence, microbial activity, pH, oxidation-reduction potential, sorption, precipitation, infiltration, dilution, dispersion, and background COI concentrations would affect the effectiveness of MNA.

Easy

Requires a comprehensive groundwater monitoring well network and sample analysis program. Requires fate and transport modeling, and long-term groundwater monitoring. Institutional controls may be required to prevent use to impacted groundwater areas until attenuation reaches cleanup levels.

Lower than Average

Costs to demonstrate MNA are lower than groundwater capture systems, depending on the number and depth of wells sampled, sampling frequency, sample analyses, and modeling efforts. Retained, MNA may

already be occurringDistal Migration

Management

Source Control(containment, treatment,

removal)



SOEP/STEP SITE




Evaluate Optionsfor Enhancing the Performanceof the Existing Capture System

Enhance the existing distal capture system by modifying the well field downgradient of a source area by replacing existing vertical wells, and/or adjusting pumping rates.

Average

Effective for hydraulically controlling plume migration. Captured groundwater would continue to be pumped to one or more process ponds and then reused at the Plant Site. Continued reduction in COI mass would be achieved in groundwater.

Easy

Optimization analysis of existing capture system would be needed. Volume of groundwater being captured may increase.

Lower than Average

May need to re-configure conveyance piping for captured groundwater. No additional operating and maintenance costs would be anticipated.

Retained, groundwater capture system is already in-place

Expand Existing Capture System with Additional Capture Wells

Expand the existing distal capture system by installing additional horizontal/vertical wells downgradient of a source area.

Above Average

Effective for hydraulically controlling plume migration, especially in certain areas where additional capture is needed and cannot be achieved by adjusting pumping rates on existing wells. Captured groundwater would continue to be pumped to one or more process ponds and then reused at the Plant Site. Continued reduction in COI mass would be achieved in groundwater.

Average

Optimization analysis of existing capture system would be needed prior to installation of new horizontal/vertical wells. Volume of groundwater being captured may increase.


Existing ponds, dams, roads, buildings and utilities would affect new well locations and installation. Costs usually increase with the depth/number of new wells. May need to re-configure conveyance piping for captured groundwater.

Retained, expand groundwater capture system that is already in-place

Recharge Barrier

Recharge aquifer with unimpacted groundwater using injection wells or infiltration basins to modify groundwater flow directions.

Average

Effective for intercepting the migration of impacted groundwater or isolating areas of impacted groundwater. Effective for redirecting unimpacted groundwater flow around impacted areas. Effective for replenishing the impacted aquifer.

Average

The injected groundwater creates a higher hydraulic head to redirect the migration of impacted groundwater or isolate impacted groundwater. Requires installation of injection wells or construction of an infiltration basin, and a supply of unimpacted groundwater. May require an injection permit.


Existing ponds, dams, roads, buildings and utilities would affect installation. Costs would vary with number and depth of wells/basins, length of operation, and distance to the source of unimpacted groundwater.

Retained

Permeable Reactive Barrier

(PRB)

Install a PRB across the groundwater flow path hydraulically downgradient from a source area (pond or cell). A PRB allows groundwater to continue flowing in the downgradient direction while reactions within the PRB inhibit COIs from passing through.

Highly dependent on COIs, aquifer characteristics and groundwater characteristics.

Multiple COIs require further evaluation to demonstrate they can be inhibited by one or more compatible reactive processes. Further evaluation would be needed if a lower confining aquifer is lacking or there is uncertainty about the total depth of impacted groundwater. Difficult hydrogeological conditions that may limit the effectiveness of PRBs, including high rates of groundwater flow, preferential flow paths, either very high or very low permeability, a high degree of aquifer heterogeneity, excessive depth to groundwater, or presence of bedrock.

Highly dependent on location, existing infrastructure and depth of the proposed PRB.

Permeable reactive media may be injected using temporary/permanent wells, or may be trenched/excavated. Greater than 45 feet to base of PRB would be beyond practical depth of trenching or excavation. Difficult to implement in bedrock. Permeable reactive media may require some frequency of reactivation/replacement.

Highly dependent on location, existing infrastructure, depth and geology.

Existing ponds, dams, roads, buildings and utilities would affect the installation and related cost. Costs usually vary with depth of installation. For injection systems, numerous overlapping injections would be required to install a high-integrity system. The required reactive media volume and/or flow-through thickness has a large influence on project cost.

Retained

Point of UseMonitoring/Treatment

Treat groundwater at the point of use to remove COIs to acceptable levels, or provide a replacement water supply.

Above Average

Effective for removing COIs from groundwater at the point of use. May require monitoring to ensure effectiveness.

Average

Requires installation and maintenance of treatment units at each point of use. May require monitoring to evaluate reactivation/replacement schedules.

Lower than Average

Costs to install water treatment units at each point of use is relatively low. Costs for routine monitoring, maintenance and reactivation/replacement is expected moderate, depending on the number of treatment units and the length of operation.

Retained

Distal Migration Management



SOEP/STEP SITE




City Ordinance

An ordinance is passed by the city council prohibiting the use of impacted groundwater within a geographic area under the city's legal jurisdiction.

Average

Effective for preventing access to impacted groundwater. However, it does not reduce COI mass or concentrations.

Average

Could be implemented as a stand-alone remedy or in combination with other alternatives. Legal requirementsand authority necessary. The state prefers that treatment or engineering controls be used to address principal threats. ICs are generally intended to supplement treatment or engineering controls versus used as the sole remedy.

Lower than Average

Retained

Deed Restriction

A restriction is placed on a property deed prohibiting the use of impacted groundwater on that property.

Average


Average

Could be implemented as a stand-alone remedy or in combination with other alternatives. Legal requirementsand authority necessary. The state prefers that treatment or engineering controls be used to address principal threats. ICs are generally intended to supplement treatment or engineering controls versus used as the sole remedy.

Lower than Average

Retained

Easement

Restrictions are placed on the use of real property agreed to by the landowner that mitigates the risk posed to public health, safety and welfare, and the environment.

Average

Effective for preventing access to impacted media at the Site. However, it does not reduce COI mass or concentrations.

Average

Could be implemented as a stand-alone remedy or in combination with other alternatives. Legal requirementsand authority necessary. The state prefers that treatment or engineering controls be used to address principal threat waste. ICs are generally intended to supplement treatment or engineering controls versus used as the sole remedy.

Lower than Average

Retained

Reservation


Average


Average


Lower than Average

Retained

Institutional Controls2

(ICs)



SOEP/STEP SITE




Covenant


Average


AverageCould be implemented as a stand-alone remedy or in combination with other alternatives. Legal requirementsand authority necessary. The state prefers that treatment or engineering controls be used to address principal threat waste. ICs are generally intended to supplement treatment or engineering controls versus used as the sole remedy.

Lower than Average

Retained

Controlled Groundwater Area

Mechanism or physical restriction for controlling present and future land use.

Average


Average


Lower than Average

Retained

Zoning

Zoning laws are passed prohibiting groundwater use within a geographic area under the zoning authority's jurisdiction.

Average


Average


Lower than Average

Retained

1 Master Plan for Coal Combustion Residual Waste Management Systems, Colstrip Steam Electric Station. Geosyntec Consultants. November 2015.2 With consideration of reasonably anticipated future uses of the Talen property and/or adjacent property where the landowner voluntarily agrees to implement institutional controls.


(ICs)


TABLE 6-1PRELIMINARY ASSEMBLY OF REMEDIAL ACTION ALTERNATIVES1

SOEP/STEP SITE

Colstrip Steam Electric StationColstrio, Montana


Remedy Evaluation Work Plan 1 of 10

Alternative 1

Alternative 2

Alternative 3

No Further Action Source Control Upgrades / Distal MNA

Source Control Upgrades /Enhance Existing Distal Capture System Operation

The Remedy Evaluation will use May 2015, the date of the SOEP/STEP Site Report (Hydrometrics, 2016), as the baseline for comparing remedial alternatives.

Alternative 1 assumes those remedial technologies in-place as of May 2015 will remain in-place and operated, but no additional remedial measures are taken. This alternative will serve as the baseline condition for this Remedy Evaluation. The remedial technologies from Table 5-1 that were in-place and operating as of May 2015 are listed below.

Same as Alternative 1 plus:● Upgrade/close cells (ponds) per Master Plan (Geosyntec, 2015).● Modify pumping rates of existing vertical source area capture wells.● Additional monitoring and modeling to demonstrate MNA of distal groundwater plumes.● Convert paste technology to dry stacking when cells are full of flyash per Master Plan (Geosyntec, 2015). ● Institutional controls.● Consider discontinuing operation of select distal capture wells, where appropriate.● Discontinue point of use monitoring when no longer needed.

Same as Alternative 2 except:● Modify pumping rates of existing vertical distal capture wells in lieu of distal MNA.

Evaluate Options for Enhancing the Performance of the Existing Capture System

Enhance the existing capture system by modifying the well field near a source area (pond or cell) by replacing existing vertical wells, and/or adjusting pumping rates.

N/A

● Modifying pumping rates of existing source area vertical capture wells (Figure 2-1) may enhance capture of COIs in groundwater adjacent to source areas (ponds or cells), and may further reduce the flux of COIs to downgradient (distal) areas.


Vertical Capture Wells

Groundwater is pumped from a vertical well or wells beneath or near a source area (pond or cell) to capture area(s) of groundwater.

● The existing capture wells (Figure 2-1) control COIs in groundwater adjacent to source areas (ponds or cells), and thereby reduce the flux of COIs to downgradient (distal) areas.



Horizontal Capture Wells

Groundwater is pumped from a horizontal well or wells beneath or near a source area (pond or cell) to capture area(s) of groundwater.

● The existing underdrain collection systems beneath the New Clearwell (Cell B) and Cell D produced no water in 2015(Hydrometrics, 2016).



Sumps or Interceptor Trenches

Groundwater is pumped from a sump or interceptor trench near a source area (pond or cell) to capture area(s) of groundwater.

● The existing drains and sumps in the Stage II Main Dam (Figure 2-1) intercepts COI-impacted groundwater, and thereby reduce the flux of COIs to downgradient (distal) areas.



Install/UpgradePond/Cell Liner

Install a geosynthetic clay liner (GCL) in the bottom of a pond or cell to reduce mass flux of COIs to groundwater.

● The existing pond/cell liners (Table 2-1) reduce the flux of COIs to groundwater.

● According to the Master Plan (Geosyntec, 2015), the liner for Old Clearwell will be upgraded, and when installed, Cell C will have a new liner system. ● The upgraded/new liners increase overall effectiveness in reducing the flux of COIs to groundwater.


Paste Technology


Polymer is added to flyash scrubber slurry to form a paste. Removes up to 90% of the free available water in the scrubber slurry prior to placement, and thus reduces hydraulic head on the pond liner. Paste solidifies (hardens) to some degree, but can still be graded.

● The existing paste plant (Figure 2-1) reduces the hydraulic head on the pond liner at Cells A, D and E, and thereby reduces seepage losses and the flux of COIs to groundwater.



Category Technology Description

Source Control


SOEP/STEP SITE




Alternative 1

Alternative 2

Alternative 3








Cap/ClosePonds/Cells

Close a pond or cell by either dewatering and removing solids, or covering ash in-place with an engineered cap. For ash disposal ponds, an engineered liner may be installed on top of the existing ash/paste prior to dry stacking (see below).

● The existing cover over the SOEP (Figure 2-1) reduces the flux of COIs to groundwater.

● According to the Master Plan (Geosyntec, 2015), Cells A and E are scheduled for closure and capping by 2021, and other cells at a later date depending on site operations.● The engineered caps increase effectiveness in reducing flux of COIs to groundwater.


Dry Stacking


Reconfigure paste plant to produce a dry flyash that is placed (stacked) on top of closed and lined flyash/paste disposal ponds. Trucks/conveyors replace piping between plant and lined disposal location. Trucks/conveyors and placement requires dust control.

N/A

● Dry stacking will reduce the volume of process water requiring storage.● Storage areas will still accumulate infiltration and captured groundwater.● Dry stacking will increase effectiveness in reducing seepage losses and flux of COIs to groundwater.



A physical barrier that surrounds or is adjacent to a source area to contain impacted groundwater within the source area and/or divert groundwater flow around the source area; typically constructed of low permeability soil-bentonite or Portland cement.

● The existing Stage I and Stage II Main Dams (Figure 2-1) reduce the migration of COI-impacted groundwater, and thereby reduce the flux of COIs to downgradient areas.● The existing Stage II Main Dam was constructed with a central core grout curtain to reduce the flux of COIs through the alluvium beneath the STEP.



Source Removal

(excavation)

Excavate coal combustion solids and impacted soils and dispose in a new location. Excavations have been implemented in limited areas (e.g., pipeline spills).

N/ANo excavations were planned as of 2015.

N/A N/A

Source Control


SOEP/STEP SITE




Alternative 1

Alternative 2

Alternative 3









(MNA)

Natural subsurface processes reduce COI concentrations to acceptable levels over time. Modeling tools are used to predict the fate and transport of COIs, which in combination with monitoring are used to demonstrate attenuation.

N/A

● Naturally occuring physical, chemical and biological processes may be effective in attenuating COIs in groundwater via immobilization, sorption, precipitation, redox reactions, dispersion and dilution.● Additional groundwater monitoring data may be needed to demonstrate effectiveness.● Long-term stability of some natural attenuating processes could be affected by changes such as pH or redox potential.

N/A


Enhance the existing capture system by modifying the well field in distal areas by replacing existing vertical wells, and/or adjusting pumping rates.

N/A N/A



Groundwater is pumped from vertical well(s) in distal area(s) to capture groundwater.

● The existing capture wells (Figure 2-1) control COIs in groundwater in downgradient (distal) areas, and provide a hydraulic barrier to the further migration of impacted groundwater away from source areas.

● The existing capture wells (Figure 2-1) control COIs in groundwater in downgradient (distal) areas, and provide a hydraulic barrier to the further migration of impacted groundwater away from source areas.● Consider shutting down some distal capture wells in locations where MNA is viable.

● The existing capture wells (Figure 2-1) control COIs in groundwater in downgradient (distal) areas, and provide a hydraulic barrier to the further migration of impacted groundwater away from source areas.

Horizontal Capture Wells/Interceptor Trenches

Groundwater is pumped from horizontal well(s) or interceptor trenches in distal area(s) to capture groundwater. N/A N/A N/A

Recharge Barrier

Recharge aquifer with unimpacted groundwater using injection wells or infiltration basins. N/A N/A N/A



SOEP/STEP SITE




Alternative 1

Alternative 2

Alternative 3








Permeable reactive barrier1

A permeable reactive barrier is installed across the groundwater flow path hydraulically downgradient from the source area to manage groundwater migration. The barrier allows passage of water while inhibiting migration of COIs by physical, chemical or biological processes.

N/A N/A N/A

Point of use monitoring / treatment or alternate water

supply

Groundwater is treated at the point of use to remove COIs to acceptable levels, or a replacement water supply is provided.

● Residents and businesses in the industrial park area (east of the STEP) were converted to city water during the mid-1990s, and many of those private wells were sealed.● Bi-annual monitoring is conducted at those private wells east of the STEP that remain.

● Residents and businesses in the industrial park area (east of the STEP) were converted to city water during the mid-1990s, and many of those private wells were sealed.● Bi-annual monitoring is conducted at those private wells east of the STEP that remain.● Discontinue point of use monitoring when not longer needed.


City Ordinance


N/A

● Effective in reducing potential exposure to COIs in groundwater ● Effective in reducing potential exposure to COIs in groundwater

Deed restrictionA restriction is placed on a property deed prohibiting the use of impacted groundwater on that property.

N/A● Effective in reducing potential exposure to COIs in groundwater ● Effective in reducing potential exposure to COIs in groundwater

Easement


N/A


Reservation


N/A





SOEP/STEP SITE




Alternative 1

Alternative 2

Alternative 3








Covenant


N/A


Controlled Groundwater AreaMechanism or physical restriction for controlling present and future land use. N/A


ZoningZoning laws are passed prohibiting groundwater use within a geographic area under the zoning authority's jurisdiction.

N/A● Effective in reducing potential exposure to COIs in groundwater ● Effective in reducing potential exposure to COIs in groundwater

1

2

N/A

With consideration of reasonably anticipated future uses of the Talen property and/or adjacent property where the landowner voluntarily agrees to implement institutional controls.Not applicable (not a remedy component for this alternative).


Permeable reactive barrier replaces alluvium extraction wells in select distal areas, but alluvium wells in other areas and bedrock extraction wells are still included.


SOEP/STEP SITE





Enhance the existing capture system by modifying the well field near a source area (pond or cell) by replacing existing vertical wells, and/or adjusting pumping rates.


Groundwater is pumped from a vertical well or wells beneath or near a source area (pond or cell) to capture area(s) of groundwater.

Horizontal Capture Wells

Groundwater is pumped from a horizontal well or wells beneath or near a source area (pond or cell) to capture area(s) of groundwater.

Sumps or Interceptor Trenches

Groundwater is pumped from a sump or interceptor trench near a source area (pond or cell) to capture area(s) of groundwater.

Install/UpgradePond/Cell Liner

Install a geosynthetic clay liner (GCL) in the bottom of a pond or cell to reduce mass flux of COIs to groundwater.

Paste Technology


Polymer is added to flyash scrubber slurry to form a paste. Removes up to 90% of the free available water in the scrubber slurry prior to placement, and thus reduces hydraulic head on the pond liner. Paste solidifies (hardens) to some degree, but can still be graded.


Source Control

Alternative 4

Alternative 5

Alternative 6

Source Control and Extraction Well UpgradesSource Control Upgrades / Enhance Capture System /

In Situ Treatment

Source Control and Extraction Well Upgrades / In Situ Treatment

Same as Alternative 3 plus:

● Install additional source area and downgradient (distal) area horizontal/vertical capture wells/interceptor trenches.


● Install PRB and/or slurry wall/grout curtain in select areas in lieu of capture wells.



● Replacing existing vertical capture wells and/or adjusting pumping rates may increase effectiveness of intercepting and capturing COIs in groundwater near source areas, and may reduce the flux of COIs to downgradient (distal) areas.

● Replacing existing vertical capture wells and/or adjusting pumping rates may increase effectiveness of intercepting and capturing COIs in groundwater near source areas, and may reduce the flux of COIs to downgradient (distal) areas.

● Adjusting pumping rates may increase effectiveness of intercepting and capturing COIs in groundwater near source areas, and may reduce the flux of COIs to downgradient (distal) areas.

● Installing additional vertical capture wells adjacent to source areas (ponds or cells) may increase the effectiveness of capturing COIs in groundwater, and may further reduce the flux of COIs to downgradient (distal) areas.



● Installing additional horizontal capture wells beneath or adjacent to source areas (ponds or cells) may increas the effectiveness of capturing COIs in groundwater, and may further reduce the flux of COIs to downgradient (distal) areas.

N/A

● Installing additional horizontal capture wells beneath or adjacent to source areas (ponds or cells) may increas the effectiveness of capturing COIs in groundwater, and may further reduce the flux of COIs to downgradient (distal) areas.

● Installing additional sumps/trenches adjacent to source areas (ponds or cells) may increase the effectivenness of capturing COIs in groundwater, and may increase effectiveness in reducing the flux of COIs to downgradient areas.

N/A

● Installing additional sumps/trenches adjacent to source areas (ponds or cells) may increase the effectivenness of capturing COIs in groundwater, and may increase effectiveness in reducing the flux of COIs to downgradient areas.








SOEP/STEP SITE





Cap/ClosePonds/Cells

Close a pond or cell by either dewatering and removing solids, or covering ash in-place with an engineered cap. For ash disposal ponds, an engineered liner may be installed on top of the existing ash/paste prior to dry stacking (see below).

Dry Stacking


Reconfigure paste plant to produce a dry flyash that is placed (stacked) on top of closed and lined flyash/paste disposal ponds. Trucks/conveyors replace piping between plant and lined disposal location. Trucks/conveyors and placement requires dust control.


A physical barrier that surrounds or is adjacent to a source area to contain impacted groundwater within the source area and/or divert groundwater flow around the source area; typically constructed of low permeability soil-bentonite or Portland cement.

Source Removal

(excavation)

Excavate coal combustion solids and impacted soils and dispose in a new location. Excavations have been implemented in limited areas (e.g., pipeline spills).

Source Control

Alternative 4

Alternative 5

Alternative 6


In Situ Treatment















● Additional slurry walls/grout curtains may increase effectiveness in reducing the migration of COI-impacted groundwater, and may further reduce the flux of COIs to downgradient areas.

● Additional slurry walls/grout curtains may increase effectiveness in reducing the migration of COI-impacted groundwater, and may further reduce the flux of COIs to downgradient areas.

N/A N/A N/A


SOEP/STEP SITE






(MNA)

Natural subsurface processes reduce COI concentrations to acceptable levels over time. Modeling tools are used to predict the fate and transport of COIs, which in combination with monitoring are used to demonstrate attenuation.


Enhance the existing capture system by modifying the well field in distal areas by replacing existing vertical wells, and/or adjusting pumping rates.


Groundwater is pumped from vertical well(s) in distal area(s) to capture groundwater.

Horizontal Capture Wells/Interceptor Trenches

Groundwater is pumped from horizontal well(s) or interceptor trenches in distal area(s) to capture groundwater.

Recharge Barrier

Recharge aquifer with unimpacted groundwater using injection wells or infiltration basins.


Alternative 4

Alternative 5

Alternative 6


In Situ Treatment








N/A N/A N/A

● Replacing existing vertical capture wells and/or adjusting pumping rates may increase effectiveness of intercepting and capturing COIs in groundwater in distal areas.



● Installing additional vertical capture wells in distal areas may increase the effectiveness of capturing COIs in groundwater.

● Installing additional vertical capture wells in distal areas may increase the effectiveness of capturing COIs in groundwater.● Cconsider shutting down some distal capture wells in locations where in situ treatment is feasible.

● Installing additional vertical capture wells in distal areas may increase the effectiveness of capturing COIs in groundwater.● Cconsider shutting down some distal capture wells in locations where in situ treatment is feasible.

● Installing additional horizontal capture wells/interceptor trenches in distal areas may increase the effectiveness of capturing COIs in groundwater. N/A

● Installing additional horizontal capture wells/interceptor trenches in distal areas may increase the effectiveness of capturing COIs in groundwater.

● Consider recharge barriers on north and south perimeters of source areas.




SOEP/STEP SITE





Permeable reactive barrier1

A permeable reactive barrier is installed across the groundwater flow path hydraulically downgradient from the source area to manage groundwater migration. The barrier allows passage of water while inhibiting migration of COIs by physical, chemical or biological processes.

Point of use monitoring / treatment or alternate water

supply

Groundwater is treated at the point of use to remove COIs to acceptable levels, or a replacement water supply is provided.

City Ordinance


Deed restrictionA restriction is placed on a property deed prohibiting the use of impacted groundwater on that property.

Easement


Reservation




Alternative 4

Alternative 5

Alternative 6


In Situ Treatment








N/A

● Installing a PRB may be effective in stopping the migration of select COIs, such as sulfate or selenium, may reduce the flux of those select COIs to downgradient areas.

● Installing a PRB may be effective in stopping the migration of select COIs, such as sulfate or selenium, may reduce the flux of those select COIs to downgradient areas.




● Effective in reducing potential exposure to COIs in groundwater ● Effective in reducing potential exposure to COIs in groundwater ● Effective in reducing potential exposure to COIs in groundwater





SOEP/STEP SITE





Covenant


Controlled Groundwater AreaMechanism or physical restriction for controlling present and future land use.

ZoningZoning laws are passed prohibiting groundwater use within a geographic area under the zoning authority's jurisdiction.

1

2

N/A

With consideration of reasonably anticipated future uses of the Talen property and/or adjacent property where the landowner voluntarily agrees to implement institutional controls.Not applicable (not a remedy component for this alternative).


Permeable reactive barrier replaces alluvium extraction wells in select distal areas, but alluvium wells in other areas and bedrock extraction wells are still included.

Alternative 4

Alternative 5

Alternative 6


In Situ Treatment












FIGURES

UNITS 1 & 2 STAGE IAND II EVAPORATION

PONDS SITE AREA

PLANTSITE

AREA

UNITS 3 & 4EFFLUENT HOLDINGPONDS SITE AREA

South Fork Cow Creek

Stocker Creek

Cow Creek

Administrative Order on Consent Site AreasColstrip Steam Electric Station

Colstrip, Montana

Figure

1-1

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6-May

-2016

Columbia, MD May 2016

1 0 10.5 Miles

³

Notes:

1. Plant Site Area digitized from Figure 1-1 of the Plant SiteReport(Hydrometrics, Inc., July 2015).2. Units 1 & 2 Stage I and II Evaporation Ponds Site Area digitized fromFigure 1-1 of the Units 1 & 2 Stage I and II Evaporation Ponds Site Report(Hydrometrics, Inc., March 2016)3. Units 3 & 4 Effluent Holding Ponds Site Area digitized from Figure 1-1 ofthe Units 3 & 4 Effluent Holding Pond Site Report (Hydrometrics, Inc., October2013).4. Aerial imagery source: National Agriculture Imagery Program (NAIP), 2013.

Legend

Plant Site Area

Units 1 & 2 Stage I and II Evaporation Ponds Site Area

Units 3 & 4 Effluent Holding Ponds Site Area

EastFork ArmellsCreek

City ofColstrip

Castle RockLake

Figure

2-1

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Legend

Columbia, Maryland June 2016

³

SOEP/STEP Process Ponds and Capture WellsColstrip Steam Electric Station

Colstrip, Montana

Notes:1. Scale shown is approximate.

2. Image source - Hydrometrics, Inc., Drawing file #1207200B002,Units1 & 2 Stage I & II Evaporation Pond Area, Capture andMonitoring Wells.

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AR-2285NSTP

NM

AR-10334

AR-11446

AR-6396

AR-7389

AR-8316

AR-9331

AR-1621

AR-334

AR-481

AR-578

AR-1213

A

B

C

D

E

F

G

H

I

J

K

Stage 1 & 2EvaporationPond Area

CastleRock Lake

City ofColstrip

Sewage Lagoons

Colstrip

PlantSite

Treated Sewage Water /Irrigation Storage Pond

PonderosaButte GolfCourse

Hydrologic RegimeEast Fork Armells CreekColstrip Steam Electric Station

Colstrip, Montana

Figure

2-3

NotesLegend

Columbia, MD

1,500 0 1,500750 Feet

³

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th:

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gpm - gallons per minutenm - not measured1. Discharge data provided by Hydrometrics, Inc. 2. Discharge data are reported in Synoptic Run reports from 1993 to 2014. 3. Discharge not measured south of AR-12 and north of AR-10.4. Image Source: ESRI World Imagery, Retrieved 2014-11-07.5. Stream Flow inputs can be via groundwaterseepage and/or tributaries.

AR-12 - Station Name 13 - Period of Record Harmonic Mean Discharge (gpm)

- Reach IDA

Str

ea

m D

isch

arg

e (

gp

m)

Year

!A Stations

Measurement Not Available

Gaining Reach

Losing Reach

Plant Site Boundary

? Groundwater Extraction Well (Approximate)

SOEP/STEP Site Conceptual Site Model

Figure

3-1Columbia, MD July 2016

Notes:

(a) Mechanism currently mitigated by closure and/or bottom liner

in some impoundments but might still affect groundwater

quality.

(b) This item is considered to be negligible at this time based upon

prior remediation of pipeline leaks.

(c) Constituents undergo dilution, dispersion, adsorption, and

possibly mineral precipitation along groundwater transport

pathway.

(d) Constituents undergo dilution, adsorption, and possibly

mineral precipitation along surface water transport pathway.

Inorganics in

Wastewater Surface

Impoundments

Wastewater Seepage

through Engineered

Barrier System to

Groundwater(a)

Groundwater

Advection toward

East Fork Armells

Creek and Supply

Wells(c)

Leaching to

Groundwater from

Soils Affected by

Leaks from

Conveyance

Systems(b)

Discharge to

Streams(d)

Aquatic Organisms,

Off-site Residents/

Recreational Users,

Trespasser, Wildlife

Groundwater Supply

Well

Livestock, Off-Site

Residents, Workers

Source Release Mechanism(s) Transport Pathway(s) Potential Exposure Point(s) Potential Receptor(s)

Maintenance WorkerShallow Trench to

Water Table


APPENDIX A

SOEP/STEP Site

Geologic Cross Sections

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CELL A

CASTLE ROCK LAKE/SURGE POND

CELL B,NEW CLEAR WELL CELL E

CELL D

STAGE I EVAPORATIONPOND (RECLAIMED)

STAGE II EVAPORATION PONDS

S to c k e r C r e e k

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SP S

SP N

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SP 3

PZ33

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OT-2

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GW-5

AR-7

AR-6

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991A

989D988D

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985A

984D

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979S

978S977A

976D

975D974D

973D

972D971D

970D969D

968D

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965D964D

963D962D

961D960D

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953D

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945A944A943A

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921A

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919D918A

917A

916A

915A

913A

912D910A

909A

908A903D

902D

901D900D

399D398D 397D

AR-9

AR-8

393D

392D

388D

387D

386D

385D

384D

383D

382A

381A376D375D

374S

373D

372D

371D

370D

369D

368D

367D

366S

364D

361D

358T

355D

354D

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351D

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GW-13

AR-11

2036D

2033D2032D

2031D

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2026D2025D2024D

2023D

2022D2021D

2019D

2018D 2012D

2011D2010D

2009D

2008D2007D 2006D2005D

2003D

2002A

121-2

PW-729PW-728

PW-727

PW-724

PW-722PW-721

PW-715

PW-713

PW-709

PW-708

PW-702

CA-12B

394D-P

391D-P

390D-P389A-P

EAP-208

EAP-205

PBR-AR10

MBMG P-11

MBMG P-12

AR-2

AR-1 North Flume

994A993D 992D

990D

987D983D

982D

928D

922A

914D

911D

907D

906D905D904D

395D

380D

379D

378A377A

362D

360A359D

358D

995DD2038A 2037D

2035A2034D

2020A

2017A

2016A 2015A2014A2013A

2004D

2001A

2000DPW-723

PW-717PW-705

942A-P941A-P

396D-P

EAP-413

EAP-411

EAP-119

PW-704D2PW-704D1

SOEP and STEP Area Map DetailsSOEP and STEP Area

CSES-Colstrip, MontanaFIGURE 2-1

0 1,200Feet

O

P:\Co

lstrip\0

4 GIS\

Projec

ts\12_P

lumes\

FIGUR

E 2-1 -

Map D

etails.

mxd

Note: 2013 NAIP Imagery

!( Monitoring Well!( Capture Well!( Montana Bureau of Mines and Gelogy (MBMG) Well

!. Deep Sub-McKay Private Wells#* Stream Gaging Station

Lithologic Cross Section Line (Shown on Figure 2-3 and 2-4)

Pond

Geologic Cross Section A- A' and B-B'SOEP AND STEP Area

Colstrip, MontanaFIGURE 2-3

Vertical Exaggeration = 10xNote: See Figure 2-1 for location of cross sections.

Elevat

ion (ft

amsl)

930C

931C

370D

373D

367D

903D

355D

900D

BSouthWest

B'Northeast

Distance (Feet)0 1,000 2,000 3,000 4,000 5,000 6,000

Cross Section C-C'

3450

3400

3350

3300

3250

3200

3150

3100

3050

3000

P:\Colstrip\04 GIS\Projects\12_Plumes\FIGURE 2-3 - Cross Section A B.mxd

STEP MAIN DAM (PROJECTED)

03,000

3,100

3,200

3,300

500 1,000 1,500 2,000 2,500 3,000 3,500

385D 361D

383D380D 393D 353D968D395D 960D 961D

962D963D 964D

965D358D 969D 970D 971D 351D

ASouth

A'North

Elevat

ion (ft

amsl)

392D

Distance (Feet)

Grout Curtain(Projected)

Cross Section D-D'

0 1,000Feet

Clinker

Coal

Sandstone

Siltstone/Shale/Claystone

AlluviumWell Screen

Feet0 500

Geologic Cross Sections C-C’ and D-D’ SOEP and STEP AreaColstrip, Montana

FIGURE 2-4

0 1,000Feet

Note: See Figure 2-1 for location of cross sections.

Elevat

ion (ft

amsl)

CNorthwest

C'Southeast

Distance (Feet)0 1,000 2,000 3,000 4,000 5,000 6,000

Stage l EvaporationPond

Cross Section B-B'Cross Section D-D'

367D 36

4D

121-2

375D 376D

946

947

354D

948M

966A

EAP-2

05

967P

3450

3400

3350

3300

3250

3200

3150

3100

3050

3000

958D

963D

2029

D

371D

924A

905D

920A

919D97

2D

966A

Eleva

tion (

ft ams

l)

DSouthwest

D'Northeast

3,100

3,300

3,000

3,400

3,200

Units 1&2 Stage 1 Main Dam

Divider Dike

Units 1&2 Stage II Main Dam

Grout Curtain

Distance (Feet)0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

918A20

21D

2016

A

Dam Core

Stage l Evaporation Pond

Cell ACell E

Cross Section A-A'

Cross Section C-C'

Fly Ash

Divider Dike

Alluvium

Gravel

Clinker

Siltstone/Claystone

Sandstone

CoalWell ScreenVertical Exaggeration = 10x

dmccammon

Rectangle


APPENDIX B

SOEP/STEP Figures Showing

Interpreted Extent of Indicator Parameters

!(!(

!(!(!(!(

!(!( !(

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!(

!(

!(

!(!(

!(!(

!(

!(

!(

!(

!(

!(

!(

!(

!(!(

!(!(

!(!(

!(

!(!(!(!( !(

!(

!(

!(!(

!(

!(

!(

!(!(!(!(

!(

!(

!(

937A1.7

935A1.9

936A1.7

2001A35.8

2002A20

2013A3.3

2014A1.9

2015A2.52016A

4

2017A10.7

2020A2.6

2028A1.9

2035A8.6

2038A6.4

360A13.5

366S5.8

374S1.8

377A12.7

378A27.3

382A4.3 908A

2909A2.2

910A1.9

913A1.4

915A2.3

916A3.2

917A0.8

918A1.1

920A19.8

921A25

922A27.1

923A20.9924A

16.8

938A1.1939A

1.3

940A1.1

941A-P1.3

942A-P2

943A1.6

944A1.1

945A2.1

94630.9

94717.8

966A37

967P21.1

977A1.7978S

1.2

985A11.1

991A1.4

994A3.2

997A10.1998A8.2

Dissolved Boron in Alluvial GroundwaterSOEP and STEP Area


!( Monitoring Well (March - December 2015)Boron Concentration (mg/L)

> 1.6 to 5.0> 5.0 to 20> 20

Service Layer Credits: Source: Esri, DigitalGlobe, GeoEye, EarthstarGeographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid,IGN, IGP, swisstopo, and the GIS User Community

P:\Co

lstrip\0

4 GIS\

Projec

ts\12_P

lumes\

FIGUR

E 2-24

- Boro

n Alluv

ium.mx

d

O0 1,400Feet

NOTE:Baseline screening level (BSL) for Boron inAlluvium = 1.6 mg/L (Neptune 2015) as ofDecember 2015

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(!(

!(!(

!(

!(

!(

!(!(

!(

!(!(!(!( !(

!(

!(

!(!(

!(

!(

!(

!(!(!(!(

!(!(

!(!(!(!(

!(!( !(

!(!(

!(

!(

!(

!(!(

!(

382A82

374S24

998A67

978S24

977A28

947101

946174

940A58

939A76

938A55

937A94

936A58

935A56

918A59

917A44

916A57

913A48

910A57

909A76378A

141

366S104

360A105

924A131

923A126

922A125

921A124

920A173

2002A104

377A86 997A73994A

36

991A25

945A70944A

34 943A74

915A75

908A78

2038A88

2035A62

2028A61

2020A63

2015A91

2014A76

2013A79

985A120

967P145

966A131

2017A105

2016A150

2001A102

942A-P84941A-P

71

!( Monitoring Well (March - December 2015)Chloride Concentration (mg/L)

> 45 -100> 100


P:\Co

lstrip\0

4 GIS\

Projec

ts\12_P

lumes\

FIGUR

E 2-25

- Chlo

ride All

uvium

.mxd

O0 1,400Feet

Chloride in Alluvial GroundwaterSOEP and STEP Area


NOTE:Baseline screening level (BSL) for Chloride inAlluvium = 45 mg/L (Neptune 2015) as ofDecember 2015

!(

!(!(!(!( !(

!(

!(

!(!(

!(

!(

!(

!(!(!(!(

!(!(

!(!(!(!(

!(!( !(

!(!(

!(

!(

!(

!(!(

!(!(

!(

!(

!(

!(

!(

!(

!(

!(

!(!(

!(!(

!(

!(

!(

!(!(

944A4610

966A10600

940A3390

941A-P4890

942A-P5540

943A5090

945A5310

9469400

9473720

967P12300

977A4940978S

4340

985A8790

991A3540

994A3760

997A5350998A5260

2001A9700

2002A9660

2013A4340

2014A3610

2015A38302016A

5640

2017A8390

2020A3920

2028A3710

2035A5040

2038A4770

360A8700

366S7350

374S4800

377A5750

378A8660

382A4280 908A3640

909A3330

910A3530

913A3380

915A5340

916A4810

917A2700

918A2730

920A9730

921A8810

922A9340

923A9140

924A10400

935A3960

936A3800

937A4750

938A3240939A

3560

!( Monitoring Well (March - December 2015)SC Concentration (umhos/cm)

> 4,270 to 7,000> 7,000 to 10,000> 10,000


P:\Co

lstrip\0

4 GIS\

Projec

ts\12_P

lumes\

FIGUR

E 2-26

- SC A

lluvium

.mxd

O0 1,400Feet

Specific Conductance in Alluvial GroundwaterSOEP and STEP Area


!(!(

!(!(!(!(

!(!( !(

!(!(

!(

!(

!(

!(!(

!(!(

!(

!(

!(

!(

!(

!(

!(

!(

!(!(

!(!(

!(!(

!(

!(!(!(!( !(

!(

!(

!(!(

!(

!(

!(

!(!(!(!(

!(

!(

!(

966A8490

2001A8390

2002A8160

2013A2420

2014A1980 2015A

21102016A3240

2017A6360

2020A2260

2028A2070

2035A3220

2038A2900

360A6530

366S5220

374S3230

377A3700

378A6840

382A2580 908A

2020909A1810

910A1800

913A1730

915A3270

916A2770

917A1350

918A1360

920A7360

921A7380

922A7660

923A7410924A

8330

938A1620939A

1970

940A1820

941A-P2790

942A-P3330

943A2900

944A2490

945A3090

9467680

9472330

967P13000

977A3330978S

2660

985A6750

991A1720

994A2010

997A3430998A3390

935A2220

936A2000 937A

2770

!( Monitoring Well (March - December 2015)Sulfate Concentration (mg/L)

> 2,600 to 6,000> 6,000 to 8,000> 8,000


P:\Co

lstrip\0

4 GIS\

Projec

ts\12_P

lumes\

FIGUR

E 2-27

- Sulfa

te Alluv

ium.mx

d

O0 1,400Feet

Sulfate in Alluvial GroundwaterSOEP and STEP Area


!(

!(!(

!(!(!(!(

!(

!(

!(

!(

!(!(

!(

!(!(

!(!(!(

!(!(

!(

!(!(

!(!(!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!( !(

!(

!(!(

!(

!(

!(

!(

!(

!(!(

!(

!(

!(

!(

!(

!(

!(!(!(

!(

!(!(

!(

!(

!( !(!(

!(

!(!(

!(

!(

!(

!(!(

!(

!(!(!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(!(

!(!(!(

!(

!(

!(

!(

!(!(

!(

!(!(

!(

!(

!(!(!(!(!(!(!(!(

!(!(!(

!(

!(!(

!(

!(

!(

!(

!(

!(!(

!(

!(

!(

!(!(

!(

!(

!(!(

929D1

926S3

370D0

350D1

368D22

2025D0

2011D1

2005D1

979S1.5

975D1.6

974D0.4

973D0.7

972D0.7

969D1.8

958D1.1

957M1.5

953D0.4

950D0.5

949D1.4

948M0.5

919D0.3

903D0.6

902D1.2

901D0.4

386D0.7

385D0.3

384D0.9

375D2.5

373D2.1

372D0.8

371D3.4

369D4.9

367D0.9

355D0.5

354D0.1

376D13.3

2032D0.3

2026D0.2

2023D0.4

2019D9.6

2018D0.6

2010D0.8

2009D0.9

121-21.1

EAP-20826.1

388D1

971D0.6

970D0.8

964D3.1

963D9.5

959D0.3

900D0.2

399D0.4

398D0.3

397D0.3

389A-P2

364D0.6

362D0.3

361D0.3

358D0.8

351D0.7

976D17.7

965D16.2

2033D2.5

2031D0.4

2022D0.3

2006D1.3

391D-P2.3

390D-P1.1

PW-7290.21

PW-7280.52

PW-7270.28

PW-7090.22

EAP-4110.3

EAP-1191.6

EAP-20516.1

2024D-216.8

!( Monitoring Well (March - December 2015)Boron Concentration (mg/L)

> 1.3 to 5.0> 5.0 to 20> 20


P:\Co

lstrip\0

4 GIS\

Projec

ts\12_P

lumes\

FIGUR

E 2-28

- Boro

n Bedr

ock.mx

d

O0 1,400Feet

!(

!(

!(

!(!(!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(!(

!(

!(

!(

!(

!(

!(

!(

!(

!(!(

!(

!(

!(

!(

!(

!(

!(

!( !(

!(!(

!(!(

!(!(

!(

!(!(

!(

!(

!(

!(

!(!(

!(

!(

!(

!(!(

!(

999D1

379D3962D

13

2027D1

996D0.3

993D1.3

992D1.2

987D0.7

984D1.2

982D1.6

981D0.6

968D9.8

964D3.1

963D9.5

961D9.3

952D1.2

951D0.3

933D0.5

932D5.7928D

1.4

927D8.7

914D1.2

912D0.4

911D8.4

905D1.7904D

0.5

395D7.3

392D1.1

383D4.1

380D4.8

361D0.3

353T0.5

989D17.9

960D12.8

907D28.5

393D10.1

2036D0.6

2029D0.62021D

4.1

2012D2.2

2008D2.8

2007D0.4

2004D2.4

2003D7.52000D

2.7

394D-P1.3

PW-7231.24

PW-7221.09

PW-7211.26

PW-7130.29

PW-7050.27

990D1.2

988D7.6

986D0.6

983D0.9

906D6.5

359D0.3

995DD0.2

934D14.4

2037D2.9 2034D

0.7

396D-P2.3

EAP-4134.3

PW-704D28.3

PW-704D14.4

Dissolved Boron in Bedrock GroundwaterSOEP and STEP Area


NOTE:Baseline screening level (BSL) for Boron inBedrock = 1.3 mg/L and in Coal Related = 1.1mg/L (Neptune 2015) as of December 2015Figure includes water quality from wellsscreened in Clinker, Overburden, McKay Coaland Sub-McKay

LOCATION

!(!(

!(

!(

!( !(!(

!(

!(!(

!(

!(

!(

!(!(

!(

!(!(!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

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!(

!(

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!(

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!(

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!(

!(

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!(

!(

!(

!(

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!(

!(

!(

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!(

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!(

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!(

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!(

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!(

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!(

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!(

!(

!(

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!(!(!(

399D7

370D2

354D8

948M8

398D10

397D13

388D15

387D26

386D25

385D20

384D28

376D83

375D26373D

10

372D28

371D20

369D22

368D52

367D46

364D14

358D29

355D12

351D21

350D28

2026D7

2025D4

979S11

976D50

975D18

974D11

973D19

972D41971D

15

964D44

959D10

958D15

957M30

953D20

950D16

949D15

929D27

926S37

919D14

903D46

902D17

901D20

900D15

391D-P5

2033D76

2032D302031D

15

2023D18

2022D33

2019D79

2018D20

2010D23

2009D24

2006D79

121-220

390D-P14

PW-72917

PW-72820

EAP-20841

EAP-205108

362D17

361D20

970D16

969D32

963D81

2011D20

2005D22

965D145

389A-P17

PW-72712

PW-70919

2024D-261

EAP-41127

EAP-11925

!( Monitoring Well (March - December 2015)Chloride Concentration (mg/L)

> 24 to 100>100


P:\Co

lstrip\0

4 GIS\

Projec

ts\12_P

lumes\

FIGUR

E 2-29

- Chlo

ride Be

drock.

mxd

O0 1,400Feet

!(!(

!(

!(

!(

!(

!(

!(

!(

!(

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!(

!(

!(

!(

!(

!(

!(

!( !(

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!(

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!(

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!(

!(

!(

!(

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!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

EAP-41367

395D66

393D59

392D17

387D26

383D54

380D56

379D53

359D18

353T21 999D

37

996D22

993D24

992D32

987D24

984D37

982D67

981D24

968D63

963D81

962D62

961D81

960D76

952D22

951D22

934D90933D

16

932D62

928D37

927D64

914D23

912D32

904D23

2036D29

2029D15

2027D40

2012D34

2008D46

2007D21

2004D44

2003D612000D

46

989D128907D

128

394D-P20

2021D105

PW-72361

PW-72232

PW-72134

PW-71310

PW-70514

990D32

988D77

986D19

983D33

906D72

905D50

2037D30

2034D26

995DD18

911D104

396D-P27

PW-704D268

PW-704D137

Chloride in Bedrock GroundwaterSOEP and STEP Area


NOTE:Baseline screening level (BSL) for Chloride inBedrock = 24 mg/L and in Coal Related = 20mg/L (Neptune 2015) as of December 2015Figure includes water quality from wellsscreened in Clinker, Overburden, McKay Coaland Sub-McKay

LOCATION

!(

!(

!(

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!(

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!(

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370D677

929D5560

926S3840

919D4400

903D5000

902D3670

901D3300900D3550

399D1590

398D1940

397D2640

388D2960

387D3660

386D2730

385D3880

384D2960

376D6320

375D2970373D

1650

372D3860

371D4760

369D3930

368D7160

367D5220

364D2570

358D4960

355D2640

354D1400

351D4130

350D3890

2025D957

979S3880

976D6870

975D3710

974D2630

973D3910

972D4630971D

4180

963D6980

959D2480

958D3770

957M4140

953D4110

950D4120

949D2940

948M1680

2033D5370

2032D42502031D

2900

2026D1870

2023D4720

2022D5240

2019D5330

2018D3630

2010D3840

2009D4120

2006D2730

121-22820

391D-P1540

390D-P3140

PW-7292770PW-728

3640PW-709

2930

EAP-2086520

EAP-2057250

362D2500

361D3360

970D4170

969D4960

965D9670964D5600

2011D4020

2005D3280

389A-P3340

PW-7272760

2024D-26170

EAP-4114030

EAP-1194520

!( Monitoring Well (March - December 2015)SC Concentration (umhos/cm)

> 3,550 to 4,470 (Exceeds Coal Related BSL)> 4,470 to 7,000> 7,000


P:\Co

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4 GIS\

Projec

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FIGUR

E 2-30

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Specific Conductance in Bedrock GroundwaterSOEP and STEP Area


NOTE:Baseline screening level (BSL) for SpecificConductance in Bedrock = 4,470 umhos/cm and inCoal Related = 3550 umhos/cm(Neptune 2015) asof December 2015

Figure includes water quality from wells screened inClinker, Overburden, McKay Coal and Sub-McKay

LOCATION

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PW-7291090

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> 2,200 to 6,000> 6,000 to 8,000


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FIGUR

E 2-31

- Sulfa

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NOTE:Baseline screening level (BSL) for Sulfate in Bedrock= 2,200 mg/L and in Coal Related = 2,061 mg/L(Neptune 2015) as of December 2015Figure includes water quality from wells screened inClinker, Overburden, McKay Coal and Sub-McKay

LOCATION


APPENDIX C

SOEP/STEP Figures Showing

Uncaptured Areas

E a s t F o r kA r m

e l l s

C r e e k

CELL A



CELL D



OLD CLEARWELL

Uncaptured Areas From Source AreasSOEP and STEP Area


0 1,200Feet

O

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FIGUR

E 6-9 -

Uncap

tured A

reas F

rom So

urce A

reas.m

xd


Starting Location For Uncaptured Particles

SOEP STEP Source Areas


e l l s

C r e e k

CELL A



CELL D



OLD CLEARWELL

Uncaptured Areas Exceeding BSLsOutside of Source Areas -Layer 2

SOEP and STEP AreaCSES-Colstrip, Montana

FIGURE 6-10

0 1,200Feet

O

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E 6-10

- Unca

ptured

L2.mx

d


Starting Locations for Uncaptured Particles Outisde of Source Areas

Extent of Groundwater Exceeding Baseline Screening Levels - Layer 2 and Layer 3

SOEP STEP Sources Areas


e l l s

C r e e k

CELL A



CELL D



OLD CLEARWELL



FIGURE 6-11

0 1,200Feet

O

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E 6-11

- Unca

ptured

L3.mx

d



Extent of Groundwater Exceeding Baseline Screening Levels - Layer 2 and Layer 3



e l l s

C r e e k

CELL A



CELL D



OLD CLEARWELL



FIGURE 6-12

0 1,200Feet

O

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E 6-12

- Unca

ptured

L4.mx

d



Extent of Groundwater Exceeding Baseline Screening Levels - Layer 4



e l l s

C r e e k

CELL A



CELL D



OLD CLEARWELL



FIGURE 6-13

0 1,200Feet

O

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FIGUR

E 6-13

- Unca

ptured

L5.mx

d



Extent of Groundwater Exceeding Baseline Screening Levels - Layer 5


Documents

Remedy Evaluation Work Plan Units 1 & 2 Stage I and II