TECHNICAL MEMORANDUM: REVIEW OF THE SUPPLEMENTAL DRAFT ENVIRONMENTAL

IMPACT STATEMENT

NorthMet Mining Project and Land Exchange

March 10, 2014

Prepared for: Minnesota Center for Environmental Advocacy

Prepared by: Tom Myers, PhD, Hydrologic Consultant, Reno NV

The U.S. Forest Service, Minnesota Dept of Natural Resources, and the U.S. Army Corps of

Engineers have released the supplemental draft environmental impact statement (SDEIS) for

the proposed Northmet Mining Project and Land Exchange. This technical memorandum

reviews that SDEIS with a focus on hydrogeology. All references simply to page numbers, tables

or figures are to that document. This author also prepared a review of the Goldsim modeling

used in support of the SDEIS (Myers 2014b) and an independent assessment of the flow and

transport at the site (Myers 2014a). These documents are referenced freely within this

document and should be considered as part of the overall review of the hydrogeology. Finally,

other Polymet documents have been reviewed as part of this review or as part of Myers (2014a

and b).

The review is structured in sections based on topics. Where the topics overlap or a comment

may apply to more than one topic, there are references to the comments at other locations.

Lack of Alternatives Analysis

The DSEIES considers effectively just the proposed action and the no action alternative. The

executive summary (p ES-42) claims that alternatives which would make the mine seep less

water have already been included, so they are not considered as an alternative. The ES also

eliminates from further consideration “alternative wet and dry closure options for the Tailings

Basin, backfilling the West Pit with Category 1 waste rock, and underground mining” (p ES-42).

West Pit Backfill was rejected without substantial argument or technical analysis because they

claim it would not offer substantial environmental benefits (p 3-151). Backfill would increase

constituent loads in the pit (p 3-151), but they do not discuss this in detail. The total load

reaching the groundwater from backfill could be substantially less than the perennial load from

the stockpile, especially if the Cat 1 stockpile containment system does not work as well as

expected. This could have considerable environmental benefits not adequately considered in

the SDEIS (the SDEIS claims the only advantage is the “opportunity to reclaim wetlands” at the

Cat 1 footprint area (p 3-152).

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Recommendation: Polymet should give the West Pit backfill option a thorough evaluation with

the Goldsim model so that there could be a comparison of concentrations with and without

backfill.

They also note there are “additional mineral resources in the West Pit that would effectively be

lost if the pit was” backfilled (p 3-149). Those resources might also be lost if they had to pump

and treat 80,000 af of pit lake water to access additional deposits. The SDEIS should consider

whether accessing those deposits would be possible once a pit lake forms.

Recommendation: Discuss the difficulties associated with pumping out a pit lake, and treating

the water, to recover ore at the bottom of the pit. Consider these difficulties in discussion and

decision as to whether to consider this as an option.

Polymet has not prepared a mining and reclamation plan (p 3-5), so the exact timing of

activities must be considered imprecise; this applies in particular to the water quality modeling

as presented throughout the SDEIS.

Recommendation: Acknowledge that the times for various management activities during

reclamation and closure are approximate because the reclamation plan has not been prepared.

The model comparisons use a Continuation of Existing Conditions scenario to compare to the

project (p 5-78). This scenario is considered for modeling only because it does not include the

potential for any future mitigation that may occur at the existing tailings impoundment. They

distinguish this alternative from the No Action alternative in that the No Action alternative

allows change including the implementation of other projects, required mitigation at existing

facilities, and climate change.

Hydrogeology of the Site

The SDEIS claims there will be a “130-ft separation between the final pit and Biwabik

Formation” (p 4-43). This is important to their analysis because they consider the Biwabik to

have a much higher permeability. They ignore the possibility of a fracture connection between

the pit and the Biwabik, which could require much higher dewatering rates. The conductivity

values (Table 4.2.2-5) are provided in a not-useful way. Without knowing the number of

samples, simply providing a range and a geometric mean is not meaningful. It would be better

for the table to simply list all observations especially since they range over several orders of

magnitude.

The SDEIS overstates the roll of gouge in filling the faults in the areas (p 4-45). The original

literature (Foose and Cooper 1978) referred only to the NE trending faults as being gouge filled.

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Shorter faults, on the order of km, could still be significant transport pathways. This literature

should not be a substitute for an adequate fracture analysis of fractures.

The SDEIS acknowledges that some tests using wells that penetrated “through the surficial

zone” (p 4-45) found “much higher average hydraulic conductivity, with values similar to the

Biwabik Formation aquifer” (Id.). The bedrock is very shallow so this suggests that there may

not be a sharp delineation between the surficial aquifer and the bedrock. Myers (2014a) found

in the development of a groundwater model for the site that the surficial aquifer by itself was

too thin to provide the needed transmissivity and that it was apparent that flow occurs through

the bedrock.

Recommendation: The analysis that relies on low permeability bedrock just under the till

should be reconsidered and potentially redone. Additional fracture analysis should be

completed to better consider the potential for fractures or faults being connected to more

distant resources.

Modeling Comments from the SDEIS

Base parameters for the SDEIS modeling were set using MODFLOW and XP-SWMM modeling

(5-26, -27). The mine site MODFLOW model is a small telescoped section of a large regional

model. The head along the mine site model was set from the head distributions predicted by

the regional model. Because the regional model had just two layers while the telescoped site

model had eight layers, this practice introduces potentially significant errors, essentially

eliminating vertical gradients among layers by setting the head in seven bedrock layers equal to

the head in one regional bedrock layer.

The SDEIS touts that an “important calibration constraint” for the MODFLOW recalibration is

that “predicted hydraulic head in the surficial aquifer would not be above ground surface” (p 5-

27). This seems like an appropriate constraint, but in reality is unnecessary and may even

introduce inaccuracies to the calibrated model. There are at least three considerations. First,

the simulation sets the upper layer as unconfined, but the reality is that some areas, especially

with perched wetlands, could be fully or partially confined. Confined conditions could cause

the head in areas in reality to be higher than ground surface. Second, the model limits the flow

to a specified layer thickness whereas the reality is that thickness is variable. Areas with

simulated head slightly above ground surface may reflect that transmissivity is too low due to

the thickness of the aquifer. Third, allowing the head to extend above the ground surface does

not in any way affect the predicted results of river discharge or drawdowns; the only time it

causes an error is if the model is simulating evapotranspiration, which is not being done herein.

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There are questions regarding the recharge value used to calibrate the model. Myers (2014 a

and b) consider this issue in detail. Calibration with a recharge that is too low causes

inaccuracies throughout the MODFLOW model which are then passed to the Goldsim model

(Myers 2014b).

Recommendation: Recalibrate the MODFLOW model based on higher recharge which would

yield more realistic conductivity parameters (see comments elsewhere).

The SDEIS claims there is very little transport in the bedrock flowpaths, primarily because of its

“very low bulk conductivity” (p 5-53). The low bulk conductivity is largely the result of the

groundwater model calibration and assumptions about bedrock conductivity, reviewed above,

based on the low recharge estimate. Because the modeling does not account for the secondary

conductivity and scale effects, it is likely that both the effective conductivity is underestimated

and that flow could take much more substantial pathways than assumed by Polymet.

Recommendation: Consider a much higher range of bedrock conductivity in the Goldsim

modeling.

The description of how they determine the P90 value from a Goldsim run, described in the third

paragraph of p 5-77, appears wrong. Goldsim is run 500 times, or “conducts 500 simulations of

a 200-year period with monthly time steps” (p 5-77). Each 200-year simulation results in a

monthly observation of each output, or one value for each month for 2400 months. That is not

the equivalent of “2,400 water quality predictions (200 years times 12 months per year) for

cobalt” (Id.). It is one prediction for each month. Running Goldsim 500 times generates 500

time series of monthly predictions. Thus the P90 is determined for any month based on the

500 generated values for that month, not based on 2400 realizations over a 200-year period (as

repeated on p 5-106). The P90 value is that which lies at number 450 in a ranking from 1 to 500

of the lowest to highest simulated values. All of the output graphs show P10, P50, and P90

values with time, by month, for 200 years, so it appears that it was done correctly using the

Goldsim output as generated.

Recommendation: Describe the processing of model output to obtain the P90 value correctly.

Partridge River Baseflow

There is controversy over the estimate and use of baseflow on the two rivers draining the

project site. Baseflow is the surface flow that consists wholly of groundwater discharge to the

river, although in reality it is difficult to separate from runoff. Baseflow varies throughout the

year depending on precipitation. After a wet period, once surface runoff ceases the remaining

flow is baseflow even though that baseflow may actually exceed peak flows from smaller runoff

events, especially those occurring after a long dry period.

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A critical water quality period in the Partridge and Embarrass Rivers occurs during baseflow

because surface water flow would consist strictly of groundwater discharge and its water

quality then would equal the water quality of the groundwater discharge. Very low flow

baseflow periods are often considered the critical periods for estimating discharge effects on

water quality. The Polymet SDEIS uses the annual 30-day low flow as the critical baseflow for

water quality considerations (SDEIS, p 4-66; Kruse 2013), which is a proper strategy.

It is not proper, however, to assume the 30-day low flow equals the average basinwide

recharge in this watershed because it is not possible to assume the entire watershed is

contributing during baseflow conditions. The baseflow may be recharge but only from a

portion of the watershed. By setting recharge equal to the 30-day low flow, Polymet’s recharge

estimate was about 0.8 in/y, which is not correct because it is based on only a small portion of

the watershed contributing baseflow.

It is common to set the average baseflow from a watershed equal to some duration, often 30

days, low flow and to then set that flow rate equal to the average recharge over the basin

(Myers 2013, 2009). This equivalence would only apply if the entire watershed is contributing

groundwater to the discharge. In small watersheds in which the flow path from the points of

recharge to discharge is substantially less than a year, the amount of watershed contributing at

any given time may be substantially less than the entire area. This would also manifest in

watersheds in which most of the precipitation is frozen for much of the year.

Myers (2014a) estimates a significantly higher recharge based on a new baseflow

reconstruction. Myers (2014b) provides substantial comments on the use of the 30-day

baseflow for recharge.

Recommendation: Polymet should reconsider their recharge estimate for modeling based on

Myers (2014a and b) and recalibrate the MODFLOW model based on that correct recharge

value.

Water Quality

Polymet relies on detailed modeling with the Goldsim model to estimate water quality

parameters resulting from this project. The model details itself are discussed in Myers (2014b)

and in the previous section. The general conclusion is that the P90 value does not exceed the

evaluation criteria at any of the groundwater, either surficial or bedrock, evaluation points for

any of the constituents (Figure 5.2.2-20, p 5-113).

Recommendation: This is a rather general recommendation in that the Goldsim modeling

should be redone with better parameterization from the MODFLOW model, as discussed in the

previous section.

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The SDEIS argues that there is no potential for saline groundwater to be drawn to the mine site

area due to changes in hydrogeology (5-113, -114). The reasoning is sound, but uncertainty

remains. The claim that dewatering pumping would capture inflow before it reaches the pit (5-

114) is not sound because saline inflow from depth would likely enter the bottom of the pit.

Also, if the transport pathway for saline groundwater is a large fracture, the inflow may be

much higher than expected and dilution may not occur as expected. The predicted relative

rates of groundwater inflow from bedrock and surficial aquifer (p 5-114) are based on very

incomplete knowledge of conditions at depth which would control inflow from depth, including

of saline groundwater.

Recommendation: Polymet should establish a series of deep groundwater monitoring wells,

screened at locations in the 500 feet below the pit bottom, to assess both changes in head and

water quality. There should be continuous electrical conductivity monitoring so that salinity

changes can be quickly detected.

Polymet’s predictions for surface water quality are that at all locations above Colby Lake and for

all constituents modeled, only sulfate and aluminum will exceed the criteria. The project does

not however increase the concentration above the continuation of existing conditions

simulation. The sulfate exceedences occur at SW005 and SW006 because the wild rice

standard applies at that point.

The SDEIS details the movement of SO4 through the system and how SO4 predictions would

occasionally violate the standard at SW005 (p 5-140 to -142). They note the exceedences are

most likely after 70 or more years because that is the travel time for SO4 through groundwater

pathways, but also note the low flow rate through these pathways limits the effect of the

groundwater discharge. They also claim that flows from some sources have sufficiently low

concentrations that the flow helps to dilute the transport from other sites. Using the Monte

Carlo sampling in Goldsim, the SDEIS claims that the project results in at most a 6.6% increased

chance of SO4 violating standards at SW005 (Figure 5.2.2-28). The model also shows that the

P50 and P10 values of SO4 actually decrease.

For aluminum, the concentrations decrease due to dilution. A few constituent (ie., arsenic,

antimony, cadmium, cobalt, copper, lead, nickel and selenium) concentrations increase due to

the project, but remain far below the criteria. The modeling assumes these constituents will be

treated but the results do not have to be as perfect as for sulfate because the background

concentrations are lower. The project will degrade water even if concentrations remain below

the evaluation criteria.

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These predictions are the result of modeling which has been reviewed elsewhere (Myers 2014 a

and b) and in this review. These results rely on perfect engineering and severe model

assumptions (Myers 2014b).

Recommendation: Reconsider the modeling of the surface water quality considering the

comments in this review, Myers (2014 a and b), and the review documents prepared by Ann

Maest, Glenn Miller, Dave Chambers, and Mike Malusis.

Contingency Plans: The SDEIS discusses contingency plans that could be employed if

groundwater monitoring below mine features shows that concentrations exceed the model

predictions (p 5-143). They basically claim that if the monitoring shows the project “would

cause or increase exceedances of the applicable evaluation criteria for sulfate, then

contingency measures could be implemented and adapted as necessary to decrease … effect on

the Partridge River prior to an actual effect occurring” (p 5-143). There are several proposed

contingency measures discussed below:

The WWTF could be changed to generate effluent with sulfate concentrations less than

9 mg/l, down to an average and maximum of 3.7 and 6.9 mg/l, respectively. Because

the discharge exceeds the groundwater flow Polymet believes this would dilute the

higher concentrations. This assumes they could time the groundwater flow and of

course that these treatments could be achieved. See the reviews of Glenn Miller and

Ann Maest for comment on whether this is possible.

Polymet could temporarily increase the discharge rate from the WWTF to dilute the

groundwater flows. Again, this assumes they could time it perfectly and also that the

WWTF discharge rate could be easily and cost effectively increased to meet this

proposed requirement.

They could install groundwater containment facilities in the various flow paths. These

are not described in the SDEIS, but there is an implication that water would be collected

and treated. Pumpback wells on large flowpaths may not be effective or may be very

costly to construct and operate. Also, the WWTF would have to be upgraded to the

higher flow rate. Once a contaminant is in the groundwater, it is very difficult to

remove; Polymet is assuming they could do it with little difficulty.

They also suggest there could be non-mechanical treatment systems along flowpaths.

Considering the flowpaths are groundwater, there is little clue as to what these could be.

Recommendation: The SDEIS should describe both the groundwater monitoring system and

better describe how the contingency plans could actually work. They should demonstrate with

their MODFLOW model that pumpback or other groundwater treatment can work. They should

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demonstrate using Goldsim that they can actually time the groundwater discharges to surface

water so as they can actually increase the WWTF discharges for dilution.

Surface water quality on the Embarrass River will change as a result of the project, though

Polymet claims no standards will be violated. Concentrations for arsenic, copper, lead, nickel

and zinc will increase because the augmentation water discharged from the WWTP will have

concentrations higher than that discharging currently from the tails (p 5-182).

Recommendation: The SDEIS should discuss why the WWTP cannot treat the water so that

degradation to the river will not occur.

Background Water Quality: Aluminum and copper increase slight in a downstream direction

(4-74). Some background, or natural groundwater concentrations (beryllium, manganese,

thallium) exceed published standards but Minnesota allows the natural level be used as the

standard. This is not unreasonable, but it should not be used to allow the project to increase

those standards, whether by discharge or by changes in the hydrogeology due to the project.

Recommendation: In cases where the natural background exceeds published standards, the

project should not be permitted to cause any increase in concentrations or further degradation

of the resource. This means any discharge (seepage) much have a lower concentration than

natural so that the net effect is dilution.

Mercury is a constituent of concern because it is on the impaired water list due to high Hg

content in fish tissue (p ES-36). Colby Lake is on 303(d) list for Hg concentrations in fish, but is

not subject to the regional TMDL because the concentrations are too high and caused by

atmospheric deposition. Polymet suggests it will stabilize in the West Pit Lake at 0.9 ng/l, or

slightly below the standard. They indicate the load to the Embarrass River will increase due to

seepage from the tailings but that reductions in the Partridge R will offset the increase for the

St Louis watershed as a whole. They also claim that 92% of the Hg in the tails will remain there;

this claim must be considered. The project would release some Hg and potentially increase

concentrations but is not predicted to exceed standards (p 5-8).

Recommendation: Polymet should better justify the claim that 92% of the mercury will remain

in the tails.

Mine Dewatering

The dewatering rate for keeping a pit dry is controlled by conductivity, storage coefficients and

gradient. The gradient changes simply by lowering the water table. The conductivity controls

the gradient and the storage coefficient controls the volume of the drawdown cone. These

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values could vary by an order of magnitude, or more. There is a significant range in dewatering

rates that could result from such a range.

The SDEIS essentially argues that the MODFLOW-predicted drawdown is inaccurate (p 5-92). If

that is correct, that drawdown estimates from MODFLOW are inaccurate, then dewatering

estimates are also inaccurate.

Recommendation: The SDEIS should acknowledge there is a large uncertainty in the estimated

dewatering, and provide an estimated range of dewatering rates.

The SDEIS does not discuss the fate of the dewatering water. Its use does not seem to come

up. The SDEIS mentions make-up water from Colby Lakes, but not the use of dewatering water.

Recommendation: The SDEIS should identify what the dewatering water will be used for or

whether it will simply be discharged. Unless there is a water quality problem, the dewatering

water should be reused onsite.

The SDEIS states that the largest change in surface water flow occurs to baseflow at year 11

(Table 5.2.2-25 and associated text). The analysis includes changes due to changes in the

drainage areas caused by the project. Myers (2014) demonstrated that the groundwater

discharge could be decreased much more than predicted in the SDEIS, therefore the predictions

in the SDEIS are likely wrong.

Recommendation: Use the MODFLOW model to predict decreases in the discharge to the river

or to predict water drawn from the tributaries or wetlands which could reduce river flow.

Dewatering-Caused Drawdown

Dewatering pits will cause drawdown in the surrounding surficial and bedrock aquifers. The

drawdown is maximum at the pit and expands away from the pit as pumping draws water from

further in the aquifer. Hydraulic conductivity and storage characteristics control the shape of

this drawdown. Polymet chose not to use their own MODFLOW-generated drawdown

predictions because similar predictions were deemed to be inaccurate at the nearby Canisteo

pit. The differences between predicted and simulated water levels at Canisteo ranged from +28

to -4 feet. They conclude that the model “clearly could not accurately estimate water level

changes of a few feet or less as would be desirable for assessing potential effects on nearby

surface features” and that therefore it was not reasonable to quantify drawdown using

MODFLOW (p 5-92). Therefore they used an analogue model based on Canisteo. Their

reasoning is wrong – the inaccuracies at Canisteo do not obviate the predicted results. With

respect to effects on surface features, drawdown of a few feet will cause those features to go

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dry. There is simply little difference to a wetland if the drawdown beneath it is 5 or 500 feet –

the wetland becomes just as dry. With respect to streams and rivers, drawdown that changes

the gradient of the groundwater connection with surface water will change the discharge rate –

it is not even necessary for there to be any drawdown at the feature.

The SDEIS outlines various differences between Northmet and Canisteo. The two most obvious

are the bedrock at the Canisteo, the Biwabik bedrock, is much more pervious than at Polymet

and the till at Canisteo is much thicker. If the bedrock is more pervious, more water would be

drawn from it more easily. Drawdown could extend further in that formation without affecting

the surficial aquifer as much.

The SDEIS claims the analogue model was based on pit lake recovery measurements, not on

actual dewatering results (p 5-92). This renders the results suspect so they should not be used

in the SDEIS.

Recommendation: The MODFLOW results for drawdown should be included in the SDEIS, either

to supplement or replace the analogue discussion.

The following figures are snapshots of drawdown maps from Polymet’s MODFLOW study (Barr

2008), showing that one-foot drawdown in the surficial aquifer would extent more than 2250

feet from the West Pit and more than several miles in the bedrock. The aquifer thickness in the

surficial aquifer limits the depth of drawdown in that aquifer. Myers (2014a) also simulated

drawdown much further from the pit than the analogue model would suggest.

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The drawdown results indicate that Polymet’s modeling does not include a significant

connection between the surficial and bedrock aquifer, because the surficial aquifer does not

drain into the bedrock. This may be deduced from the fact that the bedrock drawdown occurs

under the surficial aquifer. This differs from the description in the SDEIS, p 5-91 to -92, which

claims that bedrock remains saturated due to the surface aquifer draining into it. The

drawdown in the bedrock is on the order of tens of feet. The surficial aquifer would desaturate

before or into the bedrock aquifer.

The SDEIS notes that the analogue drawdown guidelines would apply during mine operations

and during reclamation, but that drawdown “should decline and essentially cease as the pits

flood” (p 5-93). However, it also notes that there would be a 20-foot and 10-foot permanent

drawdown in the West and East Pits, respectively. These are due to a planned lowering of the

long-term water table using outlet structures. However, the SDEIS does not disclose the long-

term, essentially permanent effects of maintaining these drawdowns. This project is in an area

where precipitation exceeds evaporation, so the pit lake would not naturally be terminal – it

would fill until it reaches a rim and begins to spillover unless groundwater outflow counters the

inflow. Polymet’s proposal must include a requirement for pumping and treating the potential

outflow, essentially forever.

Recommendation: Polymet should run their MODFLOW model in steady state mode with the

water level in the pits controlled at the proposed levels. Polymet should also disclose in the

SDEIS that maintaining the water level below the natural levels will require pumping/diversions

from the pits essentially forever; this would include a requirement to treat the water.

Hydrology Changes due to Seepage from the Tailings Impoundment

Polymet will install a seepage containment system around the tailings impoundment that is

designed to reduce seepage along the groundwater pathway to the Embarrass River from 209

to 21 gpm while removing 3171 gpm for treatment. The amount removed is diverted from

either the existing seeps or from groundwater flow. The description of the reduction in

seepage to the Embarrass River watershed during closure is very misleading – it claims there

will be a reduction from the estimated current rate of 2020 gpm to 1320 gpm but that since the

groundwater containment system remains in place there will be only 21 gpm of seepage

bypassing the containment (p 5-160). It is therefore inaccurate to claim that 1320 gpm will

reach the watershed.

Recommendation: Change the discussion of tailings seepage to acknowledge that the

containment system prevents seepage from reaching the watershed.

The SDEIS claims the reduction in average annual flow rate along the three groundwater flow

paths to Mud Lake Creek, Trimble Creek, and Unnamed Creek will be 37% (North Flowpath),

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65% (Northwest Flowpath), and 46% (West Flowpath). Polymet proposes to augment the flows

so that the maximum reduction is about 20% with water from the WWTP plant. This review is

not considering the sources but is considering the efficacy of discharging this augmentation

water into the streams. One consideration is that the water flows through the shallow

groundwater and wetlands to reach streams. Presumably (there is no description) the

augmentation would be pumped to the streams thereby bypassing (and allowing to dry) the

wetlands. Also, the augmentation will reduce the sulfate concentrations downstream in the

Embarrass River.

Recommendation: The SDEIS should describe the flow augmentation in Embarrass River better

so that its effectiveness can be considered and the proposal can be assessed as to whether it

will cause the wetlands to dry.

Wetland Hydrology

Wetlands cover 43 percent of the mine site and many are primarily supported by direct

precipitation due to the lack of interaction between surficial and bedrock aquifers. Polymet

expects a variable connection between wetland areas and bedrock depending “on the

characteristics of the wetlands and the localized hydraulic conductivity and degree of bedrock

fracturing” (4-150). The wetlands receive minimal runoff because the soil texture allows rapid

infiltration, but the interaction between the surficial and bedrock aquifers is assumed to be

insignificant (p 4-149). The SDEIS bases this idea on a reference to Siegel and Ericson, but their

interpretation is wrong.

Water in near-surface bedrock aquifers is under unconfined conditions except where the bedrock is overlain by drift of low permeability. Water in the bedrock occurs in secondary openings such as joints, fractures, and leached zones. The bedrock generally has extremely low primary hydraulic conductivity and yields little or no water unless secondary openings exist. Fractures and joints in the Duluth Complex may extend to considerable depths but are more extensive in the upper 200 to 300 feet of the bedrock. (Siegel and Ericson 1981, p 7)

These extensive fractures would enhance a connection between the bedrock and surficial

aquifers, not render it insignificant. The SDEIS assumes there is little connection throughout,

but is incorrect. See also Myers (2014a).

The SDEIS discusses a five-year wetlands hydrology observation period. They interpreted “large

fluctuations in water levels” (p 4-151) as being “indicative of wetlands supported by

precipitation and local surface runoff”. The SDEIS does not define “large fluctuation”. They

note that “shrub swamp wetlands located near the downstream portion of the project

generally show more stable water levels due to larger watershed areas and some apparent

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groundwater inflow” (Id.). There is no context to the description “more stable”. They point out

that groundwater flow paths between recharge and discharge are generally short, a description

that applies throughout the project area.

The SDEIS lists six things that could affect wetlands (P ES-38). This review considers two of

them – the changes in wetland hydrology due to groundwater drawdown and changes in

wetland water quality related to leakage from mine dumps or seepage from mine pits.

The SDEIS suggests that wetland hydrology monitoring would be conducted only during the

operations phase. This is not correct because pit lake development will draw water from the

wetlands far beyond the end of dewatering. This applies whether the Polymet pumps the pit

full, as described, or whether it fills naturally, as analyzed by Myers (2014a). It might be

possible to reach a point where wetland hydrology monitoring is no longer necessary, after the

pit lake is totally developed. However, below the tailings impoundment, because of the

drawdown to be caused by decreasing the seepage from the tails, wetlands could be dried for

decades into the future (Myers 2014a).

The SDEIS claims that the extensive wetlands have only “minimal hydraulic connection to the

underlying groundwater” (p 4-46). This assumption was based on the mapping of ombrotrophic

bogs rather than much hydrologic survey. Myers (2014a) found that recharge over the

watersheds was several inches per year. It is very possible that the connectivity is highly

heterogeneous, but the SDEIS has not sufficiently considered these issues.The SDEIS has also

not adequately considered the hydraulic connections between the wetlands and underlying

aquifers.

Recommendation: The agencies should require Polymet to install shallow piezometers near

and at distance from the mine site to consider whether there is a layer separating the surface

from the aquifers.

Reliance on Perfect Engineering

There are many sources of potential contamination caused by this proposed project, including

various waste rock and or stockpiles and ponds. The following figure is a snapshot from the

SDEIS showing those sources at the mine site. The tailings impoundment is the primary source

at the plant site. Table 5.2.2-19 summarizes the sources in Polymet’s modeling.

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The following passage from the SDEIS describes how Polymet relies on perfect implementation

of engineering designs to prevent their discharges from exceeding standards. It has to work

perfectly or the project may exceed many standards (Myers 2014b).

For MPCA-recommended wild rice waters, the engineering controls would prevent an increase in sulfate concentrations in the Partridge River and would decrease sulfate concentrations in the Embarrass River. The proposed engineering controls would provide a high degree of reliability and flexibility to ensure that the evaluation criteria would continue to be met in the future, where nearly all contact/process water at the NorthMet Project area would be treated at the WWTF or the WWTP before release to the environment. At the Mine Site, only about 10 gpm of untreated water would be released during closure (all related to groundwater seepage), which represents less than 5 percent of total Mine Site water releases. At the Tailings Basin, only about 21 gpm of untreated water would be released during closure (all Tailings Basin seepage that bypasses the groundwater containment system), which represents less than 1 percent of total Tailings Basin water releases. The NorthMet Project Proposed Action is also not predicted to result in any significant changes to groundwater and surface water flows when compared to existing conditions. (p 5-8, emphasis added)

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The 10 gpm untreated release from the mine site would be from the West Pit into

downgradient pathways and also small amounts of seepage that pass from waste rock

stockpiles to downgradient groundwater pathways. The release from the West Pit is extremely

low because Polymet has determined the downgradient conductivity in bedrock is extremely

low. The other releases require perfect construction and design of stockpile liners. The 21 gpm

release from the tails depends on the liner under the new tails and the containment systems,

described above, operating perfectly. The modeling used to present results in the SDEIS relies

on these designs working perfectly, with small variations considered to account for

imperfections. If these fail at either site, their predictions are wrong and the project will

degrade waters.

Table 5.2.2-35 further demonstrates that most of the discharge from the site will be treated

water from the WWTF. The WWTF discharge to the river is 290 gpm only after year 40. The

flow rate from various sources is a small fraction of that and any load from those sources may

take from 55 to 160 years to reach the river. Because the WWTF discharges to surface

pathways, it dilutes any of the groundwater flowpaths.

Recommendation: Polymet should expand their probabilistic modeling to include results

assuming the liners/containment systems work poorly. Instead of 99% capture, they should

test what occurs with as little as 10% captured. Also, they should consider what occurs if the

capture fails for a given time period; this would be the equivalent of considering what would

occur if a significant slug of contaminants escapes for a reasonable time period.

The SDEIS claims that any water that escapes the Cat 1 stockpile would migrate to the West Pit.

Based on the fact that there is a groundwater divide separating the upper reaches of the

Partridge River from the lower reaches (near the confluence with the S Partridge River) and

based on modeling in Myers (2014a), a portion of seepage escaping from the Cat 1 stockpile

could flow to the upstream end of the river rather than to the pit.

Recommendation: The modeling should include a flowpath north from the Cat 1 stockpile to

the Partridge River. The flow path might be temporary, only existing early during operations

and later during closure. Polymet should use their MODFLOW model to assess these time

periods. See Myers (2014a).

The Cat 1 stockpile and the tailings impoundment will be encircled by a groundwater

containment system that would capture “at least 90 percent of seepage”. At the tailings

impoundment, the SDEIS describes the containment system as “a slurry wall keyed into bedrock

and an upstream collection trench with permeable backfill and piping” (p 5-6). This design

depends on the collection trench working perfectly, the bedrock being low permeability

without fractures that would allow seepage to bypass the wall, and the wall being perfectly

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keyed into the bedrock. If these fail, the mine’s plans will fail and more contaminants than

predicted will reach the river.

The containment system depends on the cutoff wall being properly constructed including its

key into the bedrock, the drainage system that removes seepage not failing, and the bedrock

actually being “low permeability”. The performance modeling reported in the SDEIS (p 3-46)

would not show the capture efficiency if the bedrock were more fractured than assumed by

Polymet. Myers (2014a) discusses why the bedrock probably has higher conductivity than

modeled by Polymet. The construction of the cutoff wall and drainage system is discussed

elsewhere (see report of Mike Malusis).

Recommendation: Transport from the tailings should be modeled with two potential failures.

The first is that the cutoff wall and/or pump system does not perform as designed. The

modeling should consider the potential for the pump system to fail temporarily. The second is

that the bedrock underlying the tailings could have higher conductivity than assumed for this

modeling.

The project proposes to use both a cutoff wall containment system and a cover to decrease

seepage from the Cat 1 stockpile (p 3-46, 5-101). The containment system would be

constructed during the first year of mining as waste rock is added to the pile. A cover would be

added started in year 14 because that is the last year that waste rock would be added to the

stockpile (it would be backfilled to the East Pit). The long-term efficacy of this project depends

mightily on both of these systems working. Myers (2014a) considers the details of transport

around the mine site if the stockpile seepage reduction does not work.

Recommendation: Polymet should model transport from the Cat 1 stockpile with two potential

failures. The first is that the cutoff wall and/or pump system does not perform as designed.

The modeling should consider the potential for the pump system to fail temporarily. The

second is that the bedrock underlying the stockpile could have higher conductivity than

assumed for this modeling.

SDEIS Figure 5.2.2-3 shows the MODFLOW modeling of the containment system around the Cat

1 stockpile. Specifically, it shows low-K cells surrounding a system of DRAIN cells that surrounds

the proposed stockpile. This is the wrong way of modeling the cutoff wall because the

thickness of the model cell would control the conductance through the cell. This figures shows

the cutoff wall was modeled as having variable thickness and conductance, which is not realistic

and is incorrect. The SDEIS does not specify the conductivity value, but if it is the value

expected for a five-foot wall, having the model treat it as much thicker would be modeling it as

much more impervious than it would be in reality. The variable size of the DRAIN cells might

also affect the modeled ability to remove water because the conductance could be set equal to

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the product of the long length of the cell and modeled conductivity, a value which could

artificially increase the model’s ability to remove water.

Recommendation: The cutoff wall should be simulated using the horizontal flow barrier

package in MODFLOW. The DRAIN cell conductance should be set specifically avoiding the

potential problems outlined herein.

The cat 2/3 and 4 stockpiles are temporary features containing the various types of waste rock

until it is backfilled into the East Pit (5-101). Polymet models these features with extremely low

seepage rates assuming that the liners at their base will prevent seepage to the underlying soil

and divert it to capture systems around the base of the stockpiles (Id.). The low predicted

sulfate loads to the rivers depend on these liners working as designed.

Recommendation: The temporary stockpiles should be simulated assuming the liner performs

poorly to set a bound on the potential contamination. The MODFLOW model should be used to

test assumptions that seepage from the Cat 4 stockpile will flow to the East Pit.

The East Pit would be backfilled, with details as described on p 5-102. Beginning in operations

year 10, stockpiled Cat 2/3 and 4 waste rock would be backfilled into the pit while groundwater

will flow into and saturate the backfill (Id.). “The pore water in the initially saturated backfill

would have relatively high solute concentrations …, but once submerged, oxygen transport

would be limited and there would be a systematic decrease in oxidation and associated

dissolution of sulfide minerals” (Id.). The Water Modeling Data Package indicates that about

50,000 tons of sulfate would be backfilled into the pit, which would cause a concentration

about ten times higher than the 2500 mg/l shown in Figure 5.2.2-18 (Myers 2014a). The limits

in that figure, although not discussed at this point in the SDEIS, are due to concentration caps

expected to be obtained by amending the backfill.

Recommendation: Polymet should run a scenario wherein the concentration caps do not

apply, because they depend on the correct addition of amendments to control the pH in the pit.

Adding amendments after backfilling is complete is very difficult.

Polymet will pump water into the West Pit during reclamation to fill it quickly, within 20 years

of the end of operations (5-81). The pumped water includes both treated and untreated water,

so water disposal may not be the purpose. It would limit oxidation of pit walls, but the

untreated water to be pumped into it contains significant loads so it may not fulfill that

objective. The SDEIS does not explain why this is being done. Myers (2014a) found that

pumping the pit full caused a gradient for pit lake water to flow into the surrounding

groundwater, but this is not considered in the Goldsim modeling. There is no real explanation

or reasoning provided as to why they are pumping the pit full.

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Recommendation: The SDEIS should compare the pros and cons of pumping the West Pit full

with a clear explanation of why they are doing it. It could be considered an alternative mine

plan.

The project relies on submergence of Cat 2, 3, and 4 waste rock in the East Pit to prevent acid

drainage, which they acknowledge is possible (p 5-5), but there are many problems with that.

Polymet will pump water into the East Pit to keep it submerged and saturated. They also

discuss that during reclamation they will pump water from the pit at 1750 gpm and treat it.

They will keep the waste saturated at the same time. Myers (2014a and b) reviews the

difficulties with achieving Polymet’s plans and presents some alternative modeling of the

results

SDEIS Figure 5.2.2-8 and associated text compares distributions of sulfide in waste rock at

Polymet to sulfide in deposits around the world. The comparison is relatively useless without

also including a comparison of the amount of AMD problems that occurred at those other sites.

The SDEIS overlaps discussion of the tailings geochemistry and modeling (p 5-61 – 63). They

apply two empirical correction factors to reduce sulfate concentrations to levels observed

downgradient. The first was to match “observed concentrations downgradient of the tailings”

(p 5-62, -63), as described in Attachment E. The second was adjustment, sometimes by

reducing the concentrations by greater than 99% of the predicted concentration, to match

observed concentrations in downgradient waters. The factors were listed in an obscure

supporting document, the Water Model Workplan, Version 6. The SDEIS suggests this shows

there is additional attenuation not accounted for by Goldsim modeling.

The SDEIS assumes that runoff from various sources will be captured and potentially treated.

For example, pond PW-OSLA would capture runoff from the overburden storage and laydown

area (OSLA) (p 5-102). This water will be discharged to various places, with treatment if the

water quality is not sufficient for discharge. The SDEIS does not describe plans for testing the

water quality in this, or other ponds. The SDEIS does not disclose how Polymet will determine

whether the water is suitable for discharge untreated.

Recommendation: The SDEIS should include a discussion about how the water quality of

temporary runoff ponds, such as pond PW-OSLA, will be monitored. This would include

frequency of sampling and constituents to be sampled.

Polymet will use “bentonite amendments to surface material during operations and closure” to

“reduce oxygen diffusion into the subsurface and thereby decrease contaminant release rates”

(p 5-63). Bentonite amendments are also referenced elsewhere (p ES-23, 3-4, 3-129, 5-81, 5-

162, 5-358, 5-562, Table 3.2-1). These are all claims as to the bentonite preventing oxygen

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from passing, but there are no literature references to where this has been done or assurances

that it will work and not even references to Polymet design documents regarding the issue.

Recommendation: Provide evidence in the form of peer-reviewed studies or other assurances

that bentonite amendments and covers can prevent the entry of oxygen to underlying material.

This should prove that it will work essentially in perpetuity since that is the claim being made by

Polymet and relied on by the agencies.

Goldsim modeling of existing conditions uses calibrated values for seepage through portions of

the tailings impoundment (Table 5.2.2-10). The calibration is from a MODFLOW model. The

problem with converting the seepage rates modeled at the bottom of the tails to a recharge

rate at the top of the tails is that it assumes vertical flow through the tails. Because there is

ponding on certain levels, it is possible there is a horizontal gradient which would cause flow

among tailings basin subareas.

Recommendation: Polymet should redo the modeling to accurately account for flow among

subareas and use the amended bottom seepage rates for inflow to the Goldsim modeling.

Evaluation Thresholds

The evaluation threshold for water quality has been chosen to be the P90 value, or 90%

exceedence (p ES-35). This means that 90% of the simulated results were less than this value.

However, the SDEIS describes it as there being “at least a 90 percent probability that a

constituent would not exceed water quality evaluation criteria” (p 5-7). This requires the huge

caveat that it applies only if all of the conceptualizations of flow and transport are accurate and

all of the probability distributions are accurate. Myers (2014) provided evidence that suggest

many of the base parameters used in the modeling are not correct. This provides a false sense

of security in the predictions.

Recommendation: The SDEIS should not call the P90 value the value for which there is a 90%

chance it will not be exceeded without adding “based on the assumption of the modeling”.

There are no standards for groundwater drawdown (5-9). The ultimate effect is whether the

drawdown affects resources on the surface, specifically wetlands or streamflow. This has been

addressed above. However, once the effects manifest on the surface, it may be difficult to

counter them.

Recommendation: The agencies should require detailed monitoring of the water levels in the

shallow aquifers to provide advance warning of any impacts as they are occurring or before

they affect the water level on the surface.

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The SDEIS lists many surface water statistics that can be tracked to assess changes due to the

project (5-13, -14). However, there are no binding compliance standards, so there is no level of

impact that exceeds standards. They recommend that stream flows not be changed more than

plus or minus 20% (5-14). This is not standard. Flow changes due to the project could cause

concentration evaluation criteria to be exceeded but the water quality modeling (Polymet

2013a and b) does not simulate changes in flows due to the project.

Recommendation: If it is not being done, the concentration modeling should account for

decreased flow in the river and streams.

Summary

The SDEIS for the proposed NorthMet Mine essentially predicts there will be no negative water

quality impacts or that the project will improve the existing water quality. It also predicts that

any negative changes in river flow rates or wetland water levels will be mitigated by flow

augmentation with water from Colby Lakes or with water from either the waste water

treatment facility or treatment plant. Or, the modeling relies on assumptions like sorption and

concentration caps to assure that the load discharging from the facility is not too high. It is all

planned to work perfectly or the modeling assumptions render many of the effects

unimportant.

The assumptions are essentially the problem with this SDEIS. The range in potential design or

construction flaws is considered using a probability distribution of seepage values, but the

range is limited to very good installation and performance. Very little consideration is given to

what could occur when something goes significantly wrong, as it inevitably does. This SDEIS

does not consider the full range of potential problems that could occur at this site, especially

considering that the design features must operate as designed in perpetuity. It does not

adequately disclose the potential range in water quality and water quantity impacts that could

result from this project over the next 200 to 500 year and beyond.

References

Barr Engineering (Barr) 2008. RS22B 0 Mine Site odel Report, Appendix B: Groundwater

Modeling of the NorthMet Mine Site. Draft02. St. Paul MN.

Foose MP, Cooper RW (1978) Preliminary geologic report on the Harris Lake area, Northeastern

Minnesota. USGS Open File Report 78-385.

Kruse G (2013) Office Memorandum Re: Partridge River Base Flow Analysis MDNR Gage

#H03155002

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Myers T (2014a) Groundwater Flow and Transport Modeling, NorthMet Mine and Plant Site.

Prepared for the Minnesota Center for Environmental Advocacy.

Myers T (2014b) Review of the Water Quality Modeling, NorthMet Mine and Plant Site,

Minnesota. Prepared for Minnesota Center for Environmental Advocacy.