SFUND RECORDS CTR

Draft - For Review Only 2072538

Reviewof

Draft Evaluation of Final CoverSunrise Mountain Landfill

Date: 12-12-2000

Prepared bySteve Dwyer, PE

13140 Blackstone Rd. NEAlbuquerque, NM 87111

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1.0 Introduction

I have reviewed the "Draft Evaluation of Final Cover, Sunrise Mountain Landfill"prepared for Republic Services of Southern Nevada, Inc., prepared by SCS Engineers,dated October 2000. I have also review the associated soil data, dated November 7,2000, file no. 10.19907.01 forwarded to Mr. David Basinger, EPA from SCS Engineers.The cover was evaluated based on in-situ soil for a depth of 2-feet. The analysisperformed was for as an Evapotranspiration (ET) Cover, not as a prescriptive RCRASubtitle 'D' Cover per 40CFR258. In general, I believe SCS Engineers have done anadequate job of modeling the existing cover; however, the following concerns areaddressed.

2.0 Summary of Findings

The UNSAT-H model with the 5-wet year method is an appropriate design tool. I believethe flux rate through the cover should be the primary criteria for acceptance of the coverprofile design. The flux rate in this case simply refers to the amount of moisture thatmoves through or percolates down through the cover into the underlying waste over agiven period of time. The flux rate units are given in terms of a velocity (mm/year), but itis different than velocity because it is dependent on a number of variables other thangravity. This theory is explained further in section 3.

The flux rates derived from SCS Engineers' modeling effort (Table 1) are unacceptablyhigh. Hence, the cover is not thick enough and the soil has inadequate storage capacity.The cover has been modeled as an ET Cover. A simple way to look at the design conceptof an ET Cover is that the cover's soil acts as a sponge. The type of soil and vegetationcovering (in this case - no vegetation) determine how absorbent the sponge is. The coverdepth determines whether the sponge is thick enough to absorb the worst case or designinfiltration event(s) until the water is removed (in this case by evaporation). Refer tosections 3 through 5 for more detail.

The cover was also modeled using the "HELP" landfill water balance program. Thisprogram is the conventional model and is prevalent in much of the regulatory community.The HELP model results allowed for comparison of the UNSAT-H results. The twoprograms concur on general trends for landfill areas with high flux rates versus those withlower flux rates. Multiple depths were modeled using HELP. The cover depth wasincreased to as thick as five-feet with very little reduction in flux rate identifying thesoil's varied soil properties from landfill cover area to landfill cover area, as well as, thelimited storage capacity of the soil. Refer to sections 3 through 6 for more detail.

The cover depth modeled by SCS Engineers is two-feet. Trenching efforts from thecover characterization effort revealed that a number of the landfill cover areas have lessthan two-feet of soil cover. Refer to section 6 for more detail.

During the recent cover characterization effort, as well as, past characterization efforts,the cover soil was shown to have a high gravel/cobble content. SCS Engineers did not

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address this gravel/cobble content in the design effort. The gravel/cobble content willreduce the water storage capacity of a cover profile because rock has no practical storagecapacity. Refer to sections 7 and 8 for more detail and design suggestions for inclusionof the gravel/cobble content.

Section 8 discusses my recommendations. I discuss what should be included in thedesign and what constitutes an acceptable design. This reflects my opinions and does notnecessarily reflect what the governing regulatory body will dictate.

3.0 ET Cover Concept

The ET Cover consists of a single, vegetated soil layer constructed to represent anoptimum mix of soil texture, soil thickness, and vegetation cover (Dwyer, 1997a). The ETCover is a monolithic soil layer that has adequate soil water storage capacity to retain anyinfiltrated water until it can be removed via ET (Figure 1).

ET,\\^V* Ar> • \ *•s Transpiration ̂Precipitation ^ Evaporation

-Runoff

Topsoil/Surface Treatment

Figure 1. Typical ET Cover Profile

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The concept relies on the soil to act like a sponge. Any infiltrated water is held in this"sponge" until it can be removed via ET. ET is defined as the combination of waterremoval due to both evaporation and transpiration through vegetation. Previous researchhas shown that a simple soil cover can be very effective at minimizing percolation anderosion, particularly in dry environments (Nyhan et al. 1990, Hauser et al. 1994, andDwyer 1998a).

An important advantage of ET Covers is that they are less expensive than their traditionalcounterparts. The soil used will, for economic reasons, generally come from a nearbyborrow site. This issue is probably the single most important driving force for thedeployment of these covers at present because they save large amounts of money duringconstruction. A comparison of constructed costs showed that the cost savings for an ETCover versus a Compacted Clay Cover is in excess of 50 percent (Dwyer 1998c).

Other advantages of an ET Cover system are low maintenance and relevant ease of repairfor such things as problems due to differential settlement. If differential settlementoccurs within an ET cover, more soil can be applied to the surface to bring the cover backto its original grade. With a prescriptive RCRA Cover that contains a geomembrane, anysignificant differential settlement may lead to tearing of the geomembrane and crackingin the underlying clay barrier layer. These potential flaws will typically occur at thelandfill's low point where ruptures or cracks act as a funnel for water to flow downwardsthrough the underlying waste.

The ET Cover is much easier to build and requires less quality assurance duringconstruction than its traditional counterpart (Dwyer 1998c). The ET Covers are usuallyless susceptible to side slope instability than a RCRA Cover containing a geomembranebecause they do not have a geomembrane which creates a slip plane. The ET Coverperformance also increases with time as the plant community becomes fully established,while the RCRA clay barrier layer continues to decline in its effectiveness due to barrierlayer deterioration (Suter et al. 1993, Waugh and Smith 1997, Mulder and Haven 1995).

A false assumption concerning the performance of traditional landfill covers (both RCRASubtitles 'C1 and Subtitle 'D' closures) is that water movement will occur under saturatedconditions. Saturated flow in the near surface, when it does occur is primarily downwardas the hydraulic gradient is largely due to gravity. Water movement however, generallyoccurs under unsaturated conditions. This is particularly the case in dry environments.For water movement in an unsaturated soil, the hydraulic gradient is comprised of bothgravity (acting downward) and matric potential differences (which can act either upwardor downward). Consequently, both upward and downward water movement is possible inunsaturated soil.

Darcy's Law can be used to represent the fundamental equation of flow for bothscenarios:

For saturated systems:Q = Ksat i A

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where: Q = flow rateKsat = saturated hydraulic conductivity

i = hydraulic gradient; /(gravity and pressure potential)A = area

For unsaturated systems:Q = Kunsat 1 A

where: Q = flow rateKunsat = unsaturated hydraulic conductivity

i = hydraulic gradient; /(gravity and matric potential)A = area

Water moves toward regions of higher water potential and is consequently governed bygravity and matric potential equilibrium for unsaturated flow. Under saturatedconditions, the soil's matric potential is zero.

M/Total = V(/grav + V|/matric + V|/s+ Y|/a

where: vj/jotai = total soil water potential= gravitational potential= matric potential

x|/s = solute potential\j/a = air pressure potential

\j/s and v|/a are generally considered to be zero for landfill cover applications, thereforethe relationship can be simplified to:

However, in the field water movement patterns are complicated by a number of otherfactors such as climatic conditions, plants, structural voids, secondary pathways, non-homogenous soils, and hysteresis.

ET provides the mechanism to remove stored water from the cover soil layer. Water canmove upward in response to matric potential gradients induced from ET drying the upperportions of the cover soil layer. Matric potential gradients can be many orders ofmagnitude greater than the gradient component due to gravity. Evaporation from thesurface will decrease the water content and increase the matric potential of the soil,resulting in an upward matric potential gradient and inducing upward flow.

4.0 UNSAT-H Modeling by SCS Engineers

The design tool SCS Engineers have chosen is a numerical model named "UNSAT-H".This is an unsaturated flow model that can be used to model an ET Cover system. Themodeled data took two different approaches. SCS Engineers modeled each areaseparately using the same weather data with soil data taken from samples from eachrespective area. Six areas were modeled: the top deck, top deck's side slopes, eastern

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perimeter, eastern southern flats, western southern flats, and the asbestos area. The firstapproach modeled the cover areas over a 30-year period using weather data from 1977 to1990 repeated. Representative soil samples from each of the six landfill cover areas wereused as input data for the unsaturated properties of the soil. The unsaturated soilproperties found in the soil data analysis provided by SCS Engineers as tested by DBStephens seem reasonable. The unsaturated properties of soils taken from the SunriseMountain Landfill cover tested several years ago (Dwyer 1997b) are within the envelopeof the data in this report. The second approach took the wettest year and ran it fiveconsecutive years. It did not add to the precipitation in any of the years, just put the samewet year five years in a row. I prefer the five wet years approach because it is moreconservative than the first approach.

The weather data (precipitation) was inputted as a daily value. The model essentiallyintegrates this daily value over the 24-hour period. This is a conservative approach forinfiltration from a modeling perspective - not necessarily from a reality perspective. Ihave summarized the output data provided by SCS Engineers in Table 1.

Upper DeckUpper Deck with WasteEastern PerimeterEastern Perimeter with WasteEastern Southern FlatsEastern Southern Flats withWasteWestern Southern FlatsWestern Southern Flats withWasteTop Deck, Side SlopesTop Deck, Side Slopes withWasteAsbestos AreaAsbestos Area with Waste

HELP (30 year)By SF Dwyer

22.9823.22

2.151.520.680.56

1.381.41

86.0585.18

45.7045.03

UNSAT-H (30-year)By SCS Engineers

12.85

7.13

2.72

9.46

33.40

43.78

UNSAT-H (5 Wet Years >By SCS Engineers

21.88

30.58

5.42

21.06

65.10

65.38

Table 1. Average Annual Flux Rates (mm/year) through 2-Foot ET Cover

5.0 HELP Modeling

I modeled the cover profiles for the six landfill cover areas using the HydrologicEvaluation of Landfill Performance (HELP) program v.3.08 (Schroeder et al 1994) forcomparison of the UNSAT-H results. The HELP Model is a popular water balanceprogram developed with EPA funding that many design engineers and regulatorsthroughout the country use to evaluate landfill systems. Each area modeled by SCSEngineers was modeled with the HELP program with just the two-foot cover profile aswell as that cover profile with underlying municipal waste. Sometimes the addition ofwaste significantly changes the results. It did not in these cases. The results shown inTable 1 are for a two-foot thick ET Cover. The weather data used for the HELP model

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was the default parameters included in the model for the Las Vegas, Nevada area. Thesoil data used as input was taken from the same soil samples used in the UNSAT-Hmodels. The UNSAT-H model uses van Genuchten parameters derived from the soilretention curves provided in the soil data report. For the HELP model, I used the samesoil retention curves and extrapolated moisture contents from the respective curves forfield capacity taken at a matric potential head of 1/3 bar or about 330 cm and wiltingpoint taken at a matric potential head of 15 bars or about 15,000 cm. These extrapolatedparameters were then inputted into the HELP model along with the respective porosities,saturated hydraulic conductivities, initial moisture contents, surface area, slope and slopelengths. The vegetation input was default parameters provided for bare soil.

The data summarized in Table 1 reveals that the general trend of high flux rate yearscompared with low flux rate years is consistent from model to model. There issignificant difference however in the actual predicted flux rates from each model. Ibelieve that the UNSAT-H model is actually a better model for this site because it modelsthe flow as unsaturated. The difficulty of using a numerical model as a design tool for acover design such as this at Sunrise Mountain Landfill is that no model takes into accountflow through macropores where preferential flowpaths occur. This type of flow isdominant at Sunrise, especially flow through surface cracking and through localized areaswhere surface ponding has occurred.

The six landfill cover areas with and without waste were also modeled using the HELPprogram for an ET Cover thickness of 3-feet, 4-feet and 5-feet, respectively (Table 2).There was not a considerable reduction in the flux rates derived from the 2-foot coverthickness. The soil data (SCS Engineers 2000b) reveals that a number of the landfillcover areas have soil that has very little water storage capacity.

WSC = (FC- PWP)where: WSC = water storage capacity

FC = moisture content @ field capacity (1/3 bar)PWP = moisture content @ permanent wilting point (15 bars)

Landfill Cover AreaUpper DeckUpper Deck with WasteEastern PerimeterEastern Perimeter with Waste

Eastern Southern FlatsEastern Southern Flats with WasteWestern Southern FlatsWestern Southern Flats with WasteTop Deck, Side SlopesTop Deck, Side Slopes with WasteAsbestos AreaAsbestos Area with Waste

2 feet22.9823.222.151.52

0.680.561.381.41

86.0585.1845.7045.03

3 feet22.4125.14

1.871.50

0.750.561.381.61

86.0585.1746.1445.57

4 feet21.7724.97

1.811.500.570.681.381.71

86.0585.1845.1544.54

5 feet21.0624.172.341.48

0.560.771.382.01

86.0585.1846.3543.90

Table 2. Annual Flux Rates (mm/year) per HELP Model.

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For example, referring to the areas with the highest average flux rates, the net waterstorage capacity of the soil is very small. The storage capacity of the top deck, side slopesoil is only 1.8% while the asbestos area soil has a storage capacity of only 3%. Addingthickness to these areas with a consistent soil type will not help reduce the flux rate verymuch. Soil with a much higher water storage capacity should be used to significantlyreduce the flux rates.

Refer to the appendices for the HELP modeling results (both input and output data).Appendix A contains the cover modeled as a 2-foot thick soil cover, Appendix B - 3-footcover profile, Appendix C - 4-foot cover profile, Appendix D - 5-foot cover profile. Thecover profiles used soil properties from the same soil samples as those modeled by SCSEngineers. The weather data used was default data for the Las Vegas, Nevada areacontained in the HELP model.

6.0 Actual Cover Depth per Cover Characterization Data

The cover depth modeled by SCS Engineers is uniformly 2-feet. As can be seen fromTable 3 (Dwyer 2000a), there are a number of locations that do not have 2-feet of coversoil on them. Six locations found the bulk of waste at depths less than two-feet, anothertwo found discolored soil at depths less than two-feet, while two more found intermittentwaste at depths less than two-feet. That equates to 50% of the locations trenched on theexisting landfill cover as questionable for having sufficient soil depth to be consideredtwo-foot thick as was modeled.

Trench

100101102103104105106107108109110111112113114115

Intermittent WasteDepth (inch)

-6--

22--

34.----6--

Discolored SoilDepth (inch)

--------

34-----

2022

Bulk of WasteDepth (inch)

22423224522214403430222513222636

Table 3. Summary of Waste Depths in Trenches (Dwyer 2000b)

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7.0 Other Observations

A key piece of information omitted from the cover evaluation report (SCS Engineers2000a) is the gravel/cobble content removed from the soil samples. The laboratorytesting performed generally requires larger stones (e.g. greater than 3/4-inch) be removed.Consequently, results are based on a soil condition without the gravel content that wasremoved. There were also large cobbles in the soil samples. The percentage of gravel/cobble removed from the soil samples must be quantified so that it can be addressed inthe cover evaluation and design.

The UNSAT-H modeling for this site is better suited than the HELP Model because of itshandling of unsaturated flow. The HELP Modeling was performed only for comparisonof the results. HELP is generally considered to be more conservative than unsaturatedmodels used.

The UNSAT-H output does have some odd results. One example centers on Runs Bl andB2. Run Bl has a flux rate of 10.1 mm/year for year 13. Run Bl is a 30-year run. RunB2 is a 5-wet year run of the same area. This flux rate of 10.1 mm/year from run Bl isgreater than all of the flux rates in run B2 even though the precipitation used for that yearin Bl is less than all of the years used in run B2.

8.0 Recommendations

I believe the modeling format approach that SCS engineers have undertaken is adequate.I think the 5-wet consecutive years approach should be used for the design basis since itis more conservative than the 30-year run method.

The flux rates revealed in the output are unacceptably high for the 2-foot cover depthmodeled. I believe that 3 mm/year is easily achievable and a minimum value that shouldbe used for acceptance of a cover depth. Refer to Table 4 below, for the flux ratesmeasured from my research site (Dwyer 2000b) in Albuquerque, NM. The ET Cover inthis project is 3.5 feet thick with typical arid vegetation covering.

Year

1997 (May 1 -Dec 31)199819992000 (Jan 1 - Jun 25)Average

Flux rates (mm/year)

Subtitle D

10.624.963.120.004.82

GCL

1.510.384.310.001.81

Subtitle C

0.120.300.040.000.13

CapillaryBarrier

1.620.820.850.000.87

AnisotropicBarrier

0.150.140.280.000.16

ET Cover

0.220.440.010.000.19

Table 4. Annual Flux Rates Measured at Alternative Cover Demonstration,Albuquerque, NM (Dwyer, 2000b)

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The cover depth should be determined as follows: take the depth determined by theUNSAT-H modeling that yields a flux rate of not more than 3 mm/year for all modeledyears - not the overall average. This depth should then be increased by the percentage ofgravel by volume determined to have been removed from the soil samples prior to theirlaboratory testing. Then take this increased depth and increase it by a factor of safety of25%. This added factor of safety would help offset inherent faults due to surfacecracking, ponding, construction inconsistencies, soil property inconsistencies, unusuallywet years, etc. The following is an example of my suggested design procedure.

EXAMPLE:

Modeling: 5 wettest year method with no annual flux rate exceeding 3 mm/year.

Depth determined = 3 ft

Gravel Content removed: 20%

Gravel content Adjusted Depth = 3 * 1.2 = 3.6 ft

Factor of safety (25%)

Design Depth = 3.6 * 1.25 = 4.5ft

References:

1. Dwyer, SF. 1997a. Large-Scale Field Study of Landfill Covers at Sandia NationalLaboratories, in Conference Proceedings: Landfill Capping in the Semi-Arid West:Problems, Perspectives, and Solutions, edited by T.D. Reynolds and R.C. Morris,held in Grand Teton National Park, Wyoming, May 1997. Sandia NationalLaboratories, Albuquerque, NM, pp. 87-107.

2. Dwyer, SF. 1997b. Sunrise Mountain Landfill Final Cover Evaluation. Report toBLM.

3. Dwyer, SF. 1998a. Alternative Landfill Covers Pass the Test, in Civil Engineering,pp. 50-52, 1998.

4. Dwyer, SF. 1998b. Construction Costs of Six Landfill Cover Designs, SAND98-1988. Sandia National Laboratories, Albuquerque, NM.

5. Dwyer, SF. 1998c. Construction Overview of Six Landfill Cover Designs, SAND99-1988, Sandia National Laboratories, Albuquerque, NM.

6. Dwyer, SF. 2000a. Sunrise Mountain Landfill Cover Characterization. DraftReport.

7. Dwyer, SF. 2000b. Alternative Landfill Cover Demonstration, FY2000 Annual DataReport. SAND2000-2427.

8. EPA. 1991. Environmental Protection Agency, Seminar Publication: Design andConstruction of RCRA/CERCLA Final Covers. EPA/625/4-91/025, May 1991.

9. Hauser, V.L., Shaw, MA, Weand, B.L. 1994. Effectiveness of Soil-VegetativeCovers for Waste Sites. In SUPERFUND XV: 15th Environmental Conference and

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Exhibition for the Hazardous Materials/Hazardous Waste Management Industry,Washington DC, Dec 1994, pp. 368-374.

10. Mulder, J. H., E. L. Haven. 1995. Solid Waste Assessment Test "SWAT" Program.Document No.: 96-1CWP, Report to the Integrated Waste Management Board,Division of Clean Water, Water Resources Control Board, California EnvironmentalProtection Agency.

11. Nyhan, J. W., T. E. Hakonson and B. J. Drennon. 1990. A Water Balance Study ofTwo Landfill Cover Designs for Semiarid Regions, in Journal of Environmental Quality,Vol. 19, pp. 281-288.

12. Schroeder, P., C. Lloyd and P. Zappi. 1994. The Hydrologic Evaluation of LandfillPerformance (HELP) Model User's Guide for Version 3.0. EPA/600/R-94/168a, U.S.Environmental Protection Agency, Cincinnati, OH.

13. SCS Engineers. 2000a. Draft Evaluation of Final Cover, Sunrise Mountain Landfill,prepared for Republic Services of Southern Nevada, Inc., prepared by SCS Engineers,dated October 2000.

14. SCS Engineers. 2000b. Draft Evaluation of Final Cover, Sunrise Mountain Landfillassociated soil data, dated November 7, 2000, file no. 10.19907.01 forwarded to Mr.David Basinger, EPA from SCS Engineers.

15. Suter, G. W., R. J. Luxmoore, and E. D. Smith. 1993. Compacted Soil Barriers atAbandoned Landfills will Fail in the Long Term, in Journal of EnvironmentalQuality, Vol. 22, pp. 217-226.

16.Waugh, W. J., G. M. Smith. 1997. Effects of Root Intrusion at the Burrell,Pennsylvania, Uranium Mill Tailings Disposal Site. GJO-97-5-TAR, GJO-LTSM-4.

Steve Dwyer <sfdwyer@uswest.net> on 04/04/2001 04:33:01 PM

To: Steve Wall/R9/USEPA/US@EPAcc:Subject: (no subject)

SMLETCoverReview.do