EXPLANATION OF SIGNIFICANT DIFFERENCES FOR SOUTH PLANTS BALANCE OF AREAS AND CENTRAL PROCESSING AREA SOIL REMEDIATION PROJECT ROCKY MOUNTAIN ARSENAL FEDERAL FACILITY SITE

1.0 INTRODUCTION 1.1 The Rocky Mountain Arsenal The Rocky Mountain Arsenal (RMA) On-Post Operable Unit is a federally owned facility located insouthern Adams County, Colorado, approximately 10 miles northeast ofdowntown Denver, directly north ofthe former Stapleton International Airport and west of Denver International Airport (Figure 1). The RMA site is27 square miles and is currently on the U.S. Environmental Protection Agency (EPA) National Priority Listfor environmental cleanup as a result of contamination released during previous RMA operations. The On-Post Record of Decision (ROD), which describes the site-wide remedy for RMA, was signed by the U.S. Army, the EPA, and the Colorado Department of Public Health and Environment (CDPHE) on June 11, 1996. The selected remedy includes 31 cleanup projects for soil, structures, and treatment ofground water contamination. When the site-wide remediation iscomplete, mostof RMA will become a National Wildlife Refuge, as provided for inPublic Law#102-402. 1.2 Documenting Changes tothe ROD Inimplementing RODs at many sites throughout the country, the EPA realized that changes will inevitably occur and has established guidelines fordocumenting these changes. Under Section 117 ofthe Comprehensive Environmental, Response, Compensation, and LiabilityAct of 1980 (CERCLA), as amended by the Superfund Amendment and Reauthorization Act of 1986, and pursuant to40 CFR Section 300.435(c)(2)(i), the lead agency is required to publish an Explanation ofSignificant Difference (ESD) when significant changes are being considered to a final remedial action plan as described ina ROD. The lead agency for RMA is the Army. Regulatory oversight is conducted by the EPA, CDPHE, and the Tri-County Health Department (TCHD). TCHD oversees local public and environmental issues inAdams, Arapahoe, and Douglas Counties. This ESD summarizes modifications to the remedy for South Plants that resultfrom new information developed by the Army since the ROD was signed. These changes enhance the effectiveness ofthe remedy, but do not alter the overall hazardous waste management approach that was selected inthe ROD. Modifications are being made tothe South Plants Central Processing Area (SPCPA) and the South Plants Balance ofAreas (SPBA). The SPCPA is composed of47 acres and is located inthe south central part ofRMA. The SPBA is composed of 378 acres and surrounds the central processing area, The southern portion ofthe SPBA isjust north ofLake Ladora, Lake Mary, Upper Derby Lake, and Lower Derby Lake, which is currently prime habitat forwildlife and a potential high use area for the future wildlife refuge (Figure 2).

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1.3 Circumstances that Led tothe South Plants Remedy ESD This ESD documents the following significant changes to the South Plants remedy: 1) Removal ofthe requirement for a 1-footcover' inthe South Plants Balance ofAreas (SPBA) inlieu of 1 -foot ofbackfill, 2) Removal ofthe requirement to excavate biota Soi12 from under the 3-foot cover area ofthe SPBA, and 3) Enhancement of construction standards for the 4-foot cover overthe SPCPA. Change in the-1-Foot Cover Area: The ROD requires construction of a 1-foot soil cover inpart ofthe SPBA. Based on design studies conducted in1999 to more fully characterize the SPBA, an additional 948 cubic yards of contaminated soil has been identified forexcavation, sothat human health exceedance Soi13 will not remain inthe SPBA. Because human health exceedance soil and biota soil will be removed from the SPBA and replaced by clean soil, there will be no technical reason (i.e., contaminated soil remaining inplace) to require a 1-footcover. Change inthe 3-Foot Cover Area: The ROD requires the excavation ofthe top foot of soil from the 3-foot cover area ofthe SPBA to remove soil that poses a risk to biota. The ROD also requires the construction ofa 3-foot soil cover inthis area and allows the excavated foot of biota soil to be used as fill material below the 3-foot cover. Because a 3-foot cover will be constructed, leaving the top foot ofbiota soil inplace inthe 3-foot cover area ofthe SPBA is consistent with the ROD remedy: biota soil is either covered with a clean soil cover or removed and placed ina consolidation area (such as Basin Aorthe SPCPA). Change inthe 4-Foot Cover Area: The ROD requires a4-foot soil cover to be constructed over the SPCPA. One purpose forthe 4-foot cover over the SPCPA is to minimize the infiltration of water through the cover into underlying contaminated soil. Inorderto better achieve this goal, the design and construction ofthe4-foot cover will employ new methods that are currently being tested throughout the Unites States for covers over hazardous waste sites. The new cover design and construction techniques are being tested at RMA under the Resource Conservation and Recover Act (RCRA)-Equivalent Cover Demonstration Project (MKC 1998). The 4-foot cover over the SPCPA will be designed and constructed using the same criteria that were used to build RCRA-Equivalent cover test plots inorder to improve the cover's ability to prevent waterfrom infiltrating through to the underlying contamination and soil.

IA cover is defined in the ROD as a layer ofclean soil that isolates contamination in place, thus preventing exposure to underlying contamination. Covers also require long term monitoring, inspection, and maintenance to ensure long term performance (FWEC 1996). 2 Soil the top 0 to 12 inches that poses a potential risk to biota (i.e., small mammals) but not to human health. 3At RMA, soil posing risk to human health is determined by concentrations ofchemicals above action levels developed in the Record of Decision forthe On-Post Operable Unit (FWEC 1996). 2 10/23/00, UAPR0JECTS\SPS01LkS0UTHPLANTs ESD.DOCEXPLANATION OF SIGNIFICANT DIFFERENCES FOR SOUTH PLANTS BALANCE OF AREAS AND CENTRAL PROCESSING AREA SOIL REMEDIATION PROJECT ROCKY MOUNTAIN ARSENAL FEDERAL FACILITY SITE This ESD is being provided for publicreview as defined in40 CFR 300.435(c)(2)(i). The ESD and supporting documentation will become a part ofthe administrative record forthe RMA and is available to the public at the JointAdministrative Record Document Facility (JARDF) located at the west gate ofRMA on 72nd Avenue and Quebec StreetinCommerce City, Colorado. The JARDF is open to the public on Monday, Wednesday, and Friday from noon to4:30 p.m.; on Tuesday and Thursday from 5:00 p.m. to 9:00 p.m.; and on Saturday from 10:00 a.m. to 4:00 p.m. The telephone number is303-289-0143. Although not required, a formal public comment period is being held forthis ESD. Additional information on the submittal ofcomments isprovided in Section 7.
2.0 BACKGROUND 2.1 Summary ofthe RIVIA Site History and Contamination Problems RMA was established in 1942 by the Army to manufacture chemical warfare agents and munitions for use inWorld War [I. Following the war and through the early 1980s, the facilities continued tobe used by the Army. Beginning in 1946, some facilities were leased to private companies to manufacture industrial and agricultural chemicals. Shell Oil Company, the principal lessee, manufactured pesticides from 1952 to 1982 at the site. Common industrial and waste disposal practices used during these years resulted incontamination ofstructures, soil, surface water, and ground water. The On-Post Operable Unit is one oftwo operable units at RMA. The On-Post Operable Unit addresses contamination within the fenced 27 square miles of RMA. The Off-Post Operable Unit addresses contamination north and northwest of RMA. The contaminated areas within the On-Post Operable Unit include approximately 3,000 acres of soil, several areas ofground water contamination, and 798 structures. The most highly contaminated sites are located inSouth Plants (the central processing area, Hex Pits, Buried M-1 Pits, and the chemical sewers), Basins Aand F, Lime Basins, and the Army and Shell trenches. The primary contaminants found insoil and ground water at these sites are pesticides, solvents, metals, and chemical warfare agent byproducts. The sites with the highest levels and/or the greatest variety ofcontaminants are located inthe central manufacturing, transport, and waste disposal areas. The highest contaminant concentrations tend to occur insoil within five feet ofthe ground surface, although exceptions are noted, particularly at sites where burial trenches, disposal basins, or manufacturing complexes are located. Ground water contamination is also present at RMA. The characteristics and locations ofthe contamination suggest that the greatest contaminant releases to the ground water have occurred from Basin Aand the Lime Settling Basins, the South Plants chemical sewer, the South Plants tank farm and production area, the Army and Shell trenches inSection 36, and the former Basin F. The Motor Pool/Rail Yard and North Plants area are other sources of contaminant releases to the ground water.

3 10/23/00, U:\PR0JECTS\SPS01L\S0UTHPLANTs ESD.DOCEXPLANATION OF SIGNIFICANT DIFFERENCES FOR SOUTH PLANTS BALANCE OF AREAS AND CENTRAL PROCESSING AREA SOIL REMEDIATION PROJECT ROCKY MOUNTAIN ARSENAL FEDERAL FACILITY SITE Since 1975, the Army and Shell have undertaken several Interim Response Actions and Remedial Actions at RMA designed to remove threats to the environment, while remaining consistent with any long-term cleanup. Continuing Interim Response Actions include ground water intercept and treatment north of RMA; ground water intercept and treatment north of Basin F; ground water intercept and treatment inthe Basin ANeck area; boundary ground water intercept and treatment operation; remediation ofother contamination sources (Motor Pool and Rail Yard ground water treatment); asbestos removal; removal of chemical process related equipment; and others. 2.2 Summary ofthe Selected On-Post Remedy Under the on-post-cleanup plans, contaminated ground waterwill be intercepted and treated at the three existing onsite treatment plants, Structures with nodesignated future use will be demolished andthe debris placed ineitherthe new, on-post hazardous waste landfill or the Basin Aconsolidation area, depending upon the degree ofcontamination. The contaminated soil at RMA will be addressed primarily through containment inan on-post hazardous waste landfill or under caps/covers, orthrough treatment depending upon the type and degree ofcontamination. Areas that have caps or covers require long-term maintenance and will be retained by the Army. These areas will notbecome part ofthe wildlife refuge. 2.3 Summary ofthe South Plants Remedy South Plants was a manufacturing area for mustard-based chemical warfare agent and pesticides. The southern portion ofthe SPBA isjust north ofLake Ladora, Lake Mary, Upper Derby Lake, and Lower Derby Lake, which is currently prime habitat forwildlife and a potential high use area for the future wildlife refuge (Figure 2). South Plants was previously characterized during the Remedial Investigation initiated in 1984. A summary ofthe information collected during the Remedial Investigation/Feasibility Study process can be found inthe Remedial Investigation Summary Report prepared by Ebasco Services Incorporated in1992 and inthe Detailed Analysis ofAlternatives Report prepared by Foster Wheeler Environmental Corporation in1995. Risk-based analysis ofdata collected during the Remedial Investigation resulted indesignation ofhuman health exceedance and biota soil in South Plants. The ROD also defined a potential chemical agent soil contamination area, based on site history. Remedial Actions and Interim Response Actions inSouth Plants have included the removal of friable asbestos, removal ofpolychlorinated biphenyls (PCBs) associated with electrical transformers, removal ofagent-related chemical process equipment and piping, removal of underground storage tanks and above-ground storage tanks, and plugging ofthe sanitary and chemical sewers. The ROD identifies the following major actions forthe South Plants remedy: " Demolition of structures. " Removal offloor slabs and foundations insoil excavation areas. Slabs that are not in

4 10/23/00, UAPR0JECTS\SPS01L\S0UTHPLANTs ESD.DOCEXPLANATION OF SIGNIFICANT DIFFERENCES FOR SOUTH PLANTS BALANCE OF AREAS AND CENTRAL PROCESSING AREA SOIL REMEDIATION PROJECT ROCKY MOUNTAIN ARSENAL FEDERAL FACILITY SITE excavation areas are to be left inplace and cracked to prevent ponding ofwater. " Collection oftwo composite samples peracre inthe SPBA prior to placing the 1 -foot cover to verify thatthe soil underthe 1 -foot soil cover does not exceed human health criteria. If the residual soil isfound to exceed these levels, the 3-foot coverwill be extended over these areas orthe exceedance soil will be excavated and landfilled. " Screening during excavation ofpotentially agent-contaminated soil followed by treatment (ifnecessary) and landfilling. " Excavation ofhuman health exceedance soil (to a maximum depth of 5 feet inthe SPCPA and to a maximum depth of 10 feet inthe SPBA) and disposal inthe onsite hazardous waste landfill. " Excavation ofthe topfoot ofsoilfrom the entire SPBA because itpresents a riskto biota. " Backfill of human health exceedance soilexcavations with biota soil orclean soil. " Placement offill that may consist of biota soil under the SPCPA soil cover. " Construction ofa 4-foot soil coverwith a biota barrier over the SPCPA. " Construction ofa 3-foot soil cover overthe area ofthe SPBA immediately surrounding the SPCPA. " Construction ofa 1 -foot soil cover overformer biota exceedance areas around the periphery ofthe SPBA. " Revegetation consistent with ROD requirements for covers. " Institutional controls consistent with ROD requirements forcovers. " Long-term maintenance ofthe 1-, 3-, and 4-foot covers. " Long-term ground water monitoring. 0 Currently, many structures inSouth Plants have been demolished as required by the ROD, and only foundations remain. Underground utilities exist but are no longerenergized. Overhead electric lines and utility poles have been removed. Railroad tracks and ties throughout the South Plants area have been removed.

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3. 0 MODIFICATIONS TO THE ROD REMEDY 3.1 Change to the 1-Foot Soil Cover Area of the SPBA Table 1summarizes the ROD-prescribed requirements forthe 1 -foot soil cover area ofthe SPBA and shows the changes to those requirements. As shown inthe table, the primary change to this area isthe deletion ofthe 1 -foot cover and replaced with 1 -foot ofbackfill. The table also shows enhancements thatwere made to other aspects ofthe ROD-prescribed remedy forthis area to ensure the overall remedy is equally protective as the remedy prescribed inthe ROD. Table 1: Changes to the 1 -Foot Cover Area ofthe SPBA ROD-Presgribed Remedy Modification Demolition ofstructures, and cracking remaining slabs and Enhancement. All structureswill bedemolished and all foundationsto preventponding ofwater. foundations and slabswill beremoved. Collection oftwo composite samples per acre priortoplacing Enhancement. An enhanced sampling program was the 1 -footcovertoverifythatthe soil underthe 1 -foot soil conducted during the design phaseoftheprojectthat cover doesnot exceed human health criteria. included 200 samplesinaddition tothe required 2samples peracre (foratotal ofmorethan 600 samples in 208acres). Excavation andlandfill ofhuman health exceedance soil. Enhancement. Excavation ofallROD-defined human health exceedance soil andexcavation ofany additional human health exceedance soilidentified during the enhanced sampling program. Allhuman health exceedance soilwill be disposed in the on-posthazardous waste landfill. Excavation ofall ROD-defined blota soil. No Change.All ROD-defined biotasoil willbeexcavated and consolidated in the SPCPA. Backfill ofhuman health exceedance soilexcavations with Enhancement. Onlycleansoil will beusedtobackfill biota soil. excavations in the 1-foot coverarea ofSPBA. Construction ofa1 -footsoilcover. Reduction. A1 -footsoilcoverwill not beconstructed, but 1 footofclean soil will beback-filled following biota soil excavation. Revegetation standards consistentwith ROD requirements for Reduction. Vegetation standards consistentwith refuge cover systems. policy will beused. Institutional controls consistentwithROD requirements for Reduction. Institutional controlsfora noncoverarea will be covers systems (e.g.,delineation and access control). implemented (e.g,, controlled access via roads andtrails). Long-term ground watermonitoring. No Change. Thesame long-termground watermonitoring program will beimplemented.

6 10/23/00, U:\PR0JECTS\SPS01L\S0UTHPLANTs ESD.DocEXPLANATION OF SIGNIFICANT DIFFERENCES FOR SOUTH PLANTS BALANCE OF AREAS AND CENTRAL PROCESSING AREA SOIL REMEDIATION PROJECT ROCKY MOUNTAIN ARSENAL FEDERAL FACILITY SITE 3.2 Change tothe 3-Foot Soil CoverArea ofthe SPBA Table 2 summarizes the ROD-prescribed requirements for the 3-foot soil cover area ofthe SPBA and shows the removal of the requirement to excavate biota soil from under the 3-foot cover area. Table 2: Changes tothe 3-Foot Cover Area ofthe SPBA ROD-Prescribed Remedy Modification

Demolition ofstructures, removal offoundations and slabs in No Change. All structures will be demolished; foundations excavation areas, and cracking remaining slabs and and slabs will be removed in excavation areas and cracked in foundations to prevent ponding or water in non-excavation non-excavation areas. areas.

Excavation and landfill ofhuman health exceedance soil. No Change. Excavation ofall ROD-deflned human health exceedance soil and disposal inthe on-post hazardous waste landfill.
Excavation ofall ROD-defined biota soil. Reduction. Not all soil posing a potential riskto biotawill be excavated, however,the biota soil will be covered with a 3- foot cover as required by the ROD.
Backfill ofhuman health exceedance soil excavations with No Change. Biota soil can be used to backfill human health biota soil. exceedance excavations inthe 3-foot coverarea of SPBA.
Construction of a 3-foot soil cover. Enhancement. The cover will be designed and constructed using concepts derived from the RCRA-Equivalent Cover Demonstration Project at RIVIA. Also, the initial constructed coverthickness will be increased to 3.25 feetto allow for erosion and will be constructed ofsoil from the clear borrow areas. Revegetation standards consistentwith ROD requirements for No Change. The same revegetation standards will be used. covers systems.
Institutional controls consistent with ROD requirements for No Change. Institutional controls for a coverarea will be covers (e.g., markers delineating the extent ofthe cover and implemented (e.g., markers delineating the extent ofthe access control). cover and access control). Long-term ground water monitoring. No Change. The same long-term ground water monitoring program will be implemented.

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3.3 Change to the 4-Foot Cover Table 3 summarizes the ROD-prescribed requirements forthe 4-foot soil cover over the SPCPA and shows the enhanced, RCRA-Equivalent, construction methods thatwill be used to construct this cover, Table 3: Changes tothe 4-Foot Cover Area ofthe SPCPA ROD-Prescribed Remedy Modification
Demolition ofstructures, removal offoundations and slabs in No Change. All structures will be demolished; foundations excavation areas, and cracking slabs and foundations to and slabswill be removed in excavation areas and cracked in prevent ponding or water in non-excavation areas. non-excavation areas.
Excavation and landfill ofhuman health exceedance soil to a No Change. All ROD-defined human health exceedance soil maximum depth of5 feet. will be excavated to a depth of5 feet and disposed in the onpost hazardous waste landfill.
Backfill ofhuman health exceedance soil excavations with No Change. Blota soil will be used to backfill human health biota soil. exceedance excavations in the 4-foot cover area of SPCPA.

Consolidation ofbiota soil excavated from the SPBA and No Change. Biota soil will be used asfill under the 4-foot placement as fill underthe 4-foot cover. cover. Construction of a 4-foot soil cover. Enhancement. The coverwill be designed and constructed to meet the same criteria asthat used for the RCRAEquivalent Cover Demonstration Project at RIVIA. Also, the initial constructed coverthickness will be increased to 4.5-feet thick to allow for erosion and will be constructed using soil from the borrow areas. Revegetation standards consistent with ROD requirements for No Change. The same revegetation standards will be used, cover systems. consistent with refuge policy.
Institutional controls consistent with refuge policies for a cover No Change. Institutional controls for a cover area will be area (e.g., markers delineating theextent ofthe cover and implemented (e.g., markers delineating the extent ofthe access control). cover and access control). Long term ground water monitoring. No Change. The same long-term ground water monitoring program will be implemented.

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4.0 BASIS FOR THE ESD 4.1 Basis for the Change tothe 1 -Foot Soil Cover Area of the SPBA As partof the South Plants remediation project, the ROD requires additional characterization of the 1 -foot cover area byrequiring the collection oftwo samples per acre to verify that soil left after excavation does not exceed human health criteria. Since the 1-footcover area lies outside the more contaminated central processing area, the potential for remaining contamination above that already identified and reported during the Remedial Investigation/Feasibility Study is expected to be minimal. While the ROD-required sampling would indicate whether human health exceedance soil was removed from the top foot, itwould not verify this at depth. Therefore, after consulting with the regulatory agencies (EPA, CDPHE, and TCHD) the Army developed an enhanced sampling program to more fully characterize the I-foot cover area at depth. This resulted insignificantly more samples than originally required by the ROD. The enhanced sampling program was implemented during the summer of 1999 and based on the sample results, there is notechnical reason toretain the 1-footcover requirement. Long term maintenance requirements forthe 1-foot cover area may be eliminated and the protectiveness ofthe remedy is not compromised. The elimination ofthe 1 -foot cover will also result ina long term cost savings, and will allow transfer ofthis area to Department of the Interior for inclusion as part ofthe wildlife refuge, Details ofthe enhanced sampling program are described below. Sample locations forthe enhanced sampling program were selected based upon site history, historical sampling data collected during the Remedial Investigation, proposed locations of future use features, and boundaries ofexcavation sites. Additional samples were placed where fewer samples had been collected inthe past, and where exceedances or near-exceedances were identified outside ofexcavation areas. Samples were placed along proposed locations for a possible future tram route, trails, and at the boundary ofthe 3-foot cover area and the 1-foot cover inthe SPBA. Samples were also placed around ROD-identified excavation areas to verify that the planned excavation would be sufficient to remove the human health exceedance soil. Discrete samples were collected at various depths at all ofthese locations. Additional samples were placed inareas of SPBA that still had a relatively low sample density. These samples were placed using a grid system. The sampling area was divided into one-half acre grid areas. Grid areas thatdid not have existing remedial investigation samples or newly identified samples were selected for collection of composite samples. The composite samples were collected from 12 inches to 18 inches below the ground surface at the grid locations. Figure 3 shows the location of samples collected during the enhanced sampling program. The resulting sampling plan called forover 600 sample locations throughout the 1 -foot cover area, approximately 200 samples more than required by the ROD. Depending on the site history inthe vicinity ofa specific sample location, samples were analyzed for a combination ofpesticides, metals, and volatile organic compounds. The results ofthe enhanced sampling program are described inthe South Plants Balance of Areas Sample Results Report prepared by FosterWheeler Environmental Corporation inMarch 2000. Ofthe approximately 600 samples collected during the enhanced sampling program, only3 9 10/23/00, U:\PR0JECTS\SPS01L\S0UTHPLANTs ESD,DOCEXPLANATION OF SIGNIFICANT DIFFERENCES FOR SOUTH PLANTS BALANCE OF AREAS AND CENTRAL PROCESSING AREA SOIL REMEDIATION PROJECT ROCKY MOUNTAIN ARSENAL FEDERAL FACILITY SITE human health exceedances were identified. Furthermore, the three exceedances are located beneath two human health exceedance soil areas thatwere already identified for excavation by the ROD and were easily incorporated intothe design as areas ofadditional excavation. During the Remedial Investigation ofthe SPBA, 1,111 samples were collected resulting in39 human health exceedances. Therefore, there is now adata set ofover 1,700 samples inthe 208 acres area that resulted in42 exceedances. No evidence of previously undetected contaminant source areas was identified. Two additional enhancements to the ROD-prescribed remedywill beimplemented inthe 1-foot cover area. All structure foundations and floor slabs will be removed rather than leaving some in place and cracking them, and only clean soil will be used to backfill the deeper human health exceedance excavations rather than using biota soil as backfill. Therefore, because human health exceedance soil and biota soil are to be removed from the entire 1 -foot area ofthe SPBA, there is no technical reason to retain the requirement fora cover.

4.2 Basis for the Change in the 3-Foot Soil Cover Area ofthe SPBA The ROD requires excavation ofthe top 1-footofsoil from the 3-foot cover area to remove potential risk tobiota. After the 1-footofbiota soil is removed, the ROD requires placement of a 3-foot cover. However, the ROD also allows biota soil to be used as fill under the 3-foot cover. The change inthis ESD removes the requirement to excavate the top 1 -foot ofbiota soil from under the 3-foot cover system. Aportion ofthe existing biota soil will be left undisturbed rather than excavated and used as fill inother sections ofthe South Plants cover areas. Any biota removed will be consolidated into the SPCPA. Acover constructed of 3-feet ofclean soil will be placed over the biota soil. Thisfinal disposition of biota soil inthe 3-foot cover area ofthe SPBA is consistent with the ROD remedy: biota soil iseither covered with clean soil orremoved and placed ina consolidation area (such as Basin Aor the SPCPA). This change results ina more cost effective remedy while maintaining the same level ofprotection required by the ROD (i.e,, biota soil that is left inplace is contained under a long term-c6ver). Long-term maintenance requirements remain unchanged. 4.3 Basis for the Change tothe 4-Foot Cover The ROD requires construction ofa 4-foot soil coverover the SPCPA after human health exceedance soil has been excavated to a maximum depth of5 feet. The ROD also requires that the cover provide long-term protectiveness bybeing designed and constructed to minimize the infiltration ofwater through the cover into remaining contamination and the underlying soil. To meet this objective, the latest technology for minimizing infiltration through covers will be employed forthe 4-foot cover over the SPCPA, byusing the same design and construction methods thatwere used forthe ongoing RCRA-Equivalent Cover Demonstration Project at RIVIA. RCRA-equivalent soil covers use an evapotranspirative soil layer inconjunction with native vegetation to obtain levels ofperformance equal to RCRA hazardous waste landfill covers. In 10 10/23/00, UAPR0JECTS\SPS01L\S0UTHPLANTs ESD.DocEXPLANATION OF SIGNIFICANT DIFFERENCES FOR SOUTH PLANTS BALANCE OF AREAS AND CENTRAL PROCESSING AREA SOIL REMEDIATION PROJECT ROCKY MOUNTAIN ARSENAL FEDERAL FACILITY SITE April 1998, the Army designed and constructed four RCRA-equivalent cover test plots at RMA, to determine ifthese alternative covers could provide a level ofperformance equal to that provided by a traditional RCRA hazardous waste cover system (typically constructed using compacted clay and geosynthetic membranes). The RCRA-equivalent cover test plots are allowed to develop a full stand of native vegetation for three years, and following successful establishment, each test cover is monitored to determine ifitcan meet strict standards for minimal infiltration ofwater (i.e. a standard of 1.3 millimeters per year). Successful test plots will be considered "RCRA-equivalent." Successful cover designs from the field test can then be used as the RCRA-equivalent cover design at sites were the ROD specifically requires the placement of RCRA-equivalent covers (such as at the Former Basin F site), and additionally, in the SPCPA. While the testing ofthe RCRA-equivalent covertest plots will not be complete fortwo years, rigorous modeling and preliminary monitoring data from the ongoing field study suggests that the covers will meet the strict standards forminimal infiltration ofwater. On this initial basis, the Army has chosen to design and construct the coverover the SPCPA to a minimum thickness of 4.5 feet, using the same specifications and qualitycontrol principles thatwere used forthe successful RCRA-equivalent covertest plot. 5.0 SUPPORTAGENCY COMMENTS The EPA, CDPHE, and TCHD have reviewed and concurred with this ESD. Comments from these agencies have been incorporated intothe document.

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6.0 STATUTORY DETERMINATIONS Considering the changes described inthis ESD and the new information, the Army, EPA and CDPHEbelieve thatthe remedy remains protective ofhuman health and the environment, complies with federal and state requirements that are legally applicable orrelevant and appropriate tothe remedial action. Signatures For U.S. Environmental Protection Agency

Date Max H.Dodson Assistant Regional Administrator Office of Ecosystems Protection and Remediation For U.S. Army

Date Charles T. Scharmann Program Manager for Rocky Mountain Arsenal For State ofColorado V\ kk- k1o

-- Crzj Date Howard Roitman Director, Hazardous Materials and Waste Management Division

7.0 PUBLIC PARTICIPATION COMPLIANCE The Army published a public notice inthe Rocky Mountain News and Denver Poston April 6, 2000, making the Draft South Plants ESD available for public review and comment. A presentation regarding the ESD was made tothe Restoration Advisory Board (RAB) on March 2, 2000. The RAB is a community group that is open to the public and meets monthly to receive information and provide input on remedial activities being conducted at the RMA. The public comment period was closed on May 8, 2000. No comments from the public were received. The requirements set out inthe National Contingency Plan, Section 300.435(c)(2)(ii), have been met. This ESD and all documents that support the changes and clarifications are part ofthe Administrative Record and available at the Joint Administrative Record Document Facility (JARDF) and the EPA Region 8 Superfund Record Center.

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8.0 REFERENCES Ebasco Services Incorporated. 1992. RemedialInvestigationSummaryReport.Final.Rocky Mountain Arsenal. Commerce City, Colorado. January. Foster Wheeler Environmental Corporation (FWEC). 1996. RecordofDecision forthe On-Post OperableUnit. Rocky Mountain Arsenal, Commerce City, Colorado. June. FWEC. 1995. Detailed Analysis of AlternativesReport.Final. Rocky Mountain Arsenal. Commerce City, Colorado. October. FWEC. 1999. South Plants Balanceof AreasSampleResultsReport. Rocky Mountain Arsenal. Commerce City, Colorado. Morrison Knudsen Corporation (MKC). 1998. RCRA-Equivalent CoverDemonstrationProject, FinalDesignPackage. Rocky Mountain Arsenal. Commerce City, Colorado. January. U.S. Environmental Protection Agency (EPA). 1999. A GuidetoPreparing SuperfundProposed Plans,RecordsofDecision,andOtherRemedySelectionDecisionDocuments. EPA 540-R98-031. Washington D.C. July. EPA. 1989. FederalFacility AgreementPursuant to CERCLA Section 120. Docket No. CERCLA Vill-89-13. Rocky Mountain Arsenal. Commerce City, Colorado. February 17.

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