## Rocky Mountain Arsenal Offpost Operable Unit Final Record of Decision Rocky Mountain Arsenal Commerce City, Colorado
Prepared for 

## Program Manager forRocky MountainAmenal Building 111, Rocky Mountain Arsenal Commerce City. Colorado 80022-2180 HLA Project No. 21905 402010 Contract No. DAAA05-92-D-0003 Delivery Order No. 0005

TIUS DOCUMENT IS IN7LNDED TO COMPLY WITH THE NATIONAL ENVIRONMENTAL POLICY ACT OF 1969. THE INFORMATION AND CONCLUSIONS PRESENTED IN TFUS REPORT REPRESENT THE OFFICIAL POSITION OF THE DEPARTMENT OF THE ARMY UNLESS EXPRESSLY MODIFMD BY A SUBSEQUENT DOCUMENT. TMS REPORT CONSTITUTES THE RELEVAN7 PORTION OF THEADMINISTRATIVE RECORD FORTIES CERCLA OPERABLE December 19, 1995 Harding Lawson Associates z 11.= 1z Engineering and Environmental Services 707 Seventeenth Street, Suite 2400 Denver, CO 80202 - (303) 292-5365 

# M9604811 RecycledPaper

## CONTENTS

DECLARATION STATEMENT ................................... **'-**-*- DS-1

I.o SITE NAME, LOCATION, AND DESCRIPTION ................................. 1-1 1.1 Environmental Setting ................................. ... 1-2 1.2 Geology ......................................... 1-2 1.3 Hydrogeology .......................................... 1-3 2.0 SIIT HISTORY AND ENFORCEMENT ACTIVITIES ............................. 2-1 2.1 Operational History ................................................ 2-1 2.2 Previous Investigations .......................................... **** 2-2 2.2.1 U.S. Environmental ProtectionAgency Study Area .................. 2-2 2.2.2 U.S. Department ofthe ArmyInvestigation ....................... 2-3 2.3 Boundary Containment Systems ................................... - - 2-4 2.3.1 North Boundary Containment System ........................... 2-5 2.3.2 Northwest Boundary Containment System ........................ 2-6 2.3.3 Irondale Containment System ................................. 2-6 2.4 Interim Response Actions .......................................... ** 2-7 2.4.1 Offpost Interim Response Action ............................... 2-7 2.5 History of CERCLA Enforcement Activities ............................... 2-8 3.0 HIGHLIGM OF COMmUNrff PARTICIPATION .............................. 3-1 4.0 SCOPE AND ROLE OF THE OFFPOST OPERABLE UNIT ......................... 4-1
s.o SUINIMARY OF SIT`E CHARACTERISTICS .................................... 5-1 5.1 Sources of Contamination ............................................ 5-1 5.2 Nature of Contamination ............................................ 5-1- 5.3 Contamination Migration Pathways .................................... 5-2 5.4 Extent of Contamination .............................................. 5-3 5.4.1 Groundwater .............................................. 5-3 5.4.2 Surface Water ............................................. 5-4 5.4.3 Stream-bottom Sediments .................................... 5-5 5.4.4 Surface and Subsurface Soil .................................. 5-5 5.4.5 Biota .................................................... 5-6 5.5 Potential Routes ofHuman and Environmental Exposure .................... 5-7
'21905402010 Harding Lawson Associates 1107121895 R02

### 6.0 SUMMARY OF SrM RISKS ................................. 6-1 6.1 Human Health Risks .......................... 6-2 6.1.1 Identification ofChemicals of Concern .......................... 6-2 6.1.2 Exposure Assessment ...................... ...6-3 6.1.2.1 Offpost StudyArea Exposure Assessment Zones ............. 6-3 6.1.2.2 Offpost Study Area Potential Exposure Points .............. 6-5 6.1.2.3 potential Exposure Pathways and Routes .................. 6-6 6.1.2.4 Estimation ofChemical Intake .......................... 6-8 6.1.3 ToxicityAssessment .......................... ...6-8 6.1.4 Risk Characterization ............................... *,*** .... 6-9 6.1.4.1 Carcinogenic Risks ................................... 6-9 6.1.4.2 Noncarcinogenic Effects .............................. 6-10 6.2 Estimation ofPotential EcologicalEffects ............................... 6-11 6.2.1 Method ................................................. 6-11 6.2.2 Results ................................................. 6-13 6.3 Conclusion ..........................................  '** * .... 6-14 7.0 DESCRIPTION OF GROUNDWATER REMMIATION ALTERNATIVES ............... 7-1 7.1 Common Elements ofAlternatives ..................................... 7-4 7.2 identification ofGroundwater Alternatives: North Plume Group .............. 7-6 7.2.1 Alternative N-1: No Action ................................... 7-6 7.2.2 Alternative N-2: Continued Operation oftheNorth Boundary Containment System With Improvements as Necessary ...... 7-6 7.2.3 Alternative N-4: Offpost Groundwater Intercept and Treatment System 7-7 7.2.4 Alternative N-5: Expansion ofthe Offpost Groundwater Intercept and Treatment System .............................. 7-10 7.3 identification ofGroundwater Alternatives: NorthwestPlume Group .......... 7-12 7.3.1 Alternative N-1: No Action .................................. 7-12 7.3.2 Alternative NW-2: Continued Operation ofthe Northwest Boundary Containment System With Improvements as Necessary ..... 7-13 8.o COMPARATIVE ANALYSIS OFALTERNATWES ............................... 8-1 8.1 Comparative Analysis ofRemedial Alternatives ........................... 8-2 8.1.1 Overall Protection ofHuman Health and the Environment ............ 8-3 8.1.2 Compliance With AppLcable or Relevant and Appropriate Requirements - 8-4 8.1.3 Long-term Effectiveness and Permanence ......................... 8-5 8.1.4 Reduction inToxicity. Mobility. or Volume ....................... 8-6 8.1.5 Short-term Effectiveness ..................................... 8-6 8.1.6 Implementability ........................................... 8-7 8.1.7 Cost .................................................... 8-9 8.1.8 State Acceptance .......................................... 8-10 8.1.9 CommunityAcceptance ..................................... 8-11 8.2 Conclusions ofthe Comparative Analysis ofAlternatives ................... 8-11

ii Harding Lawson Associates 21905 402010 1107121895 R02

### 9.o IDENTIFICATION OFTHE SELECT REMEDY ............................... 9-1 9.1 Alternative N-4: Offpost Groundwater Intercept and Treatment System ......... 9-1 9.2 Alternative NW-2: Continued Operation oftheNorthwest Boundary Containment System with Improvementsas Necessary ...................... 9-3 9.3 Additional Components ofthe Selected Remedy ........................... 9-5 9.4 Cost ofSelectedRemedy ............................................ 9-6 9.5 Limitations ....................................................... 9-6 9.6 Criteria for Shutting Down Boundary and Offpost.Containment Groundwater Systems ......................................................... 9-7 10.0 STATUTORY DETERMINATIONS ......................................... 10-1 10.1 Consistency with the Statutory Requirements ofCERCLA in Section 121 ....... 10-1 10.1.1 Protection ofHinnan.Health andthe Environment ................. 10-1 10-1.2 Compliance With Applicable or Relevant and Appropriate Requirements 10-2 10.1.3 Cost Effectiveness ......................................... 10-3 10-1.4 Utilization cfPermanent Solutions to the Maximum Extent Practicable . 10-4 10.2 Consistencywith the National Contingency Plan ......................... 10-4 10.3 Summary ....................................................... 10-5 11.0 DOCUMENTATION OF SIGNIFICANT CHANGES ............................. 11-1 12.0 GLOSSARY .......................................................... 12-1 13.0 BIBUOGRALPHY ....................................................... 13-1 TABLES 6.1 Offj--551 ();-'rdble Urlit Groundwater Chemicals of Concern 6.2 Offt"m ()I--rdble Unit Surface-Water Chemicals of Concern 6.3 DI--r-Alle Unit Sediment Chemicals of Concern in First Creek 6.4 0ifix,st ()I--rable Unit Sod Chemicals of Concern 6.5 Suruman of Ldnd-Use Scenarios and Exposure Routes by Zone 6.6 R-1--r--n, - I)-,u-s and Slope Factors for Chemicals ofConcern 6.7 Surrinjar%of Rt-asonable Maximum Exposure Carcinogenic Risks by Zone and Exposure Route 6.8 Sunimar%of Adult Reasonable Maximum Exposure Noncarcinogenic Han d.Indices by Target Orgd,l dn I Lxposure Assessment Zone 7.1 COntdinnient System Remediation Coals for the Offpost Groundwater Intercept and Treatment S,.,stem 7.2 Containment Svstem Remediation Goals for the North Boundary Containment System 7.3 Containment System Remediation Coals for the Northwest Boundary Containment System 7.4 Groundwater Alternatives for the North and Northwest Plume Groups 8.1 Summarv ofthe Detailed Analysis and Ranking of Groundwater Alternatives for the North Plume Group 8.2 Summary ofthe Detailed Analysis and Ranking of Groundwater Alternatives for the Northwest Plume Group

'21905 402010 Harding Lawson Associates iii 1107121995 R02

### 9.1 Estimated Costs ofthe Offpost OperableUnit SelectedRemedy 10.1 SummaryEvaluation ofChemical-specific and Other Applicable -orRelevant and Appropriate Requirements forthe Offpost Operable Unit 10.2 Summary Evaluation ofAction-specific Applicable or Relevant and Appropriate Requirements for the Offpo'st Operable Unit 10.3 SummaryEvaluation ofLocation-specificApplicable or Relevant and Appropriate Requirements forthe Oftpost Operable Unit FIGURES 1.1 RockyMountain Arsenal Operable Units and Offpost Study Area 2.1 Locations of Contaminant Source Areas 5.1 Contaminant Migration Pathways 6.1 Offpost Study Area Exposure Assessment Zones 6.2 Site Conceptual Model for the Offpost Study Area 6.3 Ecological Site Conceptual Model for the Offpost Study Area 7.1 Alternative N-4 Remedial System Components 7.2 Alternative N-5 Remedial System Components 9.1 Offpost Area ofRevegetation APPENDIXES A RESPONSWENESS SUMMARY B INSTITUTIONAL CONTROLS C WELL CLOSURE CRITERIA

iv Harding Lawson Associates '21905 402010 1107121995 R02DECLARATION STATEMENT Rocky Mountain Arsenal

Offpost Operable Unit Commerce City, Colorado

### DECLARATION FORTHE RECORD OF DECISION SITE NAME AND LOCATION Rocky MountainArsenal Offpost Operable Unit CommerceCity,Adams County, Colorado STATEMENT OF BASIS AND PURPOSE This decision documentpresents the selectedremedial action forthe Rocky MountainArsenal (RMA) Offpost Operable Unit (OU) in southern Adams County,east ofCommerce City,Colorado, chosenin accordance with the Comprehensive Environmental Response, Compensation,and Uability Act (CERCLA) of 1980, as amended bythe Superfund Amendments and ReauthorizationAct (SARA) of 1986, and theNational Environmental PolicyAct (NEPA), and, to the extent practicable,the National Oil and Hazardous SubstancesPollution ContingencyPlan (NCP). This decision is based onthe administrative record file for the Offpo--t OU. and this document explains thebasis andpurpose of the selected remedyfor the Offpost OU. ASSESSMENT OFTHE SITE The Offpost Study Arearisk assessment showed that evenwithout remedial action, the baseline cumulative cancerrisks from contamination in surface water, soil, sediment, air, and groundwater are within the acceptable cancerrisk range established bythe U.S. Environmental Protection Agency (EPA). However,several site-specific factors suggest that remedial alternatives for groundwater should be developed. These site-specific factors are: (1)groundwater contributes a maximum of 2x 10'4. or approximately 75 percent ofthe total carcmogenicrisk, (2)maximum contaminant levels (MCLs). maximum contaminant level goals (NICLCs). and Colorado Basic Standards for Groundwater (CBSGs) are exceeded for somegroundwater contaminants. and (3)hazard indices (HIs) for children exceed 1.0 in Zones 2,3.and 4. 

### Although the haziud indices exceed 1.0 inZones 2, 3,and 4, the bulk ofthe HI value iscontributed through an assumed domestic use ofalluvial groundwater,which is not presently occurringand under tlus remedy is not intendedto occurinthe future. The elevated

'21905 402010 

### Harding Lawson Associates DS-1 0711121895 R02Declaration forthe Record ofDecision Ms occuronly when considering the contribution ofgroundwater. Therefore, groundwater contamination is the focus ofthis decisiondocument.
Actual or threatened releases ofhazardous substances from this site, ifnot addressedby implementingthe response action selected in this ROD, may present a potentialthreat to public health, welfare,orthe environment. DESCRIPTION OF THE REMEDY The Offpost OU is one oftwo OUs at RMA. The Onpost OU addresses the contamination within the 27 square miles ofRMA. The Offpost OU addresses groundwater contamination north ofRMA that migrated (1) beforethe RMAboundary groundwater extraction and treatment systems were installed, and (2) around the boundary systemsprior to recent improvements. The selected remedy described in this Record ofDecision (ROD) will permanently address contaminants at the sitethrough treatment to reduce the toxicity,mobility, or volume ofcontaminants. Groundwater containment system remediation goals arebased onthe risk assessment and onfederal Safe Drinking Water Act MCLs, proposed MCLs, nonzero MCLGs, and CBSGs. Action levels alsomeetthose state drinking water standards found to be applicable or relevant and appropriate requirements (ARARs).

The majorcomponents ofthe selected remedy are as follows: Operation ofthe Offpost Groundwater Intercept and Treatment System Removal of contaminated groundwater from the alluvial and the weathered upper portion ofthe Denver Formation Jhereafter called theunconfined flow system [UFSI) north ofthe RMA boundary Ln the First Creek and northern paleochannels using groundwater extraction wells Treatment ofthe organic chemicals ofconcern (CoCs)present in the groundwater using carbon adsorption Recharge oftreated groundwater to the UFS using wells and trenches Natural attenuation ofinorganic chloride and sulfate concentrations tomeet applicable standards for groundwater in a manner consistent with the Onpost remedial action

DS-2 Harding Lawson Associates '21905402010 0711121895 R02Declaration forthe Record ofDecision
Continued operation oftheNorth Boundary Containment System (NBCS) and the Northwest BoundaryContainment System (NWBCS) -In addition, theIrondale Contaminant System (ICS)will continue to operate, as required,for onpost contaminants consistent with the Irondale Interim Response Action (IRA). These containment systems willbe operated to the requirements ofSection 2.7 ofthe FFA, theAgreement for a Conceptual Remedyfor the Cleanup ofthe Rocky Mountain Arsenal (Conceptual Remedy Agreement), and the onpost ROD, when itis signed. Cessation may occur as provided in Sections 35.3 and 35.4 ofthe FFA and paragraph 20 oftheConceptual RemedyAgreement. 0 Improvements to theNBCS, NWBCS, ICS, and the Offpost Groundwater Intercept and Treatment System as necessary 0 Long-term groundwater monitoring (includingmonitoring after groundwater treatment has ceased to assure continued compliance withthe groundwater containment systemremediation goals ) & Five-year sitereviews 0 Exposure control/provision ofalternate water supply as follows: As ofthe date ofthe Onpost ROD, and based on a .392 parts perbillion (ppb) detectionlimit,the U.S. Army will usethe last available quarterly monitoring results to determine the DIMP plume footprint As part ofthe Onpost ROD, the U.S. Army and Shell Oil company willpay for the extension of, and hook-up to, the current distribution system for all existing well owners withinthe DR-AP plume footprint referenced above. Existing domestic well owners outside ofthe DlMP plume footprint as ofthe date of the Onpost ROD where it is later determined that levels ofDR" are eight ppb or greater (or other relevant CBSG at the time)will be hookedup at the U.S. Army and Shell Oil Company's expenseto the SACWSD distribution system orprovided a deep well or other permanent solution. For new domestic wells with DMIP levels ofeight ppb or greater (or otherrelevant CBSG atthe time), the Offpost ROD institutional controls willprovide that the U.S. Army and Shell Oil Company will pay for hook-up to the distribution system or provided a deep well or other permanent solution. Any user ofa domestic well withinthe Offpost Operable Unit that contains groundwater contaminants denved from RMA at concentrations that exceed the greater of theremediation goals in Tables 7.1 through 7.3 ortheARARs in Table im winbe provided an altemative water supply. Bottled water willbe provided for cooking and dnnkmg until a permanent alternative water supply is provided. Permanent alternative water supplies could include installation of a deep uncontaminated wen or connection to a municipal potable water-supply system. This commitment applies to both users of existing domestic wells and users ofwellsthat are lawfully drilled in the future. Institutional controls to prevent the use ofgroundwater exceedingremediation goals. Closure ofpoorly constructed wells within the Offpost StudyArea that could be acting as migrationpathways for contaminants found in the Arapahoe Aquifer.

'21905 402010 Harding Lawson Associates DS-3 0711121895 R02Declaration forthe Record ofDecision 

### The U.S. Army and Shell Oil Companyagreeto continue monitoringand to complete an assessment ofthe NDMA plumebyJune 13, 1996,using a20 ppt method detection limit The U.S. Army and Shell Oil Company agreeto prepare afeasibility study ofpotential actions,bothonpost and at theboundary,or adjacentto theboundaryin orderto achieve NDMA remediation goals atthe RMAboundary and touse 7.0 ppt PRG or acertified alytical detection levelreadily available at acertified commercial laboratory (currently 33 ppt). The U.S. Army and Shell Oil Companyagree torevegetate approximately IL60 acres located in the southeast portionof Section 14 and the southwestportion ofSection 13 as depicted in Figure 9.1. Revegetation will involvetilling and seeding. No samplingwill beconducted before orafter revegetation. Existing soil.risks inthe aretoberevegetated fall withinEPA;s establish acceptablerisk range andrevegetation isnot necessary. However,the U.S. Army and Shell Oil Company agreeto therevegetationprogram as part ofthe offpost settlement. TheArmy will treat any contaminated extractedgroundwaterprior todischarge orreinjection so that it meetsthe current water quality standards establishedin the ColoradoBasic Standards for Groundwater and the Colorado Basic Standards andMethodologies for Surface Water. As part ofthe Onpost remedy,the U.S. Armyand ShellOil Companywill pay for and provide, or arrange forthe provision,of4000 acre-feet ofwaterto SACWSD. STATUTORY DETERNINATIONS The selected remedyis protective ofhuman health and the environment, complieswith federal and state requirements that are legally applicable orrelevant and appropriate tothe remedial action, and is cost-effective. Theremedyuses permanent solutionsand alternativetreatmenttechnologies to the maximum extent practicable. The remedysatisfies the statutorypreference forremedies that employ treatment thatreduces toxicity,mobility, orvolume as a principal element
Because this remedy mayresult inhazardous substances remaining in the groundwater oftheOffpost OU for more than five years,areview will be conducted within five years after commencementof remedial action to ensure that theremedy contmues to adequatelyprotecthuman health and the environment.

DS-4 Harding Lawson Associates '21905402010 0711121995 R02Declaration for the Record of Decision 

### SIGNATURE PACE

am P.Yellowtau /Re)kionalMministrator,RegionVM (/1111ited States Environmental ProtectionAgency

Raymb'aFatz ActingDeputy Assistant Secretary oftheArmy (Environment,Safety and Occupational Health)
STATE OF COLORADO CONCURRENCE

TomLoob Director, Office ofEnvironmzý4 ColoradoDepartment ofPublic Health and Environment

'21905 402010 

### Harding Lawson Associates DS-5 0711121895 R02DECISION SUMMARY FOR THE RECORD OF DECISION

### 1.0 SITE NAME, LOCATION, AND DESCRIPTION The RockyMountain Arsenal (RMA) National Priorities List (NPL) site is comprised oftwo Operable Units (OUs): Onpost and Offpost. As shownin Figure 1.1, the Offpost StudyArea occupies approximately 27 square miles in southernAdams County, Colorado, and lies northofthe Denvermetropolitan area and east ofCommerceCity, Colorado. The Offpost StudyAreais defined as the area southeast ofthe South Platte River, north of 80th Avenue, southwestofSecond Creek, and north of the north and northwestboundaries ofRMA. Additionally,the Offpost StudyArea includes the surface waters ofO'Brian Canal and Burlington Ditch as they extend northeast from Second Creekto Barr Lake and the surface waters ofFirst Creek and Barr Lake. The Offpost OU (also shown in Figure 1.1) is defined bythe RMA Federal Facility Agreement (FFA) as that portion ofthe Offpost Study Area wherehazardous substances, pollutants, or contaminants fromRMA are found and are subjectto remediation. The Offpost OU encompassesrural residential, agricultural, and commercial and industrial areas locatednorth and northwest ofRMA.
Areas within the Offpost OU are used forrangeland, dryland farming, and irrigated farming with some rural residential areas and scattered areas ofintensive agricultuxal use. Parts ofthe Offpost OU
are currently zoned and developed for commercial/industrial activities. CommerceCity, located west ofRMA, is the only urban area in the immediate vicinity ofthe Offpost OU and hasrecently annexed lands within the Offpost OU.
On the basis ofan evaluation ofplanning information provided bythe Adams County Planning Commission, it is projectedthat areas ofcommercial. industrial, andurban residential land use will increase in the Offpost OU (Adams County Planrung Commission, 1987). Rural residential (including agricultural) land use is expected to decrease in the Offpost OU because anticipated increases in property values are expected to preclude increased traditional crop and livestockproduction land use, including hobbyfarming as discussed in the Airport Environs Plan (Adams County, City of Aurora, CityofBrighton, City ofCommerce City, 1990).
'21905 402010 

### Harding Lawson Associates 0711121495 

### R02Site Harn*, Locationg and Description 

### 1.1 Environmental Setting The topography ofthe Offpost StudyArea consists ofstream-valley lowlands separated by gently rolling uplands. The maximum local topographicreliefin the Offpost Study Area is approximately 100 feet. The elevation above mean sea level ranges from approximately 5140 feet at the northern and northwestern boundary ofRMA to approximately 5030 feet at the South Platte River. Cropland and rangeland provide habitat for numerous animal species. Lake and wetland areas at Barr Lake provide feeding, breeding, and roosting areas for waterfowl and endangered species, including thebald eagle. The climate ofthe Offpost Study Area is characterized by sunny, semiarid conditions.
The regional surface drainage is tothe northwest toward the South Platte River. Suxface water originating south ofRMA, on RMA, or in the Offpost Study Area flows toward the South Platte River. Two major canals, O'Brian Canal and Burlington Ditch, and several smaller ditches flow from southwest to northeast between RMA and the South Platte River. O'Brian Canal receives some drainage from the Offpost Study Area and RNIA wherethe canal intercepts First Creek. Burlington Ditch mav receive surface water infrequently from First Creek. 1.2 Geology Sediment at the land surface in the Offpost Study Area consists ofunconsolidated alluvial and eohan deposits. The composition ofthe unconsolidated sediment varies from clays to coarse gravels, and the thickness varies from less than 10 feet to approximately 100 feet. The thickest deposits of unconsolidated sediment occur in paleochannels eroded into the underlying Denver Formation.

The Denver Formation consists of 250 to 300 feet ofinterbedded 

### shale, claystone, siltstone,and sandstone, with aregional dip of1/2to 1degreetothe southeast. Thepresenceofpaleoch;;nnels in the DenverFormation surface impacts groundwater flow inthe unconsolidatedsediment and the upper weathered portion ofthe Denver Formation. Three such paleochannels, the First Creek, 1-2 Harding Lawson Associates '21905402010 0711121495 R02Site Name, Location, and Description northern, and northwestern paleochannels, are present in the Offpost Study Area. Coarse, unconsolidated materials commonly found within these paleochannels provide preferential pathways for groundwater movement. Groundwater contaminant plumes that have historically migrated across the RMA boundaries to the Offpost OU contain the highest concentrations of contaminants in and near these paleochannels. The Arapahoe Formation lies beneath the Denver Formation at depths of230 to 300 feet at the RMA north boundary and has a regional dip of 1/2 to 1 degree to the southeast. The formation consists of400 to 700 feet of interbedded conglomerate, sandstone, siltstone, and shale. The upper portion ofthe Arapahoe Formation consists predominantly of 200 to 300 feet ofblue to gray shale with some conglomerate and sandstone beds. The lower portion consists largely of sandstone and conglomerate with less prevalent beds of shale. The lower portion is a source zone for many water-supply wells in the area. A thick, impermeable claystoneunit is variously assigned to the lower Denver formation and the upper Arapahoe Formation. The claystone unit is called the -Buffer Zone" and is approximately 50-ft. thick. This unit further isolates the underlying Arapahoe aquifer from any localized contamination in the Denver confined flow system. The Arapahoe Formation is the oldest geologic unit present beneath the Offpost Study Area that was investigated durLn7,the Offpost Remedial Investigation program.
Aliuviai and eolian deposits form the ground surface in the Offpost Study Area. The Denver Formation and Arapahoe Formation are not prest-rit at the ground surface anywhere in the Offpost Study Area. 1.3 Hydrogeology Tiie two principal water-bearing units in th- Of1post Stu,i%-Area that have been impacted by contaminants originating from RMA are th,- unconsolidated alluvial deposits and the underlying Denver Formation. The hydraulic propert ies of thf!se two units. including hydraulic conductivity, porosity, and associated groundwater flow velocities. are distinctly different. The low permeability of the Denver Formation and upper Arapahoe Formation limit contaminant transport into the lower

'21905 402010 Harding Lawson Associates .1-3 0711121495 R02Site Ham*, Location, and Description Arapahoe Formation. Hydraulically, the two units generallybehave as two distinct hydrostratigraphic units: the unconfined flow system (UFS) and the confined flow system (CFS).

TheUFS includes groundwater present in the unconsolidated materials overlyingthe Denver Formation, the weathered upper portion ofthe Denver Formation, and, where the Denver Formation is missingnear the South Platte River, the weathered upper portion ofthe Arapahoe Formation. The CFS includes the deeper portions ofthe Denver Formation and the underlying Arapahoe Formation. On the basis ofan evaluation ofthe distribution ofcontaminant plumes in the Offpost Study Area, the UFS is considered theprincipal migrationroute for groundwater contaminants from RMA to the Offpost Study Area, although some contaminant are present in the CFS. Although low-level contamination maybe present in isolated portions ofthe Denver Formation CFS, this formation has low productivity as a groundwater resource.

1-4 Harding Lawson Associates P21905 402010 0711121495 R02

### 2.0 SITEHISTORY AND ENFORCEMENT ACTIVMES 2.1 Operational History Congress established RMA in 1942. The United States acquired land included within the boundaries oftheArsenal for chemical weapons manufacturing, constructed a base, and cornmencedArmy weapons production and ancillary activities in 1943. From 1945 to 1950, RMA distilled available stocks ofmustard, demilitarized several millionrounds ofmustard-filled shells and incendiarv munitions, and test-fired mortar rounds filled with smoke and high explosives. Also, many different types ofobsolete World War IIordnance were destroyed by detonation orburning.
After the conclusion ofWorld War H,selected surplus facilities were leased tonongoverrunent entities as warehouses and for the manufacture ofagricultural chemicals. Colorado Fuel and lron (CF&I) leased facilities atRMA in 1946. Julius Hyman &Company (Hyman) firstleased facilities in 1947 and succeeded to the CF&I leasehold interest, with somemodifications and additions in 1949. Shell Oil Companv (Shell) acquired a majorityinterest in Hyman in 1952 and operated theplant as the Julius H%-man Company until 1954. when the operation becamethe Shell Chemical Company - Den%-er Plant. R.\L-%%,6-asselected as the site for construction0f d 

### facility to produce Sarin, a nerve agent. The facility was completed in 1953. with the manufac4uring operation continuing until 1957 and the murutions-filling operations continuing un.1 lat- 1969 From 1970 until 1984, the primary operation at RNL-%was the disposal ofchemical warfar- nijt-,ria! Disposal practices included incinerating TX anticrop agent and mustard agent explos;%,-(, compon-nlS drid destroying Sarin and related munitions casings by caustic neutralization.
Chemicals were introduced to the RNUN envuonment primarily bythe burial or surface disposal of solid wastes, discharge ofwastewater to basins. and leakage ofwastewater and industrial fluid from chemical and sanitary sewer systems. Munitions were destroyed and disposed in trenches.
'21905 402010 

### Harding Lawson Associates 2-1 0711121495 R02Site History and Enforcement Activities Wastewater generated bythe U.S. Department ofthe Army (Army)and private industry in the South Plants and North Plants areas was discharged to a series ofunlined evaporation and holding basins (BasinsA, B, C, D, and E) and to asphalt-lined Basin F at various timesthroughout the history of RMA operations. The locations ofthese source areas are shown inFigure 2.1.
The primary areas that have contributed to groundwater contamination at RMA include (1) former manufacturing facilities, (2) former waste storage basins, (3) solid waste disposal areas, (4) the chemical sewer system, (5) locations within the rail classification yard, and (6) the motor pool area. 2.2 Previous Investigations From 1975 to the present,numerous groundwater monitoring programs havebeen conducted at RMA, both onpost and offpost,by the Army. The U.S. Environmental ProtectionAgency (EPA) has also conducted several offpost investigations. The Army designed and implemented monitoring programs to monitor regional groundwater and surface-water quality. TheArmy also designed and implemented the boundary system monitoring program to support the operation ofthe boundary groundwater containment systems. 2.2.1 U.S. Environmental Protection Agency Study Area Several orgaruc chemicals were detected in South Adams County Water and Sanitation District (SACIVSD) wells in 1981. as part of a random national survey of drinking water systems conducted bv EPA. Additional sampling in 1982 and 1985 confirmed these initial findings. As a result, EPA began a remedial investigatiori,'feasibility stud%-(RLTSIof an area west of RNIA and south ofthe Offpost Studv Area (Figure 1.1).
RNIA was suspected as one ofthe possible sources of contaminants in the EPA study area because of RNiA*s historical waste disposal practices. To mitigate the groundwater contamination problem,the Army and EPA built a water-supply system for SACWSD. Further investigation by EPA's Field Investigation Team indicated that source areas in addition to RMA contributed to groundwater
2-2 Harding Lawson Associates '21905402010 0711121495 R02Sift History and Enforcement Activities
contamination detected withinthe EPA study area. Groundwater monitoring wells installed on the Chemical Sales Company (CSQ property have since identified CSC as a significant source of groundwater contamination in the EPA study area. Recent investigations by EPA and the Army have detected the presence ofa trichloroethene plume enteringRMA at Section9, Township 3S, . Range 67W along the southern boundary ofRMA, as described in the Western Tier Report, the Stapleton Airport Environmental Assessment (Camp Dresser &McKee, Inc., 1993), and the CSC ROD (EPA, 1991a, 1991b, 1992). (Ebasco Services, Inc., 1988), 2.2.2 U.S. Department of the Army Investigation Because chemicals were detected in the Offpost Study Area, the Army initiated a regional hydrogeologic surveillance program requiring the quarterly coliection and analysis of samples from more than 100 onpost and offpost wells and surface-water stations. The program was carried out under the direction ofthe RMA Contamination Control Program, established in 1974 to ensure compliance with federal and state environmental laws. The objectives ofthe program were to (1) evaluate the nature and extent of contamination and (2)develop response actions to control contaminant migration. Potential and actual contaminant sources were assessed, and contaminant migration pathways -,vereevaluated.
From 1975 to the present, numerous groundwater morutoring programs have been conducted at RMA. The Army designed and implemented the 360 DegeeMorutoring Program to monitor regional groundwater and surface water. TheArmy designed and implemented a boundary system monitoring program to support the operation of the boundary groundwater containment systems. Studies conducted at RMA to assess groundwater and suffdC+--Wdter conditions are discussed below.
The RMA Offpost Contamination Assessment Report (CAR) (Environmental Science and Engineering, Inc. [ESE], 1987a) incorporated data from several studies to define the concentrations and distribution of offpost contamination north and northwest ofRMA. The scope ofthe CAR investigation

'21905 402010 Harding Lawson Associates 2-3 0711121495 R02Site History and Enforcement Activities

was intended to address critical data gaps required to evaluate a comprehensive set ofmultimedia exposure pathways.
Thepotential for contamination ofprivate wells was investigated in themid-1980s during the Consumptive Use (Cq Studies, Phases 1,11, and M. The CU Phases I and 11 studies addressed the Offpost Study Area. In the CU Phase III study,the Army conducted an inventory ofprivately-owned drinking water wells in an area bound by East 80thAvenue onthe south, East 96th Avenue on the north, the South Platte River on the west, and RMA on the east. The objectives ofthe CU Phase III studywere as follows: Locate all shallow domesticwells (less than 100 feet) in the Offpost StudyArea. Sample a representative number ofthe located wells. Assess the groundwater quality ofthe shallow alluvial aquifer. The Army developed the Comprehensive Monitoring Program (CMP), a long-term multimedia monitoring program designed to provide data to facilitate evaluation ofresponse actions, in the mid1980s. Sample collectionunder the CMP commenced in 1987 and is continuing as the Groundwater Momtoring Program (GMP). An RI was initiated in 1985 by the Armv in the Offpost Study Area. The primary objectives ofthe Offpost RI were as follows: Collect additional data to refine the current understanding ofgroundwater flow and surfacewater patterns and the nature and extent of contaminants offpost ofRMA. Evaluate the potential for chemical migration to the Offpost StudyArea in various environmental media, such as groundwater. surface water. sediment, air, andbiota. Followingcompletion ofthe RI, it was apparent that additional data were needed before evaluation and selection of a remedial alternative could occur. Therefore, a second RI was initiated in 1988 to collect additional data for groundwater, surface water, soil, sediment, and biota (plants and animals). The results ofthe second RI are reported in the Offpost Operable Unit Remedial Investigation,Final Addendum (HLA, 1992b). 2-4 Harding Lawson Associates '21905402010 0711121495 R02Site History and Enforcement Activities
2.3 Boundary Containment Systems Concurrent with and as a result ofthe EPA and Armyinvestigations, the Army constructed three boundary containment systems (the North Boundary Containment System [NBCS], the Northwest Boundary Containment System [NVVBCS), and the Irondale Containment System [ICSI at the north, northwestern, and western boundaries ofRMA, respectively)to minimize offpost discharge ofRMA chemicals via groundwater. Thelocations ofthese containment systems are shown in Figure 1.1. All three svstems currently intercept and treat contaminated groundwater and recharge treated water to the UTS. 2.3.1 North Boundary Containment System The NBCS is just south ofthe RMA northboundary in Sections 23 and 24. The NBCS consists of (1) a svstem of extraction wells thatremove contaminated groundwater from the UFS, (2) a soilbentorute barrier that impedes migration ofcontaminated groundwater to the Offpost Study Area, (3) a carbon-adsorption treatment systemthat removes organic contaminan from extracted Sroundwatpr. and (4) a system ofrecharge wells and trenches that return treated groundwater to the UTS. Tii, Nli(--S ;ii-,I svst-ým became operational in 1978. The pilot system was expanded approximately 1400 1-4 1 in. %%-stand 3840 feet to the east in 1981 during the second phase of construction. Sý.-veral imp: -%-*:*.--:jI5 have been made to the NBCS smce 1981: ten recharge trenches were added to II!i-- svs1em and became operational in December 1988, and five additional recharge i. :if)the east end oi th,ý systeni in 1q90. Currentlv, the soil-bentonite barrier is I-' I i \ttiidtelv 3 feet widt:. and varies in depth from 20 feet at the western end to more thin 40 1-1 ilong the eastern extension. The barrier is anchored in the Deaver Formation.
Review of groundwater contaminant distribution patterns indicates that the NBCS is having a significant effect on the distribution of organic compounds in the Offpost Study Area. Monitoring program data indicate that contaminant concentrations doomgradient ofthe NBCS are decreasing. '21905 402010 Harding Lawson Associates 2-5 0711121495 R02Site History and EnforcementActivities Activated carbonis being used to effectivelyremove the organic contaminants from the extracted groundwater to meetcontain- ent system remediationgoals. Organic contaminant concentrations are generallybelowcertified reportinglimits (CRLs) in system effluent 2.3.2 Northwest Boundary Containment System The NVVBCS is alongthe northwest boundary ofRMA in the southeast quarter of Section 22. Constructon ofthe NWBCS began in 1983, and the system became operational in 1984. The NWBCS originally consisted of(1) 15 extraction wells, (2) a soil-bentonite-barrier approximately 1600 feet in length, (3) a carbon adsorption treatment system, and (4) a system of21 downgradient recharge wells. The carbon adsorption systemwas designed to intercept and removedibromochloropropane and other organic compounds from a plume ofcontaminated groundwater originating onpost.

.pass was observed at the southwest and northeast ends ofthe NV%rBCS 1988. Contaminant by in An interim resonse action (IRA) to improve the NWBCS was initiated in 1989. in April 1990, the NWBCS ImprovementsIRA was divided into two phases: NMCS Short-term Improvements ER.A and NWBCS Long-term Improvements IRA. Under the NWBCS Short-term Improvements IRA, which was completed in 1991, the existing slurry wall was extended 665 feet to the northeast to prevent contaminant bypass. and two additional extraction wells were added at the northeast end ofthe extraction well alignment. Three additional extraction wells and four additional recharge wells were installed in Section 27. southwest ofthe NWBCS in August 1991. The NWBCS Long-term Improvements IRA is being used to assess the NV%BCS and its short-term improvements by reviewing groundwater monitoring data. 2.3.3 Irondale Containment System The ICS, which became operational in 1981. is at the southern end of the RMA northwest boundary within Section 33 and consists of(1) a hvdraulic control system of extraction and recharge wells, and (2) a carbon adsorption treatment system. The ICS was originally developed to intercept the migration of dibromochloropropane(DBCP) at the RNIA boundary. There have been no downgradient
2-6 Harding Lawson Associates '21905 402010 0711121495 R02Site History and Enforcement Activities 

### detections ofDBCP after thefirst twoyears ofoperation. The majority ofthearea downgradient of the ICS is containedwithin the EPA study area, although portions ofthe downgradient area are within the confines ofthe Offpost StudyArea. Therefore,the design and operation ofthe ICS was not included in the evaluation ofalternatives; however,the continued operation ofthe ICS, as required, for onpost contaminants consistent with the Irondale 1R.A remains an integral part ofthe Army's offpost contaminant reduction program to meet onpost cleanup goals defined in the Irondale IRA. Cessation ofoperation oftheICS willbe in accordance with paragraphs 35.2 and 35.4 ofthe FFA and paragraph 20 ofthe Conceptual RemedyAgreement. 2.4 Interim Response Actions As part ofthe Army's compliance with the requirements ofthe Comprehensive Environmental Response, Compensation,and Liability Act (CERCLA), and as describedin the FFA, the Army has instituted several IRAs that have beenperformed concurrently withthe ongoing onpost and offpost RI programs. IRAs, which are designedto be compatible withthe final remedy, are actions taken before the signing ofthe Record ofDecision (ROD) and are expeditedremedial measures to contain,
remove. or treat wastes before the final remedv is selected. Numerous IRAs havebeenimplemented to mitigate contamination both onpost and offpost. As indicated in the previous sections, some portions of the boundary containment systems have been constructed as IR.As. The Offpost IRA is discussed in the following section. 2.4.1 Offpost Interim Response Action The Offpost IRA addresses groundwater contaminant migration north ofRMA and downgradient of the NBCS along two primary contamindnt pathwai-s. defined by theFirst Creek and northern paleocharinels. Evaluation and selection ofthe collection and treatment system components that comprise IRAA, referred to as the Offpost Groundwater Intercept and Treatment System,began in 1988. The Offpost Groundwater Intercept and Treatment System DecisionDocument (HLA, 1989) presents the basisfor

'21905 402010 

### Harding Lawson Associates 2-7 0711121495 R02site History and Enforcement Activities systemplacement to address remediation ofcontamination in alluvial groundwater in the First Creek and northern paleoch-annels. The system was designedto intercept and extract contaminated groundwater fromthe UFS, treat the groundwater for organics, and recharge treated water to the UFS. Construction ofthe Offpost GroundwaterIntercept and Treatment Systembegan in November 1991 and was completed inJune 1993. Groundwater extraction is accomplished through a network of extraction wells. The organic contaminants in extracted groundwater aretreated using activated carbon adsorption, and the treated water is then recharged to the UFS using a combination of recharge wells and trenches.
The Offpost Groundwater Intercept andTreatment System was designed tobe flexible and tobe compatible with the final remedy, consistent withEPA guidance and the FFA. 2.5 History of CERCLA Enforcement Activilties Most of RMA was added to the National Priorities List (NPL) in 1987; Basin F was added in 1989. As such. RMA is subject to compliance w4th CERCLA (also known as Superfimd). A facility is subjectto compli.iric,-woh CERCLA when a release or a threat ofa release ofhazardous substances from the facilit%- ha% and when response costs have been incurred. In some cases, the potentially responsibl. parti--1PRPs) either cannot respond or cannot befound, so funding for the response como,s irr.n. It. 6:, %. rnment fund called Superfund. At RMA, the Army and Shell were identified as PRPs an.] v- Itin 1m; the cleanup.
On Ft-h.jar%I viAh. -iproposed Consent Decree was filed in the case of U.S. v. Shefl Oil Company ,.%-ith th. t'S D;V71 ". Court in Denver. Colorado. A modified version ofthe Consent Decree was filed
on June 7. 1IM8, T1w Consent Decree was entered by the U.S. District Court on February 12, 1993. On Februan-17. 1989. an FFA was executed by the Army. Shell, EPA, the U.S. Department ofthe Interior (DOI), the U.S. Department ofJustice (DOJ). and the U.S. Agency for Toxic Substances and Disease Registry (ATSDR). The FFA sets forth the procedures to be followed bythe Organizations (i.e., signatories to the FFA) to cooperate in the assessment, selection, and implementation of 2-8 Harding Lawson Associates '21905 402010 0711121495 R02Site History and Enforcement Activities
response actions resulting from the release or threat ofrelease of contaminants from Rlý.IA. The FFA designates the Army as the lead agency.

'21905 402010 Harding Lawson Associates 2-9 0711121495 R02

### 3.0 HIGHLIGHTS OF COMMUNITY PARTICIPATION Community participation opportunities wereprovided during theremedy selectionprocess to fulfill the requirements of CERCLA Sections 113(k)(2)(B)(i-v) and 117.
The RI, RI Addendum, Endangerment Assessment/Feasibility Study (EA/FS), and Proposed Plan for the Offpost OU werereleased to the publicon March 21, 1993. The documents were made available to the public in theAdministrative Record (located atthejoint Administrative Record Document Facilitv atthe west entrance to RMA at72nd Avenue and Quebec Street), in an information repositorymaintained atthe EPADocketRoom in RegionVM, and at theAdams County, Aurora, Commerce City, Denver,Lakewood, Montbel.lo, and Thornton Public Libraries. The notice of availabilitv forthese four documents waspublished in the Denver Post and RockyMountain News newspapers. An expanded Community Relations outreach was implemented to ensure community membershad the opportunity to commenton the Proposed Plan for the Offpost CU. Community outreach started in lanuarv 1993 wýith the announcement that all documents supporting an impending Proposed Plan werp available for review in local libraries. Adirect mailingto more than 1200 local citizens was made.
In March 1993, a press release was made and a legal notice was published announcing that a public meetingwas scheduled forApril 28. 1993. at Dupont Elementary School, CommerceCity, Colorado, to address the Proposed Plan. A separate letter was sent to citizens informing them ofthe documents availabilitv in the libraries. The letter also included abrieffact sheet summarizing the Proposed Plan. Originally. the ý)Iblic meetingwas scheduled for April 21, 1993, at RMA. The Army received requests to hold the meeting on a different day and offpost. Because ofthese factors and Earth Day eventsin Denver for April 21, the meetingwas movedto April 28, 1993.

'21905 402010 

### Harding Lawson Associates 3-1 0711121495 R02Highlights of Community Participation 

### A Media Daywas held the day ofthe public meeting toprovide localmedia information onthe Army'sproposal. Bothprint and video mediarepresentatives attended. Knowing the importance ofthe public meeting,the Announcement was expandedto include display advertising in 12 local and weeklynewspapers inthe Denvermetropolitan area. This was in addition to thenormal press release and Media Day event.
As a result ofcommentsreceived atthe public meeting concerning the official commentperiod, the Army published a legal notice and sentletters to citizensannouncing that the commentperiod was extended to June 21, 1993.
At the April 28, 1993, public meeting,representatives from the Army,EPA, and the State ofColorado answered questions regarding issues at the site and theremedial alternatives under consideration. Responses to commentsreceived duringthe public comment period are included inthe Responsiveness Summary, which is part ofthis ROD (Appendix A). This decision documentpresents the selectedremedial action for theRMA Offpost OU in Adams County, Colorado, chosen in accordance with CERCL.,%, as amended bythe Supe-fund Amendments and ReauthorizationAct (SARA), and with the NEPA. and, to the extent practicable. with the National Oil and Hazardous Substances PoUution Contingency Plan (NCP). The decision for this site is based on theAdministrative Record for the Offpost OU.
Additionaliv. settlementdiscussions involving municipalities. local health departments, special districts, and citizen groups were held from Idle 1q94 until April 1,1995, to discuss the final remedies forboth Onpost and Offpost OUs. The Draft Final ROD (December 7, 1993) wasrevised taking into account commentspresented bythe public. local communities, and the Parties.

3-2 Harding Lawson Associates 

### '21905 402010 0711121495 R02

### 4.0 SCOPE AND ROLE OFTHE OFFPOST OPERABLE UNIT Three RMA boundary containment systems currently intercept,treat, and recharge groundwater at the RMA north,northwest, and westboundaries. Theseboundary systems, alongwiththe physical boundaries ofRMIA, provide a logical dehneationbetween OUs. Therefore, theFFA divided the work intothe followingtwo OUs: Onpost OU: Media requiring remediation withinthe Onpost StudyArea (withinRM.A boundaries) Offpost OU: Media requiring remediation withinthe Offpost StudyArea (outside RMA boundaries) The Offpost OU addresses contamination in the groundwater north and northwest ofRMA. As discussed in Section 6.0 ofthis ROD, groundwater contamination inthe UFS poses theprincipal potential threat to human health because ofthe risks from possibleexposure to groundwater. Although health risks arepossible, the estimated risk levelsare within theacceptable riskrange established bvEPA. The purpose ofthe remedv is to (1)reduce groundwater contaminant concentiations. (2)reduce risk to human health and the environment, and (3)reduce the potential human exposure to contaminated LJFSgrouMwater.
The potential risks to ecological receptors were also evaluated. Wildlife are not exposed to contaminated groundwater: therefore. there are no risks to wildlife fromthe groundwater exposure. Wildlife exposures to soil and surface water and pot--ntial livesiock exposure to contaminated groundwater were evaluated. However,the potential risLs a%snriat--d with these exposures were showntobe negligible. Therefore. the selected remedy for the Offpost OU addresses the reduction ofpotential human exposure to contaminated UFS groundwater.

'21905 402010 

### Harding Lawson Associates 4-1 0711121495 R02

### 5.0 SUMMARY OF SITE CHARACTERISTICS Six mediawere evaluated in the RI for the Offpost StudyArea: groundwater, soil, surface water, sediment, air, andbiota. Each medium was evaluated inthe OffpostEA withrespect to (1)the nature and extent ofcontamination and (2)potential exposure pathways and associated riskto humans and the environment. A map delineatingtheboundaries ofthe Offpost Study Area is included as Figure 1.1. The site characteristics are more fully describedin the Offpost Operable Unit Remedial Investigation Report (ESE, 1988a) and the Offpost Operable Unit Remedial Investigation, Final Addend= (HLA, 1992b). 5.1 Sources of Contamination As described in Section 2.1, chemicals wereintroduced to the RMA environment primarilyby the burial or surface disposal ofsolid wastes, discharge ofwastewater tobasins, and leakage ofwastewater and industrial fluid from chemical and sanitary sewer systems. Chemicals migrated to the Offpost Study Area primarilyby shallow (i.e., shallow or unconfined) groundwater and airborne pathways. Contaminant transport in the shallow orunconfined groundwater hasbeen'controlledby construction ofthe boundary containment systems and improvementsto these systems (completed as IRAs). Offpost Studv Area surface water was contaminated primarily bythe natural interactionwith offpost groundwater. Offpost Study Area surface soil was contaminated by the deposition ofairborne contaminants, non-RNIA-related agricultural application ofpesticides, and irrigation practices. Agricultural sources ofpesticides are discussed in the Final Offpost RI Addendum (HIA, 1992b). Air morutoring data indicate that the air pathway does not contribute to human exposure. 5.2 Nature of Contamination Several chemicals ofconcern (COCs) are present in offpost groundwater, surface water, sediment, and soil (see Tables 6.1 through 6.4). COCs include organochlorinepesticides (OCPs), halogenated aliphatics, aromatic hydrocarbons. diisopropylmethyl phosphonate (DINT), sulfur-containing organic chemicals, arsenic, and dissolved salts.

'21905 402010 

### Harding Lawson Associates 5-1 0711121495 R02Summary of Site Characteristics 

### The COCs exhibit greatvariability intheir mobilityand persistence in environmental media. OCPs
areless mobile than the other COCs and morepersistent, tendingto associate with soil and sediment and tobiomagnify in thefood chain. Most ofthe remaining COCs are mobile in groundwater, and the aromatics and aliphatics are volatile in surface water. The fate properties ofthe COCs tend to determme their distribution inthe Offpost Study Area. All COCs weredetected in groundwater,but the more mobilechemicals are more widelydistributed. The OCPs are virtuallythe only COCs detected at concentrations abovebackground levels in soil and sediment. Thevolatile compounds
were not significantly elevated abovebackground levels in surface water and, in fact, were rarely detected. S.3 Contamination Migration Pathways The RI programs have shownthat there are three groundwatermigrationpathways in the Offpost Study Area. These migration pathways (shownin Figure 5.1) are referredto as the northern paleochannel, due north ofthe RMA northboundary; the First Creekpaleochannel, paralleling First Creek to the northwest fromthe RNIA north boundary; and the northwest paleochannel, northwest of the RNIA northwest boundary. The northern and First Creek paleochannels compose the North Plume Group. and the northwest paleochannel composesthe Northwest Plume Group. These two plume groups encompass an area of approximately 590 acres in the Offpost Study Area. The alluvial flow system transports most of the contamination in paleochannels characterizedby coarser sediment. Some ofthe groundwater traveling through the First Creek paleochannel discharges to FLrst Creek, probably seasonally, resulting in transfer ofcontaminants to First Creek. Figure 5.1 also presents the offpost surface-water ieatures. Theprimary surface-water pathway is First Creek. wtuch flows northwest from the northern RMA boundary. First Creek empties into O'Brian Canal, which flows northeast and empties into Barr Lake. Burlington Ditch,which parallels O'Brian Canal, also flows into Barr Lake. The majority ofthe surface-water contaminationis located in First Creek, with some contamination in O'Brian Canal do%rnstream ofthe confluence withFirst

5-2 Harding Lawson Associates '21905 402010 0711121495 R02Summary of Site Characteristics
Creek and Burlington Ditch. Barr Lake has notbeen shown tobe contaminated with RMA-related chemicals greater thannaturally occurring background levels.

In addition to the contaminant migration pathways ofgroundwater and surface water, prevailing winds transport onpost surface soilto offpost locations, and sediment provides a potential contaminant source for aquatic species. 5.4 E)ctent of Contamination Varying levels of contamination exist in the following fivemedia in the Offpost OU: groundwater, surface water, stream-bottom sediment, surface and subsurface soil, and biota. More detailed discussions ofthe offpost contaminant concentrations, along with figures showing concentration distributions are found in Sections 3.0, 4.0, 5.0, and 6.0 ofthe Final Offpost RI Addendum (HLA, 1992b). 5.4.1 Groundwater Table 6.1 presents the groundwater COCs and the exposure point concentrations used in the Endangerment Assessment. The most widespread RMA-related groundwater COCin the Offpost Studv Area is DIMP, which is present in the UFS at varyin concentrations in a band from the west end of the NIVBCS to the east end of the NBCS. and from the RMA north and northwest boundaries to the South Platte River. The other primary contaminants present in the offpost UFS are chdoroform, chlorobenzene,trichloroethene, tetrachloroethene, dibromochloropropane (DBCP), dieldrin, endrin. dicyclopentadiene (DCPD), arseruc, chloride. fluoride and sulfate.
The fughest concentrations ofDIMP observed in the past three years are in the First Creek paleochannel. Concentrations ofDIMP are lower in the northern paleochannel and lower still in the northwestern paleochannel. The maximum concentrations ofDIMP in the Offpost StudyArea have decreased by approximately 50 percent over the past 10 years. The NBCS is currently operating and has been operated in the past to remove multiple contaminants. DIMP concentrations arebeing

'21905 402010 Harding Lawson Associates 5-3 0711121495 R02Summaryof Site Characteristics 

### reduced toless than 8 ppb. Cut-offofgroundwater contaminants at.the NBCS and recharge ofthe treated groundwater has resultedin the observeddecrease in DMEP concentrations specifically, as wellas the othercontaminants found offpost.
The highestcontaminant levels downgradient fromthe NBCS occurupgradient ofthe O'Brian Canal. Certain volatile compounds such as chlorobenzene, chloroform,trichloroethene, and DBCP have been detected at low concentrations downgradient fromthe canals, butwell-defined plumes do not exist in this area and these detectionsmay be anomalous. Semivolatile organic compounds such as dieldrin and other OCPs are present almost exclusively upgradient ofthe canals. Maximum concentrations of the OCPs (i.e., aldrin, isodrin, chlordane, 2,2-bis[p-chlorophenyl]-1,1-dichloroethene[DDE], and 2.2-bis[p-chlorophenyl]-1,1,1-trichloroethane[DDII) generally occur in the First Creekpaleochannel witlun 500 to 1000 feet oftheNBCS. Only sporadic and isolated occurrences ofOCPs are observed northwest oftheRMA northwesternboundary.
Contaminants found downgradient from the NWBCS are primarilychlorobenzene, chloroform, DMEP, and dieldrin. The highest concentrations ofchloroform occurdowngradient ofthe RMA boundary. Detections ofchlorobenzenenear the NVVBCS maybe anomalous. In 1989, semivolatile compounds such as dieldrin and possiblyDINIP appeared to have bypassed the NWBCS atthe northeast and southwest ends. Subsequently, the N%VBCS IRA was initiated that included improvements and operational changes to correct thebypass. Recent modifications to the NBCS and NWBCS, in addition to the remedial action selected in this ROD. are expected to furtherreduce contaminant levels dowrigradient ofthe RMA boundaries. S.4.2 Surface Water Table 6.2 presents the surface water COCs and the exposure point concentrations used inthe EndangermentAssessment. The principal organic compounds identified in Offpost Study Area surface-water samples are DIMP and dieldrin. In general, the highest concentrations ofthe organic and inorganic analytes were detected in First Creek. DIMP concentrations in First Creekwere highest 5-4 Harding Lawson Associates '21905 402010 0711121495 R02Summary of Site Characteristics
in the area 100 to 200 feet upstream of O'Brian Canal where groundwater discharges to First Creek. DIMP was not detected in Burlington Ditch or O'Brian Canal upstream ofthe confluenceurith First Creek. DIMP was detected in Barr Lake in only one of 20 samples collected from 1985 to 1990 and

was not detected in the duphcate sample collected at the same time. This one detection is anomalous and not considered representative of conditions at Barr Lake.

The highest concentrations of arsenic were detected in First Creek near the northern RMA boundary. These detections are likely associated with discharge from the onpost sewage treatment plant. Mercury and arsenic were detected in surface water in O'Brian Canal upstream ofthe confluence %,ý

ith First Creek, suggesting that sources ofthese contaminants other than RNIA probably exist. Some contaminants identified in O'Brian Canal and Burlington Ditch may originate from the diversion of treated sewage effluent from Denver. 5.4.3 Stream-bottom Sediments Table 6.3 presents the sediment COCs and the exposure point concentrations used in the Endangermeni Assessment. The most commo-n-1v detected contaminants in stream-bottom sediment in the Offpost Study Area were dieldrin, arsenic, and mercury. The highest concentration of dieldrin was found in First Creek immediately north of the northern RMA boundary. Additional contaminants
were detected in O'Brian Canal and Burlington Ditch upstream of the confluence with First Creek, suggesting that sources of these contaminants other than RMA probably exist such as diversion of treated sewage effluent from Denver. 5.4.4 Surface and Subsurface Soil Table 6.4 presents the soil COCs and the exposure point concentrations used in the Endangerment Assessment. Approximately 100 soil samples were collected as part ofthe RI Addendum investigation and were analyzed for OCPs, arsenic. and mercury. Dieldrin was the most frequently detected OCP (in approximately 90 percent ofthe samples) with a maximum concentration located approxi-

'21905 402010 Harding Lawson Associates S-5 0711121495 R02

### Summaryof Site Characteristics mately100 to 200 feet north ofthe northern RMAboundary. DDT,DDE, aldrin, endrin, and chlordane were detectedless frequently.
The distribution ofOCPsin Offpost StudyArea soil appears to correlate with the dominant wind patterns atRMA. Thegreatest number and highestcontaminant concentrations are observedin samples collected immediately north ofthenorthern RMAboundary, consistent with the prevalent wind direction ofsouthto north. Isolated elevatedconcentrations ofOCPs observedbetweenthe northern RMAboundary and O'Brian Canal maybe the result oflocalresidential and/or commercial
use ofpesticides and notrelated to migration fromRMA. Anomalously high concentrations of dieldrin, DDE, and DDTwere also detected approximately 1.5 milesnorthwest ofRMA. These detections are considered tobe agricultural-related and notRMA-related because thearea is currently and has historicallybeen a farming community. -
The uneven distribution ofarsenic and mercury in Offpost Study Area surface soil suggeststhat the
occurrence ofthese inorganic contaminants is not related to RMA activities. 5.4.5 Biota The RI Addendum biota monitoring program provided additional data to assess thepotential impacts
on plants and arumals in the Offpost Study Area. During the RI Addendum study,biota samples
were analyzed foraldrin, dieldrin. endrin. DDE. DDT. DBCP. arsenic, and mercury. Dieldrin,the contaminant most often found in Offpost Study Area biota (36 percent ofsamples), was detected in cattle, chicken, fish, earthworm. deer mouse. prairie dog. and pheasant samples. Arsenic and mercury were detected less frequently (19 and 14 percent. respectively). DDE was detected only once,and aldrin, endrin, DDT, and DBCP were not detected in anybiota samples fromthe Offpost Studv Area. Contaminants identified in the Offpost Study Area biota survey are similarto those found onpost, although the concentrations detected in the Offpost Study Area biota are considerably lower than concentrations detected in the onpost biota.
5-6 Harding Lawson Associates '21905402010 0711121495 R02Summary of Site Characteristics
The Offpost StudyArea is knownto contain suitable habitat for endangered species such as the bald eagle. A nesting pair ofeagles was identified during offpost assessment activities. Contaminants (mercury, dieldrin, and DDE) were detected in a bald eagle egg collected in 1988 from a nest at Barr Lake. According tothe U.S. Fish and Wildlife Service, the concentrations ofthese contaminants
were tvpical ofbald eagle egg contamination throughout the United States. S.S Potential Routes of Human and Environmental Exposure Based on the current land uses in the Offpost Study Area, a review oflocal city and county planning and zoning ordinances, and consultation with local planning authorities, three primary land uses
were considered in estimatingthe risks to human health. These land uses are urban residential, rural residential, and commercial and industrial. The exposure routes and pathways considered for the Offpost Study Area include the following: Ingestion of groundwater Ingestion of soil Ingestion of sediment Ingestion of vegetables Ingestion of dairy products Ingestion of eggs Ingestion of meat Ingestion of surface water Inhalation ofvolatile chemicals in groundwater Inhalation of dust Dermal contact with soil Dermal contact with sediment Dermal contact with surface water Dermal contact with groundwater

'21905 402010 Harding Lawson Associates So7 0711121495 R02

### 6.0 SUMMARY OF SITE RISKS Therisks estimated in the EA and siimmarizedin this sectionare baselinerisks corresponding to current conditions and are, therefore, pre-remediation risk estimates. Implementation ofthe selected remedypresented (Section9.0) will lowerthepotential risks. The estimated maximum cumulative potential cancer riskto humans in the Offpost StudyArea is 3 x 10-4 

### (or 3 in 10,000 people) on the basis ofthe reasonable maximum exposure (RNE)risks presentedin theFinal EA (Volume M, Section 4.0, and Volume IV, Appendix G). This estimated potential risklevel is within the acceptableriskrange established by EPA (1x 10-'* to 5x 101;letter from EPA to Army dated February21, 1992). A cancer risk estimate of3 in 10,000 indicates anupperbound estimate ofrisk. Actual cancer risks are likelv tobe belowthis level and maybe as lowas zero. These carcinogenic risks are usuallvtermed "excesslifetime cancer risks,"which means there is an increased chance ofan individual developing cancer over70 years ofexposure to the carcinogenic chemicals in excess ofthe normal cancerrate. The background cancerrate determinedbythe American Cancer Societyis about 1 in 3.
Because the Offpost Study Area cumulative risk isless than the upper risklevel established byEPA, remedial action in the Offpost Study Area is not requLred. The Army,nevertheless, recognizesthat several site-specific factors suggest that remediation ofthe groundwater is preferableto no action in the Offpost OU. These site-specific factors are: (1)groundwater contributes a maximumrisk of 2x 10'.or approximately 75 percent ofthe 101di 

### carcinogenic risk, (2)maximum contaminant levels (NICLs), maximum contaminant level goals (MCI.Gs). and Colorado Basic Standards for Groundwater (CBSGs) areexceeded for some groundwater contaminants, and (3)hazard indices (1-11s) for children exceed 1.0 in Zones 2. 3,and 4. Although the estimated child hazard indices exceed 1.0 inZones 2, 3, and 4, the bulkofthe HI value is contributed through an assumed domestic use ofalluvial groundwater,which isnot presently occurring in the Offpost OU. Treatment ofgroundwater tothe containment svstem remediation goals will reduce (1)the total estimaterisk to less than 1 x 10' and

'21905 402010 

### Harding Lawson Associates 6-1 0711121495 R02summary of Sit* Risks
toward I x 10" and (2) the lUs to less than 1.0 in Zones 2, 3, and 4. Soil, surface water, and sediment do notrequire remecliation because ofthe low risk attributable to these media. Air was not identified as a medium of concern on thebasis ofair monitoring data and initial risk screening.

Protection ofbiota was evaluated through development ofecological exposure criteria for the protection of species potentially at rislr- The ecological assessmentindicated that the potential for adverse ecological effects is minimal. 6.1 Human Health Risks Human health risks in the Offpost Study Area were calculated in four steps: identification of COCs, exposure assessment, toxicity assessment, and risk characterization. It should be noted that many of the exposures evaluated do not currently exist and therefore do not represent existing exposures.
6.1.1 Identification of Chemicals of Concern A data set consisting ofgroundwater, surface water, sediment, soil, air, and biota data collected betwPen 1985 dnd 1991 was used to evaluate wtuch chemicals were of concernto human health and the envirnnniýnt. A trend of declining contaminant concentrations in groundwater since 1985 was noted in pomions of the Offpost Study Area. particularly near the north boundary ofRMA and This trend is due to the operation and improvement ofthe boundary systoriv.iii i ii.st-iral attenuation processes. Considering this trend, only the most recent groundwater data h -- .Ir. n. 11IM" through 1991) were used to estimate groundwater exposure point concentrations.

Daia for ili. 11. 1 nW fid were also considered. and only the data resulting from analytical methods sensitive vir-ug', ti, d-tect low concentrations were used. Data were also compared statistically with background concentrations consistent with EPA guidance presented in Risk Assessment Guidance for Superfund (EPA. 1989a). Statistical procedures included the Wilcoxon rank sum test and the Method of Proportions. These procedures are discussed in Section 1.2 ofthe Final Offpost EA/FS (HIA, 1992a).
6.2 Harding Lawson Associat" '21905 402010 0711121495 R02Summary of Site Risks 

### The primarycriterion for identifying COCs was that the chemical concentrations at locations of expected maximum concentration (i.e., near the RMA borders) mustbe significantly greater than concentrations found atbackground locations (i.e., no RMA-related contamination present). By applying statistical methods,Offpost StudyArea contaminant concentrations were compared to background concentrations atreference locations. Ifstatistical analysis indicated that Offpost Study Area concentrations were significantlyhigherthan thebackground concentrations, the presence of the chemical in the Offpost StudyArea was considered tobe RMA-related and the chemicalwas designated as a COC. This procedure was followed for eachenvironmental medium. Tables 6.1 through 6.4list the COCs for groundwater, surface water, sediment, and soil,respectively. The exposure point concentration associated with each COC is also shown in the tables.
To select COCs forbiota (plants and animals), analytical data obtained fromthe onpost biota RI were compared tobackground chemicalconcentrations available in the scientific literature. This procedure was less precise butnonetheless indicated that two chemicals (dieldrin and arsenic)may be elevated. although in low concentrations. in the tissues ofanimals located inthe Offpost OU. 6.1.2 Exposure Assessment

6.1.2.1 Ottpost StudyArea Exposure Assessment Zones The Offpost Study Area is a large. heterogeneousarea with a variety ofcharacteristics that can affect exposure levels. Specifically. distinct zc;nes of the Offpost Study Area exhibit different exposure concentrations ofCOCs in groundwater. surfacp and surface soil, including hot spots where contaminant levels are higherthan the average for the entire Offpost StudyArea. In addition, population density, land use. and water use varies throughout the Offpost StudyArea. Therefore, to avoid diluting or averaging contaminant concentrations overthe entire Offpost Study Area, the Offpost Study Area was subdivided into six zones (Figure 6.1) with different exposure conditions. The primary factor used to define the exposure zones was the pattern ofCOC concentrations in

'21905 402010 

### Harding Lawson Associates 6-3 0711121495 R02Summary of S1to Risks groundwater. The six zones, and the land use and populations evaluated within each zone, are describedbelow.
Zone 1is an area with relativelylow levels ofCOCs in groundwater and surface soil. Rural residential land use, which includes consumptionofhomegrownvegetables, milk, meat, and eggs, is the current and potential future population characteristic.
Zone 2is an area ofrelativelyhigh levelsof COCs in groundwater, low levels of COCs in surface soil, and no permanent surface-water features. Arural residential land-use scenario, identical to Zone 1,
was evaluated.
Zones 3 and 4 are similar. Zone 3 is an area ofrelatively high levels ofpesticide COCs in groundwater. surface water, and surface soil. Zone4 is an area ofrelatively high levels ofCOCs in groundwater and surface water. but relatively lowlevels ofCOCs in surface soil. BothZones 3 and 4 have recently been purchased by Shell Dil Company and are expected to be unoccupied at least until completion of offpost remediation. Plans for improvement of 96th Avenue as an access road for the
new Denver International Airport may -esult in predominantly commercial and industrial land use in these zones. An urban residential land use forZones 3 and 4 is considered possible and was selected for evaluation because this land use would result in higher exposures than the current land
use. Urban land use assumes that exposure to meat. dairy, and eggs would not occur, butthat local planting and consumption ofvegetables are possible.

Zone 5 is an area with moderate levels of COCs in groundwater and relatively low levels ofCOCs in surface soil. A commercial and industrial land use for Zone 5 was evaluated. Zone 5 is zoned for industrial use over the majority of its area. is currently developed for industrial use, and is projected
as industrial land use for the future.

6-4 Harding Lawson Associates '21905 402010 0711121495R02Summary of Site Risks

Zone 6 is an area with moderate levels of COCs in groundwater and-relativelylow levels of COCs in surface soil. Because farm residences currently exist in Zone 6, a rural residential land use was evaluated that is identical to the land use (ruralresidential) in Zones 1 and 2.
6.1.2.2 Ottpost Study Area Potential Exposure Points There are several potential exposure points iia the Offpost Study Area. The most significant routes of exposure have alreadybeen mitigated by exposure controls in areas with the highest groundwater COC concentrations (e.g., the UFS is no longerused in Zones 3 and 4). Exposure to COCs in surface soil has also been mitigated byrelocating residents from the area near the intersection of 96th Avenue and Peoria Street where soil contaminant concentrations are highest. Additionally, the Armv and Shell Oil Company have agreed to till and revegetate approximately 160 acres located in the southeast portion of Section 14 and the southwest portion of Section 13 in accordance with Paragraph 22 ofthe Conceptual Remedy Agreement (see Figure 9.1). Shell Oil Company and the U.S. Army believe that existing soil risk in the revegetated area falls within EPA's established acceptable risk range and that remediation is not necessary. However, Shell Oil company and the U.S. Army aigree to the revegetation program as part ofthe remedy.

Concentrations of surface-water contaminants were lugher in First Creek than other surface-water bodies during 1986 through 1990, creating a potential exposure point for nonhuman receptors and a direct-contact human pathway associated with wading. First Creek does not support a recreational fisherv: Barr Lake is the most likelY point of human exposure to bioaccumulated residues in fish tissue. Because COCs are not elevated in Barr Lake. with the exception of a single DIMP detection that was not verified in duplicate or later sdmpling events, consumption of contaminated fish was not evaluated.

'21905 402010 Harding Lawson Associates 6-S 0711121495 R02

### summary of-SiteRisks 6.1.2.3 Potential Exposure Pathways and Routes An exposure pathway consists offour elements: (1) a source and mechanism ofrelease, (2) a transport medium, (3) a point ofpotential contact withthe contaminated medium, and (4) an exposure route, such as ingestion,at the contact point.
The Site Conceptual Model (Figure 6-2) presents the potential exposure pathways identified in the Offpost StudyArea. The Site Conceptual Model also indicates which exposure routes were quantitatively evaluated for risk. Because ofthe variations in land use and the presence or absence of surface water in the six zones, not all exposure routes are applicableto all zones. Table 6.5 summarizes the exposure zones by land-use category and identifies the exposure routes quantified in each zone. InhalationRoute On the basis ofrisk screening evaluations conducted according to EPA guidance, therelease of volatile chemicals from groundwater used in the home forall purposes (e.g., showering, dishwashing, laundry, toilets) was determined to result in potentially significant exposures bythe inhalation route. Therefore. inhalation ofvolatile chemicals resulting from domestic use was quantified. Other potential sources ofexposure, such as the inhalation ofcontaminated dust particles, and inhalation ofvapors resulting from volatilization from underlying groundwater, were found tobe very minor contributors to the overall exposure potential. Dermal Route Dermal contact with surface soil is likely and was quantified for all potential land uses. Dermal contact with sediment in First Creek was quantified. Dermal contact with sediment ofBarr Lake is not feasible. considering the depth ofthe water and the prohibition of swimming.
Dermal contact with surface water in First Creek wasquantified. However, dermal contactwith canal water is expected tobe unlikely and. in the worst case. infrequent; therefore, dermal contact
6-6 Harding Lawson Associates '21q05402010 0711121495 R02Summary of Site Risks

was not quantified for the canals. Direct contact recreation is prohibited in Barr Lake; therefore, the dermal contact pathwaywas not quantified for Barr LakeDermal contact with groundwater used domestically is likely. However, dermal intake during showering is approximately 0.15 percent of the intake resulting from ingestion of groundwater. Potential exposures from direct ingestion and inhalation will be much higher than from dermal contact. Therefore, the dermal intake resulting from domestic use was not quantified. EPA guidance (EPA, 1989a) allows for certain pathways tobe eliminated from evaluation ifother pathways have much higher exposure. Ingestion Route Incidental ingestion of surface soil is likely under all potential land uses; therefore,this pathway was quantified. Incidental ingestion of First Greek sediment is possible in association with wading or recreational activities; therefore, this pathway was also quantified.

Cattle and other livestock raised for human consumption may bioaccumulate COCs from (1) surface water or igroundwater used for watering livestock, (2) forage grown in contaminated surface soil or irrigated by contaminated surface water or groundwater, and (3) direct ingestion of soil while grazing. Tfus pathway was quantified, using cattle as the representative species for development of a biodccumulat ion model. Additionallv. bioaccumulation resulting in dieldrin contamination of chicken eggs was quantified in the EA.

Vegetable crops grown for human consumption may contain COCs because of uptake of COCs from contaminated surface soil and surface water or groundwater for irrigation. Ingestion of vegetable
crops was quantified.

Although ingestion of the shallow groundwater is unlikely, this exposure pathway was quantified. It has been conservativelv assumed that ingestion of untreated alluvial groundwater might occur even
'21905 402010 Harding Lawson Associates .6-7 0711121495 R02Summary of Site Risks 

### though there is insufficient water inportions ofthe UFS contaminated abovegroundwater containment system remediationgoals to supply a municipal water system. 6.1.2.4 Estimation ofCbemical Intake Analytical data from each media within each ofthe six exposure assessment zones (Section 6.1.2.1) was identified. Exposure point concentrations wereselected suchthat theyrepresent an RME concentration. The RME exposure point concentrations were calculated asthe upper 95 percent confidence limit onthe arithmetic mean ofthe data. The RNE values forthe COCs in each media
are presented inTables 6.1 through 6.4. Exposure point concentrations were combined with standard EPA intake assumptions and variables to estimate theintake ofeach COCby each exposuxe route.
To estimatethe exposure point concentration forfood products (e.g., meat,eggs,vegetables), several models were used to estimate the plant and animaluptake ofa chemicalfrom soil orwater and the resultant concentration in the edible portion oftheplant or animal. All ofthe uptake and partitiorung coefficients were selected so that theresultant COCconcentration in the foodwould also represent an RMEvalue. A completediscussion ofthe plant and animal chemical uptake models is provided in the Offpost EA/FS. 6.1.3 Toxicity Assessment The toxicity ofchemicals is evaluated in terms ofcarcinogenic and noncarcinogenic effects. Cancer slope factors and reference doses are used to evaluate potential risks posedby the exposure to carcinogerucand noncarcinogeruc chemicals. respectively.
EPA-established slope factors for inhalation and ingestion exposures toCOCs are presented in Table 6.6. The slope factor for a given compound is multiplied by the estimated intake toobtain the carcinogenicrisk estimate. The individual risks from each compound in a particular exposure

6-8 

### Harding Lawson Associates '21905 402010 0711121495 R02Summary of Site Risks pathway are then summed to obtain an estimate ofthe overall carcinogenic risk for each pathway and for all pathways combined.

The reference doses (RfDs) used in the EA for inhalation and ingestion exposures are presented in Table 6.6. The estimated intake is divided by the RfD for a given compound to obtain its hazard quotient 

## (HQ). For each exposure pathway, chemicals were segregated bytheir target organ. For each target organ group, the HQs for each chemical were then summed to obtain a hazard index (HI) for each pathway and for all pathways comb4ned. When the HQ and/or the HI exceed 1.0, there may be concern for potential noncarci.nogenic health effects.
6.1.4 Risk Characteriza-lion Follo,ýving the estimation of exposure point concentrations and chemical intakes, the slope factors and RfDs are used to estimate carcinogenic risks and the potential for noncarcinogenic effects. The folio-wing sections discuss the results ofthis procedure.
6.1.4.1 Carcinogenic Risks Table 6.7 summarizes the estimated current carcinogenic risks correspondingto existing exposures by exposure assessment zone and exposure route. The total carcinogenic risks range from 1 X 104 to 3 x 10' (1 to 3 in 10.000) in Zones 1 through 4. 3 x 10-5 (3 in 100,000) in Zone 5, and 7 x 10-' (7 in 100.000) in Zone 6. The total carcinogeruc risks for each of the six exposure assessment zones are within the acceptable risk range established by EPA. The hypothetical risks in Zones 3 and 4 are h.ighly conservative in that they are based on an urban residential land-use scenario and there are no humans currently living in Zones 3 and 4. Additionally. the risks estimated for a portion of Zone 1 and Zone 2 are not current risks, because resvients in these areas do not use UFS groundwater for domestic use. Because there are no current residents in Zones 3 and 4, and the current residents in Zone 5 have water supplies other than shdlIOW wells. the estimated risks from residential use in these zones are conservative because they do not represent existing exposures.

'21905 402010 Harding Lawson Associates 6-9 0711121495 R02Summary of Site Risks 

### Groundwater usage (either domestic arid/or agricultural) is the primary contributor tocarcinogenic risk, accounting for 45 to 99 percent ofthetotal riskestimated for each zone. This indicates the major role ofthe groundwater-related exposure pathways. Risks relatedto chemicals in soil are less than 1 in 10,000 (1x 10'),and the risksresulting fromthe surface-water and sediment exposure pathways are less than 1 in 100,000 (1x 10"). Because ofthe importance ofthe groundwater pathway, the remediation ofgroundwater will have the greatesteffect in reducingpotential offpost risks. Dieldrin contributes themostto the total carcinogenic risk,followedby arsenic, chloroform, and atrazine. All of the estimated risks from dieldrin are conservative in that the dieldrin concentrations
were consideredtobe constantthroughout the exposure period (30 years). The natural reduction in dieldrin concentrations overtime was not considered. Additionally, not all ofthe total carcinogenic risks for each zone are attributable to RMA activities. Background concentrations of clieldrin in soil attributable to agricultural practices may contribute up to 50 percent ofthetotal carcinogenic risk in
some zones based on abackground concentration for dieldrin ofapproximately 8 mg/kg. Naturally occurring arsenic in groundwater maybe responsible for a riskofapproximately 4 in 100,000 (4 x 

### 10"). based on a background concentration ofarsenic in groundwater ofapproximately 3Jig./l. 6.1.4.2 Honcarcinogenic Effects As presented in Section 6.1.3, HIs are derived by comparing the estimated daily chemical intake to the estimated acceptable intake. Acute. or short-term. effects were evaluated for childrenbecause children would have the highest chemical intake perbody weight and wouldbe expected tobethe most sensitive to the chemical. The EA concludedthat there is a low potential for adverse health effects in children from hypothetical short-term exposuresto dieldrin in groundwater inZones 2, 3, and 4. 

### The HI exceeds 1 in Zones 2,3. and 4. 

### with a maximum HI of4 in Zone 3. Dieldrin is the primary contributorto the HI.

6-10 Harding Lawson Associates '21905 402010 0711121495 R02Summary of Site Risks

HIs were also estimated for long-term exposures for both children and adults. The risk characterization presented in the EA found that, with the exception of ingestion ofDuvip in groundwater in Zone 4, no single chemical or exposure pathway resulted in an M greater than 1. 111s were also calculated on the basis oftarget organ effects and the mechanism oftoxic action. For children,both liver and central nervous system (CNS) toxicants were found to exceed an III of 1. For liver toxicants, the HI exceeds 1 in Zones 2, 3, and 4, with a maximum HI of 2 in Zone 2, predominately attxibutable to inhalation and ingestion of chloroform. The FU for CNS effects exceeds 1 in Zones 2 and 4, with a maximum 1-H of 3.7 in Zone 4. The primary contributors to the estimation of CNS effects are DIMP and manganese. Direct ingestion of groundwater and ingestion ofvegetable crops irrigated .%rith groundwater are the two primary exposure pathways for DIMP and manganese.

Adult future Hls are all less than the child HIs. Table 6.8 summarizes the adult IUs segregated by target organ. When segregated for liver toxicants, the highest FIl is 1.3 in Zone 3. The HI for CNS effects also eXCI-'eLIS 1.0, where DIMP is the major contributor to an HI of 2.4 in Zone 4.
6.2 Estimation of Potential Ecological Effects 6.2.1 Method An Oftjr,-t St - i%Ar,-,i ecological risk assessment was performed to evaluate potential adverse effects

to t - nf- -rix i: dMi nonhuman receptors as a result of potential exposure to chemicals migrating from otil- .%I -ýtirr-s The two natural ecosvs1ems occurring in the Offpost OU are terrestrial and dquatr: fi.:m pr.-sents the ecological site conceptual model and presents the potential exposure pathwa,.. % The chemic,-Is selected for evaluation of potential effects on the terrestrial and aouatic r, w.-r-- limited to RMA-related chemicals found in surface water, suxface soil, and sediment. Ch-littý:.jls identified in ground%vaterwere used to evaluate agricultural receptors (e.g., crops, livestock) because of the potential for exposure through irrigation and livestock watering. The chemicals evaluated for potential ecological effects were aldrin, arsenic, dieldrin, endrin, DDE, DDT, and mercury.

'21905 402010 Harding Lawson Associates 6-11 0711121495 R02SUMMary ofSite Risks Two methods ofexposure were evaluated: direct exposure andbiomagaification. Direct exposure is
a result of contact with the original source ofthe chemical (e.g., ingestion of surface wateror soil, ingestion ofgroundwater, orfish swimmingin contaminated surface water). Biomagnification occurs whenthe tissue concentrations of a chemical increase with progressionup the foodchain. Over time, the concentrations ofchemicals in tissues mayreach a level detrimentalto the organism's health.
The evaluation ofecological effects via direct exposure is analogous to the evaluation ofhuman effects. Direct toxicity was evaluated by comparingthe estimated daily intake ofa receptorto the estimated toxicityreference value for a receptor. Thetoxicity referencevalues are similar to human RfDs in their derivation and use. These toxicityreference values were animal- and chemical-specific values, or, in the case ofaquatic life, federal AmbientWater Quality Criteria values established to protect aquatic life.
To evaluate the potential effects ofbiomagrafication. the estimatedtissue concentrations resulting from biomagnification were compared to residue concentrations known tobe without deleterious effects. On1v the top indicator species were selected to evaluate the effects ofbiomagnification. These species were the bald eagle, great blue heron, and mallard duck.

In coordination with the U.S, Fish and Wildlife Service. it was agreed that screeninglevels, developed to ensure compliance with enforceable remediation levels, would meet the requirements of the federal Endangered Species Act, the Migratory BLrd TreatyAct, and the Bald and Golden Eagle Protection Act. These screening levelswere not exceeded m the Offpos.t OU. These levels are presented in the Final Offpost Operable Unit Endangerment Assessment/Feasibility Study in Table 3.3.3-1 (ToxicityReference Values for Avian and Terrestrial Vertebrate Species ofConcern Identified at Rocky Mountain Arsenal) ofVolume 11 andTable H5-1 (Maximum AJlowable Tissue Concentration [M.ATC] Values for the Offpost EA EcologicalAssessment) ofAppendix H in
6-12 Harding Lawson Associates '21905 402010 0711121495 R02Summaryof Site Risks 

### VolumerV. Ifthe screening levels areexceeded oreffects are observedinthe future, enforceable remediation levelswillbe developed consistent with CERCLA, the Endangered Species Act, the Migratory Bird TreatyAct, and theBald and Golden EagleProtection Act.
Potential effects onwetlands and critical habitats werealso evaluated. This assessment is presented in Appendix Bofthe Final Offpost EA/FS (H1A, 1992a). The U.S. Fish and Wildlife Service (USFWS) National Wetlands Office identified approximately 300 acres ofwetlands along First Creek 

### from the northboundary ofRMA to O'Brian Canal. Potential effects ofconstruction ofthe Offpost Groundwater Intercept and Treatment System included temporary dewatering during excavation of recharge trenches and pipelines near First Creek. 6.2.2 Results Underwater aquatic life was evaluated onthebasis ofdirect toxicitybycomparing water concentrations to aquatic reference concentrations. Chlordane, dieldrin, fluoride, and DDT appeared to present a potential for an adverse effect to aquatic life in First Creek. However,because First Creek isdr%- much ofthe year and does not support a stable and ongoingfish population, adverse effects to

acua! ic I ife are 

### expected to be minimal. Because of 

### interaction between groundwater and First Creek, remedial actions taken to reduce the concentration of COCs north ofthe NBCS 

### will also reduce concentrations of

### COCs in First Creek.
Agricultural life was evaluated in Zones 1. 2. and 6 (rural residential). The results ofthe direct toxicitv P%*aluation indicated no potentidl ai%-,-rs-- impdcts to poultry from soil contaminants or to cattle from ingestion of contaminated soil arid groundwater.

The ecological risk assessment concluded that for animals in the terrestrial and aquatic food webs, there is mirumal potential for adverse effects- However, the Armv and Shell Oil Company have agreed to till and revegetate approximately 160 acres located in the southeast portion of Section 14 and the south,.%,est portion of Section 13 (see Figure 9.1). Shell Oil Company and the U.S. Army
'21905 402010 Harding Lawson Associates 6-13 0711121495 R02Summaryof Site Risks
believe that existing soil risk in the revegetated area falls withinEPA's established acceptable risk range and that remediation is not necessary. However, Shell Oil Company and the U.S. Army agree to the revegetation program as part ofthe remedy.

Construction ofthe Offpost Groundwater Intercept and Treatment Systemwas coordinated with USFWS to minimize the potential impacts on wetlands and habitat. Although the wetlands area has been slightly altered because of construction ofroads in the area, the wetlands still exist, dewatering is no longer occurring, and the amount of recharged groundwater is equal to the amount of extracted groundwater. thereby maintaining the stability ofthe wetlands area. 6.3 Conclusion Actual or threatened releases ofhazardous substances from this site, if not addressed by implementing the response action selected in this ROD, may present a potential threat to public health, welfare. or the environment.

6-14 Harding Lawson Associates '21905 402010 0711121495 R02

### 7.0 DESCRIPTION OF GROUNDWATER REMEDIATION ALTERNATIVES An FS wasconducted to develop and evaluate remedial alternatives forthe Offpost OU. The first task performed during the FS wasto identify media thatrequire remedial action and correspondingly require development and evaluationofremedial alternatives. Risks calculated in the EA were compared to acceptable risk levels establishedbyEPA inthe NCP and other guidance. The Army has closelyfollowed EPA guidance and the National ContingencyPlan (NCP) regarding theuse ofthe

10-4 

### riskthreshold to assess whether remediationis necessary. Guidance states that ifthe cumulative
cancer risk to an individual is less than 10',remedial action maynot bewarranted unless certain site-specific conditions exist. Ifremedial action is warranted,then the 10-4 to 10-' riskrangemust 

### be achieved, with an initial preference foi the 10' end. EPA guidance further states that the upper boundary oftheriskrange is not an absolute at I x 10',but rather,the acceptable riskrange can extend to 5x 10-4. Thecumulative offpost cancer riskis a maximum of3x 10',which is within the acceptableriskrange.
In explaining the use ofthe point ofdeparture. the EPA. in thepreamble to the NCP, states The use of 10' expresses EPA's preference for remedial actionsthat result in risks atthe more protective end ofthe risk range, but does not reflect a presumptionthat the final remedial action should attain such arisk level (55 FR 8718). The operation ofthe Offpost Groundwater Intercopi and Treatment System reflectsthe Army's goal of further reducing the potential risks toward -, he 10' levpl. Using conservative assumptions, including several exposure pathways that do nct currenIly exist. the maximum cumulative cancerrisk in the Offpost OU was estimated to be 3in 10,000. wfuch is within the acceptable riskrange established by EPA. Although the maximum offpost cumulative carcinogenic risk is belowthe acceptablerisk level, remediation ofgroundwater is preferable to no action forthe followingreasons:

'21905 301040 

### Harding Lawson Associates 7-1 0711121495 R02Description of Groundwater Rem*diation Altemativ*s Groundwater concentrations exceed National primary Drinking Water Standards maximum contaminant levels (MCLs) and CBSGs; in some areas ofthe Offpost OU. Groundwater isthe greatest contributor to cancerrisk and contributes a maximum risk of2 in 10,000 (or approximately 75 percent) to the cumulativeriskin zones 2, 3, and 4. Evaluation ofpotential noncarcinogenic health effects indicate that lEs calculated for groundwater contarninant concentrations in zones 2, 3, and 4 are slightly greater than 1.0. Soil, surface water, sediment, and air contribute maximum cancer risks less than 1 in 10,000 in
zones 1 through 6. Soil, surface water, sediment, and air do notrequire remediationbecause ofthe low risks contributed bythese media tothe total risk. Remedial alternatives were developed and evaluated to address contaminated groundwater in the Offpost OU North and Northwest Plunie Groups. Additionally, as part ofthe Conceptual RemedyAgreement, theArmy and Shell Oil Company have agreed to till and revegetate approximately 160 acres located in the southeast portion of Section 14 and southwest portion ofSection 13.
Remedial alternatives for groundwater were developed by (1) establishing groundwater containment svstem remediation goals (2) identifying the areas of groundwater exceedances of containment svstem remediation goals and (3) assemblingcombinations ofremedialprocess options into remedial alternatives.

Contai=ent system remediation goals (Tabie 7.1.. 7.2. and 7.3 were established on the basis of chemical-specific applicable or relevant and appropriate requirements (ARARs), health-based criteria (HBQ. exposure factors, and the statutory rwiuaf-ments stated in Section 121 ofCERCIA. ARA.Rs
were used as groundwater containment system remediation goals for contaminants with promulgated standards. and HBCbased on a risk of I x 10' caiCUldied using RME assumptions were used for carcinogensurithout ARARs. A risk level of 1x 10" was selected to correspond to the point of departure as defined in the NCP. The promulgated standards adopted as containment system remediation goals for Offpost OU groundwater include MCLs and CBSGs. In addition, containment svstem remediation goals for several contaminants with promulgated standards were adjusted
7-2 Harding Lawson Associatos '21905 402010 0711121495 R02Description of Groundwater Rernediation Alternatives
downward to reduce risk corresponding to the containment system remediation goals. For some analytes, the certified reporting limit (CRL) or the practical quarititation limit (PQL) are higher than the containment system remediation goal- The CRL and PQLrepresentthe lowerpractical limit for quantitation.
Attainment ofthe groundwater containment system remediation goals developed forthe site will reduce the estimated total hypothetical cancer risks to less than 1x 10-4toward the 1 x 10' level. Because the total cancer risk assumes that all chemicals are present in groundwater at all locations, and since groundwater contamination is variable throughout the OU, the estimated risk reduction may be greater. Attainment ofthe groundwater containment systemremediation goals developed for the site will also reduce HIs discussed in Section 6.1.4.2 to below 1.0 for all target organ groups and receptors. Again, variability in contaminants present in groundwater may increase the estimated risk reduction from that estimated by extrapolating directly fromthe risk assessment
Groundwater requiring remediation in the Offpost Study Area was identified by comparing groundwater containment svstem remediationgoals to the areal extent of groundwater contamination. Groundwater containment system remediation goals are exceeded forthe carcinogens arsenic, chloroform. DBCP. tetrachloroethviene. LrichJoroethylene. and dieldrin. Groundwater containment system remediation goals are also exceeded for the noncarcinogens chlorobenzene, dicyclopentadiene, and DIMP. The area of groundwater exceeding containment system remediation goals (and thus the Offpost 012.) encompasses approximately 5qO acres of the Offpost Study Area.
Groundwater alternatives were developed and evaluated using two groundwater models. The models simulated groundwater flow and contaminant transport for the North and Northwest Plume Groups. Groundwater modeling was used for the following purposes: developing conceptual designs for sizing and locating groundwater extraction,recharge, and treatment systems; estimating future contaminant transport, evaluating the relative merits ofremediation alternatives; and estimatingthe

'21905 301040 Harding Lawson Associates 7-3 0711121495 R02Description ofGroundwater Romodlation Alternatives time

### required to clean upthe contaminated groundwater. Because ofthe approximate nature and inherent uncertainties ofthe models,none ofthemodelresults shouldbe interpreted as an accurate prediction offuture conditions. Thepredictedremediation timeframes are estimates. Accordingly, estimatedremediation time frames were onlyused to assess the relative effectiveness ofthe groundwater alternatives.
Remedial alternatives were initially screened onthe basis ofeffectiveness, implementability, cost, and attainment ofARARs. The alternativespassing the initial screening werethen evaluated on the basis ofnine criteriarequired by theNCP. In addition toremedial alternatives, the NCP requires that
a No Action alternative be considered at everysite. TheNo Actionalternative serves primarily as a point ofcomparison for other alternatives.
A total ofsix alternatives for the North Plume Group and four remedial alternativesfor the Northwest Plume Group were developedfor analysis. Followingthe initial screening analysis in theFS, four remedial alternatives for the North Plume Group (N-1, N-2, N-4, and N-5) and tworemedial alternatives for the Northwest Plume Group (NW-1 and NW-2) remained for evaluation duringthe detailed analysis ofalternatives. These alternatives are describedbelowwith the original alternative numbering sequence from the FS report. 7.1 Common Elements of Alternatives All ofthe alternatives developed included the foliowing elements: Groundwater and surface-water monitoring: Samples willbe collected periodicallyfrom groundwater monitoring wells and surface-water locations throughout the Offpost Study Area and analyzed to assess changes in groundwater and surface-water quality during and after remediation. 0 Site review: In accordance with CERCLA. a site reviewwillbe conducted at least every five yearsuntil groundwater contai=ent system remediation goals are achievedto assure that human health and the environment are protected during and after remediation. The site reviewwill use monitoring program data to assess whether additional remedial action would be warranted.
7-4 Harding Lawson Associates '21905402010 0711121495 R02Description ofGroundwater Rernediation Alternatives Except for the No Action alternative, each alternative alsoincludes the following activities: Exposure control/provision ofalternate water supply as described below: As ofthe date ofthe Onpost ROD, and based on a .392 parts perbillion (ppb) detection limit,the U.S. Army %%,illuse the last available quarterly monitoring results to determine the DRAPplume footprint. As part ofthe Onpost ROD, the U.S. Army and Shell Oil Company will pay for the extension of, and hook-up to, the current distribution systemfor all existing well owners withinthe DRAP plume footprint referenced above. Existing domestic well owners outside ofthe DROP plume footprint as ofthe date of the On-post ROD where it is later determined that levels ofDRVIP are eight ppb or greater (or other relevant CBSG at the time)will behooked up at the U.S. Army and Shell Oil Company's expense to the SACWSD distribution system or provided a deep well or otherpermanent solution. For new domestic wells with levels of eight ppb or greater (or other relevant CBSG at the time), the Offpost ROD institutional controls will provide that the U.S. Army and Shell Oil Company will pay for hook-up tothe distribution system or provided a deep well or other permanent solution. Any user of a domestic well within the Offpost Operable Unit that contains groundwater contaminants derived from RMA at concentrations that exceed the greater of the remediation goalsin Tables 7.1 through 7.3 or the ARARs in Table 10.1 willbe provided an alternative water supply. Bottled water willbe provided for cooking and drinking until a permanent alternative water supply is provided. Permanent alternative water supplies could include installation of a deep uncontaminated well or connection to a municipal potable water-supply system. This commitment applies to both users of existing domestic wells and users of wells that are lawfully drilled in the future. As part of the Onpost ROD, the U.S. Army and Shell Oil Company agree to pay for, and provide or arrange for the provision of4,000 acre feet ofwater,the details of which .vill be worked out between the U.S. Armv, Shell Oil Company, and SACWSD. Ifsuch water is not available. the U.S. ,Vmvand Shell Oil Company will provide payment of an agreed upon amount ofmoney in lieu ofwater. Use of institutional controls to prevent the future use ofgroundwater exceeding remediation pals. Institutional controls are reflected in Appendix B. Continued operation oftEo existing boundary containment systems - The NBCS and NVVBCS will continue to operate and improvements will be made, as necessary, to assure that offpost groundwater containment system remediation goals are not exceeded. In addition, the ICS will continue to operate, as required. for onpost contaminants consistent with the Irondale IRA. These containment svstems will be operated to the requirements of Section 2.7 ofthe FFA. the Conceptual Remedy Agreement, and the Onpost ROD, when itis signed. Cessation may occur as provided in Sections 35.3 and 35.4 of the FFA and paragraph 20 ofthe Conceptual Remedy Agreement. Currently, approximately 125 million gallons per year are treated at the NBCS, 450 million gallons per year are treated at the N`VVBCS, and 45 minion gallons per vear are treated at the ICS.

'21905 301040 Harding Lawson Associates -7-5 0711121495 R02Description of Groundwater Rem*diatlon Alternatives Closure ofpoorly constructed wells withinthe Offpost Study Area - Wells that couldbe acting as migration pathways for contaminant in the Arapahoe Aquifer wiU be closedusing approved methods. The pertinent criteria are presented in Appendix C - Well Closure Criteria. 7.2 Identification of Groundwater AlternatWes: North Plume Group Alternatives developed for remediation.ofgroundwater in the NorthPlume Group are described below. Table 7.4 presents the alternatives corresponding to the North Plume Group and identifies process options, numbers ofwells and trenches, flowrates, estimatedremediation time frames, treatment facility location, and process residuals generated. 7.2.1 - Alternative N-1: No Action Under Alternative N-1, the operation ofthe NBCS would be discontinued. Alternative N-1 would therefore not providefor active remediation ofaffected groundwaterwithin the North Plume Group. Ceasing operation ofthe NBCS -wouldlikely cause an increase in contaminant concentrations within the North Plume Group. Natural fate processes. including degradation and attenuation, would be the onh,mechanisms that would reduce contaminant concentrations in groundwater within the North Plume Group. The majorcomponents ofAlternative N-1 include the following: Long-term groundwater and surface-water monitoring Site re,.-iews A long-term groundwater and surface-water morutonng program wouldbe implemented. The purpose ofthe monitoringprogram is to assess changing UFS and CFS aquifer and surface-water conditions during and after remedial action. As parl of Alternative N-1, a site review would be conducted at least every five years until containment system remediation goals are achieved.

The total present worth cost estimate for Alternative N-1 ranges from $4,061,000 to $6,102,000. This includes long-term operation and maintenance costs for performing site reviews, groundwater and surface-water monitoring,and regulatory oversite activities.

7-6 Harding Lawson Associates '21905402010 0711121495 R02Description of Groundwater Remediation Alternatives
7.2.2 Aftemative N-2: Continued Operation ofthe North Boundary Containment System With Improvements as Necessary Alternative N-2 would provide for active remediation ofaffected groundwater approaching the north boundary ofRMAthrough continued remediation ofgroundwater at the NBCS. The major components ofAlternative N-2 are as follows: Continued operation ofthe NBCS Improvements to the NBCS as necessary Long-term groundwater and surface-water monitoring Site reviews Exposure control/provision of alternate water supply as described in Section 7.1 Well closure in conformance with criteria listed in Appendix C, pages C-1 and C-2 Institutional controls as described in Appendix B Under Alternative N-2, the NBCS would continue to contain, extract,treat, and recharge approximatelv 125 million gallons of groundwater per year. Improvements would be made to the NBCS if it
was determined that the system was allowing groundwater containing COCs at concentrations exceeding offpost groundwater containment system remediation goals to mi atefromRMAtothe North Plume Group.
As part of Alternative N-2, an alternative water supply would be provided to any user of a domestic well in accordance with the provisions described in Section 7.1. The long-term groundwater and surface-xvater monitoring and site reviewremedial components under Alternative N-2 would be identical to those proposed under Alternative N-1.
The total present worth cost estimate for Alternative N-2 ranges from $30,600,000 to $32,500,000. This incudes long-term operation and maintenance costs for the NBCS and the cost oflong-term groundwater monitoring and site review components included under Alternative N-1.

'21905 301040 Harding Lawson Associates 7-7 0711121495 R02Description ofGroundwater Rem*diation Alternatives 

### 7.2.3 Alternative N-4: Offpost Groundwater Intercept and Treatment System Under Alternative N-4, the NBCS would continue to operate, and the Offpost Groundwater Intercept and Treatment System wouldbeconstructed and operatedto contain, remove,treat, andrecharge groundwater exceeding containment system remediation goals inthe First Creek and northern paleochannels downgradient ofthe NBCS. Detailed information concerningthe Offpost Groundwater Intercept and Treatment Systemis presented in the Final ImplementationDocument forthe Groundwater Intercept andTreatment SystemNorth ofRocky MountainArsenal (HLA, 1991). The Offpost Groundwater Intercept andTreatment System hasbeen in operation since early 1993. The major components ofAlternative N-4 are as follows: Removal ofcontaminated UFS groundwater north ofthe RMAboundary in the First Creek and northern paleoch-annelsusing Offpost GroundwaterIntercept and Treatment System groundwater extraction wells Treatment oforganic contaminants in extracted groundwaterusing carbon adsorption Recharge oftreated groundwaterto the UFS using Offpost Groundwater Intercept and Treatment Systemrecharge wells and trenches Continued operation ofthe NBOS Improvements to the NBCS and Offpost Groundwater Intercept and Treatment System as necessary Long-term groundwater and surface-water monitoring Site reviews Exposure control/provision ofalternate water supply as described in Section 7.1 Well closure in conformance with cnteria listed in Appendix C, pages C-1 and C-2 Institutional controls as described in Appendix B Alternative N-4 would remediate UFS groundwater in the First Creek and northern paleochannels that is contaminated with organic COCs at concentrations exceeding groundwater containment system remediation goals.

7-8 Harding Lawson Associates '21905402010 0711121495 R02Description of Groundwater Rernediation Alternatives
Extraction wells would beused in the Offpost Groundwater Intercept and Treatment System to
remove contaminated groundwater. Extracted groundwater would be conveyedto the treatment facilityvia double-contained polyvinyl chloride (PVC) pipelines.
Based on the results ofthe groundwater modeling, the configuration offive extraction wells and six recharge trenches shownin Figure 7.1 would capture and remove contaminants axially in the First Creek paleochannel. The recharge trenches wouldbe placedboth downgradient ofthe extraction wells and along the outer boundaries ofthe First Creekpaleochannel. In this manner, therecharge trenches would provide both lateral hydraulic containment ofthe First Creek paleochannel and water flushang for enhancing the removal ofcontaminants. Capture would be attained using a transverse svstem of 12 extraction and 24recharge wells directly downgradient ofthe extraction wells in the northern paleochannel system. The Offpost Groundwater Intercept and Treatment System would contain, extract, treat, and recharge approximately 480 gallons per minute (gpm). Construction of this system began in November 1991 and was completed in June 1993.
Extracted groundwater from both the First Creek and northern paleochannels would be conveyedby pipeline to a central carbon adsorption treatment facility on land in the Offpost Study Area that was previously purchased by Shell. Activated carbon adsorption is a well-developed technologythat is ,widelvused in removing organic contaminants from liquid hazardous waste streams and offgas airstreams. The waste stream comes in contact with granular activated carbon (GAC) by flowing through one or more packed-bed reactors. Organic chemicals and, to some degree, inorganic chemicals. are adsorbed onto the internal pores ofthe carbon granules by surface-attractive phenomena. Activated carbon removes many nondegradable organic compounds and is most effective for nonpolar, slightly soluble compounds.
Carbon adsorption is readily implementable. Carbon adsorption is a demonstrated, proventechnology documented to be effective at the NIVBCS, NBCS, and ICS. Activated carbon treatment would

'21905 301040 Harding Lawson Associates 7-9 0711121495 R02Description of Groundwater Remediation Alternatives achieve groundwater containment system remediation goalsfor organic contarninan before discharge via therecharge systems.
An intensive short-term monitoring component wouldbe included inAlternative N-4 as part ofthe long-term monitoringprogram. The intensive short-termprogram would consist ofmonitoring approximately 60 wells in a network that would be finalized through implementation ofthe alternative. Two years ofdata would be collected during the period commencing with Offpost Groundwater Intercept and Treatment System operations start-up. Such a program is necessaryto evaluate the performance ofthe NBCS and the Offpost Groundwater Intercept and Treatment System and would provide an increased understanding ofcontaminant transport, an estimated time to achieve groundwater containment system remediationgoals, and to determine whether improvementstothe Offpost Groundwater Intercept and Treatment System are warranted.
The total present worth cost estimate for Alternative N-4 ranges from $56,500,000 to $63,100,000. This includes the capital and long-term operation and maintenance cost for construction, operation, and performance monitoring ofthe Offpost Groundwater Intercept and Treatment System. This cost estimate also includes the continued operation ofthe NBCS, long-term groundwater monitoring, site review, and exposure control components ofAlternative N-2. 7.2.4 Alternative N-5: Expansion of the Oftpost Groundwater Intercept and Treatment System Similar to Alternative N-4. this alternative would remediate the First Creek paleochannel and northern paleochannel groundwater do%-.-ngradient ofthe NBCS. Based onthe results ofthe groundwater modeling. the configuration of extraction wells and recharge systems proposedunder Alternative N-5 would place additional extraction wells in locations where the hmiting hydrogeologic and contaminant characteristics are controlling remediation time frames. Two additional extraction wells and four rechargetrenches would be installed in the area ofrelatively slower groundwater velocityand high dieldrin concentrations in the First Creek paleochannel. One additional extraction

7-10 Harding Lawson Associates '21905 402010 0711121495 R02Description of Groundwater Rernodiation Alternatives
well and two recharge trenches wouldbe installed in an area oflow hydraulic conductivity in the northern paleochannel. The major components ofAlternative N-5 are as follows: Removal of contaminated UFS groundwater north ofthe RMA boundary in the First Creek and northern paleochannels, using Offpost Groundwater Intercept and Treatment System groundwater extraction wells Expansion ofthe Offpost Groundwater Intercept and Treatment System extraction and recharge systems Treatment of extracted groundwater using carbon adsorption Recharge oftreated groundwater to the UFS, usingrecharge wells and trenches Continued operation ofthe NBCS Improvements to the NBCS as necessary Long-term groundwater and surface-water monitoring Site reviews Exposure control/provision of alternate water supply as described in Section 7.1 Well closure in conformance with criteria listed in Appendix C, pages C-1 and C-2 Institutional controls as described in Appendix B The expansvin (,f thf, Offpost Groundwater Intercept and Treatment System is shown in Figure 7.2. The thro-- dIlitiorwl extraction wells would each pump 30 gpm (90 gpm additional), and the additional ir. !,.-%would recharge the same volume. Thus, Alternative N-5 would extract and treat
a total (,I in ý.t,nipared to 480 gpm for Alternative N-4. Other remedial components under Alternati% - % %,-,,ulJ be identical to those proposed under Alternative N-4.

The total %vorth cost estimate for Alternative N-5 ranges from $56,200,000 to $63,000,000. This includ--s th- rdpital and operation and maintenance costs ofthe expansion systemsto the Offpost Groundwater Intercept and Treatment Svstem and the cost components ofAlternative N-4.

'21905 301040 Harding Lawson Associates 7-11 0711121495 R02Description ofGroundwater Rernediation Alternatives 7.3 Identification of Groundwater Altematives: Northwest Plume Group The following subsections identifythe alternatives developed for theNorthwest Plume Group. Table 7.4 preseýts the alternatives corresponding to the Northwest Plume Group and identifies process options, numbers ofwells and trenches, flowrate, estimated remediation time frames, treatment facility location, and process residuals generated. 7.3.1 Altemative N-1: No Action Under Alternative NW-1, the operation ofthe NWBCS would be discontinued. Alternative NW-1 would not provide for active remediation ofaffected groundwater withinthe Northwest Plume Group. Ceasing operation ofthe NWBCS wouldlikely cause an increase in contaminant concentrations within the Northwest Plume Group. Natural fate processes,including degradation and attenuation, would be the only mechanisms that would reduce contaminant concentrations in groundwater within the Northwest Plume Group. The major components ofAlternative NW-1 are as follows: Long-term groundwater monitoring Site reviews A long-termpgroundwater monitoring program would be implemented. The purpose ofthe monitoring program would be to assess changing UFS and CIFS aquifer conditions during and after remedial action. As part ofAlterriative NW-1. a site review would be conducted at least every five years until contairizient svstem remediation goals are achieved.
The total present worth cost estimate for Allernative NW-1 ranges from $608,000 to $1,260,000. This includes long-term operation and maintenance costs for performing site reviews, groundwater morutoring. and regulatory oversite activities.

7.12 Harding Lawson Associates '21905 402010 0711121495 R02Description ofGroundwater Rernediation Alternatives
7.3.2 Altemative NW-2: Continued Operation of the Northwest Boundary Containment System With Improvements as Necessary Alternative NW-2 would provide for active remediation of affected groundwater approaching the northwest boundary ofRMA through continuedremediation of groundwater at the NWBCS. The major components ofAlternative NW-2 are as follows: Continued operation ofthe NWBCS Improvements to the NWBCS as necessary Long-term groundwater monitoring Site reviews Exposure control/provision of alternate water supply as described in Section 7.1 Well closure in conformance %rithcriteria listed in Appendix C, pages C-1 and C-2 Institutional controls as described in Appendix B Under Alternative NW-2. the NT"rBCS would continue to contain, extract, treat, and recharge approximately 450 million gallons ofgroundwater per year. Improvements would be made to the N'1%7'BCS if it was determined that the system was allowing groundwater containing COCs at concentrations exceeding offpost groundwater containment svsternremediation goals to migrate from RINUN to the Northwest Plume Group.
As pail ofAlternative NRV-2, an alternatwe water supply would be provided to any user of a domestic well that contains groundwater contaminants at concentrations exceeding containment svstem remediation goals . Other retneital : zipont-nis under Alternative NW-2 would be identical to those proposed under Alternative N%V-1.
The total present worth cost estimate for Allerndli%-e NIV-2 ranges from $12,400,000 to $13,100,000. This includes long-term operation and maintenance costs for the MVBCS and the long-term groundwater monitoring, site reviews. and exposure control components ofAlternative NW-1.

'21905 301040 Harding Lawson Associates 7-13 0711121495 R02

### 8.0 COMPARATIVE ANALYSIS OFALTERNATIVES Theremedial alternatives wereevaluated with respect to nine threshold, primarybalancing, and modifying criteriaas required bythe NCP. Thecriteria are as follows: ThresholdCriteria Overall protection ofhuman health and the environment Compliance withARARs Primary BalancingCriteria Long-termeffectiveness and permanence Reductionoftoxicity,mobility, orvolume Short-term effectiveness Implementability Cost Modifying Criteria State acceptance Community acceptance Threshold criteria must be satisfied by the selected alterative. Primarybalancing criteria are used to ,veigh trade-offs among alternatives. Modifying criteria may beused to alter a proposed remedial alternative. Briefdescriptions ofthe evaluation criteria and the items considered when assessing alternatives withrespect to each criterion are presentpd in the summary ofthe comparative analysis ofalternatives.
Themodels simulating UFS groundwater flo-,%-and dissolved chemical transport wereprepared for the analysis ofalternatives and are approximate in nature. Because detailed models were not needed to comparethe benefitsofeach remedial alternative, attempts weremade to produce modelsthat incorporate general features ofgroundwater flow and associated transport phenomena in the Offpost
'21905 301040 

### Harding Lawson Associates 0711121495 R02comparative Analysis ofAlternatives Study Area. Nonetheless,the resulting models predicted flow and chemicaltransport phenomena that agree with historical and current hydrogeologic data and observedcontarninan distributions. Because ofthe approximate nature ofthe models and the considerable uncertainty in the conceptual model and hydrogeologic parameters, none ofthe modeling results shouldbe construed as accurate predictions offuture contaminant distribution. Rather, the models and modeling results should be viewed as tools for assessing the relative merits ofremedial alternatives. Although there are inherent uncertainties in the groundwater model, it is the toolbeing used to evaluate the alternatives, and predicted differences in remediation time frames are considered with respect to evaluating alternative effectiveness.
For the North Model, the following remedial action scenarios were simulated: (1) continued operation ofthe NBCS with improvements as necessary (AlternativeN-2), (2) Offpost Groundwater Intercept and Treatment System (Alternative N-4), and (3) expansion ofthe Offpost Groundwater Intercept and Treatment System (Alternative N-5). The results ofthese simulations were evaluated
on the basis of estimated remediation times measured on maximum concentrations versus time graphs. The range ofestimated remediation times was based on attainment ofthe groundwater cleanup standards for DIMP, chloroform. and dieldrin, using a range ofretardation factors. Although
some remediation goals have changed since modelling was performed, these changes do not affect the assessment ofthe relative merits ofthe remedial alternatives.
Forthe Northwest Model, the remedial action scenario for continued operation ofthe NWBCS with improvements as necessary (Alternative NRV-2) was simulated. 8.1 Comparative Analysis of Remedial Alternatives The purpose ofthe comparative analysis is to identify the advantages and disadvantages ofeach alternative relative to the others. Critical tradeoffs were identified and used to assist in selection of the preferred remedy. Summaries ofthe detailed analysis oftheNorth Plume Group and Northwest Plume Group alternatives are presented inTables 8.1 and 8.2, respectively. A briefdescription ofthe 8-2 Harding Lawson Associates '21905 402010 0711121495 R02Comparative Analysis ofAlternatives
evaluation criteria and a comparison ofeach alternative with respectto the evaluation criteria is presented below. Components commonto all ofthe alternatives werenot evaluated in the comparative analysis. 8.1.1 Overall Protection ofHuman Health and the Environment The criterion of overall protection ofhuman health and the environment serves as a final check in assessing whether each alternative provides adequate protection ofhuman health and the environment. This criterionwas also used to evaluate how risks wouldbe eliminated, reduced, or controlled through treatment, engineering, institutional controls, or otherremedial activities. North PlumeGroupAlternatives Overall protection ofhuman health and the environment would be provided by all alternatives with the exception ofAlternative N-1. Alternatives N-4 and N-5 would provide greater protection than Alternative N-2 because extraction, truatment, and recharge systems within the North Plume Group would decrease organic contaminant concentrations and reduce potentialrisks within a shorter time period. Although groundwater modeling estimates that Alternative N-5 would achieve containment svstein remediation goals in a shorter time period than Alternative N-4 (10-20 years for Alternatives N-5 versus 15-30 vears forAlternative N-4), the two alternatives are essentially equivalent with respect to providing protection ofhuman health and the environment for the followingreasons: Alternatives N-4 and N-5 both provide for active remediation ofthe First Creek and Northern paleochannel groundwater in approximately the same time period throughremoval of contaminated UFS groundwater. treatment ofthe organics in the contaminated groundwater using carbon absorption, and recharge ofthe treated groundwater using recharge wells and trenches. Both alternatives also provide a sigruficant reduction in potential risk in approximately the same time period through orgaruc contaminant removal and treatment by the Offpost Groundwater Intercept and Treatment System. NorthwestPlume GroupAlternatives Alternative NW-1 would notbe protective ofhuman health and the environment because the NWBCS would cease operation. Overall protection of human health and the environment wouldbe provided

'21905 301040 Harding Lawson Associates 8-3 0711121495 R02Comparative AnalyAs ofAlternatives byAlternative NW-2. Alternative NW-2 would decrease contaminant concentrations and reduce potential risks associated with groundwater entering the Offpost Study Area north ofthe NWBCS. Recharge ofgroundwater treated atthe NWBCS wouldreduce contaminant concentrations inthe Northwest Plume Group through flushing withtreated groundwater. Groundwater modeling estimates that Alternative NW-2 would achieve groundwater containment systemremediation goals in approximately three to eight years. Alternative NW-1 would not likely achieve groundwater containment system remediation goals because operation ofthe NWBCS would cease. 8.1.2 Compliance With Applicable or Relevant and Appropriate Requirements The criterion of compliance withARARs is used to assess whether each alternative will attain ARARs. The comparative analysis describes how each alternative exceeds, attains, or does not attain these requirements. Other information such as advisories, criteria, or guidance documents havebeen considered where appropriate during the ARARs analysis (see Section 10.0). NorthPlumeGroupAlternatives Compliance with chemical-specific ARARs would be achieved by all alternatives with the exception ofAlternative N-1 - Cleanup standards for Offpost OU groundwater include Safe Drinking Water Act MCLs and CBSGs. Groundwater modeling estimates that chemical-specificARARs would be aclueved in the shortesttimebv Alternative N-5. followed by Alternative N-4, followed by .AJternative N-2. Compliance with location-specific and action-specific ARARs will be achieved by all treatment alternatives. Because no remediation would take place under Alternative N-1, there would be no federal and state location- or action-specific ARARs. Inorganic standards for chloride and sulfate will be met bv natural attenuation consistent with the onpost remedial action. NorthwestPlume Group Alternatives Compliance with chemical-specific ARARs would be achieved onlyby Alternative NW-2. Groundwater modeling indicates that Alternative NIV-2 would achieve chemical-specificARARs in approxi8-4 Harding Lawson Associates '21905 402010 0711121895 R02Comparative Analysis of Alternatives matelythree to eight years. Alternative NW-2 would complywithlocation- and action-specific ARARs. 8.1.3 Long-term Effectiveness and Permanence The long-term effectiveness and permanence criterion addresses the riskremaining at the site after response objectiveshavebeen met. Components ofthe criterion that were addressed for each alternative are as follows: Magnitude ofresidual risk at the end ofremedial activities Adequacy and reliability of controls that are used to manage either treatment residuals or untreated materials thatremain at the site NorthPiume GroupAftematives Comparison ofNorth Plume Group alternatives withrespect to long-termeffectiveness and permanence indicates that each alternative, Except Alternative N-1, provides a high degree ofeffectiveness and permanence. However, Alternative N-4 is superior to Alternative N-5 because using full-scale operating dald as the basis for identifying the need for placing additional wells and trenches and identifYin; lb- oplimum locations will enhance long-term system performance. A.11 of the alternatives %vill. it,.. - xf:--ption of the No Action alternative would reduce potential risk and address expostir- pallma%--I-%- reducing COC concentrations in the North Plume Group. UndertheNo Action all. ,t, %I;% - 11,,lential risks would likely increase after ceasing operation ofthe NBCS.
NorthwestPlume Group Alternatives Compari% i. I t:.- N rth%%-est Plume Group alternatives with respect to long-term effectiveness and permaiwi, - it, ii it.-s that Alternative NW-2 reduces potential risk and addresses exposure pathways by reduciti;z C(K: t.oncentrations in the North%vesl Plume Group. Under the No Action alternative, potential risks would likely increase afte.- ceasing operation of the NWBCS.

'21905 30IG40 Harding Lawson Associates 8-5 0711121495 R02Comparative Analysis ofAlternatives 

### 8.1.4 Reduction in Toxicity, Mobility, or Volume The reductionoftoxicity, mobility,orvolume criterion addresses the statutory preference for selecting remedial actions thatpermanently and significantly reducetoxicity, mobility,or volume of hazardous materials atthe site. This preference is satisfied when treatment is usedtoreduce principal risks through destruction orirreversiblereductions oftoxicity, mobility,and/or volume. NorthPlumeGroupAlternatives All North Plume Group alternatives with the exception oftheNo Action alternative would reduce the toxicity, mobility,and volume ofcontaminated groundwater entering the Offpost OU north ofthe NBCS. Groundwater contaminant concentrationsunder the No Action alternative would likely increase. Alternatives N-4 and N-5 would provide the greatest reduction in toxicity,mobility, and volumeofcontaminated groundwater, through extraction, treatment, and recharge within the North Plume Group. As stated previously,the uncertaintyassociated withthe remediationtime frames estimated by the groundwater modeling suggests that. in practical terms, the estimatedtime frames forbothAlternatives N-4 and N-5 are essentially equivalent. Further, the intensive short-term groundwater monitoring component ofAlternative N-4 would allowfor full-scale performance data regarding the reduction ofcontaminant concentrations. Such data would be necessaryto assess the need for and optimum location ofany modifications to AlternativeN-4. NorthwestPlumeGroupAlternatives Alternative N'W-2 would reduce the toxicity. mobility.and volume ofcontaminated groundwater entering the Offpost OU northwest ofthe NWBCS through extraction, treatment, and recharge. Groundwater contaminant concentrations under the No Action alternative would likelyincrease; thus toxicity,mobility, or volumewould not be reduced. 8.1.5 Short-term Effectiveness Theshort-term effectiveness criterion addresses the protection ofhuman health and the environment during the construction and implementation phase. The followingfactors were addressed during the evaluation process: 6_6 Harding Lawson Associates '21905 402010 0711121495 R02Comparative Analysis ofAlternatives
Protection ofthe community during remedial actions -This factor addresses anyrisk that results from implementation cfthe proposed remedial alternative, such as dust from excavation or transportation ofhazardous material. Protection ofthe workers during remedial actions - This factor assesses threats that maybe posed to workers and the effectiveness andreliability ofmeasures to betaken. Environmental impacts ofthe remedial action -This factor addresses the potential adverse environmental impactsthat mayresult from construction and implementation ofa remedial alternative and evaluates the reliability ofmitigation measures, if necessary,to prevent or reduce potential impacts. Time lapse before achievement ofresponse objectives - This factor includes an estimate of the time required to achieve protection for the site. NorthPlume GroupAlternatives Assessment ofthe North Plume Group alternatives withrespect to protection ofthe community and workers, short-term adverse environmental impacts, and implementation period indicates that the No Action alternative and Alternative N-2 are slightlybetter than the alternatives with active remediation components. However, during the implementation period, Alternatives N-4 and N-5 wouldbe ableto mirumize adverse short-term impacts through standard engineering controls and adherence to standard health and safety practices. Northwest Plume GroupAlternatives The assessment of the two Northwest Plume Group alternatives with respect to protection ofthe community and workers, short-term adverse environmental impacts, and implementation period shows that the No Action alternative and Alternative NIV-2 are essentially equivalent exceptthat the discontinued operation ofthe NWBCS, as part of the No Action alternative, has an adverse environmental impact. Neither alternative. %%"Lththe exception noted above,has significant short-term effectiveness issues. 8.1.6 Implementability The implementability criterion evaluates the techmcal and administrative feasibility ofimplementing each alternative, and it addresses the availability ofrequired services and materials during

'21905 301040 Harding Lawson Associates 8-7 0711121495 R02Comparadv* Analysis ofAlternativets implementation ofthe alternative. The following factors were addressed during the evaluation process:Construction and operation -This factor considersthe technical difficulties and the unknowns associated withthe technology. Reliability ofthe technology- This factor considers the likelihoodthat problems associated with implementation mayresult in schedule delays. Implementing additional remedial action -This factor is not applicable to the alternatives developed becausethe alternatives considered are not interim measures. Monitoring the effectiveness ofthe remedy -This factor addresses the ability to evaluate the effectiveness ofthe remedy and includes an evaluation oftherisks ofexposure should monitoringbe insufficient to detect a system failure. Coordination with otheroffices and agencies needed to implement remedial alternatives (e.g., obtainmS necessary permits for offsite activities) Availabilitv ofnecessary equipment, specialists, services and materials, and adequate offsite treatment. storage, and dispcýsal services North Plume GroupAfternatives All North Plume Group alternatives evaluated would be technicallyfeasible to implement. The No Action ili,-mat1%-4- and Alternative N-2 would bethe easiest to implementwith respect to technical feasibilit%b-rdUSIý the monitoring wells have already been installed and the NBCS system is currenfly (,I---tdii()ndl. Alternative N-4 is constructed and is fully operational. However,Altemative N-5 we,ul-Ii-.juise additional design and construction. All treatment alternatives would use carbon. -ittro-alment. which has been demonstrated at the boundary containment systems to be d re-hat... p, im ]waier treatment process option. Groundwater monitoringis a componentof all foLL- -m,l would provide information regarding the effectivenessof each alternative.

All allernati%---s with the exception oftht!No Action alternative wouldbe administratively feasible. It is unlikely 1hdt the regulatory agencies or the public would accept shutdown ofthe NBCS as proposed under the No Action alternative. Additionally, the Army will not cease operating the NBCS until cleanup certification. Each ofthe threetreatment alternatives would meet federal and state substantive requirements for recharging the treated groundwater to the UFS. 8-8 Harding Lawson Associattits '21905 402010 0711121495 R02Comparative Analysis ofAlternatives
The No Action alternative and Alternative N-2 would notrequire additional equipment and services. The implementation ofAlternative N-5 would not be limited with respect to availability of services and materials. Contractors with the equipment and knowledge to construct and implement this alternative arereadily available. The remedial systems ofAlternative N-4 were completed in June 1993. NorthwestPlume GroupAlternatives Both Northwest Plume Group alternatives evaluated wouldbe technically feasible to implement. The No Action alternative and Alternative NW-2 would be implementable withrespect to technical feasibility because the monitoring wells have already been installed and the NWBCS is currently operational. Alternative NW-2 would use carbon adsorption treatment, which has been demonstrated at the boundarv containment svstems to be a reliable groundwater treatment process option. Groundwater monitoring is a component ofboth alternatives and would provide informationregarding the effectiveness of each alternative.
The No Action alternative would not be admirustrativelv feasible. It is unlikely that the regulatory agencies or the public would accept shutdown of the NVVBCS as proposed under the No Action alternative. Additionallv, the Armv does not intend to cease operating the NWBCS. .AJternative N-W-2 would meet federal and state substantive requirements for rechargingthe treated groundwater to the UFS. Neither alternative would require additional equipment and services. 8.1.7 Cost The cost criterion evaluates both c-ipital costs and any long-term costs required to operate and maintain an alternative. Cost estimates for each alternative were based on vendorinformation, cost estimating guides, review ofpublished cost data at previous sites, and operation and maintenance costs at the boundarv containment sN,stems.

'21905 301040 Harding Lawson Associates 8-9 0711121495 R02Comparative Analysis ofAlternaffivos 

### NorthPlumeGroupAlternatives The total present worth costs range from $4.1 to $6.0 millionfor Alternative N-1 to $56.5 to $63.1 million for Alternative N-4. The present worth costs are nearly identical for Alternatives N-4 and N-5 because the additional capital expenditures required forAlternative N-5 arebalanced bythe additional operation and maintenance (O&M) costs incurred through the estimated 10-year differences in remediation timeframe for Alternative N-4.

The additional capital expenditure of approximately $2.7 million forAlternative N-5 as compared to Alternative N-4 points out the importance ofcollecting additional full-scale operating data to aid in decision-making regarding any necessary expansion ofthe Offpost Groundwater Intercept and Treatment System. Collection offull-scale data on contaminant transport and actual plume remediation time frames through the intensive short-term monitoring program is cuxrently being conducted. This monitoring program will provide data for use in any system expansion decisionmaking regarding the potential need for and placement of improvements to Alternative N-4 to reduce the remediation timeframe and/or efficiencv. Northwest Plume Group Alternatives The total present worth costs rant,ze from $0.6 to $1.3 million for Alternative NW-1 to $12.4 to $13.1 million for Alternative NW-2. 8.1.8 State Acceptance State acceptance evaluates technical and admirustrative concerns the State may communicate in its Comments concerrung each alternative. The State has been actively involved throughout the RI/FS and remedv selection process for the Offpost OU. The State was provided the opportunity to comment on the RVFS document and proposed plan. and took part in the public meeting held to inform the public on the proposed plan. Written comments from the state received during the public comment period indicate that the State prefers Alternative N-5 or a slightly modified version of Alternative N-5 over Alternative N-4 because of the addition of several wells and trenches for
8-10 Harding Lawson Associatos '21905 402010 0711121495 R02Comparative Analysis ofAlternatives
enhanced contaminant removal. Responses to the State's concerns onthis and otherissues are provided in AppendixA - Responsiveness Summary.
Additional discussions were heldbetween the U.S. Army, Shell Oil Company, the State ofColorado, the U.S. EPA, and theUS. Fish and Wildlife Service followingtheissuance ofthe Offpost proposed plan regarding theremedy forboth the Offpost and Onpost OUs. As a result ofthese discussions, the State of Colorado and the otherparties have agreed to theremedy as described inAgreement fora Conceptual Remedyfor the CleanupoftheRockyMountainArsenal(Conceptual RemedyAgreement). Each party has agreed to support the conceptual remedyas the preferred remedial alternative and to support the proposed plan based on the elements ofthe conceptual remedy. 8.1.9 CommunityAcceptance The preferred alternative forthe Offpost OU was presented to the public in a Proposed Plan, which provides a brief summary of all ofthe alternatives evaluated during the detailed analysis ofalternatives in the FS. In accordance with the NCP, the public had an opportunity to review and comment
on the selected remedial alternatives presented in the Proposed Plan. The concerns expressed included (1) soil remediation issues. (2) DIMP groundwater cleanup standard, (3) why expansion of the Offpost Groundwater Intercept and Treatment System was not selected, (4) the presence ofDIMP immediately do%%mgraclient ofthe Off4post Groundwater Intercept and Treatment Systemin the First Creek area, and (5) the classification ofpotential future land use. The public's comments are addressed in the attached responsiveness summary (Appendix A). Community participation was also included during the Conceptual Remedy Agreement negotiations. 8.2 Conclusions of the Comparative Analysis ofAlternatives The conclusions ofthe comparative analysis of groundwater alternatives for the North and Northwest Plume Groups are summarized below.

'21905 402010 Harding Lawson Associates 8-11 0711121895 R02Comparative Analysis of Altema6v*s In terms ofoverallprotection ofhuman health and the environment,comphance withARARs, effectiveness, and reduction oftoxicity,mobility,orvolume,Alternatives N-4 and N-5 are superior to Alternatives N-1 and N-2. Alternative N-4 is equal to Alternative N-2 in implementability. Alternative N-4 is morereadily implementablethan Alternatives N-1 and N-5 because Alternative N-1 would not be administratively feasible, and Alterative N-5 wouldrequire a second design and construction phase. Alternatives N-4 and N-5 are approximately equal in cost when compared to each other and higher in cost when compared to Alternatives N-1 and N-2. Therefore, Alternatives N-4 and N-5 were identified as being superior to Alternatives N-1 and N-2. Alternatives N-4 and N-5
are essentially equivalent with respectto evaluation ofcompliance withARARs, short-term effectiveness, and reduction in toxicity, mobility, orvolume.
Alternative N-4 was demonstrated to be superior to Alternative N-5 with respectto the detailed analvsis criteria for the followingreasons: The remedial system in Alternative N-4 is designed to effectivelyaddress groundwater contamination within the North Plume Group on the basis of all available data. The Offpost Groundwater Intercept and Treatment System is designed similar to the existingboundary containment systems in that monitoring data is being evaluated to assess whether any improvements are necessary. The intensive short-term groundwater monitoring program included under Alternative N--%. adds flexibility through providing information that will be used to identify any necessary or beneficial improvements to the Offpost Groundwater Intercept and Treatment Svstem and provides information about the optimal location of additional wells or trenches. Because the estunates ofremediation time frames developed for the groundwater alternatives are uncertain. additional capital expenditures are notjustified until actual full-scale data is available. Alternative N-4 is superior to Alternative N-5 with respect to long-term effectiveness and permanence. The combination offull-scdle operational data from the Offpost Groundwater Intercept and Treatment System and future possible system modifications will resultin an optimized treatment system. Immediate pldcement of additional wells and trenches in Alternative N-5, baseý on groundwater modeling results, would not be based onthe more accurate empirical data. Alternative N-4 is more readily implementable than Alternative N-5 because implementation ofAlternative N-5 would require additional remedial design and construction. Operation of Alternative N-4 would start immediateiv. Alternative MV-2 ranks aboveAlternative NW-1 in all criteria except cost;however,the additional costs are not prohibitive in light ofthe reduction in time for remediation. 8-12 Harding Lawson Associates '21905 402010 0711121495 R02

### 9.0 IDENTIFICATION OFTHE SELECTED REMEDY The selectedremedyforthe Offpost OU consists ofimplementing Alternative N-4 for remediation of groundwater in the NorthPlume Group, AlternativeNW-2 forremediation ofgroundwaterin the Northwest Plume Group, and continued operation ofthe Irondale Containment System consistent withthe Irondale IRA. Theselected alternatives are described indetail in Section 7 and the Declaration to the ROD. 9.1 Alternative N-4: Oftpost Groundwater Intercept and Treatment System Construction ofthe Offpost Groundwater Intercept andTreatment Systembegan inNovember 1991 and full-scale system operationbegan inJune 1993. Additional detail concerningdesign specifics is contained in the Final ImplementationDocumentforthe Groundwater Intercept and Treatment SystemNorth ofRMA (HLA, 1991). The major componentsofthis alternative are as follows: Operation ofthe Offpost Groundwater Intercept andTreatment System

- Removal ofcontarninated UFS groundwater northofthe RMAboundary in the First Creek and northern paleochannels, using Offpost Groundwater Intercept andTreatment Systemgroundwater extraction wells

- Treatment ofthe extracted groundwater, using carbon adsorption

- Recharge oftreated groundwater to the UFS, using Offpost Groundwater Intercept and Treatment System recharge wells and trenches

- Natural attenuation ofinorganic chloride and sulfate concentrations to meet applicable standards for groundwater in a manner consistent with the on-Post remedial action Continued operation ofthe NBCS Improvementsto Offpost Groundwater Intercept and Treatment System and the NBCS, as necessary Long-term groundwater and surface-water morutonng Sitereviews Exposure control/provision ofalternate water supply as described in Section 7.1 Well closure in conformance with criteria listed in Appendix C,pages C-1 and C-2

'21905 402010 

### Harding Lawson Associates 9-1 1107121895 R02Idontification ofthe SoWeted Rornody institutional controls for the selectedremedy are reflected inAppendix B. These institutional controls are intended to preventthe future use ofgroundwater exceeding remediation goals.
The Offpost Groundwater Intercept and Treatment System is an array ofextraction wells and recharge trenches in the northern and First Creek paleorhannel . The system is configured to extract and treat UFS groundwaterthat exceeds containmentsystem remediationgoals and to recharge the treated groundwater. Figure 7.1 presents the placement of extraction wells and recharge wells in the northern paleochannel and the placement ofextraction wells and rechargetrenches in the First Creek paleochannel. The location ofthe treatinent facility is also shown in Figure 7.1. The northern paleochannel collection system consists of 12 extraction wells spaced approximately 200 feet apart across the paleochannel, perpendicular to the direction ofgroundwater flow. Therecharge system inthe northern paleochannel consists of 24recharge wells spaced 100 feetapart and placed parallel to and approximately 300 feet downgradient ofthe collection system. The First Creekpaleochannel collection system consists of5 extraction wells spaced 200 to 500 feet apart along the axis ofthe paleochannel. Recharge trenches are placed such that four ofthe six trenches are parallelto theflow axis and located onthe margins ofthe paleochannel, with the remainingtwotrenches located downgradient ofthe extraction well system and oriented perpendicular to the flow axis.

The system is designed to extract and treat an average flow of300 gpm fromthe northern paleochannel, an average flow of 180 gpm from the First Creek paleochannel, and a peak flow of 1.5 times the average flow. The treatment facilitybasic process flow includes influent storage, pumping,bag filtration for particulate removal. carbon adsorption. multimediafiltration, treated water storage, treated water pumping, and final bag filtration.
A total of approximately 250 million gallons per year would betreated bythe Offpost Groundwater Intercept and Treatment System at the average flows. In addition, operation ofthe NBCS component ofthis alternative will treat approximately 125 milhon gallons per year. Thus, a total ofapproxi9-2 Harding Lawson Associate5 '21905 402010 0711121895 R02Identification ofthe Selected Remedy mately 375 million gallons ofUFS groundwater willbe treated annuallyto attain Offpost OU containment systemremediation goals (Tables 7.1, 7.2, and 7.3) underthis alternative.

An intensive short-term monitoring component willbe included inAlternative N-4 as part ofthe long-term monitoring program. For costing purposes, itis assumedthat this program will consist ofa network ofapproximately 60 wellsto be sampled semiannually fortwo to three years,beginning with the Offpost Groundwater Intercept and Treatment System start-up. The intensive monitoring program will allow the collection and subsequent interpretation ofperformance data for the full-scale operation ofboth the Offpost Groundwater Intercept and Treatment System and the NBCS. The data will also beused to assess the need for any improvements to the systems. The acquisition of such data will allow for increased accuracy in assessing the response ofthe UFS groundwater to the NBCS and Offpost Groundwater Intercept and Treatment System remediation systems.
In addition, the preferred alternative includes long-term monitoring ofoffpost groundwater and surface water to assess contaminant ccncentration reduction and remedy performance. Groundwater monitoring will continue utilizingboth monitoring wells and private drinking water wells. Selected surface-water monitoring locations will be included to evaluate the effect ofgroundwater treatment on surface water quahty. Monitoring will continue after system shut-offto assure continued compliance with containment system remediation goals. TheArmy will present the scope ofthese ongoing monitoring programs in an Implementation Plan tobe submitted within 90 days following issuance of the ROD. A schedule for compliance with the containment system remediationgoals will be included in the Implementatior Plan. 9.2 Alternative NW-2: Continued Operation of the Northwest Boundary Containment System with Improvements as Necessary This section summarizes Alternative NW-2, the continued operation ofthe NWBCS with improvements as necessary. For additional details ofthe extraction/recharge systems, the recentupgrades to the system, and the treatment facility at the NWBCS, the reader is referred to the following reports:

'21905 402010 Harding Lawson Associates 9-3 1107121895 R02Identification of the Selected Remedy 

### Final implementationDocumentforNWBCS Short-term ImprovementsIRA (Morrison-Knudsen Environmental Services [MKES], 1990a); NVVBCS Long-termImprovements IRAB(ii) FinalAssessmentDocument(Woodward-Cayde [wwq, 1991a); ProposedDecisionDocument NWBCS RMA Longterm ImprovementsIRA (WWC, 1991b); Report ofField Investigations,Assessment, and Final DecisionDocument forthe NWBCS Short-termImprovements IRA (NflCES, 1990b); Implementation Document forthe NorthwestBoundary System long-termImprovementsIRAFinal Report (MKES, 1992); and NorthwestBoundary Containment SystemLong-term ImprovementsIRA One-year Evaluation Report (MKES, 1993). Themajorcomponents ofthis alternative are asfollows: Continued operationofthe NWBCS Improvementsto theNWBCS as necessary Long-term groundwater monitoring Site reviews Exposure control/provision ofalternate water supply as described inSection 7.1 Well closure in conformance with criteria listed in Appendix C,pages C-1, and C-2 Institutional controls as described in Appendix B In addition. the preferred alternative 4.ncludes long-term monitoring to assess contaminant concentration reduction and remedyperformance. After attainment ofgroundwater containment system remediation goals and system shut-off, groundwater monitoring willcontinue to assure continued compliance with containment systemremediation goals. The Army will present the scope ofthese monitoringprograms in implementation plans tobe submitted following issuance ofthe ROD.
The MVBCS began operation in 1984. The NWBCS collection system consists of 20 extraction wells and a sodbentonitebarrier approximately 2300 feet in length. Therecharge system consists of25 downgradient recharge wells.

9-4 Harding Lawson Associates '21905 402010 0711121895 R02Identification ofthe Selected Remedy
9.3 Additional Components ofthe Selected Remedy In accordancewith the NCP, the public had an opportunity to review and comment on the selected remedial alternatives presented in the ProposedPlan. In response, the Parties held additional discussions to determine howbest to address these comments. These discussions resulted in clarifications and minor technical changes -thatdo not significantly alter the overall scope, performance, or cost ofthe Offpost preferred alternative.
Because the mainfocus ofthe Offpost preferred alternative is unchanged, and the additional actions only clarify and enhance the preferred alternative, the changes werenot considered to be significant.
The discussions also involved broader issues which wereresolved in a document entitled "Agreement for a Conceptual Remedy forthe Cleanup ofthe Rocky Mountain Arsenal,' dated June 13, 1995. With regard tothe Offpost area, the Pzrties agreed to several additional components which are an integral part ofthe overall remedybut are proposed for inclusion in the Onpost ROD. Many ofthese components are inthe Onpost Proposed Plan, which is available for public comment from October 16, 1995, through January 19, 1995.
The additional components added in response to public comment and as part ofthe Conceptual RemedyAgreement discussions include: TheU.S. Army and Shell Oil Company agreeto continue monitoring and to complete an assessment ofthe NDMA plume by June 13, 1996, using a 20 ppt methoddetection limit. The U.S. Army and Shell Oil Company agreeto prepare a feasibility study of potential actions, both onpost and at the boundary. or ad)acent to the boundary in orderto achieve NDMA remediation goals at the RMA boundary and to use 7.0 ppt PRG or a certified analytical detection level readily available at a certified commercial laboratory (currently 33 ppt). The U.S. Army and Shell Oil Company agree to revegetate approximately 160 acres located in the southeast portion ofSection 14 aný the southwest portion of Section 13 as depicted in Figure 9.1. Revegetationwill involve tilling and seeding. No sampling will be conducted before or after revegetation. Existing soil risks in the area to be revegetatedfall withinEPA's established acceptable riskrange and revegetation is notnecessary. However, the U.S. Army and Shell Oil Company agree to the revegetation program as part ofthe Offpost settlement.

'21905 402010 Harding Lawson Associates 9-5 1107121995 R02identification ofthe Selected Remedy The Army will treat any contaminated extracted groundwater priorto discharge or reinjection so that it meets the current water quality standards established in the Colorado Basic Standards for Groundwater and the Colorado Basic Standards and Methodologies for Surface Water. As ofthe date ofthe OnpostROD, and based on a 0.392 parts per billion (ppb)detection limit,the U.S. Armywill usethe last available quarterly monitoringresultsto determine the DRAP plume footprint The Army and Shell Oil Companywill pay for the extension of, and hook-up to, the current water distribution system for all existing well owners within the DRAP plume footprint referenced above. Existing domestic well owners outside ofthe DIMP plume footprint as ofthe date ofthe Onpost ROD where it is later determined that levels ofDMV areeight ppb or greater (or otherrelevant CBSG at the time)will be hookedup atthe U.S. Army and Shell Oil Company's expenseto the SACWSD distribution system orprovided a deep well or other permanent solution. For new domestic wellswith levels ofeight ppb or greater forother relevant CBSG at the time), the Offpost ROD institutional controlswill provide thatthe U.S. Army and Shell Oil Companywill pay for hook-up to the distribution system or provided a deep well or other permanent solution. The parties to the Conceptual RemedyAgreement committo good faithbestefforts to establish a trust fund for the operations and maintenance ofthe remedy,including habitat and surficial soil. The parties recognize.however,that establishment ofsuch a trust fund requires special legislation and there are restrictions on the actions federal agencies can take with respect to proposing legislation and supporting proposed legislation. As part ofthe Onpost remedy, the U.S. Army and Shell Oil Companywill pay for and provide. or arrange for the provision, of4000 acre-feet ofwater to SACWSD. 9.4 Cost of Selected Remedy A detailed cost summary forthe selected remedy is presented in Table 9.1. Thetotal estimated cost ranges from approximately $69 to $76 million. This cost does not include implementation ofthe additional components discussed in Section 9.3. However,these additional components would be included in all the alternatives evaluated (except the No Action alternative); therefore, therelative relationship ofthe cost ofthe various alternatives will not change. 9.5 Limitations It should be recognized that U.S. Environmental Protection Agency(EPA) studies (EPA, 1988b) have indicated that it maynot alwaysbe possible to reach MCJs or proposed MCIs through currently available technology. If itbecomes apparent during implementation or operation ofthe selected 9-6 Harding Lawson Associates '21905 402010Identification of the Selected Remedy remedythat contaminant levels are remaining constant for a significant amount oftime at levels higher than the groundwater containment systemremediation goals delineated inthe ROD, the containment systemremediation goals and theremedy willbereevaluated. Further, the NCP requires a formal review ofthe effectiveness ofthe selected remedyat least every five years. As needed, the operational design ofthe selectedremedy willbe reviewedto achievethe groundwater containment system remediation goals . 9.6 Criteria for Shutting Down Boundary and Offpost Containment Groundwater Systems Existing wells within the boundary and offpost containment systems canberemoved from production when concentrations of constituents detected in the well are less than applicable orrelevant and appropriate requirements (ARARs) established in the ROD and/orit can be demonstrated that discoiatinumg operation ofa well mill notjeopardizethe containment objective ofthe systems. Wells removed from production, and monitoring wellsupgradient and downgradient ofthe boundary and offpost containment systems,will be monitored quarterly for a period offive years to determine if contaminants reappear. Wells turned aff for hydraulic purposes will notbe subject to the quarterly morutoring requirements. Boundary and offpost containment system extraction wells removed from production for water quality reasons will be placed back into productionif contaminant concentrations exce--d tia- AFLALRs established in the ROD. Wells with concentrations less than ARARs established in the ROD can remain in production if additional hydraulic control isrequired.

'21905 402010 Harding Lawson Associates 9-7 1107121895 R0210.0 

### STATUTORY DETERMINATIONS A description ofhowthe selected remedymeets statutory requirements,compliance with the requirements ofCERCLA, and consistencywith theNCP is presented in this section. 10.1 Consistencywith the StatutoryRequirements ofCERCLA in Section 121 The statutory requirements ofCERCLA Section 121, as describedbelow, and the statutory preference fortreatment are metthrough implementation ofthe selected remedy. 10.1.1 Protection ofHuman Health and the Environment The selected remedy willresult in the remediation ofthe Offpost OU groundwater consistent with remedial action objectivesand containment systemremediation goals established for the site. Contaminated groundwater in the North and Northwest Plume Groups willbe addressed by implementingthe selected remedythrough groundwater extraction, treatment,and recharge.
The groundwaterremedial actions proposed under Alternatives N-4 and NW-2 will permanently address the primary threat to human health and the environment forthe Offpost Study Area through ca.rbon adsorption treatment to reduce the toxicity. mobility.and volume ofcontaminated groundwater Contaminant levels in Offpost Study Area groundwater wil-lbe reduced to orbelow groundwater containment svstem remediation goals foliowing treatment. Reduction ofgroundwater contaminant concentrations to these goals %%-illfurther reduce the groundwater cumulative excess
cancer risk toward 10'. Following groundwater remedial action, the M fornoncarcinogens will be less than 1.
It should berecognized, however, that studies con3ucted at other sites (byEPA and others) have indicated that it may not always be possible tn reach groundwater containment systemremediation goalsbecause ofthe limitations ofthe technologyused to assess groundwater hydrogeological. properties. the technologyused to estimate aquifer remediationtime frames, and the technologyused to extract and recharge groundwater. Ifitbecomesapparent during operation ofthe groundwater

'21905 402010 

### Harding Lawson Associates 10-1Statutory Determinations
treatment systemsthat groundwater contaminant levelsare remaining constant at levels higher than the Offpost OU groundwater containment system remediation goals ,the selected remedyprovides for improvements to the proposed remeclial systems as necessary. An alternative water supply will be provided to anyuser ofa domesticwell in accordance with theprovisions in Section 7.1. Institutional controls that are part ofthis remedy are intended to preventthe future domestic use of groundwater exceeding the containment system remediation goals.
Ofthe alternatives evaluated for cleaning up the groundwater, the selected remedyprovides the highest degree ofprotection ofhuman health without adverse impact to the environment. No unacceptable short-term risks will be caused by implementingthis remedy.

Potential ecological impacts duxing remediation will be continually evaluated. Maintenance of existing habitats and ecosystems are important. Although the Federal Endangered Species Act, the Migratory Bird Treaty Act, and the Bald Eagle Protection Act were not considered as ARARs, the FFA requLres their application. Remediation goals consistent with the substantive requirements ofthese Acts are being met and will be assured through close interaction with the U.S. Fish and Wildlife Service. In coordination with the U.S. Fish and Wildlife Service, it was agreed that screening levels, developed to ensure compliance with enforceable remediation levels, meet the requirements ofthe federal Endangered Species Act. the Migratory Bud Treaty Act, and the Bald and Golden Eagle Protection Act. These screening levels were not exceeded in the Offpost OU. These levels are presented in the Final Offpost Operable Unit Endangerment Assessment/Feasibility Study in Table 3.3.3-1 (ToxicityReference Values for Avian and Terrestrial Vertebrate Species ofConcern Identified at Rocky Mountain Arsenal) ofVolume 11 and Table 1-15-1 (MaximumAllowable Tissue Concentration [1AATC] Values for the Offpost E.A. Ecological Assessment) ofAppendix H in Volume I'V. Ifthe screening levels are exceeded or effects are observed in the future, enforceable remediation levels urill be developed consistent with CERCLA, the Endangered Species Act, the Migratory Bird Treaty Act, and the Bald and Golden Eagle Protection Act.
10-2 Harding Lawson Associates '21905 402010 1107121495 R02Statutory Determinations
10.1.2 Compliance With Applicable or Relevant and Appropriate Requirements Under Section 121(d)(1) ofCERCLA, remedial actions must attain standards, requirements, limitations, or criteria that are applicable orrelevant and appropriate under the circumstances ofthe release at a site. ARARs would be met or exceeded upon completion ofthe selectedremedy at the Offpost OU. Chernicai-specific ARARs Groundwater containment system remediation goals are based on chemical-specific ARARs for those chemicals having promulgated standards and on HBC for those chemicals withoutARARs (Tables 7.1, 7.2, and 7.3). The preferred site%ide alternative is expectedto attain or exceed chemical-specificARARs. A summary of the chemical-specific and otherARARs that have been assessed to be applicable or relevant and appropriate is presented in Table 10.1. Action-specificARARs The selected remedywill comply witý action-specific ARARs. A summary ofthe action-specific AR.ALRs that have been assessed tobe applicable or relevant and appropriate is presented in Table 10.2. Location-specific ARAPIS The selected remedy xvill comply %%ithlocation-specific AFkRs. A summary ofthe location-specific ARARs that have been assessed to be applicable or reievant and appropriate is presented in Table 10.3. 10.1.3 Cost Effectiveness The selected remedv is cost-effective in mitiv,,itinz the risks posed at the siteby contaminated groundwater. Cost-effectiveness is determined b%evaluating three ofthe fivebalancing criteria to determine overall effectiveness: long-term eftecuveness and permanence; reduction oftoxicity, mobility, or volume through treatment. and short-term effectiveness. Overall effectiveness is then compared to costto ensure that the remedy is cost-effective.

'21905 402010 Harding Lawson Associates 1-0.3 1107121495 R02Statutory Determinations The selectedremedyfor groundwater providesthe best overall effectiveness ofal.1 alternatives considered proportional to its cost. The selected remedy will greatlyreduce the toxicity,mobility, and volume ofgroundwater exceeding containment systemremediation goals. Also the implementation ofthis remedywW result in long-term effectivenessby reducingresidual carcinogenic risks through permanent treatment. Through the groundwater monitoringprogram, the Army can more accurately assess the contaminant removal rates as a function oftime, using the full-scale data available during operation ofthe Offpost Groundwater Intercept and Treatment System, the NBCS, and the NVVBCS. The analysis ofthis data will allow for cost-effective decisions regarding any future improvements that maybe required for the remedial systems. 10.1.4 Utilization ofPermanent Solutions to the Maximum Extent Practicable The selected remedy for the Offpost OU represents the maximum extent to which permanent solutions and treatment technologies can be used in a cost-effective mannerto remediate groundwater at the site. Ofthose alternatives that are protective ofhuman health andthe environment and comply,with ARARs, the selected remedy (Alternatives N-4 and NW-2) will providethe bestbalance oftrade-offs in terms oflong-term effectiveness and permanence; reduction oftoxicity, mobility,or volume through treatment. short-term effectiveness: implementability; cost; the statutory preference for treatment as a principal element. and state and community acceptance. 10.2 Consistency with the National Contingency Plan The NCP requires that the following two features b-ý present in the remedy selectionprocess: The nine criteria used to evaluate alternatives Ln the detailed analysis are used to select a remedy. Selected Superfund remedies must employ the rune criteria to make the fol.lowing four determinations: Each remedial action selected shall be protective ofhuman health and the environment. 10-4 Harding Lawson Associates '21905 402010 1107121495 R02Statutory Determinations
Onsiteremedial actions selected in a ROD must attain ARARs or provide grounds for invoking awaiver. Each remedial action selected shall be cost effective,provided that itfirst satisfies the threshold criteria (defined in Section 8.0). Eachremedial action shall use permanent solutions to the maximum extent practicable. The preferred sitewide alternative is fully consistent with the NCP, as is the selection processused to arrive at the preferred alternative. Alternatives were developed and screened, and the detailed analysis ofalternatives was performed in a manner consistent with the NCP. 10.3 Summary The preferred sitewide alternative for iemediation ofthe Offpost OU is the combination ofAlternatives N-4 and NW-2. The preferred alternative was selected in accordance withthe requirements of CERCLA and the NCP. The remedial actions that composethe sitewide preferred alternative will permanently address the principal threats through groundwater extraction and treatment to reduce the toxicity. mobility, or volume of contaminants for protectionofhuman health and the environment. Although the requirements for provision of an alternate water supply and hookup to the SACWSD
are part of the Onpost remedy. these actions will also significantly reduce the potential for exposure to offpost groundwater.

'21905 402010 Harding Lawson Associates 10-5 1107121495 R0211.0 

### DOCUMENTATION OF SIGNIFICANT CHANGES The Proposed Planfor theRockyMountain Arsenal Offpost Operable Unit was released for public commentin March 1993. TheProposed Plan identified Alternative N-4 (Offpost Groundwater Intercept and Treatment System) as the preferred alternative for groundwater inthe North Plume Group and AlternativeNW-2 (Continued Operation oftheNorthwest Boundary Containment System With Improvements as Necessary) as the preferred alternative for groundwater in the Northwest Plume Group. TheArmy receivedwritten comments fromthe U.S. Environmental Protection Agency,the State ofColorado, the Tri-County Health Department, cityand county governments, environmental action groups, and citizens. After review ofthese comments,it was determined that
no sigruficant changes to the preferred alternative, as itwas originallyidentified inthe Proposed Plan, werenecessary. As indicated earlier in Section 8.1.8. followingthe issuance ofthe Offpost Proposed Plan, additional discussions wereheld betweenthe Parties regarding the implementation ofthe preferred alternative for the Offpost OU and the remedies forthe Onpost OU. The main components ofthe preferred alternative for the Offpost OU remain intact. These components are: Operation (and improvement. if necessary) ofthe Offpost Groundwater Intercept and Treatment Svstem Continued operation (and impmvern,,nt. ifri-ý:--ssarv) ofthe NBCS and NWBCS Long-term groundwater and surface-wat-,r nionooring Five-year site review Well closure Provision ofalternate water suppli-i.-in,l iniplnuentationofinstitutional controls intended to prevent future use ofcontaminatt-d groun iwater. The Conceptual RemedyAgreement pro,-idesmore specific criteria forthe provision of alternate water supplies to current and future well owners. specific criteria for continued operation ofand requirements for shutdown ofthe groundwater treatment systems, and additional requirements for

'21905 402010 

### Harding Lawson Associates 11-1 1107121495 R02Documentation of Significant Changes the Army and Shell Oil Company such as tiffing and revegetationof surface soil and additional study requirements. Because the main focus ofthe preferred alternative isunchanged by the Conceptual RemedyAgreement, and the additional actions specified in the Conceptual RemedyAgreement only clarify and enhance the preferred alternative, the Conceptual RemedyAgreement was not considered tobe a significant change. Therefore, thepreferred alternative as presented in the Proposed Plan, and additional actions to enhance the preferred alternative as outlined in the Conceptual Remedy Agreement, is the selectedremedy.

11-2 Harding Lawson Associates '21,005 402010 1107121895 R02

### 12.0 GLOSSARY ARAR Applicable orrelevant and appropriate requirement Army U.S. Department oftheArmy . ATSDR U.S. AgencyforToxic Substances and DiseaseRegistry CAR Cont-ination Assessment Report CBSG Colorado Basic Standards for Groundwater CBSM Colorado Basic Standards and Methodologiesfor Surface Water CERCLA Comprehensive Environmental Response, Compensation, and Liability Act CF&I Colorado Fuel and lron CFS Confined flow system CNIP Comprehensive Monitoring Program CNS Central nervous system COC Chemicals ofconcern CRL Certified reporting limit CSC Chemical Sales Company CU Consumptive use DBCP D ibromochloropropane DCPD Dicyclopentadiene DDE 2.2-bis(p-Chlorophenvl)-1.1 -dick-Joroethene DDT 2.2-bis(p-Chlorophenvi)- I.I.I-trichloroethane DINIP Diisopropy1methyl j:h otrh,)nit,- DOI U.S. Department of InIvrirr DOJ U.S. Department ofjustic-- FATS Endangerment dSSeSSm,-nL 

### f-asibilitv study EPA U.S. Envuoranental I'vPtectionAgency ESE Environmental Science and Engineering, Inc. FFA Federal FacilitN.Agreement
'21905 402010 

### Harding Lawson Associates 12-1 1107121495 R02Glossary GMT Groundwater M3nitoring Program gpm Gallons per minute HBC Health-based criteria HI Hazard index HQ Hazard quotient Hyman Julius Hyman &Company ICS Irondale Containment System IRA Interim response action MCL Maximum contaminant level MCLC Maximum contaminant level goal MKES Morrison-Knudsen Enviro=ental Services NBCS North Boundary Containment System NDMA N-nitrosodimetliylamine NCP National 00 and Hazardous Substances Pollution Contingency Plan NEPA National Environmental PolicyAct NTL National Priorities List NR%'BCS Northwest Boundary Containment System OW Operation and maintenance OCP Organochlorine pesticide ou Operable unit PRP Potentially responsible party PVC Polyvinyl chloride RfD Reference dose RWFS Remedial investigatiorx/feasibilitystudy RM.A Rocky Mountain Arsenal R11v1E Reasonable maximum exposure ROD Record ofDecision
12-2 Harding Lawson Associates '21905 402010 1107121495 R02Glossary
SACWSD South Adams County Water and Sanitation District SARA Superfund Amendments and Reauthorization Act Shell Shell Oil Company UFS Unconfined flow system WWC Woodward-Clycie ,vg/I Micrograms per liter jjgýkg Micrograms per kilogram

'21905 402010 Harding Lawson Associates 12-3 1107121495 R02

### 13.0 BIBLIOGRAPHY Adams County,City ofAurora, City ofBrighton,City ofCommerce City. 1990. Airport Environs Concept Plan, Brighton, Colorado. Adams County Planning Cominission. 1987. Adams CountyFuture Land Use Plan, Brighton,CO. Camp Dresser &McKee,Inc. 1993. Stapleton International Airport Environmental Assessment, Northern Zone. Preparedfor Stapleton International Airport, City and County ofDenver.June 15. Ebasco Services,Inc. 1988. WesternTier TCE Soil Gas Investigation, Final SummaryReport. January. Ebasco Services,Inc. 1989. Technical Support for RockyMountainArsenal Draft Final Water Remedial Investigation Report. Contract Nos.DAAK11-84-DO016 and DAAA15-88-DO024. Denver.CO. Environmental Science and Engineering, Inc. 1985. Rocky MountainArsenal OffpostAssessment: Ground Water Quality Report (Consumptive Use - Phase 1)for SamplingPeriod December 1984 throughJanuary 1985. Prepared for U.S. ArmyToxic and Hazardous MaterialsAgency. Denver,CO. Environmental Science and Engineering, Inc. 1986. RockyMountain Arsenal OffpostAssessment: Ground Water Quality Report (Consumptive Use- Phase II)for Sampling Period September through October 1985. Contract No. DAAK-11-D-007, Task Order 0006. Denver, CO. Environmental Science and Engineering, Inc. 1987a. Rocky MountainArsenal OffpostAssessment, Contamination Assessment Report: Draft Final, Denver prepared for the Office ofthe Program Manager. RockyMountain Arsenal. Enviroruxiental Science and Engineering, Inc. 1987b. Rocky Mountain Arsenal Offpost Assess-ment: Ground Water Qualitv Report (DomesticUse - Phase III) for Sampling Period Septemberthrough October 1986 and Februarv 1987. Contract No. DAAK-11-83-D-007, Task Order 0006. Enviro=,ýntal Science and Engineering. Inc. 1988a. Offpost Operable Unit Remedial Investigation and Chemical Specific Applicable or Relevant and Appropriate Requirements, Final Report.Prepared bv Environmental Science and Engineering. Inc.. Harding Lawson Associates; and Applied Environmental. Inc.. Rockv Klountain Arsenal. CommerceCity. CO. 3 volumes. Environmental Science and Engineering. Inc. 1988b. Air Remedial Investigation Report, Final. Prepared frr Office of the Program Manager. Denver. CO. 

### Environmental Science and Engineering. Inc. 1989. Draft Final Endangerment Assessment/ Fe,isibility Study and Applicable or Relevant and Appropriate Requirements. Prepared for Office of the Program Manager. Denver. CO. Federal Facility Agreement. 1989. Pursuant v, CERCLA,Section 120, DocketNo. CERCLA VIII-89-13, signed by U.S. Department ofthe Arm\-. U.S. Environmental Protection Agency,U.S. Environmental Protection Agency RegionVIII, U.S. Department ofInterior, U.S. Fish and Wildlife Service, Agency forToxic Substances and DiseaseRegistry. U.S. D-partment ofJustice, and Shell Oil Company. Harding Lawson Associates 1989. Final Decision Document forthe Groundwater Intercept and Treatment Svstem North ofRocky hlountain Arsenal, Interim Response Action.

'21905 402010 

### Harding Lawson Associates 13-1 1107121495 R02Bibliography Harding LawsonAssociates. 1991. Final Implementation Documentfor the Groundwater Intercept and Treatment SystemNorth ofRocky Mountain Arsenal, Interim Response Action. Harding Lawson Associates. 1992a. Offpost Operable Unit, Endangerment Assessment/Feasibility Study, Final Report, 8 volumes. Harding Lawson Associates. 1992b. Offpost Operable Unit Remedial Investigation, Final Addendum, 2 volumes. Morrison-Knudsen Environmental Services. 1990a. ImplementationDocument for Northwest Boundary System Short-term Improvements IRA. June. Morrison-Knudsen Environmental Services. 1990b. Report ofField Investigations, Assessment, and Final Decision Document forthe Northwest Boundary System Short-term Improvements Interim Response Action, RMA. June. Morrison-Knudsen Environmental Services. 1992. Implementation Documentfor the Northwest Boundary System Long-term Improvements ERA Final Report. January Morrison-Knudsen Environmental SerArices. 1993. Northwest Boundary Containment System Longterm Improvements IRA One-year Evaluation Report. July. Mullins, D.E., Johnsen, R.E., and Starr, R.I. 1971. Persistence ofOrganoclorine Insecticide Residues in Agricultural Soils of Colorado.Pest. Monit. J.5: 268-275. R.L. Stollar &Associates, Inc. 1990. Comprehensive Monitoring Program; Air Quality Data Assessment Report for 1989. Contract Number DAAA15-87-0095. Prepared for U.S. Army Program Manager for Rocky Mountain Arsenal. Commerce City. CO. U.S. Environmental Protection Agency. 1988a. CERCLA Compliance with Other Laws Manual Draft, OSWER DLrective 9234.1-01, August. U.S. Envi:onmental Protection Agency. 1988b. Guidance on Remedial Actions for Contaminated Grcundwater at Superfund Sites. OSWER Directive 9283.1-2. August. U.S. En-.-Lro=ental Protection Agency. 1988c. Guidance for Conducting RemedialInvestigations and Feasibility Studies Under CERCLA. Interim Final. EPA/540/G-89004. October. U.S. Environmental Protection Agency. 1989a. Risk Assessment Guidance for Superfund, Volume 1., Human Health Evaluation Manual (Part A). Interim Final. EPA/540/1-89/002. U.S. Environmental Protection Agency. 1989b Exposure Factors Handbook.Final Report, Office of Health and Environmental Assessment. EPk/600/8-89/043. U.S. Environmental Protection Agency. 1991 a. Record ofDecision forChemical Sales Company, Operable Unit 2. June 27, 1991. U.S. Environmental Protection Agency. 1991b. Record ofDecision for Chemical Sales Company, Operable Unit 3, June 27. 1991. U.S. Environmental ProtectionAgency. 1992. Record of Decision forChemical Sales Company, Operable Unit 2, December 29. 1992. 13-2 Harding Lawson Associates '21905 402010 1107121495 R02Sibliograph Woodward-Clyde. 1991a. Northwest Boundary System Long-term Improvements Interim Response ActionB(ii) Final Assessment Document. June. Woodward-Clyde. 1991b. Proposed Decision Document Northwest Boundary System Rocky Mountain Arsenal Long-term Improvements Interim Response Action. June.

'21905 402010 Harding Lawson Associates 13-3 1107121495 R02

### Table 6.1: Offpost Operable Unit Groundwater Chemicals ofConcern Exposure Point Concentration ("011 Chemicals ofConcern Zone 1 Zone 2 Zone 3 Zone4 Zone 5 Zone 6 Aldrin 0.029* 0.045* 0.050* 0.12* 0.039* 0.030* Arsenic 2.15 1.63 - 2.78* 2.68* --- Atrazine 2.87 5.31* 12.9* 7.36* - 4.48* Benzene 0.61 0.64 0.75 0.93 --- --- Carbon tetrachloride --- 0.76* --- - Chlordane --- 0.18* 0.19* 0.54* -- --- Chloride 120,000 205,000 487,000* 660,000* 262,000* 191,000 Chlorobenzene 1.02 1.78 1.77 4.51 1.09 1.27 Chloroform 0.68 67.5* 5.01 1.51 12.0* 3.33 CPMSCI --- 14.5 10.4 7.68 --- --- CPMSO, 4.35 6.63 5.09 --- --- Dibromochloropropane --- 0.44* 0.14 0.15 0.10 1,2-Dichloroethane --- 0.77* 0.92* 7.32* --- --- Dicyclopentadiene --- 3.64 163* 66.6* --- --- DDi 0.029 0.029 0.22* 0.085 --- --- DDT 0.037 0.033 0.11* 0.10 --- Dichlorobenzene --- 5.1 --- 2.9 --- --- DIMP 63.3* 713* 590* 4950* 7.68 4.67 Dieldrin 0.034* 0.035* 0.21* 0.055* 0.071* 0.039* Dithiane - - 1.97 4.22 --- --- Endrin 0.033 0.037 0.73* 0.058 Eth,.,Ibenzene -- - --- 0.57 --- -- Fluoride 1830 2210* 3510* 3290* 1810 2230* Hexach.lorocyclopentadiene 0.029 0.033 0.044 0.043 0.035 --- Isodrin 0.028 0.035 0.047 0.057 --- 0.040 Nlalati-uon --- 0.26 0.38 0.32 --- --- Nlanganese --- 1580 --- 1250 670 --- Oxathaane --- - 1.32 2.21 --- --- Sulfate 340.000' 636.000* 909.000' 1,118,000* 148,000 213,000 Tetrachioroethene 0.70 10.1 20.7* 6.09* 0.75 1.67 Toluene --- --- 1.28 1.18 --- --- Trichloroethene -- 0.64 0.51 2.70 --- 4.04* Xvlene 0.75 --- --- 1.11 ---
Not a chemical ofconcern in this zone CPMSCI 4-chlorophenylmethyl sulfoxide CPMSCI, 4-chlorophenylmethylsulfone DDE 2,2-bis(p-chlorophenyl)-I,l -dichloroethene DDT 2.2-bis(p-clilorophenyl)-1,1.1 -trichloroethane DIMP Diisopropylmethyl phosphonate Pg/1 Micrograms perfiter Exceeds groundwater containment system remediation goal listed in Tables 7.1, 7.2, and 7.3. All exposure point concentrations represent the upper 95 percent confidence limiton the arithmetic mean of measured concentrations in monitoring and private wells. '21905 402010 

### Harding Lawson Associates 1107121895 R02

### Table 6.2: Of1post Operable Unit Surface-Water Chemicals ofConcern Exposure PointConcentration (ugjj)' Chemicals ofConcem Fftst Creek lnig&tion CAnAl Arsenic 18 NE Chlordane 0.18 NE Chloride 206,000 NE Dicyclopentadiene 10 NE DDE 0.089 NE DDT 0.046 NE Dieldrin 2.6 NE DRvfP 230 20 Fluoride 2550 970 Sulfate 438,000 NE

DDE 2,2-bis(p-chlorophenyl)-1,1-clichloroethene DDT 2,2-bis(p-chlorophenyl)-1,1.1-trichloroethane DIMP Diisopropy1methyl phosphonate NE Chemical not significantly elevated above background levels in the irrigation canals JjS!1 Micrograms per liter All exposure point concentrations represent the upper 95 percent confidence limit on the arithmetic mean ofmeasured concentrations in unfiltered surface-water samples.

'21905 402010 Harding Lawson Associates 1107121495 R02

### Table 6.3: Offpost Operable Unit Sediment Chemicals ofConcern in First Creek Exposure Point Chemicals ofConcern Concentration (mg/kg)a 

### Aldrin 0.011 Dibromochloropropane 0.099 Dieldrin 0.134 Endrin 0.0038 DDE 0.0005 DDT 0.0084

DDE 2,2-bis(p-chlorophenyl)-1,1-clichloroethene DDT 2,2-bis(p-chlorophenvl)-1,1,1-tricWoroethane mg!l-,g Milligrams per kilogram All exposure point concentrations represent the upper 95 percent confidence limit onthe arithmetic mean ofmeasured concentrations in sediment.

'21905 402010 

### Harding Lawson Associates 1107121495 R02

### Table GA: Offlaost Operable Unit Soil Chemicals ofConcern Exposure Point Concentration fm-ARIýChemicals ofConcern Zone 3 Outside Zone 3 

### Aldrin 0.014 0.0021 

### Chlordane 0.049 

### ND Dieldrin 0.112 0.018 

### Endrm 0.032 0.0042 

### DDE 0.024 0.015 

### DDT 0.063 0.030

DDE 2,2-bis(p-chlorophenyl)-1,1-dichloroethene DDT 2.2-bis(p-chlorophenyl)-1,1,1-trichloroethane mg/kg Milligrams per kilogram ND Chlordane not detected in soil outside zone 3 All exposure point concentrations represent the upper 95 percent confidence hmit on the arithmetic mean ofmeasured concentrations in sod.

'21905 402010 

### Harding Lawson Associates 1107121495 R02

### Table 6.5: Summary ofLand-Use Scenarios and Exposure Routes by Zone
Scenario Zone Exposure Routes Quantified Rural residential 1,2,6 Dermal, soil inhalation, groundwater Oral, dairy Oral, eggs Oral, groundwater Oral, meat Oral, soil Oral, vegetables Urban residential 3,4 Dermal, soil Dermal, sediment Dermal, surface water Inhalation, groundwater Oral, groundwater Oral, sediment Oral, soil Oral, vegetables Commercial and industrial 5 Dermal, soil Inhalation, groundwater Oral, groundwater Oral, soil

'21905 402010 

### Harding Lawson Associates 1107121495 R02

### Table 6.6: Reference Doses and Slope Factors for Chemicals ofConcern Noncarcinogenic Carcinogenic Chronic RMD Slope Factor sL (myAxLday)' fmp-/k dayl Chemicals Carcinogenic ofConcern Oral Inhalation Oral Inhalation Weight-of-Evidence Aldrin 3E-5 NE 1.7E+1 1.7E+1 B2 Arsenic 3E-4 NE 1.75 5.0E+1 A Atrazine 5E-3 NE 2.2E-1 NE C Benzene 2E-2 NE 2.9E-2 2.9E-2 A Carbon tetrachloride 7E-4 NE 1.3E-1 5.313-2 B2 Chlordane 6E-5 NE 1.3 1.3 B2 Chloride 7.1 NE NA NA NA Chlorobenzene 2E-2 5E-3 NA NA NA Chloroform 1E-2 NE 6.1E-3 8.iE-2 B2 CP?VlS0 2E-2" NE NA NA NA CPNIS02 

### 2E-2a-b NE NA NA NA Dibromochloropropane 5E-3 5.7E-5 -1.4 2AE-3 B2 Dichloroberizenes (as 1,2-) 9E-2 4E-2 2AE-2 NE C DDE 5E-4 NE 3AE-1 3-4E-1 B2 DDT 5E-4 NE 3AE-1 3-4E-1 B2 1,2-Dichloroethane 7E-2 NE 9.1E-2 9.1E-2 B2 Dicyclopentadiene 3E-2 6E-5 NA NA NA Dieldrin 5E-5 NE 1.6E-i 1.6E+ 1 B2 DIMP 8E-2' NE NA NA NA 1.4-Dittuane 3E-16 NE NA NA NA Endrin 3E-4 NIE NA NA NA Ethvlbenzene iE-1 3E-i NA NA NA Fluoride 6E-2 N-E NA NA NA HexacWorocyclopentadiene 7E-3 NE NA NA NA Isodrin 7E-5' NE NA NA NA Malath.ion 2E-2 NT NA NA NA Manganese 1E-1 1 1 F-4 NA NA NA 1,4-Oxathiane 3E-1' NE NA NA NA Sulfate 1-1E+1 \E NA NA NA Tetrachloroethene 1E-2 IE-2 5 iE-2 1.8E-3 B2 Toluene 2E-1 1-ff-i NA NA NA Trichloroethene 4E-14 4F.1 i iE-2 1.7E-2 B2 Xvlene 2 8.6E-21 NA NA NA Weight ofEvidence Classification

A = Human carcinogen Bi or B2 = Probable human carcinogen. B1 indi::al"s that limited human data are available. B2 indicates sufficient evidence in animals and inadequate orno evidence in humans. C Possible human carcinogen

'21905 402010 

### Harding Lawson Associates I of 2 1107121495 R02Table 6.6 (continued) CPMSO 4-chlorophenvimethyl sulfoxide CPMS02 4-chlorophenylmethyl sulfone DDE 2,2-bis(p-chlorophenyl)-1,1-dichloroethene DDT 2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane DIMP Diisopropylmethvi phosphonate mg/kg/day Milligrams per Idiogram.per day NA Not applicable NE Not established RfD Reference dose a. Derived from scientific literature or obtained from agencies otherthan EPA. b. Subsequent to this assessment, a Region VIII Health Advisory was issued (see letter dated januarv 27, 1994). This Health Advisory has notbeen reviewedbythe other parties. The other parties may provide comments to this Health Advisory in the future. Reference to these values from EPA Region VIII's Health Advisory in this document does not constitute agreement by other parties. The Region VIII Health Advisory values are as follows: 10-Dav Longer-term Child 0.2 nigil 0.02 mg/1 Adult 0.6 ni&I 0-06 mo c. This RFD is taken from the 1989 EPA Health Advisory for DIMP.

2 of2 Harding Lawson Associates '21905 402010 1107121495 R02

### Table 6.7: Summary ofReasonable Maximum Exposure Carcinogenic Risks by Zone and Exposure Route ExposureRoute Exposure Assessment Zone Ingestion Inhalation Dermal Total 1A* 1.1E-4 8.7E-7 1.OE-7 1.1E-4 1B* 1.3E-4 8.7E-7 1-OE-7 1.3E-4 1C* mE-4 8.7E-7 1.OE-7 1.113-4 2 1.6E-4 6.6E-5 1.OE-7 2.3E-4 3 2.5E-4 6.5E-6 1.3E-6 2.613-4 4 2.1E-4 1.OE-5 7.3E-7 2.2E-4 5 2AE-5 3AE-6 6.7E-8 2.7E-5 6 6.9E-5 4.OE-6 1.OE-7 7.3E-5

Zone 1is subdivided onthe basisofthe presence ofsurface water and whetherthe ditch water used for irrigation is collected upstream or downstream ofthe mouth ofFirst Creek.

'21905 402010 

### Harding Lawson Associates 1107121495 R02

### Table 6.8: Summary ofAdult Reasonable Maximum Exposure Noncarcinogenic Hazard Indices byTarqet Organ and Exposure Assessment Zone Exposure Assessment Zone Target Organ 1A 1B ic 2 3 4 5 6 Blood 1.7E-3 1.8E-3 1.7E-3 1.9E-3 2AE-3 2.9E-3 Cardiovasculax 1.6E-2 2.OE-2 1.6E-2 3.8E-2 9.OE-2 5-4E-2 --- 2.5E-2 CNS 2.4E-2 2.6E-2 2.3E-2 8.4E-1 2.4E-1 2.4E+O 6.6E-2 1.6E-3 Gastrointestinal 1.5E-4 3.1E-4 1.5E-4 3.5E-4 4.3E-4 4.2E-4 4.9E-5 --- Hepatic 1.8E-1 2.1E-1 1.8E-1 1.1E+0 1.3E+o q.oE-1 7.2E-2 2.OE-1 Ocular --- --- 3.1E-4 2.8E-4 --- --- Renal 7.OE-3 7.4E-3 7.OE-3 2.3E-1 8.1E-2 1.1E-1 2.OE-2 8.8E-1 RespLratory 2.4E-4 2.4E-4 2.4E-4 2.3E-4 5.8E-4 2.8E-3 Skin 2.OE-1 2.3E-1 2.OE-1 1.7E-1 2.3E-2 3.1E-1 8.7E-2 ---

--- Chemicals forthis target organ not detected in this zone CNS Central nervous system

'21905 402010 

### Harding Lawson Associates 1107121495 R02

### Table 7.1: Containment System Remediation Goals for the Offpost Groundwater Intercept andTreatment System
Rural Residential CSRG Hypothetical Analyte Ujg/l) Source 

## PQLa Cancer Riskb 

### 1,2-Dichloroethane 0.4 CBSG 1.Or 9.1 x 10-1 1,3-Dichlorobenzene 6.5 HBC NA 1,4-Oxathiane 160 HBC NA 10-7 

### Aldrin 0.002 CBSG 0.05d 4.0 x Atrazine 3 MCL, CBSG NA Benzene 3 HBC 2.0x 10" Carbon tetrachloride 0.3 CBSG 0.99d 

### 7.9 x 10-7 

### Chlordane 0.03 CBSG 0.095d 

### 5.7 x 10*7 

### Chlorobenzene 25 HIBC NA Chloroform 6 CBSG 6.4 x 10-6 CPMS 

### 30 HBC NA CPMSO 36 HBC NA CPMS02 36 HBC NA DBCP 0.2 MCL, CBSG 3.8 x 10'6 

### DCPD 46 HBC NA DDE 0.1 CBSG 8.5 x 10-7 

### DDT 0.1 CBSG 4.1 x 10,7 

### Dieldrin 0.002 CBSC 0.05d 

### 1.2 x 104 DIMP 8 CBSC NA Dithiane 18 HBC NA Endrin 0.2 CBSC NA EthN-lbenzene 200 HBC NA Hexachl-orocyclopentadiene 0.23 HBC NA lsod-rin 0.06 HBC NA Nialathion 100 HBC NA N'DN tA 0.007 1*,) 0.033 1.0 X 10-5 

### TeLrachloroethylene 5 MCL. CBSC 4.0 x 104 Toluene 1.000 %1CL.CBSC NA Trichloroethviene 3 fiRC 9.9 X 10,7 

### XvIenes 1.000 lific NA Arseruc 2.35 1 IBC 5.6 x 10" Cli.loride 250.000' CBS(" NA Fluoride 2.000 CBSG NA Sulfate 250.OOV CH.SG NA Totalh 8.8 x 10*'

'21905 402010 

### Harding Lawson Associates 1107121495 R02

### Table 7.1 (continued)

The following chemical have ARARsthat wereadjusted downward to reduce overallrisk: arsenic benzene, chlorobenzene, 1,3-dichlorobenzene, trichloroethylene, and xylene. CBSG Colorado Basic Standards for Groundwater CPMS 4-chlorophemylmethvl sulfide CPMSO 4-chlorophenvlmethyl sulfoxide CPMS02 

### 4-chiorophenvimethyl sulfone CSRG Containment systemremediation goal DBCP Dibromochloropropane DCPD Dicyclopentadiene DDE 2,2-bis(p-chlorophenyl)-1,1-dichloroethene DDT 2.2-bis(p-chlorophenyl)-l .1,1-trichloroethane DIMP Diisopropy1methylphosphonate HBC Health-based criteria NICL Maximum containment level NA Not applicable NDMA N-nitrosodimethylamine PQL Practical quantitation limit .us/, Micrograms per liter a. Practical quantitation limit;presented only when the PQLis greaterthan the CSRG. b. Based on the CSRG. c. PQLlisted in the CBSG standards d. PQL attainable bythe U.S. Army e. Theremediation goal for NDNtA was established at 0.007 parts per trillion (ppt)in the Conceptual RemedyAgreement. The current PQLreadilv available is 0.033 ppt. The estimated risk associated with NI)MA is based on a 70-year residential exposure duration. f. Inorganic standard for chloride wi!I be met bvnatural attenuation consistent withthe onpost remedial action. S. Inorgaruc standard for sulfate may bethe natural background concentration, which will be established and met by natural attenuation consistent with onpostremedial action. h. Because ofthe variability in contaminant distribution and concentration,the maximum risk associated with the groundwater cleanup concentrations is not expected to occur at anyone location.

2 of2 Harding Lawson Associates '21905 402010 1107121495 

### R02

### Table 7.2: Containment System Remedlation Goals forthe North BoundaryContainment System
Rural Residential CSRG Hypothetical Analyte (119/1) Source PQV Cancer Risk' 1,2-Dichloroethane 0.4 CBSG I.Oc 9.1 x 10-1 1,2-Dichloroethylene 70 CBSG NA 1,4-Oxathiane 160 H13C NA Aldrin 0.002 CBSG 0.05d 4.0x 10-' Atrazine 3 MCL CBSG NA Benzene 3 BBC 2.0 x 10'6 Carbon tetrachloride 0.3 CBSG 0 .99d 

### 7.9 x 10-7 

### Chloroform 6 CBSG 6.4 x 10-6 CPMS 30 HBC NA CPMSO 36 HBC NA CPMS02 36 HBC NA DBCP 0.2 MCL CBSG 3.8 x 10-6 

### DCPD 46 HBC NA Dieldrin 0.002 CBSC 0.05d 

### 1.2x 10-6 DIMP 8 CBSC NA Dithiane 18 HBC NA Endrin 0.2 CBSC NA Isodrin 0.06 KBC NA Malath.ion 100 HBC NA - MethvIene chloride 5.0 MCL. CBSC NA NDNL-% 0.007 (e) 0.033 1.0x 10" Tet.rachloroethvlene 5 MCL. CBSC 4.0 x 10-6 

### Toluene 1.000 MCL. CBSC NA Trichloroethvlene 3 HBC 9.9 X 10.7 

### XvIenes 1.000 IIBC NA Arsenic 2.35 IIBC 5.6 x 10-' Chloride 250.00W C.BSC. NA Fluoride 2.000 CBSG NA Sulfate 250.0001 CBS(; NA Totalh 8.0 x 10-'

Nlethvlene chloride is a common laboraton,cn.-.Iiminant and analytical anomalies are anticipated during compliance monitoring. The following chemical have ARARs that wpre at]justed downward to reduce overall risk: arsenic benzene,chlorobenzene, trichloroethylene.,ind xylene.
'21905 402010 

### Harding Lawson Associates 1107121495 R02

### Table 7.2 (continued) CBSG Colorado Basic Standards for Groundwater CPMS 4-chlorophenylmethyl sulfide CPMSO 4-chlorophenylmethyl sulfoxide CPMS02 

### 4-chlorophenylmethyl sulfone CSRG Containment systemremediation goal DBCP Dibromochloropropane DCPD Dicyclopentadiene DIMP Diisopropylmethyl phosphonate HBC Health-based criteria MCL Maximum containmentlevel NA Not applicable NIDMA N-rutrosodimethylamine PQL Practical quantitation limit ilg1l Micrograms per liter a. Practical quantitation limit; presented only when the PQL is greaterthan the CSRG. b. Based on the CSRG c. PQLlisted in the CBSG standards d. PQLattainable bythe U.S. Army e. Theremediation goal forNDMA was established at 0.007 parts pertrillion (ppt)in the Conceptual Remedy Agreement. The current PQL readilyavailable is0.033 ppt. The estimated risk associated witfi NDMA is based on a 70-year residential exposure duration. f. Inorganic standard for chloride -willbe met by natural attenuation consistentwith the onpost remedial action. g. Inorganic standard for sulfate maybe the natural background concentration,whichwill be established and met bv natural attenuation consistent with onpostremedial action. h. Because ofthe variabilitv in contaminant distribution and concentration,the maximum risk associated -withthe groundwater cleanup concentrations is not expected to occur at any one location.

2 of2 Harding Lawson Associates '21905 402010 1107121495 R02

### Table 7.3: Containment System Remediation Goalsforthe Northwest Boundary Containment System Rural Residential CSRG Hypothetical Analyte Rg/l Source POLa Cancer Riskb 

### Chloroform 6 CBSG 6,4x 10-6 

### DIMP 8 CBSG NA Dieldrin 0.002 CBSG 0.050 1.2x 10-6 Endrin 2.0 CBSG NA Isodrin 0.06 HBC NA NDMA 0.007 (d) 0.033 1.0x 10-5 Trichloroethylene 3 HBC 9.9 x 10-7 

### Arsenic 2.35 HBC 5.6x 10-5 Totale 7.5x 10-5 

### The following chemicals have ARARsthatwere adjusted downward to reduce overall risk. arsenicand trichloroethene CBSG Colorado Basic Standards forGroundwater DIMP Diisopropylmethyl phosphonate HBC Health-based criteria MCL Maximum contaminant level NA Not applicable NDMA N-nitrosodimethylarnine PQL Practical quanititation limit j.tg/l Micrograms perliter a. Practical quanititation limit; presented onlywhenthe PQLis greaterthan CSRG. b. Based on onthe CSRG c. PQLattainable by theU.S. Army d. The remediation goal forNDMA was established at0.007 ttg/l (7 partspertrillion (ppt))inthe Conceptual Remedy Agreement. Thecurrent PQLreadily available is 0.033 l.Lg/l (33ppt). The estimated riskassociated with NDMA isbased on a70-year residential exposure duration. e. Because ofthe variability incontaminant distribution and concentration, the maximum risk associated withthe groundwater cleanup concentrations is notexpected to occur atany one location.

21905402010 1107121495 R02 Revised 4/23/01 G9900667Table 7.4: Groundwater Alternatives for the North and Northwest Plume Groups
Rochargo ExIniclions Wolls/1'renchos Roniodiolion Troalmont Wells (tolal onoilior/ Flow Role Tiniefriinio Facility Residuals AlInrualiva' l1rocaq.-i ()pIhmq Ilidmiclionnol (lotal numbor) 14111111longl1i) (gliIII) (years) Location Gonorntool

NOTIII 111mim (;rotili N-1 No aclloii NI(Illiloillig still I-(:, N Norio Norio N/A Unknown NIA Norio Flivillws N-2 Coillilmod o1mratism (if lho NBCS Nll(:s olintalloo FC. N No adilitloital No addillonal 240 15 to 30+ NBCS No additional W illi 11111IFovolikoills as 1111cossary (%4,11 1141111olillo 1'.1111or. catboll ad'I'l IIIloo) N 4 ()fflloýl 0-1.1 .11ill 1,111.111114.111 Call'oo Ailsorlillon R" 5 1;trovichos/15I)o foot 180 15 to 30 T2S, R67W, Spent carbon SYS111111 NII(.S opol,1111111 N 12 300 Sec. 14, NE 1/4 Sec. N. 5 Expalisloll of 1111% 0111,05t IIIIIIII-01.1 Callioll nikorl,11,111 H; 7 10 trenches/ 240 10to 20 T2S, ROM Sprint carbon lid I Svýlmo 2700 foot Sec. 14, a NIICS oliot,01(m N 13 2 Irmichos/600 foot 330 NE 1/4 Sec. Nor I hwost I'limin Grolill NW-1 No aclioo Moniloring silo NW Norio Norio N/A Unknown N/A Norio ravlows NW-2 Con(iound oparallon of tire NW13CS NWBCS operation NW No additional No additional 850 3 to 8 NWBCS No additional Willi IInprovoll1011 Is as iiricossary
M First Crook 81)111 Gallons per minuto N/A Not applicable N Noriliorn NBCS North Botindary Contalieniont System NW Norlliwost NWBCS Nor(liwest Boundary Cooldilluio"t Syste", * All n1tornatives inchido gromidwator monitoring and site reviews.
'21905 402010 1107121495 R02Table 8.1: Summary of the Detailed Analysis and Ranking of Groundwater Alternatives for the Horth Plume Group

Alloritioliva N-2 co-Aiinuail olipernflion if( Iloo Ncirlit Hoostoodatery Allarnistiva N-4 Alternative N-5 Alloruntivoli N- I cionininniont System Willi Offloost Intercept And Expansion 1 to Interim CrIlarla Nip Acllf)n &t% Nocnspinry Tronituant Systant Response Acliun A
Ovorall proloclimi of I'llis alloillativ(W %voillilllill Tl&; alitimativii loriMiliv; 11milod ovorall loioloc- This allornalive reduces polontial risk This alternative rodiices potontial risk I II

I of

Iitf1 1141.1 It II a II(I provillo piroloclion of 1111111nil 11(111 of 1111111all 111talill fillet life$ flilvitiollifloill by Rill] provides prollicilon of both 1111111an still provides protection ofboth human fill, AlIvirollninill lillidill atoll title folivilmlilloild. pinvolitills Illigiallon of Collin11111131115 flout health and ilia environment by remedla- health anti ilia environment by MKIA lie dole IMlifi%l Stiffly Atom n(irth oftitle thig North Mallon Group gromidwalor romedialing North Plunin Groiire NOCS. llointillal risk nssiocialml with and groundwater migrating front RMA to groundwater anti groundwator FtIO411111W.11411' lit Ilin Nonl1i I'limait Gamilp wmild Ilin Offliost Study Arna. migrating from RMA to ilia Offroost (IM-roa%" oval 1111611. Sliidy Area. 011111111.111(ol lVith 1111,6 dil-.111.011v" 1, $fill C.6-iiiii oil pecific ARAIN wootild lie allahind lit ChninicnI-sporicific ARARs wnuld 110 GI- Chomical-specific ARARs wotil(i be ARAM% 4.%l I-I I.,Al 4111,1(11411114114V 15 to :111 1111"1 Vents. As asilinalatl laino(l In approximately 15 to 30 years. Attained In Approximately 10 to 20 yeilipAiii it 1-. Ill. AKARý I.v as estimated by grourldwalor modeling. ars, as estimated by groundwater otiodeling. Linig-torin MI'm live- 1111-t All-111411va in's-11.1 Iva 1111%4110111.111vis %V11111.1 1414111111 146,14111ill THA 'rills n1loornallve woidd rOduCn residual Through treatment. this allernnilve 114154 Allot 1-111. 0 Ilia 1-1,11141 0.11 4-opf 141pol witli Nuilli Pitima Ontilp risk associated with North Plume Group would reduce residual risk Associated is

5-.

1.

" 1.110.1 %. 11Is of ... IlA% .11-f 1111-1161111w4l"t 11V 11111viosill"R groundwalor, throtigh operation of do" with North Plume Group groundwater wilitiAlinot atIlin NIICS Allot c(toillotooliog toichargo Nll('.S slid ilia Olfpost Intercept anti through operation ofIlia NBCS, ilia off Ill'.114,11 Ftollioll.1wallit lit 1111%11 Tr6alment System and Improvolmonts to Offposl Intercept and Treatment lie lhop Mollie ['fifteen Giciiiii. both systems as necessary. System. and file Expansion I system. l1whicilon ofinobil- 'rills allorlinlivo wollIll not 'rills all(Itilillivo wollid radlice I(Imicity, Thl-ough treatment, this alternative Through treatment, this alternative Ity, toxicity, or 01111110Y ally lefoatillout mobility, still volume of groundwater migrating would reduce toxicity, mobility, still would reduce the toxicity, mobility. volumn procoss Options Allot woltill front RMA to Ilia Offposl Study Area. volume ofgroundwater within the North and volume ofgroundwater within the nol rediice toxicity, mobility, Plume Group said groundwater migrating North Plume Group and groundwater or volmno ofgromidwah,r front RMA to ilia Offposl Study Area. migrating from RMA to the Offpwithin (lie North Illume cest Study Area. Grnulo or gioundwalor migrating from RMA to Ilia Offliost Study Area.

'21909 402010 1 of 1107121495 R02Table 8.1 (continued)

Allitrowlive N-2 01111illund ()IOnrilli(In oil thit Notrilt Ilsoustilory Allornalivii N-4 Alternative N-5 Allpritiolivo N- I 0111111iiIIII0111 SY-111311 Willi Offliost Intercept and Expansion. I too Interins Criteria No At lime n%Npcot%siory Trovillin"DI System Response Action A Sholl-torin Offoclivoo- 1141callsil Ito 14,111(illial ;it lifill %voidil Im titi shmi-itirot Impacts bocalis" C0111111111111y and workors wore protected Community and workers would be pro11OSS WO%11(1 bn polfolinfid. 111olo Iliff NOCS 1,;alinativ (iiiiiialitig. Thoro would by adhering to standard health and locled during construction through woldil Im 114) "11(ill Wrill III) Ito iIIIIII(IIII"Illilliou polioll. safety practices. The Implementation adhering to standard health and safety Impacts. There would 1111 Ito porlool Is coinpliflo and the sysiont is practices. The Implementation period hoplonlolitatioll 111111o.l. fully operational. would be approximately 14 months. 11111114.111flolabilliv '141,1111ir'll fol.1sillilliv %V"111.1 This n1fortintivo ii roashly inijilmotaiiiahlo. TLIS alternative Is readily This alternative Is readily I... Itifth Iho itivo '14whilical aild .111111illklialivo folasibillty woul4l I Inp101110111 abio. Technical and imploutentablo. However, Ilia f".1'Ohilliv %Volilil1.0 110%v. 110 111811. adukinisli-1111VO feasibility would be high. construction would be conductod it) Iwo time perlods due to the design phass far the expansion. Technical atid administrative feasibility wmild be high. I"SlItIvilod cOft 11 Tiptal Capilal Cost = S -D. Total Capital Cost = $16.7 million 'rotal capital cost - $19.4 million uoog two, ()&M Total Long-term O&M Cost - $30.6 to 32.5 Total Long-term O&M Cost = $39.8 to Total Long-term O&M Cost Cost = $4.1 to (HI milhou million 46.4 million $36.9 to 43.6 million 'rotal Ptosent Worth Total Prosoul Worth Total Present Worth Total Present Worth Cost = $4.1 to 6.0 milliou Cost = $30.6 to 32.5 million Cost = $56.5 to 63.1 million Cost = S.96.2 to 63 millinn

ARAR Applicable or relevant and appropriate requirement NBCS North Boundary Containment SYSIOIII 0&M Operation and maintenance RMA Rocky Mountain Arsenal

'21905 402010 2 of 2 1107121495 R02Table 8.2: Summary of the Detailed Analysis and Ranking of Groundwater Alternatives for the Northwest Plume Group

Allarunfivoi NW-2 Continued Oporalitin (ifthe Northwest AlIntitalivo N%V-I 1114oundary Ciontninniont System With Crilorin Nov Actitin Improvements as Necassnry I M niall Pr(Ilecti(pli rof 1111111all 1111,11111This .1111orlialivoi %voillil loot liToV1110 prolection of This allooriiative would provide protection ofhuman healthand Iliaanviand Ibe 1-1,11VII0111110111 Immaii health aitil 0sn 4invirimmmil. riviiiiiiiiii by preventing migration ofcoutnuilnatitsfrom RMA to theOffpost Sludy Area north ofilia NWHCS. Poloinlini risksassociate(] with iliaNorthwest I'linnoGroup groundwater would be substantially reduced through coidbitiod operationofIlia NWB(;S and finprovotmovids as necessary. Coempli.m(p Mob ARAR-t 'I111,ollolljoilve 1,ejoii w4pa,Intl too sublown TIM allarlinliviv IsOX1100041 to innal or exceed chnnilcal-specific ARARs In

.lwiow.ol qlwwiflo-ARAR-%. approxinialoly flegs toeight years, asost!uialod bygroundwater modelltie.

F114" .111.1 11.1, AllefetAllve W40061.1 11111 11111114 a lite sn.14111al This alternative would reduce residual risk associated wilhgroundwater 114-01.01 It.&4-ft 1.01 ...

Iwilh 114DIO-1111.11 ItI41411141walell OXIHI- williles iliaNc)rlltwosi I'linnot Gump throtigh preventing conlantinant migration nt titsNWBCS and recharging Iroolod gromadwalor to Rush conlainhianig Its Ilm Northwest I'luato Group. 16(11101011 f)f 111vil. liv. 11.1-411-1 16.111vto S."0111.1 11411 101111DIVIV.111V Ign.111110111 This alternative would reduce toxicity, mobility,and volumeorgroundwater

V4111111141 1-1- 0'. 411alloplis 41141 w(pilld lifol r1141114-0ilia migrating from RMA toIliaOffliost Study Area. Groundwater conlaudsiont JIV. V1111111141 lifFtI41111146.11tif concentrations would be reduced within ilia Northwest Plume Group by wilhinlite Noilliwesl I'llinin (4111111 (ifItill"Itil- flushing provided by recharge oftreated water at liteNWBCS. waint sisilltalhig fitim KNIA to IliaOffilost Study Alan. Shorl-lorui E.11roiclivoinoss Because no remedial action wotild be performed. Therewould beno short-term luipacls. There would heno Implementation there would be no shriti-Intin Impacts. There period. would be no Implementation parind. luiplonientability Thetechnical feasibility would hehigh. The This alternative Isreadily implemontablov. Technical and administrative administrative feasibility would be low. feasibility would behigh. Estimated cost Total Capital Cost = $-0- Total CaplialCost - $-0- Total Long-term O&M Cost =$0.6 to 1.3 million Total Lmig-torm O&MCost =$12.4 to 13.1 million Total F'Fosont Worth Cost =$0.6 to 1.3 million Total Present Worth Cost = $12.4 to 13.1 million

'21905 402010 1of2 1107121495 R02Table 8.2 (continued)

ARAR Applicaldo 1), ....... . .. NWIICS NoillmwA hollo-1-11 %, C-11-oll-11-1-1 (Am ()IIIII-al loll aI ol lo. RMA Rocky Momilalo Aiýno.sl

'21905 402010 2 of2 1107121495 R02

### Table 9.1: Estimated Costs ofthe offpost Operable Unit Selected Remedy Cost'Component Alternative N-4 Alternative NW-2' Capital Costs Monitoring well system $ 908,000 NA Offpost Intercept andTreatment 4,593,000 NA System extractionJrecharge system 4,106,000 NA Treatment facility Startup costs 341,000 NA Indirect costs 6.715.000 NA Total estimated capital costs $16,663,000 $0 Annual Operation and Maintenance Costs $ 352.000 $ 134,000 Groundwater monitoring 150,000 150,000 Site reviews North and northwest boundary system operations 1,724,000 769,000 Offpost Intercept and Treatment 522.000 NA System facility O&M Offpost Intercept and Treatment System carbon replacementb 0 to 315 vears 817.000 NA 3/5 vears to systemshutdown 227.000 NA Total estimated Annual O&M Costs 0to 3/5 vears S 4.618.000 3,15 vears to svstem shutdown S 4.028.000 $ 1,053,000 Nonconservative' Conservative' Total remedv costs S68.911,000 $76,143,000 Dl1,,1P Dusopropylinethylphosphonate NA Not applicable O&M Operation and maintenance a. There are no capital costs forAlternative %NV-2 because the remedial systems are currently operational. b. The carbon usage rate is assumed to (i.,'.rvas- %*,,rtime ds a result ofexpected decreases in influent DINIP concentration. Th--duralr,n tJ It.ris-bofore adecrease in carbon usage rate is expected to occur witiun three t(, ti,-- C. A range of total costs has been -islimai---l ,ii 1b. basis of the range ofexpected remediation. tirueframes as estimated bythe Sroun-i%%-.i1--r ruo-W r'-sults.

'21905 402010 

### Harding Lawson Associates 1107121495 R02Table 10.1: Summary Evaluation ofChemical-specifIc and Other Applicable or Relevant and Appropriate Requirements for the Off post Operable Unit

Applicable/ Relovank and Slandliril, Roiluiraninot Appropriate Critnria, or Hinihilloo D"Scriplion Requirement Comment Ch"Inical-spocific AKAK% Safn Dtinkitig Walsit Act 411 CFI( 1-:,iablishos lotlotary M(.I.s for public walor- No/Yos Grousidwalor fit the vicinity ofilia site Pail 141 Is holng used or may he used as a source of water far puhlic water sysioul or privalo supply wells. Therefore, those pritnary MCI.s that are inore stringont than ilia Colorado Primary Drinklug Water Rogulations (hecause, Colorado has primary ouforcomont authority) are rolevald and appropriate. 411( If( l-'%talillhos (iionntifutconhle health guals) No[Yes Gromidwalor fit ilia vichilty ofilia silo S... 141 '-it im ImIslic %valo,r svshimi. Is heing used or may he used as a A11-1 14 1 .1 source of water for n public water system or private supply wells. Thereforo, fit accordance with the NCP, nonzoro MCLG9 are considered to he relevant and appropriate. other AKARs Colorado Basic Standards 5CGR 1002-8 I-Istalillshes slatowido standards for waters ofilia Yos/No State standards that are more siringoat for Gromidwater: Soclimi 3.11.0 at soq-1. state. than federal slandards are considorod Colorado Basic Standards Socilon 3.1.0 at so(l. applicablo. and Molhodologios for Surface Water

'21905 402010 1107121495 R02Table 10.2: Summary Evaluation of Action-specific Applicable or Relevant and Appropriate Requirements for the Offpost Operable Unit
Applicable/ Rolovoill and AppropFlolo Adion-spocific Siondoril, Roydrooov,"I cirlinrlit. or I-loolhill--n 4 iWom IJP%oriplion Comment
Federal ARARq Safo Drinking Wiloi Art 42 1ISC Sol lillivi 111111i In :111111, 7 loloOlost 411 CFR PaI1 14.1lo 147 Foislillsheit ilidudards (fir construction and Yos/No Applicable Ifreinjection wells/Ironclios are Coultul Kiiiiiiintion-i III it( 1110".111111 will I S/l gonc. bos usod for discliargn oftreated water: rolovai.1 and appropriate if some ollier ineiliod of reinjection is used. IIndor tbo provisions of40 CFR 144.13(l.). EPA lins doloruilned that Ilia reinjection wolls/itenclios used In conjunction wilb Ilia barrier Irealmen( system do not endanger underground sources ofdrinking walor. Tlio level oftreatment prior to reinjection, offliost alinirnativo water supplies, and otbor remedies are sufficient to meal ilia requirements ofIlia UIC program. Colfitailo Air Quailly CHS Sni-Iiiiii,; 25-7-101 In Slall(InFds 25-7-800

Wor Emission Colorado Ali Qiiallty Sots IIIIIIIS oil emission ofodorous air Yos/No Applicable to remedial action for tbe Offpost Rogulallons Control Rogidallon No. 2 contaminants 0U.

ARAR Applicable or relevant and appropriate requirement CFR Code of Foderal Regulations CRS Colorado Revised Statues oti Operable unit EPA U.S. Environmental Protection Agoncy UIC . USC United States Code voc Volatile organic compound
'21905 402010 1107121495 R02Table 10.3: Summary Evaluation of Location-specific Applicable or Relevant and Appropriate Requirements for the Offpost Operable Unit
Applicnidn/ Relovant and Appropriula Location-specific Slandord, Rniluirmoont Roquiromouts Crilorlit, fir I.Indlislion Citalifill Description Comment Foilaral ARARx EXIOCIIING 01,10F IORR 411 CFH I'aII11. D114" 1,4 1`4111111,11 agnitclosto avold long- ofshort- Yes/No Requires a Soo-year floodplain to beIdenliflod Flood P1.011 1`61.111.1g"1110111 Alplitiodix A l"1111 11111141 I'sOýV)4.11116641 wilh of-1:11111111cy nod and considorod In scoping any remedial 08,40.11114 4111111 fir 4 (ICH1411114ill. ad lons.

F110" Illiva 01016.1 1Prul 411 CIN hliwini7ws ilip dostioction. Ifiss. or dogradailoii of Yos/No Roquirninoos associated with this order 1'4et If.Apl-e..hs A wotild ho applicabla to niq remedial actions that cmild affod lho exisliog wellands.

ARAR Applicallin for folow-lol .111-1 CFR Codel of 1`4116141 Holtol-Ili"11%

'21905 402010 1107121495 R028 

### Explanation Offpost StudyArea Offpost Operable Unit

............

........... .

.............. 

# P110

........... -"'NorthBoundary

containment System NorthwestBounder ContainmentSystem Onpost Operable

nit Mfl Ave. Irondal* containment system EPA Study Rocky Mountain Area Arsenal CoffrMceCey RWA-Wo-u;ýda;;

owww 

### 70 ScW* in Stapleton International Airport

Figure 1.1 Rocky Mountain Arsenal Operable Units and Offpost Study AreaNorth Boundary ContainmentSystem 8

-- -- -- - - -- -- -- -- - - -- -- --
Northwest Boundary .22 North Bog Containrnent S am 23 /24 19 20 0, lmndaie Containment North Plants 25 System 27 29 //28

. . ............ 36 31 32 33 34 35

A I

4 3 6 5 If RWI Lake II ciassirication Lax@ ;:4ý t.. JL Malyll _J Yard SunedLake Buned Lake ow., ludgeArea Sluage Area ýfj7ý Owby Rodand GunClub 9 10 12 Poncr 7

-- -- - - - - --- . .. .. .. Explanation MaptpotentW contaminant source area Sanstary sewer system

...... Chemocai sewer system 0 0. 1.0 Scale in miles

Figure 2.1 Locations ofContaminant Source Areas8 

### Barr Lake N st 

### =dya 0 1 2 ScWe inmHes
First Creek Pakmhannel Cc? Northern Paleochannei rthwest Pakiochannel ýh Boundary Containment System Northwest Bounds Containment System 

### Onpost Study 80thAve. Irondale Area Containment System Explanation Groundwaler flow cfrecbm E]Northwest plume group North plume group

Rgure 5.1 Contaminant Migration Pathways

### 0 3500 7000 Scaleinteet % %

%

%Sul. 109

North Boundary Containment System Northwest % Boundary Containment System

%P- Q ExpLarurtion Zone I ;kosw tewcoma. iand use Zone 2 Rurw resecwcai Land use Zone 3 urimn resadiencaj land use Zone 4 Urban residential land use Zone 5 Commercial and industrial Land use Zone 6 Rural residential land use OtfPost Study area boundary Figure 6.1 Otipost Study Area Exposure Assessment Zones

### Primary Primary Primary Secondury Secondary Secondary Medium Receptor Sources Release Transport Sources Release Transport Mechanism Processes Mechanism Processes Exposure Residents CommercialHouto Child Adult Industrial

Particular** Dispersion NO NO NO Ale and/or lion Inhalation Dogrodo Vapors I ollpoel Deposition Sell Ingestion a 0 0
AMA Depoollion sell Degradation Dermal 0 Sail Blois Ingestion 0 0 NO Vagelables Ingestion NO NO NQ

Blois I.Wealso 30 slocon- E got Ingestion 0 a NO cooliallon Ingestion a a NQ Beal Ingestion 0 a NO Lagoons Wildids Ingestion NO NO NO Ingestion a Q Dispersion c1loundwolor Dermal NO NO NO Irrigation lailliestion 4E ý ] Degradation 10 Inhalation a 0 t 0lioundwolov Dispersion Ground- Discharge Ingestion NO NO NO surface Wale( to Surface Sorption jýý Sorption WellIF Water Degradation Degradation Dermal Q a NO Deposition AMA Aunoll Rosusponsion Inge tion Cl a NO Surface Wale, Deposill:1 III i :ýH JA

- NO and Sedinve., W.I., 3111 Dermal A p 5,19nil Su"4::d

-- qS.rroaurA sorption Sediment -AU-0,fj a :n
EXPLANATION a Quantified NO Not quantilied Figure 6.2 Site Conceptual Model Offpost Study AreaSecondary Secondary Secondary Exposure Exposure Receptors Quantified? Source Release Transport Media Routes Mechanism Process Ingestion 11-- NJ Livestock Yes From Ollsile Groundwal Pumping Groundwater Flow, Disc a(ge Sorption Onpost Groundwater to Surface Water Plant Uptake Crops Yes Ingestion L1, tock Yes Fugitive Cr sj From Ollsile -)o Deposition Soil Dermal Crops No Dust ;Ir Wi Onpost Soils Entrainmeni Inhalation erresiriaLlWildlif No Ingestion Livestock Yes From 

### Ollsile Oeposillon]H I Plant 0-plake Crops Yes 1110 Surface Wale Yes Onpost -- )0 Surlace Wale( Runoll Sorption Direct Contact Aauallc Life Sediment Ingestion TerrestrialWildlife Yes Ingestion AquaticLife Yes Sediment De mal errestrialWildlife Yes L Biola Terrestrial Wildlile Yes

"Plants Ingestion "Animals 0 LAquatic Life Yes



### Figure 6.3 Ecological Site Conceptual Model forthe011post StudyArea.V RC-111 4; 0 'CNE4 RC-24 NE-121 . ..... ... Norftm Paleochannel -J, p N, N :'First Creek Paleochannell E-3 -4A A FE-2 T., ý-ýTreatment Facdi ty T-6 FE-S .&FF-.i T-3 T

N ,-ýNortll Boundary Contmnment System Explanation Otfpost intercept and Treatment System extracton well Oftpost Intercept andTreatment System recharge well Offpost intercept and Treatmen: System recharge trench Pipeline Unsaturated alluvium 0 ISOO M Scaw in lost

Figure 7.1 Aftemative N-4 Remedial System ComponentsC

.....

...

..... . .......

:IX AP-3 AT-6 I AT-5 Northern Paleochannel V

........... -

1---.-;ýirst Creek PaleochanneU TreatrnentFacility AT-4 Ir AT-3 AT-2 AP-1

N
,, ýN.M Bounaary Containment System Explanation M Additional extraction well unoe, Alternative N-5 Additonal recharge trencn ur-oef Afternative N-5 Otfpost Intercept and Treat-nent Sys-zern extraction well Offpost Intercept and Treanent Systern recharge well Offpost Intercept anc Treatment System recharge trench 0 1500 Pipeline SaMe in 1"t Unsaturated allu%nurn
Figure 7.2 Afterrialive N-5 Remedial System Components

## SHELL OFTPOST PROPERTIES VEGETAUON CLASSHICATION with 1"5EMMATED SURFACE SOM DIELDRINDlSrRUlUTloN ými 21850M 

## NafivePeftIUtial ýaUland 1 2ý 

## Wetland TmeGmve ýeckUýea HomUWadSOW 13 Baý dOrý UWJatiUfiUl SM[ ýýIllcs 

## Die[IkcOI>-.Wug/g(1"5) ýItW Bý
1616`ý ýJ,U IOU ýFWIUm 224 49'Ol U6,jUsI T.c;,,OU 476 ToiwIIwstMU 4LOP 1995 BOMA UIUUUIUd 

## MOHWNINUDO COMMON hiýmw ýicmWpm

SCý 1 7," 0 I= IOU Figurg 9.1 Offpost AfimI of RevegMation