Rocky Mountain Arsenal

Offpost Operable Unit

Final Record of Decision

Rocky Mountain Arsenal

Commerce City, Colorado

Prepared for

Program Manager for Rocky Mountain Amenal

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 THE ADMINISTRATIVE RECORD FOR TIES

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

Recycled Paper
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 Protection Agency Study Area .................. 2-2

2.2.2 U.S. Department of the Army Investigation ....................... 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 Contam ination ............................................ 5-1

5.2 Nature of Contam ination ............................................ 5-1-

5.3 Contamination Migration Pathways .................................... 5-2

5.4 Extent of Contam ination .............................................. 5-3

5.4.1 Groundw ater .............................................. 5-3

5.4.2 Surface W ater ............................................. 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 of Human and Environmental Exposure .................... 5-7

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6.0 SUMMARY OF SrM RISKS ................................. 6-1

6.1 Human Health Risks .......................... 6-2

6.1.1 Identification of Chemicals of Concern .......................... 6-2

6.1.2 Exposure Assessment ...................... ... 6-3

6.1.2.1 Offpost Study Area 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 of Chemical Intake .......................... 6-8

6.1.3 Toxicity Assessment .......................... ... 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 of Potential Ecological Effects ............................... 6-11

6.2.1 M ethod ................................................. 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 of Alternatives ..................................... 7-4

7.2 identification of Groundwater Alternatives: North Plume Group .............. 7-6

7.2.1 Alternative N-1: No Action ................................... 7-6

7.2.2 Alternative N-2: Continued Operation of the North

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 of the Offpost Groundwater

Intercept and Treatment System .............................. 7-10

7.3 identification of Groundwater Alternatives: Northwest Plume Group .......... 7-12

7.3.1 Alternative N-1: No Action .................................. 7-12

7.3.2 Alternative NW-2: Continued Operation of the Northwest

Boundary Containment System With Improvements as Necessary ..... 7-13

8.o COMPARATIVE ANALYSIS OFALTERNATWES ............................... 8-1

8.1 Comparative Analysis of Remedial Alternatives ........................... 8-2

8.1.1 Overall Protection of Human 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 in Toxicity. Mobility. or Volume ....................... 8-6

8.1.5 Short-term Effectiveness ..................................... 8-6

8.1.6 Im plementability ........................................... 8-7

8.1.7 Cost .................................................... 8-9

8.1.8 State Acceptance .......................................... 8-10

8.1.9 Community Acceptance ..................................... 8-11

8.2 Conclusions of the Comparative Analysis of Alternatives ................... 8-11

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9.o IDENTIFICATION OF THE SELECT REMEDY ............................... 9-1

9.1 Alternative N-4: Offpost Groundwater Intercept and Treatment System ......... 9-1

9.2 Alternative NW-2: Continued Operation of the Northwest Boundary

Containment System with Improvements as Necessary ...................... 9-3

9.3 Additional Components of the Selected Remedy ........................... 9-5

9.4 Cost of Selected Remedy ............................................ 9-6

9.5 Lim itations ....................................................... 9-6

9.6 Criteria for Shutting Down Boundary and Offpost. Containment Groundwater

System s ......................................................... 9-7

10.0 STATUTORY DETERMINATIONS ......................................... 10-1

10.1 Consistency with the Statutory Requirements of CERCLA in Section 121 ....... 10-1

10.1.1 Protection of Hinnan. Health and the 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 cf Permanent Solutions to the Maximum Extent Practicable . 10-4

10.2 Consistency with the National Contingency Plan ......................... 10-4

10.3 Sum m ary ....................................................... 10-5

11.0 DOCUMENTATION OF SIGNIFICANT CHANGES ............................. 11-1

12.0 G LO SSARY .......................................................... 12-1

13.0 BIBU OGRALPHY ....................................................... 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 of Concern

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 of the Detailed Analysis and Ranking of Groundwater Alternatives for the North

Plume Group

8.2 Summary of the Detailed Analysis and Ranking of Groundwater Alternatives for the

Northwest Plume Group

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9.1 Estimated Costs of the Offpost Operable Unit Selected Remedy

10.1 Summary Evaluation of Chemical-specific and Other Applicable -or Relevant and Appropriate

Requirements for the Offpost Operable Unit

10.2 Summary Evaluation of Action-specific Applicable or Relevant and Appropriate Requirements

for the Offpo'st Operable Unit

10.3 Summary Evaluation of Location-specificApplicable or Relevant and Appropriate

Requirements for the Oftpost Operable Unit

FIGURES

1.1 Rocky Mountain 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 of Revegetation

APPENDIXES

A RESPONSWENESS SUMMARY

B INSTITUTIONAL CONTROLS

C WELL CLOSURE CRITERIA

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DECLARATION STATEMENT

Rocky Mountain Arsenal

Offpost Operable Unit

Commerce City, Colorado
DECLARATION FOR THE RECORD OF DECISION

SITE NAME AND LOCATION

Rocky Mountain Arsenal

Offpost Operable Unit

Commerce City, Adams County, Colorado

STATEMENT OF BASIS AND PURPOSE

This decision document presents the selected remedial action for the Rocky Mountain Arsenal (RMA)

Offpost Operable Unit (OU) in southern Adams County, east of Commerce City, Colorado, chosen in

accordance with the Comprehensive Environmental Response, Compensation, and Uability Act

(CERCLA) of 19 80, as amended by the Superfund Amendments and Reauthorization Act (SARA) of

1986, and the National Environmental Policy Act (NEPA), and, to the extent practicable, the National

Oil and Hazardous Substances Pollution Contingency Plan (NCP). This decision is based on the

administrative record file for the Offpo--t OU. and this document explains the basis and purpose of

the selected remedy for the Offpost OU.

ASSESSMENT OF THE SITE

The Offpost Study Area risk assessment showed that even without remedial action, the baseline

cumulative cancer risks from contamination in surface water, soil, sediment, air, and groundwater are

within the acceptable cancer risk range established by the 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

2 x 10'4. or approximately 75 percent of the total carcmogenic risk, (2) maximum contaminant levels

(MCLs). maximum contaminant level goals (NICLCs). and Colorado Basic Standards for Groundwater

(CBSGs) are exceeded for some groundwater contaminants. and (3) hazard indices (HIs) for children

exceed 1.0 in Zones 2, 3. and 4. Although the haziud indices exceed 1.0 in Zones 2, 3, and 4, the

bulk of the HI value is contributed through an assumed domestic use of alluvial groundwater, which

is not presently occurring and under tlus remedy is not intended to occur in the future. The elevated

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Declaration for the Record of Decision

Ms occur only when considering the contribution of groundwater. Therefore, groundwater contamination is the focus of this decision document.

Actual or threatened releases of hazardous 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.

DESCRIPTION OF THE REMEDY

The Offpost OU is one of two OUs at RMA. The Onpost OU addresses the contamination within the

27 square miles of RMA. The Offpost OU addresses groundwater contamination north of RMA that

migrated (1) before the RMA boundary groundwater extraction and treatment systems were installed,

and (2) around the boundary systems prior to recent improvements. The selected remedy described

in this Record of Decision (ROD) will permanently address contaminants at the site through

treatment to reduce the toxicity, mobility, or volume of contaminants. Groundwater containment

system remediation goals are based on the risk assessment and on federal Safe Drinking Water Act

MCLs, proposed MCLs, nonzero MCLGs, and CBSGs. Action levels also meet those state drinking

water standards found to be applicable or relevant and appropriate requirements (ARARs).

The major components of the selected remedy are as follows:

Operation of the Offpost Groundwater Intercept and Treatment System

Removal of contaminated groundwater from the alluvial and the weathered upper

portion of the Denver Formation J hereafter called the unconfined flow system [UFSI)

north of the RMA boundary Ln the First Creek and northern paleochannels using

groundwater extraction wells

Treatment of the organic chemicals of concern (CoCs) present in the groundwater

using carbon adsorption

Recharge of treated groundwater to the UFS using wells and trenches

Natural attenuation of inorganic chloride and sulfate concentrations to meet applicable

standards for groundwater in a manner consistent with the Onpost remedial action

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Declaration for the Record of Decision

Continued operation of the North Boundary Containment System (NBCS) and the Northwest

Boundary Containment System (NWBCS) - In addition, the Irondale Contaminant System

(ICS) will continue to operate, as required, for onpost contaminants consistent with the

Irondale Interim Response Action (IRA). These containment systems will be operated to the

requirements of Section 2.7 of the FFA, the Agreement for a Conceptual Remedy for the

Cleanup of the Rocky Mountain Arsenal (Conceptual Remedy Agreement), and the onpost

ROD, when it is signed. Cessation may occur as provided in Sections 35.3 and 35.4 of the

FFA and paragraph 20 of the Conceptual Remedy Agreement.

0 Improvements to the NBCS, NWBCS, ICS, and the Offpost Groundwater Intercept and

Treatment System as necessary

0 Long-term groundwater monitoring (including monitoring after groundwater treatment has

ceased to assure continued compliance with the groundwater containment system remediation goals )

& Five-year site reviews

0 Exposure control/provision of alternate water supply as follows:

As of the date of the Onpost ROD, and based on a .392 parts per billion (ppb) detection limit, the U.S. Army will use the last available quarterly monitoring results to

determine the DIMP plume footprint

As part of the Onpost ROD, the U.S. Army and Shell Oil company will pay for the

extension of, and hook-up to, the current distribution system for all existing well

owners within the DR-AP plume footprint referenced above.

Existing domestic well owners outside of the DlMP plume footprint as of the date of

the Onpost ROD where it is later determined that levels of DR" are eight ppb or

greater (or other relevant CBSG at the time) will be hooked up at the U.S. Army and

Shell Oil Company's expense to the SACWSD distribution system or provided a deep

well or other permanent solution.

For new domestic wells with DMIP 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 to the 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 denved from RMA at concentrations that exceed the greater of

the remediation goals in Tables 7.1 through 7.3 or the ARARs in Table im win be

provided an altemative water supply. Bottled water will be 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 of wells that are lawfully drilled in

the future.

Institutional controls to prevent the use of groundwater exceeding remediation goals.

Closure of poorly constructed wells within the Offpost Study Area that could be acting as

migration pathways for contaminants found in the Arapahoe Aquifer.

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Declaration for the Record of Decision

The U.S. Army and Shell Oil Company agree to continue monitoring and to complete an

assessment of the NDMA plume by June 13, 1996, using a 20 ppt method detection limit

The U.S. Army and Shell Oil Company agree to prepare a feasibility study of potential

actions, both onpost and at the boundary, or adjacent to the boundary in order to achieve

NDMA remediation goals at the RMA boundary and to use 7.0 ppt PRG or a certified

alytical detection level readily available at a certified commercial laboratory (currently

33 ppt).

The U.S. Army and Shell Oil Company agree to revegetate approximately IL60 acres located in

the southeast portion of Section 14 and the southwest portion of Section 13 as depicted in

Figure 9.1. Revegetation will involve tilling and seeding. No sampling will be conducted

before or after revegetation. Existing soil. risks in the are to be revegetated fall within EPA;s

establish acceptable risk range and revegetation is not necessary. However, the U.S. Army

and Shell Oil Company agree to the revegetation program as part of the offpost settlement.

The Army will treat any contaminated extracted groundwater prior to 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 part of the Onpost remedy, the U.S. Army and Shell Oil Company will pay for and

provide, or arrange for the provision, of 4000 acre-feet of water to SACWSD.

STATUTORY DETERNINATIONS

The selected remedy is protective of human health and the environment, complies with federal and

state requirements that are legally applicable or relevant and appropriate to the remedial action, and

is cost-effective. The remedy uses permanent solutions and alternative treatment technologies to the

maximum extent practicable. The remedy satisfies the statutory preference for remedies that employ

treatment that reduces toxicity, mobility, or volume as a principal element

Because this remedy may result in hazardous substances remaining in the groundwater of the Offpost

OU for more than five years, a review will be conducted within five years after commencementof

remedial action to ensure that the remedy contmues to adequately protect human health and the

environment.

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Declaration for the Record of Decision

SIGNATURE PACE

am P. Yellowtau

/Re)kional Mministrator, Region VM

(/1111ited States Environmental Protection Agency

Raymb'aFatz

Acting Deputy Assistant Secretary of the Army

(Environment, Safety and Occupational Health)

STATE OF COLORADO CONCURRENCE

Tom Loob

Director, Office of Environmzý4

Colorado Department of Public Health and Environment

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DECISION SUMMARY FOR THE RECORD OF DECISION
1.0 SITE NAME, LOCATION, AND DESCRIPTION

The Rocky Mountain Arsenal (RMA) National Priorities List (NPL) site is comprised of two Operable

Units (OUs): Onpost and Offpost. As shown in Figure 1.1, the Offpost Study Area occupies approximately 27 square miles in southern Adams County, Colorado, and lies north of the Denver metropolitan area and east of Commerce City, Colorado. The Offpost Study Area is defined as the area

southeast of the South Platte River, north of 80th Avenue, southwest of Second Creek, and north of

the north and northwest boundaries of RMA. Additionally, the Offpost Study Area includes the

surface waters of O'Brian Canal and Burlington Ditch as they extend northeast from Second Creek to

Barr Lake and the surface waters of First Creek and Barr Lake. The Offpost OU (also shown in

Figure 1.1) is defined by the RMA Federal Facility Agreement (FFA) as that portion of the Offpost

Study Area where hazardous substances, pollutants, or contaminants from RMA are found and are

subject to remediation. The Offpost OU encompasses rural residential, agricultural, and commercial

and industrial areas located north and northwest of RMA.

Areas within the Offpost OU are used for rangeland, dryland farming, and irrigated farming with

some rural residential areas and scattered areas of intensive agricultuxal use. Parts of the Offpost OU

are currently zoned and developed for commercial/industrial activities. Commerce City, located west

of RMA, is the only urban area in the immediate vicinity of the Offpost OU and has recently ann exed

lands within the Offpost OU.

On the basis of an evaluation of planning information provided by the Adams County Planning

Commission, it is projected that areas of commercial. industrial, and urban 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 livestock production land

use, including hobby farming as discussed in the Airport Environs Plan (Adams County, City of

Aurora, City of Brighton, City of Commerce City, 1990).

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Site Harn*, Locationg and Description

1.1 Environmental Setting

The topography of the Offpost Study Area consists of stream-valley lowlands separated by gently

rolling uplands. The maximum local topographic relief in 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 of RMA 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 the bald eagle. The climate of the Offpost Study Area is characterized by sunny, semiarid

conditions.

The regional surface drainage is to the northwest toward the South Platte River. Suxface water

originating south of RMA, 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 where the 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 of unconsolidated alluvial and eohan

deposits. The composition of the 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 of interbedded shale, claystone, siltstone, and

sandstone, with a regional dip of 1/2 to 1 degree to the southeast. The presence of paleoch;;nnels in

the Denver Formation surface impacts groundwater flow in the unconsolidated sediment and the

upper weathered portion of the Denver Formation. Three such paleochannels, the First Creek,

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Site 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 of 230 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 of 400 to 700 feet of interbedded conglomerate, sandstone, siltstone, and shale.

The upper portion of the Arapahoe Formation consists predominantly of 200 to 300 feet of blue 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 claystone unit 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 t wo 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

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Site Ham*, Location, and Description

Arapahoe Formation. Hydraulically, the two units generally behave as two distinct hydrostratigraphic units: the unconfined flow system (UFS) and the confined flow system (CFS).

The UFS includes groundwater present in the unconsolidated materials overlying the Denver

Formation, the weathered upper portion of the Denver Formation, and, where the Denver Formation

is missing near the South Platte River, the weathered upper portion of the Arapahoe Formation. The

CFS includes the deeper portions of the Denver Formation and the underlying Arapahoe Formation.

On the basis of an evaluation of the distribution of contaminant plumes in the Offpost Study Area,

the UFS is considered the principal migration route for groundwater contaminants from RMA to the

Offpost Study Area, although some contaminant are present in the CFS. Although low-level

contamination may be present in isolated portions of the Denver Formation CFS, this formation has

low productivity as a groundwater resource.

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2.0 SITE HISTORY AND ENFORCEMENT ACTIVMES

2.1 Operational History

Congress established RMA in 1942. The United States acquired land included within the boundaries

of the Arsenal for chemical weapons manufacturing, constructed a base, and corn m enced Army

weapons production and ancillary activities in 1943. From 1945 to 1950, RMA distilled available

stocks of mustard, demilitarized several million rounds of mustard-filled shells and incendiarv

munitions, and test-fired mortar rounds filled with smoke and high explosives. Also, many different

types of obsolete World War II ordnance were destroyed by detonation or burning.

After the conclusion of World War H, selected surplus facilities were leased to nongoverrunent

entities as warehouses and for the manufacture of agricultural chemicals. Colorado Fuel and lron

(CF&I) leased facilities at RMA in 1946. Julius Hyman & Company (Hyman) first leased facilities in

1947 and succeeded to the CF&I leasehold interest, with some modifications and additions in 1949.

Shell Oil Companv (Shell) acquired a majority interest in Hyman in 1952 and operated the plant as

the Julius H%-man Company until 1954. when the operation became the Shell Chemical Company -

Den%-er Plant.

R.\L-% %,6-as selected 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 of chemical 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 by the burial or surface disposal of

solid wastes, discharge of wastewater to basins. and leakage of wastewater and industrial fluid from

chemical and sanitary sewer systems. Munitions were destroyed and disposed in trenches.

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Site History and Enforcement Activities

Wastewater generated by the U.S. Department of the Army (Army) and private industry in the South

Plants and North Plants areas was discharged to a series of unlined evaporation and holding basins

(Basins A, B, C, D, and E) and to asphalt-lined Basin F at various times throughout the history of

RMA operations. The locations of these source areas are shown in Figure 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 have been conducted at RMA,

both onpost and offpost, by the Army. The U.S. Environmental Protection Agency (EPA) has also

conducted several offpost investigations. The Army designed and implemented monitoring programs

to monitor regional groundwater and surface-water quality. The Army also designed and

implemented the boundary system monitoring program to support the operation of the 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%- (RLTS I of an area west of RNIA and south of the

Offpost Studv Area (Figure 1.1).

RNIA was suspected as one of the 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

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contamination detected within the 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 of a trichloroethene plume entering RMA at Section 9, Township 3S, .

Range 67W along the southern boundary of RMA, 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 of the RMA Contamination Control Program, established in 1974 to ensure compliance

with federal and state environmental laws. The objectives of the 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 -,vere evaluated.

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. The Army 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 of RMA. The scope of the CAR investigation

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was intended to address critical data gaps required to evaluate a comprehensive set of multimedia

exposure pathways.

The potential for contamination of private wells was investigated in the mid-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 of privately-owned

drinking water wells in an area bound by East 80th Avenue on the south, East 96th Avenue on the

north, the South Platte River on the west, and RMA on the east. The objectives of the CU Phase III

study were as follows:

Locate all shallow domestic wells (less than 100 feet) in the Offpost Study Area.

Sample a representative number of the located wells.

Assess the groundwater quality of the shallow alluvial aquifer.

The Army developed the Comprehensive Monitoring Program (CMP), a long-term multimedia

monitoring program designed to provide data to facilitate evaluation of response actions, in the mid1980s. Sample collection under 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 of the

Offpost RI were as follows:

Collect additional data to refine the current understanding of groundwater flow and surfacewater patterns and the nature and extent of contaminants offpost of RMA.

Evaluate the potential for chemical migration to the Offpost Study Area in various environmental media, such as groundwater. surface water. sediment, air, and biota.

Following completion of the 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 of the second RI are reported in the Offpost Operable Unit Remedial Investigation, Final

Addendum (HLA, 1992b).

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2.3 Boundary Containment Systems

Concurrent with and as a result of the EPA and Army investigations, 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 of RMA, respectively) to minimize offpost discharge of RMA

chemicals via groundwater. The locations of these 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 of the RMA north boundary in Sections 23 and 24. The NBCS consists of

(1) a svstem of extraction wells that remove contaminated groundwater from the UFS, (2) a soilbentorute barrier that impedes migration of contaminated groundwater to the Offpost Study Area,

(3) a carbon-adsorption treatment system that removes organic contaminan from extracted

Sroundwatpr. and (4) a system of recharge 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. %%-st and 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

I I!i-- svs1em and became operational in December 1988, and five additional recharge

i. : if) the east end oi th,ý systeni in 1 q90. 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 of the NBCS are decreasing.

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Activated carbon is being used to effectively remove the organic contaminants from the extracted

groundwater to meet contain- ent system remediation goals. Organic contaminant concentrations are

generally below certified reporting limits (CRLs) in system effluent

2.3.2 Northwest Boundary Containment System

The NVVBCS is along the northwest boundary of RMA in the southeast quarter of Section 22.

Constructon of the 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 of 21 downgradient recharge wells.

The carbon adsorption system was designed to intercept and remove dibromochloropropane and

other organic compounds from a plume of contaminated groundwater originating onpost.

. pass was observed at the southwest and northeast ends of the NV%rBCS 1988.

Contaminant by in An

interim resonse action (IRA) to improve the NWBCS was initiated in 1989. in April 1990, the

NWBCS Improvements IRA 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 of the

extraction well alignment. Three additional extraction wells and four additional recharge wells were

installed in Section 27. southwest of the 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 dib romochloropro pane (DBCP) at the RNIA boundary. There have been no downgradient

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detections of DBCP after the first two years of operation. The majority of the area downgradient of

the ICS is contained within the EPA study area, although portions of the downgradient area are

within the confines of the Offpost Study Area. Therefore, the design and operation of the ICS was

not included in the evaluation of alternatives; however, the continued operation of the ICS, as

required, for onpost contaminants consistent with the Irondale 1R.A remains an integral part of the

Army's offpost contaminant reduction program to meet onpost cleanup goals defined in the Irondale

IRA. Cessation of operation of the ICS will be in accordance with paragraphs 35.2 and 35.4 of the

FFA and paragraph 20 of the Conceptual Remedy Agreement.

2.4 Interim Response Actions

As part of the Army's compliance with the requirements of the Comprehensive Environmental

Response, Compensation, and Liability Act (CERCLA), and as described in the FFA, the Army has

instituted several IRAs that have been performed concurrently with the ongoing onpost and offpost

RI programs. IRAs, which are designed to be compatible with the final remedy, are actions taken

before the signing of the Record of Decision (ROD) and are expedited remedial measures to contain,

remove. or treat wastes before the final remedv is selected. Numerous IRAs have been implemented

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 of RMA and downgradient of

the NBCS along two primary contamindnt pathwai-s. defined by the First Creek and northern

paleocharinels.

Evaluation and selection of the collection and treatment system components that comprise IRA A,

referred to as the Offpost Groundwater Intercept and Treatment System, began in 1988. The Offpost

Groundwater Intercept and Treatment System Decision Document (HLA, 1989) presents the basis for

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system placement to address remediation of contamination in alluvial groundwater in the First Creek

and northern paleoch-annels. The system was designed to intercept and extract contaminated

groundwater from the UFS, treat the groundwater for organics, and recharge treated water to the UFS.

Construction of the Offpost Groundwater Intercept and Treatment System began in November 1991

and was completed in June 1993. Groundwater extraction is accomplished through a network of

extraction wells. The organic contaminants in extracted groundwater are treated 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 and Treatment System was designed to be flexible and to be

compatible with the final remedy, consistent with EPA 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 subject to

compli.iric,- woh CERCLA when a release or a threat of a release of hazardous 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 be found, 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. -i proposed 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 of the Consent Decree was filed

on June 7. 1 IM8, 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 of the

Interior (DOI), the U.S. Department of Justice (DOJ). and the U.S. Agency for Toxic Substances and

Disease Registry (ATSDR). The FFA sets forth the procedures to be followed by the Organizations

(i.e., signatories to the FFA) to cooperate in the assessment, selection, and implementation of

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response actions resulting from the release or threat of release of contaminants from Rlý.IA. The FFA

designates the Army as the lead agency.

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3.0 HIGHLIGHTS OF COMMUNITY PARTICIPATION

Community participation opportunities were provided during the remedy selection process 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 were released to the public on March 21, 1993. The documents were made available

to the public in the Administrative Record (located at the joint Administrative Record Document

Facilitv at the west entrance to RMA at 72nd Avenue and Quebec Street), in an information

repository maintained at the EPA Docket Room in Region VM, and at the Adams County, Aurora,

Commerce City, Denver, Lakewood, Montbel.lo, and Thornton Public Libraries. The notice of

availabilitv for these four documents was published in the Denver Post and Rocky Mountain News

newspapers.

An expanded Community Relations outreach was implemented to ensure community members had

the opportunity to comment on 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. A direct mailing to 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 for April 28. 1993. at Dupont Elementary School, Commerce City, Colorado,

to address the Proposed Plan. A separate letter was sent to citizens informing them of the documents

availabilitv in the libraries. The letter also included a brief fact 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 of these factors and Earth Day

events in Denver for April 21, the meeting was moved to April 28, 1993.

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A Media Day was held the day of the public meeting to provide local media information on the

Army's proposal. Both print and video media representatives attended.

Knowing the importance of the public meeting, the Announcement was expanded to include display

advertising in 12 local and weekly newspapers in the Denver metropolitan area. This was in

addition to the normal press release and Media Day event.

As a result of comments received at the public meeting concerning the official comment period, the

Army published a legal notice and sent letters to citizens announcing that the comment period was

extended to June 21, 1993.

At the April 28, 1993, public meeting, representatives from the Army, EPA, and the State of Colorado

answered questions regarding issues at the site and the remedial alternatives under consideration.

Responses to comments received during the public comment period are included in the Responsiveness Summary, which is part of this ROD (Appendix A). This decision document presents the

selected remedial action for the RMA Offpost OU in Adams County, Colorado, chosen in accordance

with CERCL.,%, as amended by the Supe-fund Amendments and Reauthorization Act (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 the Administrative Record

for the Offpost OU.

Additionaliv. settlement discussions involving municipalities. local health departments, special

districts, and citizen groups were held from Idle 1q94 until April 1, 1995, to discuss the final

remedies for both Onpost and Offpost OUs. The Draft Final ROD (December 7, 1993) was revised

taking into account comments presented by the public. local communities, and the Parties.

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4.0 SCOPE AND ROLE OF THE OFFPOST OPERABLE UNIT

Three RMA boundary containment systems currently intercept, treat, and recharge groundwater at

the RMA north, northwest, and west boundaries. These boundary systems, along with the physical

boundaries of RMIA, provide a logical dehneation between OUs. Therefore, the FFA divided the work

into the following two OUs:

Onpost OU: Media requiring remediation within the Onpost Study Area (within RM.A

boundaries)

Offpost OU: Media requiring remediation within the Offpost Study Area (outside RMA

boundaries)

The Offpost OU addresses contamination in the groundwater north and northwest of RMA. As

discussed in Section 6.0 of this ROD, groundwater contamination in the UFS poses the principal

potential threat to human health because of the risks from possible exposure to groundwater.

Although health risks are possible, the estimated risk levels are within the acceptable risk range

established bv EPA. The purpose of the 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 LJFS grouMwater.

The potential risks to ecological receptors were also evaluated. Wildlife are not exposed to contaminated groundwater: therefore. there are no risks to wildlife from the 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 shown to be

negligible. Therefore. the selected remedy for the Offpost OU addresses the reduction of potential

human exposure to contaminated UFS groundwater.

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5.0 SUMMARY OF SITE CHARACTERISTICS

Six media were evaluated in the RI for the Offpost Study Area: groundwater, soil, surface water,

sediment, air, and biota. Each medium was evaluated in the Offpost EA with respect to (1) the

nature and extent of contamination and (2) potential exposure pathways and associated risk to

humans and the environment. A map delineating the boundaries of the Offpost Study Area is

included as Figure 1.1. The site characteristics are more fully described in 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 were introduced to the RMA environment primarily by the

burial or surface disposal of solid wastes, discharge of wastewater to basins, and leakage of wastewater and industrial fluid from chemical and sanitary sewer systems. Chemicals migrated to the

Offpost Study Area primarily by shallow (i.e., shallow or unconfined) groundwater and airborne

pathways. Contaminant transport in the shallow or unconfined groundwater has been 'controlled by

construction of the boundary containment systems and improvements to these systems (completed as

IRAs). Offpost Studv Area surface water was contaminated primarily by the natural interaction with

offpost groundwater. Offpost Study Area surface soil was contaminated by the deposition of airborne

contaminants, non-RNIA-related agricultural application of pesticides, and irrigation practices.

Agricultural sources of pesticides 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 of concern (COCs) are present in offpost groundwater, surface water, sediment, and

soil (see Tables 6.1 through 6.4). COCs include organochlorine pesticides (OCPs), halogenated

aliphatics, aromatic hydrocarbons. diisopropylmethyl phosphonate (DINT), sulfur-containing organic

chemicals, arsenic, and dissolved salts.

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The COCs exhibit great variability in their mobility and persistence in environmental media. OCPs

are less mobile than the other COCs and more persistent, tending to associate with soil and sediment

and to biomagnify in the food chain. Most of the remaining COCs are mobile in groundwater, and

the aromatics and aliphatics are volatile in surface water. The fate properties of the COCs tend to

determme their distribution in the Offpost Study Area. All COCs were detected in groundwater, but

the more mobile chemicals are more widely distributed. The OCPs are virtually the only COCs

detected at concentrations above background levels in soil and sediment. The volatile compounds

were not significantly elevated above background levels in surface water and, in fact, were rarely

detected.

S.3 Contamination Migration Pathways

The RI programs have shown that there are three groundwater migration pathways in the Offpost

Study Area. These migration pathways (shown in Figure 5.1) are referred to as the northern

paleochannel, due north of the RMA north boundary; the First Creek paleochannel, paralleling First

Creek to the northwest from the 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 composes the 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 characterized by coarser

sediment. Some of the groundwater traveling through the First Creek paleochannel discharges to

FLrst Creek, probably seasonally, resulting in transfer of contaminants to First Creek.

Figure 5.1 also presents the offpost surface-water ieatures. The primary 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 of the surface-water contamination is located

in First Creek, with some contamination in O'Brian Canal do%rnstream of the confluence with First

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Creek and Burlington Ditch. Barr Lake has not been shown to be contaminated with RMA-related

chemicals greater than naturally occurring background levels.

In addition to the contaminant migration pathways of groundwater and surface water, prevailing

winds transport onpost surface soil to 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 five media in the Offpost OU: groundwater,

surface water, stream-bottom sediment, surface and subsurface soil, and biota. More detailed

discussions of the offpost contaminant concentrations, along with figures showing concentration

distributions are found in Sections 3.0, 4.0, 5.0, and 6.0 of the 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 COC in 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 of DIMP observed in the past three years are in the First Creek paleochannel. Concentrations of DIMP are lower in the northern paleochannel and lower still in the

northwestern paleochannel. The maximum concentrations of DIMP in the Offpost Study Area 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 are being

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reduced to less than 8 ppb. Cut-off of groundwater contaminants at. the NBCS and recharge of the

treated groundwater has resulted in the observed decrease in DMEP concentrations specifically, as

well as the other contaminants found offpost.

The highest contaminant levels downgradient from the NBCS occur upgradient of the O'Brian Canal.

Certain volatile compounds such as chlorobenzene, chloroform, trichloroethene, and DBCP have been

detected at low concentrations downgradient from the canals, but well-defined plumes do not exist in

this area and these detections may be anomalous. Semivolatile organic compounds such as dieldrin

and other OCPs are present almost exclusively upgradient of the 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 Creek paleochannel

witlun 500 to 1000 feet of the NBCS. Only sporadic and isolated occurrences of OCPs are observed

northwest of the RMA northwestern boundary.

Contaminants found downgradient from the NWBCS are primarily chlorobenzene, chloroform, DMEP,

and dieldrin. The highest concentrations of chloroform occur downgradient of the RMA boundary.

Detections of chlorobenzene near the NVVBCS maybe anomalous. In 1989, semivolatile compounds

such as dieldrin and possibly DINIP appeared to have bypassed the NWBCS at the northeast and

southwest ends. Subsequently, the N%VBCS IRA was initiated that included improvements and

operational changes to correct the bypass. Recent modifications to the NBCS and NWBCS, in

addition to the remedial action selected in this ROD. are expected to further reduce contaminant

levels dowrigradient of the RMA boundaries.

S.4.2 Surface Water

Table 6.2 presents the surface water COCs and the exposure point concentrations used in the

Endangerment Assessment. The principal organic compounds identified in Offpost Study Area

surface-water samples are DIMP and dieldrin. In general, the highest concentrations of the organic

and inorganic analytes were detected in First Creek. DIMP concentrations in First Creek were highest

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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 of the 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 of the confluence

%,ý ith First Creek, suggesting that sources of these 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 of the RI Addendum investigation and were analyzed for OCPs, arsenic. and mercury. Dieldrin was the most frequently detected

OCP (in approximately 90 percent of the samples) with a maximum concentration located approxi-

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mately 100 to 200 feet north of the northern RMA boundary. DDT, DDE, aldrin, endrin, and

chlordane were detected less frequently.

The distribution of OCPs in Offpost Study Area soil appears to correlate with the dominant wind

patterns at RMA. The greatest number and highest contaminant concentrations are observed in

samples collected immediately north of the northern RMA boundary, consistent with the prevalent

wind direction of south to north. Isolated elevated concentrations of OCPs observed between the

northern RMA boundary and O'Brian Canal may be the result of local residential and/or commercial

use of pesticides and not related to migration from RMA. Anomalously high concentrations of

dieldrin, DDE, and DDT were also detected approximately 1.5 miles northwest of RMA. These

detections are considered to be agricultural-related and not RMA-related because the area is currently

and has historically been a farming community. -

The uneven distribution of arsenic and mercury in Offpost Study Area surface soil suggests that the

occurrence of these inorganic contaminants is not related to RMA activities.

5.4.5 Biota

The RI Addendum biota monitoring program provided additional data to assess the potential impacts

on plants and arumals in the Offpost Study Area. During the RI Addendum study, biota samples

were analyzed for aldrin, dieldrin. endrin. DDE. DDT. DBCP. arsenic, and mercury. Dieldrin, the

contaminant most often found in Offpost Study Area biota (36 percent of samples), 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 any biota samples from the Offpost

Studv Area. Contaminants identified in the Offpost Study Area biota survey are similar to those

found onpost, although the concentrations detected in the Offpost Study Area biota are considerably

lower than concentrations detected in the onpost biota.

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The Offpost Study Area is known to contain suitable habitat for endangered species such as the bald

eagle. A nesting pair of eagles 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 to the U.S. Fish and Wildlife Service, the concentrations of these contaminants

were tvpical of bald 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 of local city and county planning

and zoning ordinances, and consultation with local planning authorities, three primary land uses

were considered in estimating the 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 of volatile chemicals in groundwater

Inhalation of dust

Dermal contact with soil

Dermal contact with sediment

Dermal contact with surface water

Dermal contact with groundwater

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6.0 SUMMARY OF SITE RISKS

The risks estimated in the EA and siimmarized in this section are baseline risks corresponding to

current conditions and are, therefore, pre-remediation risk estimates. Implementation of the selected

remedy presented (Section 9.0) will lower the potential risks. The estimated maximum cumulative

potential cancer risk to humans in the Offpost Study Area is 3 x 10-4 (or 3 in 10,000 people) on the

basis of the reasonable maximum exposure (RNE) risks presented in the Final EA (Volume M,

Section 4.0, and Volume IV, Appendix G). This estimated potential risk level is within the acceptable risk range established by EPA (1 x 10-'* to 5 x 101; letter from EPA to Army dated February 21,

1992). A cancer risk estimate of 3 in 10,000 indicates an upperbound estimate of risk. Actual cancer

risks are likelv to be below this level and may be as low as zero. These carcinogenic risks are

usuallv termed "excess lifetime cancer risks," which means there is an increased chance of an

individual developing cancer over 70 years of exposure to the carcinogenic chemicals in excess of the

normal cancer rate. The background cancer rate determined by the American Cancer Society is

about 1 in 3.

Because the Offpost Study Area cumulative risk is less than the upper risk level established by EPA,

remedial action in the Offpost Study Area is not requLred. The Army, nevertheless, recognizes that

several site-specific factors suggest that remediation of the groundwater is preferable to no action in

the Offpost OU. These site-specific factors are: (1) groundwater contributes a maximum risk of

2 x 10'. or approximately 75 percent of the 101di carcinogenic risk, (2) maximum contaminant levels

(NICLs), maximum contaminant level goals (MCI.Gs). and Colorado Basic Standards for Groundwater

(CBSGs) are exceeded 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 in Zones 2,

3, and 4, the bulk of the HI value is contributed through an assumed domestic use of alluvial

groundwater, which is not presently occurring in the Offpost OU. Treatment of groundwater to the

containment svstem remediation goals will reduce (1) the total estimate risk to less than 1 x 10' and

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summary 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 not require remecliation because of the low risk attributable to these media. Air was not

identified as a medium of concern on the basis of air monitoring data and initial risk screening.

Protection of biota was evaluated through development of ecological exposure criteria for the

protection of species potentially at rislr- The ecological assessment indicated 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 of groundwater, surface water, sediment, soil, air, and biota data collected

betwPen 1985 dnd 1991 was used to evaluate wtuch chemicals were of concern to 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 of RMA and

This trend is due to the operation and improvement of the 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. 1 1. 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 of the Final Offpost EA/FS (HIA,

1992a).

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The primary criterion for identifying COCs was that the chemical concentrations at locations of

expected maximum concentration (i.e., near the RMA borders) must be significantly greater than

concentrations found at background locations (i.e., no RMA-related contamination present). By

applying statistical methods, Offpost Study Area contaminant concentrations were compared to

background concentrations at reference locations. If statistical analysis indicated that Offpost Study

Area concentrations were significantly higher than the background concentrations, the presence of

the chemical in the Offpost Study Area was considered to be RMA-related and the chemical was

designated as a COC. This procedure was followed for each environmental medium. Tables 6.1

through 6.4 list 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 for biota (plants and animals), analytical data obtained from the onpost biota RI were

compared to background chemical concentrations available in the scientific literature. This

procedure was less precise but nonetheless indicated that two chemicals (dieldrin and arsenic) may

be elevated. although in low concentrations. in the tissues of animals located in the Offpost OU.

6.1.2 Exposure Assessment

6.1.2.1 Ottpost Study Area Exposure Assessment Zones

The Offpost Study Area is a large. heterogeneousarea with a variety of characteristics that can affect

exposure levels. Specifically. distinct zc;nes of the Offpost Study Area exhibit different exposure

concentrations of COCs in groundwater. surfacp and surface soil, including hot spots where

contaminant levels are higher than the average for the entire Offpost Study Area. In addition,

population density, land use. and water use varies throughout the Offpost Study Area. Therefore, to

avoid diluting or averaging contaminant concentrations over the 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 of COC concentrations in

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groundwater. The six zones, and the land use and populations evaluated within each zone, are

described below.

Zone 1 is an area with relatively low levels of COCs in groundwater and surface soil. Rural

residential land use, which includes consumption of homegrown vegetables, milk, meat, and eggs, is

the current and potential future population characteristic.

Zone 2 is an area of relatively high levels of COCs in groundwater, low levels of COCs in surface soil,

and no permanent surface-water features. A rural residential land-use scenario, identical to Zone 1,

was evaluated.

Zones 3 and 4 are similar. Zone 3 is an area of relatively high levels of pesticide COCs in groundwater. surface water, and surface soil. Zone 4 is an area of relatively high levels of COCs in

groundwater and surface water. but relatively low levels of COCs in surface soil. Both Zones 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 for Zones 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, but that local

planting and consumption of vegetables are possible.

Zone 5 is an area with moderate levels of COCs in groundwater and relatively low levels of COCs 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.

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Zone 6 is an area with moderate levels of COCs in groundwater and -relatively low 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 (rural residential) 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 already been mitigated by exposure controls in areas with the highest groundwater

COC concentrations (e.g., the UFS is no longer used in Zones 3 and 4). Exposure to COCs in surface

soil has also been mitigated by relocating 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 of the 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 of the 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.

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6.1.2.3 Potential Exposure Pathways and Routes

An exposure pathway consists of four elements: (1) a source and mechanism of release, (2) a

transport medium, (3) a point of potential contact with the 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 Study Area. The Site Conceptual Model also indicates which exposure routes were

quantitatively evaluated for risk. Because of the variations in land use and the presence or absence

of surface water in the six zones, not all exposure routes are applicable to all zones. Table 6.5

summarizes the exposure zones by land-use category and identifies the exposure routes quantified in

each zone.

Inhalation Route

On the basis of risk screening evaluations conducted according to EPA guidance, the release of

volatile chemicals from groundwater used in the home for all purposes (e.g., showering, dishwashing,

laundry, toilets) was determined to result in potentially significant exposures by the inhalation route.

Therefore. inhalation of volatile chemicals resulting from domestic use was quantified. Other

potential sources of exposure, such as the inhalation of contaminated dust particles, and inhalation

of vapors resulting from volatilization from underlying groundwater, were found to be 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 of Barr Lake is

not feasible. considering the depth of the water and the prohibition of swimming.

Dermal contact with surface water in First Creek was quantified. However, dermal contact with

canal water is expected to be unlikely and. in the worst case. infrequent; therefore, dermal contact

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was not quantified for the canals. Direct contact recreation is prohibited in Barr Lake; therefore, the

dermal contact pathway was 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 to be eliminated from evaluation if other 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. b i oaccumu lat ion 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

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Summary of Site Risks

though there is insufficient water in portions of the UFS contaminated above groundwater containment system remediation goals to supply a municipal water system.

6.1.2.4 Estimation of Cbemical Intake

Analytical data from each media within each of the six exposure assessment zones (Section 6.1.2.1)

was identified. Exposure point concentrations were selected such that they represent an RME

concentration. The RME exposure point concentrations were calculated as the upper 95 percent

confidence limit on the arithmetic mean of the data. The RNE values for the COCs in each media

are presented in Tables 6.1 through 6.4. Exposure point concentrations were combined with

standard EPA intake assumptions and variables to estimate the intake of each COC by each exposuxe

route.

To estimate the exposure point concentration for food products (e.g., meat, eggs, vegetables), several

models were used to estimate the plant and animal uptake of a chemical from soil or water and the

resultant concentration in the edible portion of the plant or animal. All of the uptake and partitiorung coefficients were selected so that the resultant COC concentration in the food would also

represent an RME value. A complete discussion of the plant and animal chemical uptake models is

provided in the Offpost EA/FS.

6.1.3 Toxicity Assessment

The toxicity of chemicals is evaluated in terms of carcinogenic and noncarcinogenic effects. Cancer

slope factors and reference doses are used to evaluate potential risks posed by the exposure to

carcinogeruc and noncarcinogeruc chemicals. respectively.

EPA-established slope factors for inhalation and ingestion exposures to COCs are presented in

Table 6.6. The slope factor for a given compound is multiplied by the estimated intake to obtain the

carcinogenic risk estimate. The individual risks from each compound in a particular exposure

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pathway are then summed to obtain an estimate of the 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 by their 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 of this 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.

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Groundwater usage (either domestic arid/or agricultural) is the primary contributor to carcinogenic

risk, accounting for 45 to 99 percent of the total risk estimated for each zone. This indicates the

major role of the groundwater-related exposure pathways. Risks related to chemicals in soil are less

than 1 in 10,000 (1 x 10'), and the risks resulting from the surface-water and sediment exposure

pathways are less than 1 in 100,000 (1 x 10"). Because of the importance of the groundwater

pathway, the remediation of groundwater will have the greatest effect in reducing potential offpost

risks.

Dieldrin contributes the most to the total carcinogenic risk, followed by arsenic, chloroform, and

atrazine. All of the estimated risks from dieldrin are conservative in that the dieldrin concentrations

were considered to be constant throughout the exposure period (30 years). The natural reduction in

dieldrin concentrations over time was not considered. Additionally, not all of the 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 of the total carcinogenic risk in

some zones based on a background concentration for dieldrin of approximately 8 mg/kg. Naturally

occurring arsenic in groundwater may be responsible for a risk of approximately 4 in 100,000

(4 x 10"). based on a background concentration of arsenic in groundwater of approximately 3 Jig./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 children because

children would have the highest chemical intake per body weight and would be expected to be the

most sensitive to the chemical. The EA concluded that there is a low potential for adverse health

effects in children from hypothetical short-term exposures to dieldrin in groundwater in Zones 2, 3,

and 4. The HI exceeds 1 in Zones 2, 3. and 4. with a maximum HI of 4 in Zone 3. Dieldrin is the

primary contributor to the HI.

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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 of Duvip 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 of target organ effects and the mechanism of toxic 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 of vegetable 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 Oft jr,-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 -ý tirr-s The two natural ecosvs1ems occurring in the Offpost OU are terrestrial and

dquat r: f i.: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.

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Two methods of exposure were evaluated: direct exposure and biomagaification. Direct exposure is

a result of contact with the original source of the chemical (e.g., ingestion of surface water or soil,

ingestion of grou ndwater, or fish swimming in contaminated surface water). Biomagnification occurs

when the tissue concentrations of a chemical increase with progression up the food chain. Over

time, the concentrations of chemicals in tissues may reach a level detrimental to the organism's

health.

The evaluation of ecological effects via direct exposure is analogous to the evaluation of human

effects. Direct toxicity was evaluated by comparing the estimated daily intake of a receptor to the

estimated toxicity reference value for a receptor. The toxicity reference values are similar to human

RfDs in their derivation and use. These toxicity reference values were animal- and chemical-specific

values, or, in the case of aquatic life, federal Ambient Water Quality Criteria values established to

protect aquatic life.

To evaluate the potential effects of biomagrafication. the estimated tissue concentrations resulting

from biomagnification were compared to residue concentrations known to be without deleterious

effects. On1v the top indicator species were selected to evaluate the effects of biomagnification.

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 screening levels,

developed to ensure compliance with enforceable remediation levels, would meet the requirements of

the federal Endangered Species Act, the Migratory BLrd Treaty Act, and the Bald and Golden Eagle

Protection Act. These screening levels were 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 (Toxicity Reference Values for Avian and Terrestrial Vertebrate Species of Concern

Identified at Rocky Mountain Arsenal) of Volume 11 and Table H5-1 (Maximum AJlowable Tissue

Concentration [M.ATC] Values for the Offpost EA Ecological Assessment) of Appendix H in

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Volume rV. If the screening levels are exceeded or effects are observed in the future, enforceable

remediation levels will be developed consistent with CERCLA, the Endangered Species Act, the

Migratory Bird Treaty Act, and the Bald and Golden Eagle Protection Act.

Potential effects on wetlands and critical habitats were also evaluated. This assessment is presented

in Appendix B of the Final Offpost EA/FS (H1A, 1992a). The U.S. Fish and Wildlife Service

(USFWS) National Wetlands Office identified approximately 300 acres of wetlands along First Creek

from the north boundary of RMA to O'Brian Canal. Potential effects of construction of the 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 on the basis of direct toxicity by comparing 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

is dr%- much of the year and does not support a stable and ongoing fish population, adverse effects to

acu a! 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 of the NBCS will also reduce

concentrations of COCs in First Creek.

Agricultural life was evaluated in Zones 1. 2. and 6 (rural residential). The results of the 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

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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 agree

to the revegetation program as part of the remedy.

Construction of the Offpost Groundwater Intercept and Treatment System was coordinated with

USFWS to minimize the potential impacts on wetlands and habitat. Although the wetlands area has

been slightly altered because of construction of roads 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 of the wetlands area.

6.3 Conclusion

Actual or threatened releases of hazardous 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.

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7.0 DESCRIPTION OF GROUNDWATER REMEDIATION ALTERNATIVES

An FS was conducted to develop and evaluate remedial alternatives for the Offpost OU. The first

task performed during the FS was to identify media that require remedial action and correspondingly

require development and evaluation of remedial alternatives. Risks calculated in the EA were

compared to acceptable risk levels established by EPA in the NCP and other guidance. The Army

has closely followed EPA guidance and the National Contingency Plan (NCP) regarding the use of the

10-4 risk threshold to assess whether remediation is necessary. Guidance states that if the cumulative

cancer risk to an individual is less than 10', remedial action may not be warranted unless certain

site-specific conditions exist. If remedial 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 of the risk range is not an absolute at I x 10', but rather, the acceptable risk range can

extend to 5 x 10-4. The cumulative offpost cancer risk is a maximum of 3 x 10', which is within the

acceptable risk range.

In explaining the use of the point of departure. the EPA. in the preamble to the NCP, states

The use of 10' expresses EPA's preference for remedial actions that result in risks at the more

protective end of the risk range, but does not reflect a presumption that the final remedial

action should attain such a risk level (55 FR 8718).

The operation of the Offpost Groundwater Intercopi and Treatment System reflects the 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 cancer risk in the

Offpost OU was estimated to be 3 in 10,000. wfuch is within the acceptable risk range established by

EPA.

Although the maximum offpost cumulative carcinogenic risk is below the acceptable risk level,

remediation of groundwater is preferable to no action for the following reasons:

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Groundwater concentrations exceed National primary Drinking Water Standards maximum

contaminant levels (MCLs) and CBSGs; in some areas of the Offpost OU.

Groundwater is the greatest contributor to cancer risk and contributes a maximum risk of 2 in

10,000 (or approximately 75 percent) to the cumulative risk in zones 2, 3, and 4.

Evaluation of potential 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 not require remediation because of the

low risks contributed by these media to the total risk. Remedial alternatives were developed and

evaluated to address contaminated groundwater in the Offpost OU North and Northwest Plunie

Groups. Additionally, as part of the Conceptual Remedy Agreement, the Army and Shell Oil

Company have agreed to till and revegetate approximately 160 acres located in the southeast portion

of Section 14 and southwest portion of Section 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) assembling combinations of remedial process 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 of CERCIA. ARA.Rs

were used as groundwater containment system remediation goals for contaminants with promulgated

standards. and HBC based on a risk of I x 10' caiCUldied using RME assumptions were used for

carcinogensurithout ARARs. A risk level of 1 x 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

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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 PQL represent the lower practical limit for

quantitation.

Attainment of the groundwater containment system remediation goals developed for the site will

reduce the estimated total hypothetical cancer risks to less than 1 x 10-4 toward 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 of the groundwater containment system remediation 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 from the risk assessment

Groundwater requiring remediation in the Offpost Study Area was identified by comparing groundwater containment svstem remediation goals to the areal extent of groundwater contamination.

Groundwater containment system remediation goals are exceeded for the 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 of remediation alternatives; and estimating the

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Description of Groundwater Romodlation Alternatives

time required to clean up the contaminated groundwater. Because of the approximate nature and

inherent uncertainties of the models, none of the model results should be interpreted as an accurate

prediction of future conditions. The predicted remediation time frames are estimates. Accordingly,

estimated remediation time frames were only used to assess the relative effectiveness of the groundwater alternatives.

Remedial alternatives were initially screened on the basis of effectiveness, implementability, cost,

and attainment of ARARs. The alternatives passing the initial screening were then evaluated on the

basis of nine criteria required by the NCP. In addition to remedial alternatives, the NCP requires that

a No Action alternative be considered at every site. The No Action alternative serves primarily as a

point of comparison for other alternatives.

A total of six alternatives for the North Plume Group and four remedial alternatives for the Northwest

Plume Group were developed for analysis. Following the initial screening analysis in the FS,

four remedial alternatives for the North Plume Group (N-1, N-2, N-4, and N-5) and two remedial

alternatives for the Northwest Plume Group (NW-1 and NW-2) remained for evaluation during the

detailed analysis of alternatives. These alternatives are described below with the original alternative

numbering sequence from the FS report.

7.1 Common Elements of Alternatives

All of the alternatives developed included the foliowing elements:

Groundwater and surface-water monitoring: Samples will be collected periodically from

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 review will be conducted at least every five

years until groundwater contai=ent system remediation goals are achieved to assure that

human health and the environment are protected during and after remediation. The site

review will use monitoring program data to assess whether additional remedial action would

be warranted.

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Description of Groundwater Rernediation Alternatives

Except for the No Action alternative, each alternative also includes the following activities:

Exposure control/provision of alternate water supply as described below:

As of the date of the Onpost ROD, and based on a .392 parts per billion (ppb)

detection limit, the U.S. Army %%,ill use the last available quarterly monitoring results

to determine the DRAP plume footprint.

As part of the Onpost ROD, the U.S. Army and Shell Oil Company will pay for the

extension of, and hook-up to, the current distribution system for all existing well

owners within the DRAP plume footprint referenced above.

Existing domestic well owners outside of the DROP plume footprint as of the date of

the On-post ROD where it is later determined that levels of DRVIP are eight ppb or

greater (or other relevant CBSG at the time) will be hooked up at the U.S. Army and

Shell Oil Company's expense to the SACWSD distribution system or provided a deep

well or other permanent 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 to the 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 goals in Tables 7.1 through 7.3 or the ARARs in Table 10. 1 will be

provided an alternative water supply. Bottled water will be 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 of 4,000 acre feet of water, the details of

which .vill be worked out between the U.S. Armv, Shell Oil Company, and SACWSD.

If such water is not available. the U.S. ,Vmv and Shell Oil Company will provide

payment of an agreed upon amount of money in lieu of water.

Use of institutional controls to prevent the future use of groundwater exceeding remediation

pals. Institutional controls are reflected in Appendix B.

Continued operation of tEo 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 of the

FFA. the Conceptual Remedy Agreement, and the Onpost ROD, when it is signed. Cessation

may occur as provided in Sections 35.3 and 35.4 of the FFA and paragraph 20 of the

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.

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Description of Groundwater Rem*diatlon Alternatives

Closure of poorly constructed wells within the Offpost Study Area - Wells that could be

acting as migration pathways for contaminant in the Arapahoe Aquifer wiU be closed using

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. of groundwater in the North Plume Group are described

below. Table 7.4 presents the alternatives corresponding to the North Plume Group and identifies

process options, numbers of wells and trenches, flow rates, estimated remediation time frames,

treatment facility location, and process residuals generated.

7.2.1 - Alternative N-1: No Action

Under Alternative N-1, the operation of the NBCS would be discontinued. Alternative N-1 would

therefore not provide for active remediation of affected groundwater within the North Plume Group.

Ceasing operation of the NBCS -would likely 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 major components of Alternative N-1 include the following:

Long-term groundwater and surface-water monitoring

Site re,.-iews

A long-term groundwater and surface-water morutonng program would be implemented. The

purpose of the monitoring program 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.

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Description of Groundwater Remediation Alternatives

7.2.2 Aftemative N-2: Continued Operation of the North Boundary

Containment System With Improvements as Necessary

Alternative N-2 would provide for active remediation of affected groundwater approaching the north

boundary of RMA through continued remediation of groundwater at the NBCS. The major components of Alternative N-2 are as follows:

Continued operation of the 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

exceed ing off post 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 -xvat er monitoring and site review remedial 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 of long-term

groundwater monitoring and site review components included under Alternative N-1.

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Description of Groundwater 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 would be constructed and operated to contain, remove, treat, and recharge

groundwater exceeding containment system remediation goals in the First Creek and northern

paleochannels downgradient of the NBCS. Detailed information concerning the Offpost Groundwater

Intercept and Treatment System is presented in the Final Implementation Document for the

Groundwater Intercept and Treatment System North of Rocky Mountain Arsenal (HLA, 1991). The

Offpost Groundwater Intercept and Treatment System has been in operation since early 1993. The

major components of Alternative N-4 are as follows:

Removal of contaminated UFS groundwater north of the RMA boundary in the First Creek

and northern paleoch-annels using Offpost Groundwater Intercept and Treatment System

groundwater extraction wells

Treatment of organic contaminants in extracted groundwater using carbon adsorption

Recharge of treated groundwater to the UFS using Offpost Groundwater Intercept and

Treatment System recharge wells and trenches

Continued operation of the 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 of alternate 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.

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Description of Groundwater Rernediation Alternatives

Extraction wells would be used in the Offpost Groundwater Intercept and Treatment System to

remove contaminated groundwater. Extracted groundwater would be conveyed to the treatment

facility via double-contained polyvinyl chloride (PVC) pipelines.

Based on the results of the groundwater modeling, the configuration of five extraction wells and six

recharge trenches shown in Figure 7.1 would capture and remove contaminants axially in the First

Creek paleochannel. The recharge trenches would be placed both downgradient of the extraction

wells and along the outer boundaries of the First Creek paleochannel. In this manner, the recharge

trenches would provide both lateral hydraulic containment of the First Creek paleochannel and water

flushang for enhancing the removal of contaminants. Capture would be attained using a transverse

svstem of 12 extraction and 24 recharge wells directly downgradient of the 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 conveyed by

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 technology that is

,widelv used 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 of the 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, proven technology documented to be effective at the NIVBCS, NBCS, and ICS. Activated carbon treatment would

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Description of Groundwater Remediation Alternatives

achieve groundwater containment system remediation goals for organic contarninan before

discharge via the recharge systems.

An intensive short-term monitoring component would be included in Alternative N-4 as part of the

long-term monitoring program. The intensive short-term program would consist of monitoring

approximately 60 wells in a network that would be finalized through implementation of the alternative. Two years of data would be collected during the period commencing with Offpost Groundwater Intercept and Treatment System operations start-up. Such a program is necessary to evaluate

the performance of the NBCS and the Offpost Groundwater Intercept and Treatment System and

would provide an increased understanding of contaminant transport, an estimated time to achieve

groundwater containment system remediation goals, and to determine whether improvements to the

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 of the Offpost Groundwater Intercept and Treatment System. This cost

estimate also includes the continued operation of the NBCS, long-term groundwater monitoring, site

review, and exposure control components of Alternative 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 of the NBCS. Based on the results of the

groundwater modeling. the configuration of extraction wells and recharge systems proposed under

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 recharge trenches would be installed in the area of relatively slower groundwater

velocity and high dieldrin concentrations in the First Creek paleochannel. One additional extraction

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Description of Groundwater Rernodiation Alternatives

well and two recharge trenches would be installed in an area of low hydraulic conductivity in the

northern paleochannel. The major components of Alternative N-5 are as follows:

Removal of contaminated UFS groundwater north of the RMA boundary in the First Creek

and northern paleochannels, using Offpost Groundwater Intercept and Treatment System

groundwater extraction wells

Expansion of the Offpost Groundwater Intercept and Treatment System extraction and

recharge systems

Treatment of extracted groundwater using carbon adsorption

Recharge of treated groundwater to the UFS, using recharge wells and trenches

Continued operation of the 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-- d I lit iorwl 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

Alternat i% - % %,-,,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 of the expansion systems to the

Offpost Groundwater Intercept and Treatment Svstem and the cost components of Alternative N-4.

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Description of Groundwater Rernediation Alternatives

7.3 Identification of Groundwater Altematives: Northwest Plume Group

The following subsections identify the alternatives developed for the Northwest Plume Group.

Table 7.4 preseýts the alternatives corresponding to the Northwest Plume Group and identifies

process options, numbers of wells and trenches, flow rate, 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 of the NWBCS would be discontinued. Alternative NW-1

would not provide for active remediation of affected groundwater within the Northwest Plume Group.

Ceasing operation of the NWBCS would likely 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 of Alternative NW-1 are as follows:

Long-term groundwater monitoring

Site reviews

A long-termpgroundwater monitoring program would be implemented. The purpose of the monitoring program would be to assess changing UFS and CIFS aquifer conditions during and after remedial

action. As part of Alterriative 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.

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Description of Groundwater 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 of RMA through continued remediation of groundwater at the NWBCS. The

major components of Alternative NW-2 are as follows:

Continued operation of the 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 %rith criteria 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 of groundwater 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 svstern remediation goals to migrate from

RINUN to the Northwest Plume Group.

As pail of Alternative 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 of Alternative NW-1.

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8.0 COMPARATIVE ANALYSIS OF ALTERNATIVES

The remedial alternatives were evaluated with respect to nine threshold, primary balancing, and

modifying criteria as required by the NCP. The criteria are as follows:

Threshold Criteria

Overall protection of human health and the environment

Compliance with ARARs

Primary Balancing Criteria

Long-term effectiveness and permanence

Reduction of toxicity, mobility, or volume

Short-term effectiveness

Implementability

Cost

Modifying Criteria

State acceptance

Community acceptance

Threshold criteria must be satisfied by the selected alterative. Primary balancing criteria are used to

,veigh trade-offs among alternatives. Modifying criteria may be used to alter a proposed remedial

alternative. Brief descriptions of the evaluation criteria and the items considered when assessing

alternatives with respect to each criterion are presentpd in the summary of the comparative analysis

of alternatives.

The models simulating UFS groundwater flo-,%- and dissolved chemical transport were prepared for

the analysis of alternatives and are approximate in nature. Because detailed models were not needed

to compare the benefits of each remedial alternative, attempts were made to produce models that

incorporate general features of groundwater flow and associated transport phenomena in the Offpost

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Study Area. Nonetheless, the resulting models predicted flow and chemical transport phenomena

that agree with historical and current hydrogeologic data and observed contarninan distributions.

Because of the approximate nature of the models and the considerable uncertainty in the conceptual

model and hydrogeologic parameters, none of the modeling results should be construed as accurate

predictions of future contaminant distribution. Rather, the models and modeling results should be

viewed as tools for assessing the relative merits of remedial alternatives. Although there are inherent

uncertainties in the groundwater model, it is the tool being 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 of the NBCS with improvements as necessary (Alternative N-2), (2) Offpost Groundwater

Intercept and Treatment System (Alternative N-4), and (3) expansion of the Offpost Groundwater

Intercept and Treatment System (Alternative N-5). The results of these simulations were evaluated

on the basis of estimated remediation times measured on maximum concentrations versus time

graphs. The range of estimated remediation times was based on attainment of the groundwater

cleanup standards for DIMP, chloroform. and dieldrin, using a range of retardation factors. Although

some remediation goals have changed since modelling was performed, these changes do not affect the

assessment of the relative merits of the remedial alternatives.

For the Northwest Model, the remedial action scenario for continued operation of the NWBCS with

improvements as necessary (Alternative NRV-2) was simulated.

8.1 Comparative Analysis of Remedial Alternatives

The purpose of the comparative analysis is to identify the advantages and disadvantages of each

alternative relative to the others. Critical tradeoffs were identified and used to assist in selection of

the preferred remedy. Summaries of the detailed analysis of the North Plume Group and Northwest

Plume Group alternatives are presented in Tables 8.1 and 8.2, respectively. A brief description of the

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evaluation criteria and a comparison of each alternative with respect to the evaluation criteria is

presented below. Components common to all of the alternatives were not evaluated in the

comparative analysis.

8.1.1 Overall Protection of Human Health and the Environment

The criterion of overall protection of human health and the environment serves as a final check in

assessing whether each alternative provides adequate protection of human health and the environment. This criterion was also used to evaluate how risks would be eliminated, reduced, or controlled

through treatment, engineering, institutional controls, or other remedial activities.

North Plume Group Alternatives

Overall protection of human health and the environment would be provided by all alternatives with

the exception of Alternative 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 potential risks 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 for Alternative N-4), the two alternatives are essentially equivalent with

respect to providing protection of human health and the environment for the following reasons:

Alternatives N-4 and N-5 both provide for active remediation of the First Creek and Northern

paleochannel groundwater in approximately the same time period through removal of

contaminated UFS groundwater. treatment of the organics in the contaminated groundwater

using carbon absorption, and recharge of the treated groundwater using recharge wells and

trenches.

Both alternatives also provide a sigruf icant reduction in potential risk in approximately the

same time period through orgaruc contaminant removal and treatment by the Offpost Groundwater Intercept and Treatment System.

Northwest Plume Group Alternatives

Alternative NW-1 would not be protective of human health and the environment because the NWBCS

would cease operation. Overall protection of human health and the environment would be provided

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Comparative AnalyAs of Alternatives

by Alternative NW-2. Alternative NW-2 would decrease contaminant concentrations and reduce

potential risks associated with groundwater entering the Offpost Study Area north of the NWBCS.

Recharge of groundwater treated at the NWBCS would reduce contaminant concentrations in the

Northwest Plume Group through flushing with treated groundwater. Groundwater modeling

estimates that Alternative NW-2 would achieve groundwater containment system remediation goals

in approximately three to eight years. Alternative NW-1 would not likely achieve groundwater

containment system remediation goals because operation of the NWBCS would cease.

8.1.2 Compliance With Applicable or Relevant and Appropriate Requirements

The criterion of compliance with ARARs 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 have been

considered where appropriate during the ARARs analysis (see Section 10.0).

North Plume Group Alternatives

Compliance with chemical-specific ARARs would be achieved by all alternatives with the exception

of Alternative 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 shortest time bv 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.

Northwest Plume Group Alternatives

Compliance with chemical-specific ARARs would be achieved only by Alternative NW-2. Groundwater modeling indicates that Alternative NIV-2 would achieve chemical-specificARARs in approxi8-4 Harding Lawson Associates '21905 402010

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Comparative Analysis of Alternatives

mately three to eight years. Alternative NW-2 would comply with location- and action-specific

ARARs.

8.1.3 Long-term Effectiveness and Permanence

The long-term effectiveness and permanence criterion addresses the risk remaining at the site after

response objectives have been met. Components of the criterion that were addressed for each

alternative are as follows:

Magnitude of residual risk at the end of remedial activities

Adequacy and reliability of controls that are used to manage either treatment residuals or

untreated materials that remain at the site

North Piume Group Aftematives

Comparison of North Plume Group alternatives with respect to long-term effectiveness and permanence indicates that each alternative, Except Alternative N-1, provides a high degree of effectiveness

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 of the NBCS.

Northwest Plume 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.

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8.1.4 Reduction in Toxicity, Mobility, or Volume

The reduction of toxicity, mobility, or volume criterion addresses the statutory preference for

selecting remedial actions that permanently and significantly reduce toxicity, mobility, or volume of

hazardous materials at the site. This preference is satisfied when treatment is used to reduce

principal risks through destruction or irreversible reductions of toxicity, mobility, and/or volume.

North Plume Group Alternatives

All North Plume Group alternatives with the exception of the No Action alternative would reduce the

toxicity, mobility, and volume of contaminated groundwater entering the Offpost OU north of the

NBCS. Groundwater contaminant concentrations under the No Action alternative would likely

increase. Alternatives N-4 and N-5 would provide the greatest reduction in toxicity, mobility, and

volume of contaminated groundwater, through extraction, treatment, and recharge within the North

Plume Group. As stated previously, the uncertainty associated with the remediation time frames

estimated by the groundwater modeling suggests that. in practical terms, the estimated time frames

for both Alternatives N-4 and N-5 are essentially equivalent. Further, the intensive short-term

groundwater monitoring component of Alternative N-4 would allow for full-scale performance data

regarding the reduction of contaminant concentrations. Such data would be necessary to assess the

need for and optimum location of any modifications to Alternative N-4.

Northwest Plume Group Alternatives

Alternative N'W-2 would reduce the toxicity. mobility. and volume of contaminated groundwater

entering the Offpost OU northwest of the NWBCS through extraction, treatment, and recharge.

Groundwater contaminant concentrations under the No Action alternative would likely increase; thus

toxicity, mobility, or volume would not be reduced.

8.1.5 Short-term Effectiveness

The short-term effectiveness criterion addresses the protection of human health and the environment

during the construction and implementation phase. The following factors were addressed during the

evaluation process:

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Protection of the community during remedial actions - This factor addresses any risk that

results from implementation cf the proposed remedial alternative, such as dust from

excavation or transportation of hazardous material.

Protection of the workers during remedial actions - This factor assesses threats that may be

posed to workers and the effectiveness and reliability of measures to be taken.

Environmental impacts of the remedial action - This factor addresses the potential adverse

environmental impacts that may result from construction and implementation of a remedial

alternative and evaluates the reliability of mitigation measures, if necessary, to prevent or

reduce potential impacts.

Time lapse before achievement of response objectives - This factor includes an estimate of

the time required to achieve protection for the site.

North Plume Group Alternatives

Assessment of the North Plume Group alternatives with respect to protection of the community and

workers, short-term adverse environmental impacts, and implementation period indicates that the No

Action alternative and Alternative N-2 are slightly better than the alternatives with active remediation

components. However, during the implementation period, Alternatives N-4 and N-5 would be able to

mirumize adverse short-term impacts through standard engineering controls and adherence to

standard health and safety practices.

Northwest Plume Group Alternatives

The assessment of the two Northwest Plume Group alternatives with respect to protection of the

community and workers, short-term adverse environmental impacts, and implementation period

shows that the No Action alternative and Alternative NIV-2 are essentially equivalent except that the

discontinued operation of the NWBCS, as part of the No Action alternative, has an adverse environmental impact. Neither alternative. %%"Lth the exception noted above, has significant short-term

effectiveness issues.

8.1.6 Implementability

The implementability criterion evaluates the techmcal and administrative feasibility of implementing

each alternative, and it addresses the availability of required services and materials during

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implementation of the alternative. The following factors were addressed during the evaluation

process:

Construction and operation - This factor considers the technical difficulties and the

unknowns associated with the technology.

Reliability of the technology - This factor considers the likelihood that problems associated

with implementation may result in schedule delays.

Implementing additional remedial action - This factor is not applicable to the alternatives

developed because the alternatives considered are not interim measures.

Monitoring the effectiveness of the remedy - This factor addresses the ability to evaluate the

effectiveness of the remedy and includes an evaluation of the risks of exposure should

monitoring be insufficient to detect a system failure.

Coordination with other offices and agencies needed to implement remedial alternatives (e.g.,

obtainmS necessary permits for offsite activities)

Availabilitv of necessary equipment, specialists, services and materials, and adequate offsite

treatment. storage, and dispcýsal services

North Plume Group Afternatives

All North Plume Group alternatives evaluated would be technically feasible to implement. The No

Action ili,-mat 1%-4- and Alternative N-2 would be the easiest to implement with 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- I i-.juise additional design and construction. All treatment alternatives would use

carbon. -it tro-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 of tht! No Action alternative would be administratively feasible. It

is unlikely 1hdt the regulatory agencies or the public would accept shutdown of the NBCS as

proposed under the No Action alternative. Additionally, the Army will not cease operating the NBCS

until cleanup certification. Each of the three treatment alternatives would meet federal and state

substantive requirements for recharging the treated groundwater to the UFS.

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The No Action alternative and Alternative N-2 would not require additional equipment and services.

The implementation of Alternative 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 are readily available. The remedial systems of Alternative N-4 were completed in June

1993.

Northwest Plume Group Alternatives

Both Northwest Plume Group alternatives evaluated would be technically feasible to implement. The

No Action alternative and Alternative NW-2 would be implementable with respect 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 of both alternatives and would provide information regarding 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.

.AJternat ive N-W-2 would meet federal and state substantive requirements for recharging the 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 vendor information, cost

estimating guides, review of published cost data at previous sites, and operation and maintenance

costs at the boundarv containment sN,stems.

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North Plume Group Alternatives

The total present worth costs range from $4.1 to $6.0 million for 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 for Alternative N-5 are balanced by the

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 for Alternative N-5 as compared to

Alternative N-4 points out the importance of collecting additional full-scale operating data to aid in

decision-making regarding any necessary expansion of the Offpost Groundwater Intercept and

Treatment System. Collection of full-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

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enhanced contaminant removal. Responses to the State's concerns on this and other issues are

provided in Appendix A - Responsiveness Summary.

Additional discussions were held between the U.S. Army, Shell Oil Company, the State of Colorado,

the U.S. EPA, and the US. Fish and Wildlife Service following the issuance of the Offpost proposed

plan regarding the remedy for both the Offpost and Onpost OUs. As a result of these discussions, the

State of Colorado and the other parties have agreed to the remedy as described in Agreement for a

Conceptual Remedyfor the Cleanup of the Rocky Mountain Arsenal (Conceptual Remedy Agreement).

Each party has agreed to support the conceptual remedy as the preferred remedial alternative and to

support the proposed plan based on the elements of the conceptual remedy.

8.1.9 Community Acceptance

The preferred alternative for the Offpost OU was presented to the public in a Proposed Plan, which

provides a brief summary of all of the alternatives evaluated during the detailed analysis of alternatives 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 of DIMP

immediately do%%mgraclient of the Off4post Groundwater Intercept and Treatment System in the First

Creek area, and (5) the classification of potential 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 of Alternatives

The conclusions of the comparative analysis of groundwater alternatives for the North and Northwest

Plume Groups are summarized below.

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In terms of overall protection of human health and the environment,comphance with ARARs,

effectiveness, and reduction of toxicity, mobility, or volume, 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 more readily implementable than Alternatives N-1 and N-5 because Alternative N-1 would not be administratively feasible, and Alterative N-5 would require 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 respect to evaluation of compliance with ARARs, short-term effectiveness, and reduction in toxicity, mobility, or volume.

Alternative N-4 was demonstrated to be superior to Alternative N-5 with respect to the detailed

analvsis criteria for the following reasons:

The remedial system in Alternative N-4 is designed to effectively address 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 existing boundary

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 of remediation time frames developed for

the groundwater alternatives are uncertain. additional capital expenditures are not justified

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 of full-scdle operational data from the Offpost Groundwater

Intercept and Treatment System and future possible system modifications will result in an

optimized treatment system. Immediate pldcement of additional wells and trenches in

Alternative N-5, baseý on groundwater modeling results, would not be based on the more

accurate empirical data.

Alternative N-4 is more readily implementable than Alternative N-5 because implementation

of Alternative N-5 would require additional remedial design and construction. Operation of

Alternative N-4 would start immediateiv.

Alternative MV-2 ranks above Alternative NW-1 in all criteria except cost; however, the additional

costs are not prohibitive in light of the reduction in time for remediation.

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9.0 IDENTIFICATION OF THE SELECTED REMEDY

The selected remedy for the Offpost OU consists of implementing Alternative N-4 for remediation of

groundwater in the North Plume Group, Alternative NW-2 for remediation of groundwater in the

Northwest Plume Group, and continued operation of the Irondale Containment System consistent

with the Irondale IRA. The selected alternatives are described in detail in Section 7 and the

Declaration to the ROD.

9.1 Alternative N-4: Oftpost Groundwater Intercept and Treatment System

Construction of the Offpost Groundwater Intercept and Treatment System began in November 1991

and full-scale system operation began in June 1993. Additional detail concerning design specifics is

contained in the Final Implementation Document for the Groundwater Intercept and Treatment

System North of RMA (HLA, 1991). The major components of this alternative are as follows:

Operation of the Offpost Groundwater Intercept and Treatment System

- Removal of contarninated UFS groundwater north of the RMA boundary in the First

Creek and northern paleochannels, using Offpost Groundwater Intercept and Treatment System groundwater extraction wells

- Treatment of the extracted groundwater, using carbon adsorption

- Recharge of treated groundwater to the UFS, using Offpost Groundwater Intercept and

Treatment System recharge wells and trenches

- Natural attenuation of inorganic chloride and sulfate concentrations to meet

applicable standards for groundwater in a manner consistent with the on-Post

remedial action

Continued operation of the NBCS

Improvements to Offpost Groundwater Intercept and Treatment System and the NBCS, as

necessary

Long-term groundwater and surface-water morutonng

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

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institutional controls for the selected remedy are reflected in Appendix B. These institutional

controls are intended to prevent the future use of groundwater exceeding remediation goals.

The Offpost Groundwater Intercept and Treatment System is an array of extraction wells and

recharge trenches in the northern and First Creek paleorhannel . The system is configured to extract

and treat UFS groundwater that exceeds containment system remediation goals 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 of extraction wells and recharge trenches in the First Creek

paleochannel. The location of the 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 of groundwater flow. The recharge system in the

northern paleochannel consists of 24 recharge wells spaced 100 feet apart and placed parallel to and

approximately 300 feet downgradient of the collection system. The First Creek paleochannel

collection system consists of 5 extraction wells spaced 200 to 500 feet apart along the axis of the

paleochannel. Recharge trenches are placed such that four of the six trenches are parallel to the flow

axis and located on the margins of the paleochannel, with the remaining two trenches located

downgradient of the extraction well system and oriented perpendicular to the flow axis.

The system is designed to extract and treat an average flow of 300 gpm from the 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 facility basic process flow includes influent storage, pumping, bag

filtration for particulate removal. carbon adsorption. multimedia filtration, treated water storage,

treated water pumping, and final bag filtration.

A total of approximately 250 million gallons per year would be treated by the Offpost Groundwater

Intercept and Treatment System at the average flows. In addition, operation of the NBCS component

of this alternative will treat approximately 125 milhon gallons per year. Thus, a total of approxi9-2 Harding Lawson Associate5 '21905 402010

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Identification of the Selected Remedy

mately 375 million gallons of UFS groundwater will be treated annually to attain Offpost OU

containment system remediation goals (Tables 7.1, 7.2, and 7.3) under this alternative.

An intensive short-term monitoring component will be included in Alternative N-4 as part of the

long-term monitoring program. For costing purposes, it is assumed that this program will consist of a

network of approximately 60 wells to be sampled semiannually for two 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 of performance data for the full-scale

operation of both the Offpost Groundwater Intercept and Treatment System and the NBCS. The data

will also be used to assess the need for any improvements to the systems. The acquisition of such

data will allow for increased accuracy in assessing the response of the UFS groundwater to the NBCS

and Offpost Groundwater Intercept and Treatment System remediation systems.

In addition, the preferred alternative includes long-term monitoring of offpost groundwater and

surface water to assess contaminant ccncentration reduction and remedy performance. Groundwater

monitoring will continue utilizing both monitoring wells and private drinking water wells. Selected

surface-water monitoring locations will be included to evaluate the effect of groundwater treatment

on surface water quahty. Monitoring will continue after system shut-off to assure continued

compliance with containment system remediation goals. The Army will present the scope of these

ongoing monitoring programs in an Implementation Plan to be submitted within 90 days following

issuance of the ROD. A schedule for compliance with the containment system remediation goals 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 of the NWBCS with improvements as necessary. For additional details of the extraction/recharge systems, the recent upgrades to

the system, and the treatment facility at the NWBCS, the reader is referred to the following reports:

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Final implementation Document for NWBCS Short-term Improvements IRA (Morrison-Knudsen

Environmental Services [MKES], 1990a); NVVBCS Long-term Improvements IRA B(ii) Final Assessment Document (Woodward-Cayde [wwq, 1991a); Proposed Decision Document NWBCS RMA Longterm Improvements IRA (WWC, 1991b); Report of Field Investigations, Assessment, and Final

Decision Document for the NWBCS Short-term Improvements IRA (NflCES, 1990b); Implementation

Document for the Northwest Boundary System long-term Improvements IRA Final Report (MKES,

1992); and Northwest Boundary Containment System Long-term Improvements IRA One-year

Evaluation Report (MKES, 1993). The major components of this alternative are as follows:

Continued operation of the 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 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 remedy performance. After attainment of groundwater containment system

remediation goals and system shut-off, groundwater monitoring will continue to assure continued

compliance with containment system remediation goals. The Army will present the scope of these

monitoring programs in implementation plans to be submitted following issuance of the ROD.

The MVBCS began operation in 1984. The NWBCS collection system consists of 20 extraction wells

and a sod bentonite barrier approximately 2300 feet in length. The recharge system consists of 25

downgradient recharge wells.

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9.3 Additional Components of the Selected Remedy

In accordance with the NCP, the public had an opportunity to review and comment on the selected

remedial alternatives presented in the Proposed Plan. In response, the Parties held additional

discussions to determine how best to address these comments. These discussions resulted in

clarifications and minor technical changes -that do not significantly alter the overall scope, performance, or cost of the Offpost preferred alternative.

Because the main focus of the Offpost preferred alternative is unchanged, and the additional actions

only clarify and enhance the preferred alternative, the changes were not considered to be significant.

The discussions also involved broader issues which were resolved in a document entitled "Agreement

for a Conceptual Remedy for the Cleanup of the Rocky Mountain Arsenal,' dated June 13, 1995.

With regard to the Offpost area, the Pzrties agreed to several additional components which are an

integral part of the overall remedy but are proposed for inclusion in the Onpost ROD. Many of these

components are in the 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 of the Conceptual

Remedy Agreement discussions include:

The U.S. Army and Shell Oil Company agree to continue monitoring and to complete an

assessment of the NDMA plume by June 13, 1996, using a 20 ppt method detection limit.

The U.S. Army and Shell Oil Company agree to prepare a feasibility study of potential

actions, both onpost and at the boundary. or ad)acent to the boundary in order to 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 of Section 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 revegetated fall within EPA's

established acceptable risk range and revegetation is not necessary. However, the U.S. Army

and Shell Oil Company agree to the revegetation program as part of the Offpost settlement.

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The Army will treat any contaminated extracted groundwater prior to 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 of the date of the Onpost ROD, and based on a 0.392 parts per billion (ppb) detection

limit, the U.S. Army will use the last available quarterly monitoring results to determine the

DRAP plume footprint

The Army and Shell Oil Company will 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 of the DIMP plume footprint as of the date of the

Onpost ROD where it is later determined that levels of DMV are eight ppb or greater (or

other relevant CBSG at the time) will be hooked up at the U.S. Army and Shell Oil Company's expense to the SACWSD distribution system or provided a deep well or other

permanent solution.

For new domestic wells with levels of eight ppb or greater for 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 to the distribution system or provided a deep well or other

permanent solution.

The parties to the Conceptual Remedy Agreement commit to good faith best efforts to

establish a trust fund for the operations and maintenance of the remedy, including habitat

and surficial soil. The parties recognize. however, that establishment of such 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 of the Onpost remedy, the U.S. Army and Shell Oil Company will pay for and

provide. or arrange for the provision, of 4000 acre-feet of water to SACWSD.

9.4 Cost of Selected Remedy

A detailed cost summary for the selected remedy is presented in Table 9.1. The total estimated cost

ranges from approximately $69 to $76 million. This cost does not include implementation of the

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, the relative

relationship of the cost of the 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 may not always be possible to reach MCJs or proposed MCIs through currently

available technology. If it becomes apparent during implementation or operation of the selected

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Identification of the Selected Remedy

remedy that contaminant levels are remaining constant for a significant amount of time at levels

higher than the groundwater containment system remediation goals delineated in the ROD, the

containment system remediation goals and the remedy will be reevaluated. Further, the NCP

requires a formal review of the effectiveness of the selected remedy at least every five years. As

needed, the operational design of the selected remedy will be reviewed to achieve the 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 can be removed from production when concentrations of constituents detected in the well are less than applicable or relevant and

appropriate requirements (ARARs) established in the ROD and/or it can be demonstrated that

discoiatinumg operation of a well mill not jeopardize the containment objective of the systems. Wells

removed from production, and monitoring wells upgradient and downgradient of the boundary and

offpost containment systems, will be monitored quarterly for a period of five years to determine if

contaminants reappear. Wells turned aff for hydraulic purposes will not be 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 production if 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 is required.

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10.0 STATUTORY DETERMINATIONS

A description of how the selected remedy meets statutory requirements, compliance with the

requirements of CERCLA, and consistency with the NCP is presented in this section.

10.1 Consistency with the Statutory Requirements of CERCLA in Section 121

The statutory requirements of CERCLA Section 121, as described below, and the statutory preference

for treatment are met through implementation of the selected remedy.

10.1.1 Protection of Human Health and the Environment

The selected remedy will result in the remediation of the Offpost OU groundwater consistent with

remedial action objectives and containment system remediation goals established for the site.

Contaminated groundwater in the North and Northwest Plume Groups will be addressed by

implementing the selected remedy through groundwater extraction, treatment, and recharge.

The groundwater remedial actions proposed under Alternatives N-4 and NW-2 will permanently

address the primary threat to human health and the environment for the Offpost Study Area through

ca.rbon adsorption treatment to reduce the toxicity. mobility. and volume of contaminated groundwater Contaminant levels in Offpost Study Area groundwater wil-l be reduced to or below groundwater containment svstem remediation goals foliowing treatment. Reduction of groundwater

contaminant concentrations to these goals %%-ill further reduce the groundwater cumulative excess

cancer risk toward 10'. Following groundwater remedial action, the M for noncarcinogens will be

less than 1.

It should be recognized, however, that studies con3ucted at other sites (by EPA and others) have

indicated that it may not always be possible tn reach groundwater containment system remediation

goals because of the limitations of the technology used to assess groundwater hydrogeological.

properties. the technology used to estimate aquifer remediation time frames, and the technologyused

to extract and recharge groundwater. If it becomes apparent during operation of the groundwater

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Statutory Determinations

treatment systems that groundwater contaminant levels are remaining constant at levels higher than

the Offpost OU groundwater containment system remediation goals , the selected remedy provides for

improvements to the proposed remeclial systems as necessary. An alternative water supply will be

provided to any user of a domestic well in accordance with the provisions in Section 7.1. Institutional controls that are part of this remedy are intended to prevent the future domestic use of

groundwater exceeding the containment system remediation goals.

Of the alternatives evaluated for cleaning up the groundwater, the selected remedy provides the

highest degree of protection of human health without adverse impact to the environment. No

unacceptable short-term risks will be caused by implementing this 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 of these

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 (Toxicity Reference Values for Avian and Terrestrial Vertebrate Species of Concern

Identified at Rocky Mountain Arsenal) of Volume 11 and Table 1-15-1 (Maximum Allowable Tissue

Concentration [1AATC] Values for the Offpost E.A. Ecological Assessment) of Appendix H in

Volume I'V. If the 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.

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Statutory Determinations

10.1.2 Compliance With Applicable or Relevant and Appropriate Requirements

Under Section 121(d)(1) of CERCLA, remedial actions must attain standards, requirements, limitations, or criteria that are applicable or relevant and appropriate under the circumstances of the

release at a site. ARARs would be met or exceeded upon completion of the selected remedy 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 without ARARs

(Tables 7.1, 7.2, and 7.3). The preferred site%ide alternative is expected to attain or exceed

chemical-specificARARs. A summary of the chemical-specific and other ARARs that have been

assessed to be applicable or relevant and appropriate is presented in Table 10.1.

Action-specific ARARs

The selected remedywill comply witý action-specific ARARs. A summary of the action-specific

AR.ALRs that have been assessed to be applicable or relevant and appropriate is presented in

Table 10.2.

Location-specific ARAPIS

The selected remedy xvill comply %%ith location-specific AFkRs. A summary of the 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,,it inz the risks posed at the site by contaminated

groundwater. Cost-effectiveness is determined b% evaluating three of the five balancing criteria to

determine overall effectiveness: long-term eftecuveness and permanence; reduction of toxicity,

mobility, or volume through treatment. and short-term effectiveness. Overall effectiveness is then

compared to cost to ensure that the remedy is cost-effective.

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The selected remedy for groundwater provides the best overall effectiveness of al.1 alternatives

considered proportional to its cost. The selected remedy will greatly reduce the toxicity, mobility,

and volume of groundwater exceeding containment system remediation goals. Also the implementation of this remedy wW result in long-term effectiveness by reducing residual carcinogenic risks

through permanent treatment.

Through the groundwater monitoring program, the Army can more accurately assess the contaminant

removal rates as a function of time, using the full-scale data available during operation of the Offpost

Groundwater Intercept and Treatment System, the NBCS, and the NVVBCS. The analysis of this data

will allow for cost-effective decisions regarding any future improvements that may be required for the

remedial systems.

10.1.4 Utilization of Permanent 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 manner to remediate groundwater at the site. Of those alternatives that are protective of human health and the environment and

comply,with ARARs, the selected remedy (Alternatives N-4 and NW-2) will provide the best balance

of trade-offs in terms of long-term effectiveness and permanence; reduction of toxicity, 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 selection process:

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 of human health and the environment.

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Statutory Determinations

Onsite remedial actions selected in a ROD must attain ARARs or provide grounds for

invoking a waiver.

Each remedial action selected shall be cost effective, provided that it first satisfies the

threshold criteria (defined in Section 8.0).

Each remedial action shall use permanent solutions to the maximum extent

practicable.

The preferred sitewide alternative is fully consistent with the NCP, as is the selection process used to

arrive at the preferred alternative. Alternatives were developed and screened, and the detailed

analysis of alternatives was performed in a manner consistent with the NCP.

10.3 Summary

The preferred sitewide alternative for iemediation of the Offpost OU is the combination of Alternatives N-4 and NW-2. The preferred alternative was selected in accordance with the requirements of

CERCLA and the NCP. The remedial actions that compose the sitewide preferred alternative will

permanently address the principal threats through groundwater extraction and treatment to reduce

the toxicity. mobility, or volume of contaminants for protection of human 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.

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11.0 DOCUMENTATION OF SIGNIFICANT CHANGES

The Proposed Plan for the Rocky Mountain Arsenal Offpost Operable Unit was released for public

comment in March 1993. The Proposed Plan identified Alternative N-4 (Offpost Groundwater

Intercept and Treatment System) as the preferred alternative for groundwater in the North Plume

Group and Alternative NW-2 (Continued Operation of the Northwest Boundary Containment System

With Improvements as Necessary) as the preferred alternative for groundwater in the Northwest

Plume Group. The Army received written comments from the U.S. Environmental Protection

Agency, the State of Colorado, the Tri-County Health Department, city and county governments,

environmental action groups, and citizens. After review of these comments, it was determined that

no sigruficant changes to the preferred alternative, as it was originally identified in the Proposed

Plan, were necessary.

As indicated earlier in Section 8.1.8. following the issuance of the Offpost Proposed Plan, additional

discussions were held between the Parties regarding the implementation of the preferred alternative

for the Offpost OU and the remedies for the Onpost OU. The main components of the preferred

alternative for the Offpost OU remain intact. These components are:

Operation (and improvement. if necessary) of the Offpost Groundwater Intercept and

Treatment Svstem

Continued operation (and impmvern,,nt. if ri-ý:--ssarv) of the NBCS and NWBCS

Long-term groundwater and surface-wat-,r nionooring

Five-year site review

Well closure

Provision of alternate water suppli-i. -in,l iniplnuentation of institutional controls intended to

prevent future use of contaminatt-d groun iwater.

The Conceptual Remedy Agreement pro, -ides more specific criteria for the provision of alternate

water supplies to current and future well owners. specific criteria for continued operation of and

requirements for shutdown of the groundwater treatment systems, and additional requirements for

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Documentation of Significant Changes

the Army and Shell Oil Company such as tiffing and revegetation of surface soil and additional study

requirements. Because the main focus of the preferred alternative is unchanged by the Conceptual

Remedy Agreement, and the additional actions specified in the Conceptual Remedy Agreement only

clarify and enhance the preferred alternative, the Conceptual Remedy Agreement was not considered

to be a significant change. Therefore, the preferred alternative as presented in the Proposed Plan,

and additional actions to enhance the preferred alternative as outlined in the Conceptual Remedy

Agreement, is the selected remedy.

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12.0 GLOSSARY

ARAR Applicable or relevant and appropriate requirement

Army U.S. Department of the Army .

ATSDR U.S. Agency for Toxic Substances and Disease Registry

CAR Cont-ination Assessment Report

CBSG Colorado Basic Standards for Groundwater

CBSM Colorado Basic Standards and Methodologies for 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 of concern

CRL Certified reporting limit

CSC Chemical Sales Company

CU Consumptive use

DBCP D ibro mochl oropro pane

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 of justic--

FATS Endangerment dSSeSSm,-nL f-asibilitv study

EPA U.S. Envuoranental I'vPtection Agency

ESE Environmental Science and Engineering, Inc.

FFA Federal FacilitN. Agreement

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Glossary

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 Policy Act

NT L 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 invest igat io rx/feas ib ility study

RM.A Rocky Mountain Arsenal

R11v1E Reasonable maximum exposure

ROD Record of Decision

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Glossary

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

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13.0 BIBLIOGRAPHY

Adams County, City of Aurora, City of Brighton, City of Commerce City. 1990. Airport Environs

Concept Plan, Brighton, Colorado.

Adams County Planning Cominission. 1987. Adams County Future Land Use Plan, Brighton, CO.

Camp Dresser & McKee, Inc. 1993. Stapleton International Airport Environmental Assessment,

Northern Zone. Prepared for Stapleton International Airport, City and County of Denver. June 15.

Ebasco Services, Inc. 1988. Western Tier TCE Soil Gas Investigation, Final Summary Report.

January.

Ebasco Services, Inc. 1989. Technical Support for Rocky Mountain Arsenal Draft Final Water

Remedial Investigation Report. Contract Nos. DAAK11-84-DO016 and DAAA15-88-DO024.

Denver. CO.

Environmental Science and Engineering, Inc. 1985. Rocky Mountain Arsenal Offpost Assessment:

Ground Water Quality Report (Consumptive Use - Phase 1) for Sampling Period December 1984

through January 1985. Prepared for U.S. Army Toxic and Hazardous Materials Agency. Denver, CO.

Environmental Science and Engineering, Inc. 1986. Rocky Mountain Arsenal Offpost Assessment:

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 Mountain Arsenal Offpost Assessment,

Contamination Assessment Report: Draft Final, Denver prepared for the Office of the Program

Manager. Rocky Mountain Arsenal.

Enviroruxiental Science and Engineering, Inc. 1987b. Rocky Mountain Arsenal Offpost Assess-ment:

Ground Water Qualitv Report (Domestic Use - Phase III) for Sampling Period September through

October 1986 and Februarv 1987. Contract No. DAAK-1 1-83-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. Commerce City. 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, Docket No. CERCLA VIII-89-13,

signed by U.S. Department of the Arm\-. U.S. Environmental Protection Agency, U.S. Environmental

Protection Agency Region VIII, U.S. Department of Interior, U.S. Fish and Wildlife Service, Agency

for Toxic Substances and Disease Registry. U.S. D-partment of Justice, and Shell Oil Company.

Harding Lawson Associates 1989. Final Decision Document for the Groundwater Intercept and

Treatment Svstem North of Rocky hlountain Arsenal, Interim Response Action.

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Bibliography

Harding Lawson Associates. 1991. Final Implementation Document for the Groundwater Intercept

and Treatment System North of Rocky 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. Implementation Document for Northwest

Boundary System Short-term Improvements IRA. June.

Morrison-Knudsen Environmental Services. 1990b. Report of Field Investigations, Assessment, and

Final Decision Document for the Northwest Boundary System Short-term Improvements Interim

Response Action, RMA. June.

Morrison-Knudsen Environmental Services. 1992. Implementation Document for 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 of Organoclorine 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 Remedial Investigations 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 of Decision for Chemical Sales Company,

Operable Unit 2. June 27, 1991.

U.S. Environmental Protection Agency. 1991b. Record of Decision for Chemical Sales Company,

Operable Unit 3, June 27. 1991.

U.S. Environmental Protection Agency. 1992. Record of Decision for Chemical Sales Company,

Operable Unit 2, December 29. 1992.

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Sibliograph

Woodward-Clyde. 1991a. Northwest Boundary System Long-term Improvements Interim Response

Action B(ii) Final Assessment Document. June.

Woodward-Clyde. 1991b. Proposed Decision Document Northwest Boundary System Rocky

Mountain Arsenal Long-term Improvements Interim Response Action. June.

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Table 6.1: Offpost Operable Unit Groundwater Chemicals of Concern

Exposure Point Concentration ("011

Chemicals of Concern Zone 1 Zone 2 Zone 3 Zone 4 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 of concern in this zone

CPMSCI 4-chlorophenylmethyl sulfoxide

CPMSCI, 4-chlorop henylm ethyl sulfone

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 per fiter

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 limit on the arithmetic mean of

measured concentrations in monitoring and private wells.

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Table 6.2: Of1post Operable Unit Surface-Water Chemicals of Concern

Exposure Point Concentration (ugjj)'

Chemicals of Concem 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 of measured concentrations in unfiltered surface-water samples.

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Table 6.3: Offpost Operable Unit Sediment Chemicals of Concern in First Creek

Exposure Point

Chemicals of Concern 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 on the

arithmetic mean of measured concentrations in sediment.

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Table GA: Offlaost Operable Unit Soil Chemicals of Concern

Exposure Point Concentration fm-ARIýChemicals of Concern 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 of measured concentrations in sod.

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Table 6.5: Summary of Land-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

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Table 6.6: Reference Doses and Slope Factors for Chemicals of Concern

Noncarcinogenic Carcinogenic

Chronic RMD Slope Factor

sL (myAxLday)' fmp-/k dayl

Chemicals Carcinogenic

of Concern 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 of Evidence 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 or no evidence in humans.

C Possible human carcinogen

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Table 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 other than EPA.

b. Subsequent to this assessment, a Region VIII Health Advisory was issued (see letter dated

januarv 27, 1994). This Health Advisory has not been reviewed by the 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.

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Table 6.7: Summary of Reasonable Maximum Exposure Carcinogenic Risks

by Zone and Exposure Route

Exposure Route

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 1 is subdivided on the basis of the presence of surface water and whether the ditch water

used for irrigation is collected upstream or downstream of the mouth of First Creek.

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Table 6.8: Summary of Adult Reasonable Maximum Exposure Noncarcinogenic

Hazard Indices by Tarqet 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 for this target organ not detected in this zone

CNS Central nervous system

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Table 7.1: Containment System Remediation Goals for the

Offpost Groundwater Intercept and Treatment System

Rural

Residential

CSRG Hypothetical

Analyte Ujg/l) Source PQLa Cancer Risk b

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 2.0 x 10"

Carbon tetrachloride 0.3 CBSG 0.99d 7.9 x 10-7

Chlordane 0.03 CBSG 0.095 d 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 0.05 d 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'D N 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*'

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Table 7.1 (continued)

The following chemical have ARARs that were adjusted downward to reduce overall risk: 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 system remediation 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 Diisopropy1methyl phosphonate

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 PQL is greater than the CSRG.

b. Based on the CSRG.

c. PQL listed in the CBSG standards

d. PQL attainable by the U.S. Army

e. The remediation goal for NDNtA was established at 0.007 parts per trillion (ppt) in the

Conceptual Remedy Agreement. The current PQL readilv 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 bv natural attenuation consistent with the onpost

remedial action.

S. Inorgaruc standard for sulfate may be the natural background concentration, which will be

established and met by natural attenuation consistent with onpost remedial action.

h. Because of the variability in contaminant distribution and concentration, the maximum risk

associated with the groundwater cleanup concentrations is not expected to occur at any one

location.

2 of 2 Harding Lawson Associates '21905 402010

1107121495 R02
Table 7.2: Containment System Remedlation Goals for the

North Boundary Containment 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 4.0 x 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 0.05 d 1.2 x 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 1.0 x 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 system remediation goal

DBCP Dibromochloropropane

DCPD Dicyclopentadiene

DIMP Diisopropylmethyl phosphonate

HBC Health-based criteria

MCL Maximum containment level

NA Not applicable

NIDMA N-rutrosodimethylamine

PQL Practical quantitation limit

ilg1l Micrograms per liter

a. Practical quantitation limit; presented only when the PQL is greater than the CSRG.

b. Based on the CSRG

c. PQL listed in the CBSG standards

d. PQL attainable by the U.S. Army

e. The remediation goal for NDMA was established at 0.007 parts per trillion (ppt) in the

Conceptual Remedy Agreement. The current PQL readily available is 0.033 ppt. The estimated

risk associated witfi NDMA is based on a 70-year residential exposure duration.

f. Inorganic standard for chloride -will be met by natural attenuation consistent with the onpost

remedial action.

g. Inorganic standard for sulfate may be the natural background concentration, which will be

established and met bv natural attenuation consistent with onpost remedial action.

h. Because of the variabilitv in contaminant distribution and concentration, the maximum risk

associated -with the groundwater cleanup concentrations is not expected to occur at any one

location.

2 of 2 Harding Lawson Associates '21905 402010

1107121495 R02
Table 7.3: Containment System Remediation Goals for the

Northwest Boundary Containment System

Rural

Residential

CSRG Hypothetical

Analyte Rg/l Source POLa Cancer Risk b

Chloroform 6 CBSG 6,4 x 10-6

DIMP 8 CBSG NA

Dieldrin 0.002 CBSG 0.050 1.2 x 10-6

Endrin 2.0 CBSG NA

Isodrin 0.06 HBC NA

NDMA 0.007 (d) 0.033 1.0 x 10-5

Trichloroethylene 3 HBC 9.9 x 10-7

Arsenic 2.35 HBC 5.6 x 10-5

Totale 7.5 x 10-5

The following chemicals have ARARs that were adjusted downward to reduce overall risk. arsenic and

trichloroethene

CBSG Colorado Basic Standards for Groundwater

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 per liter

a. Practical quanititation limit; presented only when the PQL is greater than CSRG.

b. Based on on the CSRG

c. PQL attainable by the U.S. Army

d. The remediation goal for NDMA was established at 0.007 ttg/l (7 parts per trillion (ppt)) in the

Conceptual Remedy Agreement. The current PQL readily available is 0.033 l.Lg/l (33 ppt). The

estimated risk associated with NDMA is based on a 70-year residential exposure duration.

e. Because of the variability in contaminant distribution and concentration, the maximum risk

associated with the groundwater cleanup concentrations is not expected to occur at any one

location.

21905 402010

1107121495 R02 Revised 4/23/01

G9900667
Table 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) 14111111 longl1i) (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 III loo)

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 I Inprovoll 1011 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 R02
Table 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 %voillil llill Tl&; alitimativii loriMiliv; 11milod ovorall loioloc- This allornalive reduces polontial risk This alternative rodiices potontial risk

I I I I of I it f 1 1141.1 It I I a I I (I provillo piroloclion of 1111111nil 11(111 of 1111111all 111talill fillet life$ flilvitiollifloill by Rill] provides prollicilon of both 1111111an still provides protection of both human

fill, AlIvirollninill lillidill atoll title folivilmlilloild. pinvolitills Illigiallon of Collin 11111131115 flout health and ilia environment by remedla- health anti ilia environment by

MKIA lie dole I Mlifi%l Stiffly Atom n(irth of title 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 :1111111"1 Vents. As asilinalatl laino(l In approximately 15 to 30 years. Attained In Approximately 10 to 20 yei lipAiii 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 %. 11 Is of ... IlA% .11-f 1111-1161111w4l"t 11V 11111viosill"R groundwalor, throtigh operation of do" with North Plume Group groundwater

wilitiAlinot at Ilin NIICS Allot c(toillotooliog toichargo Nll('.S slid ilia Olfpost Intercept anti through operation of Ilia 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 of inobil- '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 of groundwater within the North and volume of groundwater within the

nol rediice toxicity, mobility, Plume Group said groundwater migrating North Plume Group and groundwater

or volmno of gromidwah,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 

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1107121495 R02
Table 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 I ho itivo '14whilical aild .111111illklialivo folasibillty woul4l I Inp 101110111 abio. Technical and imploutentablo. However, Ilia

f".1'Ohilliv %Volilil 1.0 110%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 

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Table 8.2: Summary of the Detailed Analysis and Ranking of Groundwater Alternatives for the Northwest Plume Group

Allarunfivoi NW-2

Continued Oporalitin (if the Northwest

AlIntitalivo N%V-I 1114oundary Ciontninniont System With

Crilorin Nov Actitin Improvements as Necassnry

I

M niall Pr(Ilecti(pli rof 1111111all 1111,11111 This .1111orlialivoi %voillil loot liToV1110 prolection of This allooriiative would provide protection of human health and Ilia anviand Ibe 1-1,11VII0111110111 Immaii health aitil 0sn 4invirimmmil. riviiiiiiiiii by preventing migration of coutnuil na tits from RMA to the Offpost

Sludy Area north of ilia NWHCS. Poloinlini risks associate(] with ilia Northwest I'linno Group groundwater would be substantially reduced through

coidbitiod operation of Ilia NWB(;S and finprovotmovids as necessary.

Coempli.m(p Mob ARAR-t 'I 111,ollolljoilve 1, ejoii w4pa, Intl too sublown TIM allarlinliviv Is OX1100041 to innal or exceed chnnilcal-specific ARARs In

. lwiow.ol qlww iflo- ARAR-%. approxinialoly flegs to eight years, as ost!uialod by groundwater modelltie.

F114" .111.1 11.1, AllefetAllve W40061.1 11111 11111114 a lite sn.14111al This alternative would reduce residual risk associated wilh groundwater

114-01.01 It.& 4-ft 1.01 ... I wilh 114DIO-1111.11 ItI41411141walell OXIHI- williles ilia Nc)rlltwosi I'linnot Gump throtigh preventing conlantinant migration

nt tits NWBCS 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 volume or groundwater

V4111111141 1-1- 0'. 411alloplis 41141 w(pilld lifol r1141114-0 ilia migrating from RMA to Ilia Offliost Study Area. Groundwater conlaudsiont

JIV. V1111111141 lif FtI41111146.11tif concentrations would be reduced within ilia Northwest Plume Group by

wilhin lite Noilliwesl I'llinin (4111111 (if Itill"Itil- flushing provided by recharge of treated water at lite NWBCS.

waint sisilltalhig fitim KNIA to Ilia Offilost Study

Alan.

Shorl-lorui E.11roiclivoinoss Because no remedial action wotild be performed. There would be no short-term luipacls. There would he no Implementation

there would be no shriti-Intin Impacts. There period.

would be no Implementation parind.

luiplonientability The technical feasibility would he high. The This alternative Is readily implemontablov. Technical and administrative

administrative feasibility would be low. feasibility would be high.

Estimated cost Total Capital Cost = $ -0- Total Caplial Cost - $ -0-

Total Long-term O&M Cost = $0.6 to 1.3 million Total Lmig-torm O&M Cost = $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 

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1107121495 R02
Table 8.2 (continued)

ARAR Applicaldo 1), ....... ...

NWIICS NoillmwA hollo-1-11 %, C-11-oll-11-1-1

(A m ()IIIII-al loll a I ol lo.

RMA Rocky Momilalo Aiýno.sl

'21905 402010 

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1107121495 R02
Table 9.1: Estimated Costs of the offpost Operable Unit Selected Remedy

Cost'Component Alternative N-4 Alternative NW-2'

Capital Costs

Monitoring well system $ 908,000 NA

Offpost Intercept and Treatment 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 system shutdown 227.000 NA

Total estimated Annual O&M Costs

0 to 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 S 68.911,000 $ 76,143,000

Dl1,,1P Dusopropylinethyl phosphonate

NA Not applicable

O&M Operation and maintenance

a. There are no capital costs for Alternative %NV-2 because the remedial systems are currently

operational.

b. The carbon usage rate is assumed to (i., '.rvas- %*,,r time ds a result of expected decreases in

influent DINIP concentration. Th-- duralr,n tJ It.ris- bofore a decrease in carbon usage rate is

expected to occur witiun three t(, t i,--

C. A range of total costs has been -islimai---l , ii 1b. basis of the range of expected remediation.

tirueframes as estimated by the Sroun-i%%-.i1--r ruo-W r'-sults.

'21905 402010 Harding Lawson Associates

1107121495 R02
Table 10.1: Summary Evaluation of Chemical-specif Ic 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 of ilia 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 of ilia silo

S... 141 '-it im ImIslic %valo,r svshimi. Is heing used or may he used as a

A 11-1 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 5 CGR 1002-8 I-Istalillshes slatowido standards for waters of ilia 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 R02
Table 10.2: Summary Evaluation of Action-specific Applicable or Relevant and Appropriate Requirements

for the Off post Operable Unit

Applicable/

Rolovoill and

AppropFlolo

Adion-spocific

Siondoril, Roydrooov,"I

cirlinrlit. or I-loolhill--n 4 iWom IJP%o riplion Comment

Federal ARARq

Safo Drinking Wiloi Art 42 1 ISC Sol lillivi 111111i In

:111111, 7

loloOlost 411 C FR Pa I 1 14 .1 lo 14 7 Foislillsheit ilidudards (fir construction and Yos/No Applicable If reinjection wells/Ironclios are

Coultul Kiiiiiiintion-i III it( 1110".111111 will I S/l g onc. bos usod for discliargn of treated water:

rolovai.1 and appropriate if some ollier

ineiliod of reinjection is used.

I Indor tbo provisions of 40 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 of drinking walor. Tlio

level of treatment prior to reinjection, offliost

alinirnativo water supplies, and otbor remedies

are sufficient to meal ilia requirements of Ilia

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 of odorous 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 R02
Table 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 I ORR 4 11 C FH I'a I I 11. D114" 1,4 1`4111111,11 agnitclos to avold long- of short- Yes/No Requires a Soo-year floodplain to be Idenliflod

Flood P1.011 1`61.111.1g"1110111 Alplitiodix A l"1111 11111141 I's OýV)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 1 Prul 411 C IN 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 R02
8

Explanation

Offpost Study Area

Offpost Operable Unit

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

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

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

P110

...........

-"'NorthBoundary

containment System

Northwest Bounder

Containment System

Onpost

Operable

nit Mfl Ave. Irondal*

containment system

EPA

Study Rocky Mountain

Area Arsenal

CoffrMce Cey

RWA-Wo-u;ýda;;

owww

70 ScW* in

Stapleton International Airport

Figure 1.1

Rocky Mountain Arsenal Operable Units and Offpost Study Area
North Boundary Containment System 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

I I ciassirication Lax@ t.. JL Malyll ;:4ý _J Yard

Suned Lake

Buned Lake ow., ludge Area

Sluage Area ýfj7ý Owby Rod and

Gun Club

9 10 12 Poncr 7

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

Explanation

Mapt potentW contaminant source area

Sanstary sewer system

...... Chemocai sewer system

0 0. 1.0

Scale in miles

Figure 2.1

Locations of Contaminant Source Areas
8

Barr Lake

N st

=dya

0 1 2

ScWe in mHes

First Creek

Pakmhannel

Cc? Northern

Paleochannei

rthwest

Pakiochannel

ýh Boundary

Containment System

Northwest Bounds

Containment System

Onpost

Study

80th Ave. Irondale Area

Containment System

Explanation

Groundwaler flow cfrecbm

E]Northwest plume group

North plume group

Rgure 5.1

Contaminant Migration Pathways
0 3500 7000

Scale in teet

% %

% % 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

Vapors Dogrodo 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 

surface Ingestion NO NO NO

Wale(

to Surface Sorption jýý Sorption Well IF 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., A p 5,19nil W.I., Su"4::d 3111 Dermal -- qS.rroaurA sorption Sediment

-AU-0,fj 

a :n

EXPLANATION

a Quantified

NO Not quantilied

Figure 6.2

Site Conceptual Model Offpost Study Area
Secondary Secondary Secondary Exposure Exposure Receptors Quantified?

Source Release Transport Media Routes

Mechanism Process

Ingestion 11-- NJ Livestock Yes

From Ollsile Groundwal Pumping Groundwater

Onpost Groundwater Flow, Disc a(ge Sorption to Surface Water Plant Uptake Crops Yes

Ingestion L1, tock Yes

Fugitive 

Cr sj

From Ollsile -)o Deposition Soil Dermal Crops No

Dust 

Onpost Soils Entrainmeni Inhalation erresiriaLlWildlif ;Ir Wi 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 Terrestrial Wildlife Yes

Ingestion Aquatic Life Yes

Sediment De mal errestrial Wildlife Yes

L

Biola Terrestrial Wildlile Yes

" Plants Ingestion

" Animals 0 LAquatic Life Yes

Figure 6.3

Ecological Site Conceptual Model

for the 011post Study Area
.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 Facdity T-6 FE-S .&FF-.i

T-3 T

N

,-ýNortll Boundary Contmnment System

Explanation

Otfpost intercept and Treatment System extracton well

Oftpost Intercept and Treatment 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 Components
C

........

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

:IX

AP-3

AT-6 I

AT-5 Northern

Paleochannel

V

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

1---.-;ýirst Creek

PaleochanneU

Treatrnent Facility

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"5 EMMATED SURFACE SOM

DIELDRIN DlSrRUlUTloN

ými

21850M

2ý Nafive PeftIUtial ýaUland 

1

Wetland

Tme Gmve

ýeckU ýea

HomUWad SOW

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 4 LOP

1995 BOMA UIUUUIUd

MOHWN INUDO COMMON

hiýmw ýicm Wpm

SCý 1 7," 0 I= IOU Figurg 9.1

Offpost AfimI of RevegMation