INTERIM RESPONSE ACTION (IRA) SUMMARY REPORT

Groundwater Intercept and Treatment System North of

Rocky Mountain Arsenal

Final

May 2000

Jointly developed and written by the

U.S. Environmental Protection Agency, Region 8,

and the U.S. Army, Rocky Mountain Arsenal 6438-1/1

00020075
Interim Response Action Summary Report

Offpost Groundwater Intercept and Treatment System

Table of Contents

1.0 Introduction .......................................................... 1

2.0 B ackground .......................................................... 2

2.1 Investigative Studies .............................................. 2

2.2 Nature and Extent of Contamination ................................. 3

2.3 D ecision Process ................................................ 4

3.0 W ork Perfonned ...................................................... 4

4.0 Operations/M onitoring Results ............................................ 5

5.0 Construction and O&M Costs ............................................ 7

6.0 Status .............................................................. 7

7.0 R eferences ........................................................... 8

UAWPD0Cs\0FFGWTS.WPD
Interim Response Action Summary Report

Offpost Groundwater Intercept and Treatment System

1.0 Introduction

In February 1989, the U.S. Environmental Protection Agency (EPA), the Department of

the Army, the Department of the Interior, the Agency for Toxic Substances and Disease

Registry (ATSDR), Shell Oil Company, and the Department of Justice (DOJ), entered into

a Federal Facility Agreement (FFA) which established procedures for implementing the

Rocky Mountain Arsenal (RMA) cleanup program as specified in the Technical Program

Plan. The FFA also incorporated many provisions of the modified proposed Consent

Decree issued June 7, 1988. The Army and Shell Oil Company shared responsibility for

the various costs of the remediation to be developed and performed under the oversight of

the EPA, with participation by the State of Colorado. The FFA specified 13 Interim

Response Actions (IRAs) determined to be necessary and appropriate while studies for the

selection of the final remedy were ongoing. The Of1post Groundwater Intercept and

Treatment System north of the Arsenal (FFA, Section XXII, paragraph 22. 1 (a)) is one of

the 13 IRAs.

The objectives of the Interim Response Action (IRA) for the offpost study area, as stated

in the IRA Decision Document (BLA, 1989) were as follows:

Mitigate migration of contaminants in alluvial groundwater as soon as

practicable.

Treat contaminated alluvial groundwater to provide a beneficial impact on

groundwater quality.

Evaluation and selection of the collection and treatment system components that comprise

this IRA began in 1988. The system was designed to intercept, extract, treat, and

recharge contaminated groundwater from the unconfined flow system (UFS). The UFS is

composed of the alluvial aquifer and the weathered portion of the underlying Denver

Formation. Construction of the Offpost Groundwater Intercept and Treatment System

began in November 1991 and was completed in June 1993. The treatment system extracts

contaminated groundwater from the UFS through a network of extraction wells; treats the

groundwater for organic contaminants with activated carbon; and then recharges the water

back into the UFS through a combination of recharge wells and trenches.

The Of1post Groundwater Intercept and Treatment System was designed to be flexible and

compatible with the final remedy and consistent with U.S. Environmental Protection

Agency (EPA) guidance and the Federal Facility Agreement (FFA).

U-\WPD0CS\0FFGWTS.WPD I
2.0 Background

RMA occupies more than 17,000 acres (approximately 27 square miles) in Adams County,

directly northeast of metropolitan Denver, Colorado (Figure 1). The property was

purchased by the U.S. government in 1942 for use in World War H to manufacture and

assemble chemical warfare materials, such as mustard and lewisite, and incendiary

munitions. Starting in 1953, RMA produced the nerve agent GB

(isopropyl methylphosphonofluoridate) until late 1957. A significant amount of chemical

warfare material destruction also took place during the 1950s and 1960s. In addition to

these military activities, between 1947 and 1982, major portions of the plant facilities were

leased to private industries, including Shell Oil Company, for the manufacture of various

insecticides and herbicides. The RMA site was proposed to the National Priorities List

(NPL) in 1984 and formally designated as a Superfund site in July 1987.

The migration of RMA contaminants via groundwater north of the RMA boundary

resulted in the designation of the Of1post Study Area. As shown in Figure 2 (BLA, 1995)

the Offpost Study Area occupies a large area 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 Of1post 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 Of1post Operable Unit

(OLJ) encompasses rural, residential, agricultural, commercial, and industrial areas within

South Adams County.

Old erosion channels, called paleochannels, provide a preferential flow path for the

contaminated groundwater. There are three main paleochannels influencing groundwater

in the Of1post OU resulting in three plumes north of RMA (Figure 3). The depth to

groundwater varies considerably between these three flow paths, being nearly at the

ground surface along the First Creek Paleochannel to approximately 30 feet below ground

surface in the Northern Paleochannel.

2.1 Investigative Studies

Initial investigations at RMA were prompted by complaints in the mid 1950s from farmers

with land adjacent to the northwestern boundary where severe crop losses resulted from

the use of shallow well water for irrigation. The farmers believed that their crops had been

damaged by chemicals manufactured at RMA.

A number of studies, such as the Basin F Monitoring Program, were conducted at RMA

between 1956 and 1974 in an attempt to define the hydrogeologic system and identify the

toxic constituent(s) in ground and surface water. In May of 1974, diisopropylmethyl

U-\WPD0CS\0FFGVrrS.WPD 2
phosphonate (DRVIP) and dicyclopentadiene (DCPD) were detected in surface water at the

northern boundary. Later that year, the Colorado Department of Health (CDH) detected

DVvIP in a well north of the site. In 1975, in response to the detection of offsite

contaminants, the CDH issued three administrative orders for RMA to cease and desist in

activities that could result in contamination of the environment.

In response to the cease and desist orders, a regional sampling and hydrogeologic

surveillance program, known as the 360 Degree Monitoring Program, was initiated by the

Army in 1976 and consisted of quarterly collection and analysis of over 100 on-post and

off-post surface water and groundwater samples. This program changed in scope several

times from its inception to its completion in 1984 in response to the geologic, hydrologic,

and chemical information obtained and changing groundwater contamination patterns.

During this period, the Army performed numerous other contaminant monitoring tasks

and initiated groundwater remediation efforts on RMA. To n-ýitigate problems associated

with contaminant migration off-post, the following three groundwater treatment systems

were installed between 1978 and 1984 at the northern and northwestern property

boundaries to intercept and treat contaminated groundwater:

I . North Containment System Pilot Plant (1978);

2. Irondale Containment System (198 1); and

3. Northwest Boundary Containment System (1984).

2.2 Nature and Extent of Contamination

Groundwater investigations conducted under the RI programs have shown that there are

three groundwater migration pathways in the Of1post Study Area. These migration

pathways (Figure 3), characterized by alluvial flow systems of coarser sediment, are

referred to as the Northern PaleochanneI, due north of the RMA north boundary; the First

Creek Paleochannel, paralleling First Creek to the northwest from the RMA north

boundary; and the Northwest Paleochannel, northwest of the RMA northwest boundary.

The Northern and First Creek Paleochannels compose what is collectively called 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 Ofrpost

Study Area.

Table I (BLA, 1995) presents the groundwater chemicals of concern (COCs) and the

exposure point concentrations used in the Endangerment Assessment. These

concentrations were based on water quality data collected from 1989 through 1991.

Figure 4 shows the zones that correspond to Table 1. The most widespread RMA-related

groundwater COC in the Oflpost Study Area is diisopropy1methylphosphonate (DUVIP),

U'\WPDOCS\OFFGWTS.WPD 3
which is present in the UFS at varying concentrations in a band from the west end of the

Northwest Boundary Containment System (NWBCS) to the east end of the North

Boundary Containment System (NBCS), and from the RMA north and northwest

boundaries to the South Platte River. The other primary contaminants present in the

of1post UFS are chloroform, chlorobenzene, trichloroethene, tetrachloroethene,

dibromochloropropane (DBCP), dieldrin, endrin, dicyclopentadiene (DCPD), arsenic,

chloride, fluoride, and sulfate.

2.3 Decision Process

An IRA for the Ofipost OU was incorporated as a necessary requirement by all the parties

(Army, EPA, US Fish and Wildlife Service, CDPHE and Shell) in the FFA (DOJ, 1989).

Consequently, an Alternatives Assessment was prepared (Harding Lawson Assoc., 1988)

which evaluated the impact of no action versus a groundwater pump and treat system.

A groundwater pump and treat system was selected and documented in the Decision

Document (HLA, 1989) followed by the design presented in the Implementation

Document (BLA, 1991). The design for the pump and treat system presented in the

Implementation Document included five extraction wells in the First Creek pathway and

twelve extraction wells in the northern pathway. The common treatment system (GAC

adsorption) was designed for a peak flow of 720 gpm and an average flow of 480 gpm.

3.0 Work Performed

The intercept and treatment system consisted of constructing an array of extraction wells

and recharge wells or recharge trenches in the Northern and First Creek Paleochannels

along with a central treatment facility. The system is configured to extract, treat, and then

reinject UFS groundwater that exceeds containment system remediation goals (CSRGs).

Figure 5 shows the location of the treatment facility, extraction and recharge wells in the

Northern Paleochannel, and the extraction wells and recharge trenches in the First Creek

Paleochannel. The Northem 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 down gradient 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 were placed so that four of the six

trenches were parallel to the flow axis and located on the margins of the paleochannel,

while the remaining two trenches were located down gradient of the extraction well

system and oriented perpendicular to the flow axis.

The Of1post Groundwater Intercept and Treatment System was designed to extract and

treat average flows of 300 and 180 gpm from the Northern and First Creek Paleochannels

UAWPDOCS\OFFGWTS.WPD 4
respectively, with a peak flow of 1.5 times the combined averages. The treatment facilities

treatment processes includes influent storage, pumping, bag filtration for particulate

removal, carbon adsorption, multimedia filtration, treated water storage, treated water

pumping, and final bag filtration. It was initially estimated that the Of1post Groundwater

Intercept and Treatment System would treat an average annual flow of approximately 250

million gallons per year.

4.0 Operations/Monitoring Results

Operations and monitoring results for the of1post treatment system are reported by the

Army in quarterly and annual reports. The quarterly reports provide results of effluent

water quality data derived from grab samples of the plant effluent. The annual of1post

treatment system operational assessment reports, which have been published since fiscal

year 1993, provide analytical data of samples taken from the plant influent and effluent

and extraction wells. The annual reports also provide summaries of system operations

and results of groundwater elevation monitoring. System operations include modification,

downtime and operational summary, flow quantities, influent/effluent water quality,

treatment plant effectiveness, contaminant mass removal, carbon usage, contaminant

concentrations in dewatering wells, and dewatering well contaminants.

Prior to operation of the Treatment System, the highest DEMP concentrations were

located in the First Creek Paleochannel, with DRAP concentrations of over 1000 ug/l at

some places. The DRAP levels in the Northern Pathway Paleochannel have have been

approaximately an order of magnitude lower.

The highest contaminant levels downgradient from the NBCS occur upgradient of the

O'Brian Canal. Certain volatile compounds such as chloroform and trichloroethene 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. Sernivolatile

organic compounds, such as dieldrin and other organochlorine pesticides (OCPs), are

present almost exclusively upgradient of the canals. Maximum concentrations of the

OCPs (i.e., aldrin, isodrin, chlordane, 2,2-bis[p-chlorophenyl]-I,I-dichloroethene[DDE],

and 2,2-bis[p-chlorophenyl]-1,1,1-tfichloroethane[DDT]) generally occur in the First

Creek Paleochannel within 500 to 1000 feet of the NBCS. Only sporadic and isolated

occurrences of OCPs are observed northwest of the RMA northwestern boundary.

The following annual operating parameters; average flow, total volume of water treated,

and down time were reported in the annual operational assessment reports for fiscal years

1994, 1995, and 1996 (1994-1996, PNffUAA).

U*\WPDOCS\OFFGWTS.WPD 5
Offpost Groundwater Intercept and Treatment System

Summary of Annual Operating Parameters

1994 1995 1996

Average flow rate (gpm) 316 318 362

Total volume of water treated (million gallons) 166 167 191

Total down time (hours) 49 68 102

Several contaminants make up the majority of contamination removed by the treatment

system from the north plume: Diisopropy1methylphosphonate (DMIP), Chloroform

(CHCL3), Tetrachloroethylene (TCLEE), and p-Chlorophenylmethyl sulfoxide (CPMSO).

The following table summarizes the reported annual mass removal for these contaminants.

Annual mass of contaminants removed (pounds)

1994 1995 1996

Total 273.4 148.9 123.9

DUVIP 243.7 138.2 103.7

CHCL3 8.8 3.9 4.3

TCLEE 4.3 2.3 2.4

CPMSO 2.6 3.5 5.2

The implementation and ongoing remediation of the UFS by the Offpost Groundwater

Intercept and Treatment System in conjunction with cutting off additional migration of

groundwater contaminants at the North Boundary Containment System have significantly

reduced DRAP concentrations in the Of1post Operable Unit. This is reflected by the

average annual DMIP influent concentrations to the Offpost Groundwater Intercept and

Treatment System as shown below:

Average Annual DIEMP Concentrations (ug/1)

1994 1995 1996

Influent 181.6 100.1 66.5

Effluent 2.4 0.9 1.2

Figures 6a - 6c show the DRVIP concentrations in the Of1post OU from 1994 through

1996. The figures show that the DEMP plume has significantly smaller concentrations,

particularly in the First Creek Pathway.

Table 2 lists the chernical-specific ARAR standards for the offpost treatment system. The

annual reports for fiscal years 1994 to 1996 show that the average organic and inorganic

U:\WPDOCS\OFFGWTS.WPD 6
analyte concentrations of the plant effluent have successfully been below ARAR standards

for each analyte.

5.0 Construction and O&M Costs

The estimated cost of the Offpost Pump and Treat System IRA was $18,800, 000 'with

estimated annual O&M costs of $375,000.

Construction of the Ofl[post Groundwater Intercept and Treatment System began in

November 1991 with full-scale treatment beginning in June 1993. Actual IRA costs were

$19,600,000. This includes design work leading up to construction.

The annual O&M costs of operating the Offpost Groundwater Intercept and Treatment

since 1993 have been approximately $300,000, with annual utility costs at approximately

$50,000.

6.0 Status

The Oflpost Groundwater Intercept & Treatment System IRA was intended to intercept

and treat RMA contaminated groundwater migrating northward from the RMA boundary.

Based on the treatment standards set in the Decision Document (BLA, 1989), this IRA

has achieved its intended purpose by effectively intercepting the north plume group and

treating it to below ARAR standards. Average concentrations of DEVIP in the Oflpost

OU have significantly reduced during each year of operation. Due to its successful

operation, the Offpost Groundwater Intercept and Treatment System has been

incorporated into both the Onpost and Oflpost Records of Decision (ROD) for continued

operation until the north plume concentrations meet ROD remediation requirements.

Sampling continues to ensure that levels of DMP do not exceed CSRGs listed in Table 3.

The system is operated and maintained to ensure the groundwater is adequately treated.

U.\WPDOCS\OFFGWTS.WPD 7
7.0 References

Department of the Army. Final Draft Assessment of Mechanisms Impacting NNitrosodimenthylanmine (NDMA) Fate Within the North Boundary Containment

System, Rocky Mountain Arsenal; June 13, 1996. Document Control Number:

3102-036-RT-BJLW.

Department of the Army. Rocky Mountain Arsenal Oflpost Treatment System

Operational Assessment, FY94 Final Report; June 1996. Document Control

Number: 3102-036-RT-BJKL.

Department of the Army. Final Startup Report For The Groundwater Intercept and

Treatment System North of Rocky Mountain Arsenal (Offpost Treatment System),

May 27, 1994.

Environmental Science and Engineering. Oflpost Operable Unit, Remedial Investigation

and Chemical Specific Applicable or Relevant and Appropriate Requirements,

Final Report; December 1988.

Harding Lawson Associates. Rocky Mountain Arsenal Offpost Operable Unit Final

Record of Decision, Rocky Mountain Arsenal, Commerce City, Colorado;

December, 1995. Document Control Number: 3102-036-RT-BHTN.

Harding Lawson Associates. Final Work Plan for the Groundwater Monitoring Plan,

Rocky Mountain Arsenal, Commerce City, Colorado; March 11, 1993.

Harding Lawson Associates. Oflpost Operable Unit, Final Addendum Volumes 1&2;

March 30, 1992.

Harding Lawson Associates. Final Implementation Document for the Ground-Water

Intercept and Treatment System North of Rocky Mountain Arsenal Interim

Response Action; January 199 1. Document Control Number: 7760-009-RTBPMW.

Harding Lawson Associates. Final Decision Document for the Ground-Water Intercept

and Treatment System North of Rocky Mountain Arsenal Interim Response

Action; July 1989. Document Control Number: TZ4-CO8001-FR-AO504.

Harding Lawson Associates. Oftpost Interim Response Action Alternatives Assessment

Draft Final Report; December 1988. Document Control Number 198-RII-RTGUCD.

UAWPD0CS\0FFGWTS.WPD 8
R.L. Stollar & Associates, INC.. Comprehensive Monitoring Program, Final Technical

Plan, Version 3.2, Ground water; June 1989.

U.S. Army Engineer waterways Experiment Station; 1982. Regional Groundwater Study

of Rocky Mountain Arsenal, Denver, Colorado- Report 1, Hydrogeological

Definition, Vicksburg, Miss.

U. S. Department of Justice. Federal Facility Agreement and Settlement Agreement to

Ensure the Continued Cleanup of Rocky Mountain Arsenal; February 17, 1989.

Document Control Number: TES.

U'.\WPDOCS\OFFGW'rS.WPD 9
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Scale In MIle3

Figure Prepared for:

LOCATION MAP OF U.S. Army Program Manager's Office

ROCKY MOUNTAIN ARSENAL For Rocky Mountain Arsenal

Aberdeen Proving Ground, Maryland
0

Explanation

Off post Study Area

Offpost Operable ETn it

C, ........ . .

zi, - 7.r ii i*K

W,

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Northwest Bounder

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EPA

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Area Arsenal

c4ernmMe Cky

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Figure 2

Rocky Mountain Arsenal Operable Units and 011post Study Area -
F7

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st

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Area

0 1 2

0cale in miles

DOI

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ortmwest

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Area

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contairdnerit System

Explanation

14;-SS Grour4walGr fow diredon

Northvvest plume group

North plume group

Figure 3

Contamirlarit migraon Pathways
0 35M 7000

GOO

All all, as

North Bo ary

Containment System

Northwest

% Boundary

Containment

System

Explanation

Zone 1 Rural residential land use

Zone 2 Ruraj residential land use

Zone 3 Urban residential land use

Zone 4 Urban residential land use

Zone 5 Commercial and industrial land use

Zone 6 Rural residential land use

OffPoSt Study area boundary Figure 4

off post Study Area Exposure Assessment Zones
RC-fý

, J.RC

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RC-N

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t

.- 3

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.2

A FE-2 Treatment Facility T-6 FE-S FE-i

T Ný 'k, T-4 ...............

. .........

Boundary 'Co'ntainment System t

."n

Explanation

Offpost Intercept and Treatment System extracton well

Offpost Intercept and -rreawnent System recharge well

Offpost Intercept and -rreatment System recharge trench

Pipeline

Unsaturated alluvium 0 0 Ism

SCZ6 ýn 1*41

Figure 5

Alternative N-4 Remedial System Components
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Table 1: Offpost Operable Unit Groundwater Chemicals of Concern

Exposure Point Concentration (,vgA)'

Chemicals of Concern 'Zone I 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

CPMSO 14.5 10.4 7.68 -

CPMS02 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*

DDE 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

Dield.rin 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

Ethylbenzene - - 0.57 --- -

Fluoride 1830 2210* 3510* 3290* 1810 2230*

Hexachlorocyclopentadiene 0.029 0.033 0.044 0.043 0.035 ---

Isodrin 0.028 0.035 0.047 0.057 0.040

Malatl-Lion --- 0.26 0.38 0.32 -

Manganese --- 1580 - 1250 670

Oxathiane - - 1.32 2.21 - -

Sulfate 340,000* 636,000- 909,000* 1,118,000* 148,000 213,000

Tetrachloroethene 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*

Xylene 0.75 1.11 -

Not a chemical of concern in this zone

CPMSO 4-chlorophenylmethylsulfoxide

CPMS02 4-chlorophenylmethyl sulfone

DDE 2,ý-bis(p-chiorophenyl)-I,I-dicWoroethene

DDT 2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane

DIMP Diisopropylmethyl phosphonate

Pg/I Micrograms per liter

Exceeds groundwater containment system remediation goal listed in Tables 7.1, 7.2, and 7.3.

All exposure point concentratioýs represent the upper 95 percent confidence limit on the arithmetic mean of

measured concentrations in monitoring and private wells.

'21905 402010 Harding Lawson Associates 1107121895 R02
Table 2

Chem ical-Sp,ec i fic ARAR Concentrations Versus FY94 OPS

Treatment Plant Averne Effluent Concentrations

ARAR Standard Average Effluent

Analyte Abbreviation Concentration. 4tg/f Concentultion. Uv/P

1,2-DicWoroethane 12DCLE 5 1, 0.33*

Arsenic AS 50 1.71

Benzene C6H6 5 0.30

Carbon tetrachloride CCLA 5 0.34*

Cadmium CD 10 2.76*

Chloroform CHCL3 100 0.28

Chlorobenzene, CLC6H5 300 0.29*

Chlordane CLDAN 0.15 0.05*

Chromium CR 50 6.76*

Diisopropylmethylphosphonate DIMP 600 2.40

Dieldrin DLDRN 0.06 0.01*

Endrin ENDRN 0.20 0.01*

Ethylbenzene ETC6H5 680 3.09*

Fluoride F 4,000 1,280.00

Mercury HG 2 0.09*

Toluene MEC6H5 2,420 3.22*

Nitrate (as N) N03 10,000 3.90

Lead PB 50 16.83

Tetrachloroethylene TCLEE 10 0.56

Trichloroethylene TRCLE 5 0.26*

All concentrations less tha-n die respective RL's

If multiple samples were taken during a quarter. an average for dut quarter was first calculated. The average for the fiscal year was then calculated by summing die averages for the quarters

and then dividing by the number of quarters.

2) Aver-ages for values less amn the reporting limit were calculated by using 117 the reporting limit.
Table 3: Containment System Remediation Goals for the

Offpost Groundwater Intercept and Treatment System

CSRG Analyte GroM/ Source Agal:qe PQv

Organophosphoras Compounds

Diisopropylmethyl phosphonate (DRvIP) 8 CBSG

Organochlorine Pesticides 

b Aldrin (AIDRN) 0.002 CBSG 0.05 b Chl.ordane (CLDAN) 0.03 CBSG 0.095

2,2-bis(p-Chlorophenyl)-1,1-dichloroethene (PPDDE) 0.1 CBSG

2,2-bis(p-Chlorophenyl)-1,1,1-trichloroethane (PPDDT) 0.1 CBSG b Dieldrin (DLDRN) 0.002 CBSG 0.05

Endrin (ENDRN) 0.2 CBSG

Hexachlorocyclopentadiene (CL6CP) 0.23 BBC

Isodrin (ISODR) 0.06 BBC

Organophosphorus; Pesticides

Atrazine (ATZ) 3 MCL, CBSG

Malathion QvETH1*4) 100 BBC

Organosulphur Compounds

1,4-Oxathiane (OXAT) 160 FIBC

4-Chorophenylmethyl sulfide (CPMS) 30 B13C

sulfo)dde (CPMSO) 36 BBC

4-Chorophenylmethyl sulfone (CPMS02) 36 BBC

Dithiane (DITH) 18 BBC

Volatile Aromatic Organic Compounds

Benzene (C6H6) 3 BBC

Ethylbenzene (ETC6H5) 200 BBC

Tofuene (NMC6H5) 1,000 MCL, CBSG

Xylenes (XYLEN) 1,000 BBC

Volatile Halogenated Organic Compounds d 1,2-Dichloroethane (12DCLE) 0.4 CBSG 1.0

1,3-Dichlorobenzene (13DCL33)' 6.5 BBC b Carbon tetrachloride (CCL4) 0.3 CBSG 0.99

Chlorobenzene (CLC6H5) 25 BBC

Chloroform (CHCL3) 6 CBSG

Tetrachloroethylene (TCLEE) 5 MCL, CBSG

Trichloroethyl;ne (TRCLE) 3 HBC

Volatile Hydrocarbon Compounds

Dicyclopentadiene (DCPD) 46 BBC

Other Organic Compounds

Dibromochloropropane (DBCP) 0.2 MCL, CBSG

N-nitrosodimethylamine (NNDNEEA) 0.007 (e) 0.033,

21914 102091 Harding Lawson Associates I of 2

0311091396 ID2
Table 3 (continued)

CSRG

Analyte GroULknalyle CU24) Source PQV

Metal

Arsenic (AS) 2.35 BBC

Anions

Chloride (CL) 250,000f CBSG

Fluoride (F) 2,000 CBSG

Sulfate (S04) 540,0W ENB

The following chemicals have ARARs that were adjusted downward to reduce overal.1 risk. arsenic,

benzene, chlorobenzene, 1,3-dichlorobenzene, trichloroethylene, and xylene.

ARAR Applicable or relevant and appropriate requirement

CBSG Colorado Basic Standards for Groundwater

CSRG Containment system remediation goal

ENB Estimated Natural Background at the north boundary of the Rocky Mountain Arsenal

(MKES and FWEC, 1996)

BBC Health-based criteria

MCL Ma)dm= containment level

PQL Practical quantitation limit

A911 Micrograms per liter

a. PQL presented only when the PQL is greater than the CSRG.

b. PQL attainable by the U.S. Army.

c. 1,3-Dichlorobenzene has not traditionally been listed as a target analyte in the Rocky Mountain

Arsenal target analyte suite; therefore, the analytical method used to analyze for volatile

halogenated organic compounds will need to be reviewed to ensure that 1,3-Dichlorobenzene can

be detected.

d. PQL listed in the CBSG standards.

e. The remediation goal for NNDMEA was established at 7 parts per triUion (ppt) in the Conceptual

Remedy Agreement. The current PQL readily available is 20 ppt.

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 has been

established as 540,000 Ig/1 at the North Boundary Containment System, and will. be met by natural

attenuation consistent with onpost remedial action. The natural ýackground concentration of

sulfate at the OQITS has not been estimated due to the limited historical data available; however,

the sulfate concentrations tend to increase naturally as grbundwater migrates northward in this

area.

2 of 2 Harding Lawson Associates 21914 102091

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