### INTERIM RESPONSE ACTION (IRA) SUMMARY REPORT

Groundwater Intercept and Treatment System North of Rocky Mountain Arsenal

Final

May 2000
Jointlydevelopedandwrittenby the U.S. Environmental Protection Agency, Region8, andthe U.S.Army, Rocky Mountain Arsenal 6438-1/1

00020075

## Interim Response Action Summary Report Offpost Groundwater Intercept and Treatment System Table ofContents
1.0 Introduction .......................................................... 1 2.0 Background .......................................................... 2 2.1 Investigative Studies .............................................. 2 2.2 Nature and Extent ofContamination ................................. 3 2.3 Decision Process ................................................ 4 3.0 WorkPerfonned ...................................................... 4 4.0 Operations/Monitoring Results ............................................ 5 5.0 Construction and O&M Costs ............................................ 7 6.0 Status .............................................................. 7 7.0 References ........................................................... 8

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## Interim Response Action Summary Report Offpost Groundwater Intercept and Treatment System

1.0 Introduction InFebruary 1989, the U.S. Environmental Protection Agency (EPA), the Department of the Army,the Department ofthe Interior, the Agency for Toxic Substances and Disease Registry (ATSDR), Shell Oil Company, and theDepartment ofJustice (DOJ), entered into aFederal Facility Agreement (FFA) which established procedures for implementing the Rocky Mountain Arsenal (RMA) cleanup program as specified inthe Technical Program Plan. TheFFA also incorporated manyprovisions ofthe modified proposed Consent Decree issued June 7, 1988. The Army and Shell Oil Company shared responsibility for the various costs ofthe remediation to be developed and performed underthe oversight of theEPA, withparticipation bythe State ofColorado. The FFA specified 13 Interim Response Actions (IRAs) determined to be necessary and appropriate while studies forthe selection ofthe final remedy were ongoing. The Of1post Groundwater Intercept and Treatment System north ofthe Arsenal (FFA, Section XXII, paragraph 22. 1 (a)) is one of the 13 IRAs. The objectives ofthe InterimResponse Action (IRA) forthe offpost study area, as stated in the IRADecision Document (BLA, 1989) were asfollows: Mitigate migration ofcontaminants inalluvial groundwater as soon as practicable. Treat contaminated alluvial groundwater to provide a beneficial impact on groundwater quality. Evaluation and selection ofthe collection andtreatment 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 ofthe alluvial aquifer and theweathered portion ofthe underlying Denver Formation. Construction ofthe Offpost Groundwater Intercept and Treatment System began in November 1991 and was completed inJune 1993. Thetreatment system extracts contaminated groundwater from the UFS through anetwork ofextraction wells; treats the groundwater for organic contaminants with activated carbon; and then recharges the water back into the UFS through a combination ofrecharge wells and trenches. The Of1post Groundwater Intercept andTreatment System was designed to beflexible 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 I2.0 Background RMA occupies morethan 17,000 acres (approximately 27 square miles) in Adams County, directly northeast ofmetropolitan Denver, Colorado (Figure 1). Theproperty was purchased bythe U.S. government in 1942 foruse 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 ofchemical warfare material destruction also tookplace during the 1950s and 1960s. In addition to these military activities, between 1947 and 1982, major portions ofthe plant facilities were leased to private industries, including Shell Oil Company, for the manufacture ofvarious insecticides and herbicides. The RMA site wasproposed to the National Priorities List (NPL) in 1984 and formally designated asa Superfund site in July 1987. The migration ofRMA contaminants via groundwater north ofthe RMA boundary resulted in the designation ofthe Of1post Study Area. As shown inFigure 2 (BLA, 1995) the Offpost Study Area occupies a large area in southern Adams County, Colorado, and lies north ofthe Denver metropolitan area and east ofCommerce City, Colorado, The Offpost StudyArea isdefined as the area southeast ofthe South Platte River, north of 80th Avenue, southwest ofSecond Creek, and north ofthe north and northwest boundaries ofRMA. 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 ofFirst Creek and BarrLake. The Of1post Operable Unit (OLJ) encompasses rural, residential, agricultural, commercial, and industrial areas within South Adams County. Old erosion channels, called paleochannels, provide apreferential flow path forthe contaminated groundwater. There arethree mainpaleochannels influencing groundwater inthe Of1post OU resulting inthree plumes north ofRMA (Figure 3). The depth to groundwater varies considerably between thesethree flowpaths, 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 ofshallow well water for irrigation. The farmers believed that their crops had been damaged by chemicals manufactured atRMA. Anumber ofstudies, such as theBasin 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. InMay of 1974, diisopropylmethyl

U-\WPD0CS\0FFGVrrS.WPD 2phosphonate (DRVIP) and dicyclopentadiene (DCPD) were detected in surface water at the northern boundary. Later that year, the ColoradoDepartment ofHealth (CDH) detected DVvIP in awell north ofthe site. In 1975, in response to the detection ofoffsite contaminants, the CDH issued three administrative orders for RMAto cease and desist in activities that could result in contamination ofthe environment. In response to the cease and desist orders, aregional sampling and hydrogeologic surveillance program, known as the 360 DegreeMonitoring Program, was initiated by the Army in 1976 and consisted ofquarterly collection and analysis ofover 100 on-post and off-post surface water and groundwater samples. This program changed in scope several times from itsinception 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 monitoringtasks and initiated groundwater remediation efforts on RMA. Ton-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 ofContamination Groundwater investigations conducted under theRI programs have shown that there are three groundwater migration pathways inthe Of1post Study Area. These migration pathways (Figure 3), characterized by alluvial flow systems ofcoarser sediment, are referred to as theNorthern PaleochanneI, due north ofthe RMA north boundary; the First CreekPaleochannel, paralleling First Creekto the northwest from the RMA north boundary; and the NorthwestPaleochannel, northwest ofthe RMA northwest boundary. TheNorthern andFirst CreekPaleochannels compose what is collectively called theNorth Plume Group, andthe Northwest Paleochannel composes theNorthwest Plume Group. Thesetwo plume groups encompass an area ofapproximately 590 acres in the Ofrpost Study Area. Table I (BLA, 1995) presents the groundwater chemicals ofconcern (COCs) and the exposure point concentrations used in the Endangerment Assessment. These concentrations werebased on water quality data collected from 1989 through 1991. Figure 4 shows the zones that correspond to Table 1. The mostwidespread RMA-related groundwater COC inthe Oflpost Study Area isdiisopropy1methylphosphonate (DUVIP),

U'\WPDOCS\OFFGWTS.WPD 3which is present inthe UFS atvarying concentrations in aband from the west end ofthe Northwest Boundary Containment System (NWBCS) to the east end ofthe North Boundary Containment System (NBCS), and from theRMA 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 forthe Ofipost OU was incorporated as a necessary requirement by all the parties (Army, EPA, US Fish and Wildlife Service, CDPHE and Shell) in theFFA (DOJ, 1989). Consequently, an Alternatives Assessment wasprepared (HardingLawson Assoc., 1988) which evaluated the impact ofno 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 bythe design presented in the Implementation Document (BLA, 1991). Thedesign forthe 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 apeak flow of720 gpm and an average flow of480 gpm. 3.0 Work Performed The intercept and treatment system consisted ofconstructing an array ofextraction wells and recharge wells orrecharge trenches inthe Northern and First Creek Paleochannels along with a centraltreatment facility. The system is configured to extract, treat, and then reinject UFS groundwater that exceeds containment system remediation goals (CSRGs). Figure 5 showsthe location ofthetreatment facility, extraction and recharge wells in the Northern Paleochannel, and the extraction wells andrecharge trenches in theFirst Creek Paleochannel. TheNorthem Paleochannel Collection System consists of 12extraction wells spaced approximately 200 feet apart across the paleochannel, perpendicular to the direction ofgroundwater flow. The recharge system in theNorthern Paleochannel consists of24 recharge wells spaced 100 feet apart and placed parallel to and approximately 300 feet down gradient ofthe collection system. TheFirst Creek Paleochannel collection system consists of5 extraction wells spaced 200 to 500 feet apart along the axis ofthe paleochannel. Recharge trenches were placed so that four ofthe six trenches were parallel to the flow axis and located on the margins ofthe paleochannel, while the remaining two trenches were located down gradient ofthe 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 of300 and 180 gpm from theNorthern andFirst Creek Paleochannels

UAWPDOCS\OFFGWTS.WPD 4

### respectively, with apeak flow of1.5 timesthe combined averages. The treatment facilities treatment processes includes influent storage, pumping, bag filtration forparticulate removal, carbon adsorption, multimediafiltration, treated water storage, treated water pumping, and final bagfiltration. It was initially estimated that the Of1post Groundwater Intercept and Treatment System would treat an average annual flow ofapproximately 250 million gallons peryear. 4.0 Operations/Monitoring Results Operations and monitoring results forthe of1posttreatment system are reported by the Army in quarterly and annualreports. The quarterlyreports provide results ofeffluent water quality data derived from grab samples ofthe plant effluent. The annual of1post treatment system operational assessment reports, whichhavebeen published since fiscal year 1993, provide analytical data ofsamplestaken from theplant influent and effluent and extractionwells. Theannual reports also provide summaries ofsystem operations and results ofgroundwater elevation monitoring. System operations include modification, downtime and operational summary, flow quantities, influent/effluent water quality, treatment plant effectiveness, contaminant mass removal, carbonusage, contaminant concentrations in dewatering wells, and dewatering well contaminants. Priorto operation ofthe Treatment System, the highest DEMP concentrations were located in the First CreekPaleochannel, with DRAP concentrations ofover 1000ug/l at some places. The DRAP levels in theNorthern Pathway Paleochannel have havebeen approaximately an order ofmagnitude lower. The highest contaminant levels downgradient fromthe NBCS occurupgradient ofthe O'BrianCanal. Certain volatile compounds such as chloroform and trichloroethene have been detected atlow 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 ofthe canals. Maximum concentrations ofthe 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 theFirst CreekPaleochannel within 500 to 1000 feet ofthe NBCS. Only sporadic and isolated occurrences ofOCPs areobserved northwest oftheRMA northwestern boundary. Thefollowing annual operating parameters; average flow, totalvolume ofwater treated, and down time were reported in the annual operational assessmentreports forfiscal years 1994, 1995, and 1996 (1994-1996, PNffUAA).

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### 5Offpost Groundwater Intercept and Treatment System Summary ofAnnual Operating Parameters 1994 1995 1996 Average flow rate (gpm) 316 318 362 Total volume ofwater treated (million gallons) 166 167 191 Total down time (hours) 49 68 102 Several contaminants makeup the majority ofcontamination removed bythe treatment system from the northplume: Diisopropy1methylphosphonate (DMIP), Chloroform (CHCL3), Tetrachloroethylene (TCLEE), and p-Chlorophenylmethyl sulfoxide (CPMSO). The following table summarizes the reported annual mass removal forthese 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 ongoingremediation oftheUFS by the Offpost Groundwater Intercept andTreatment System in conjunction with cutting offadditional migration of groundwater contaminants atthe NorthBoundary Containment System have significantly reduced DRAP concentrations in the Of1post Operable Unit. This isreflected bythe average annual DMIP influent concentrations to the Offpost Groundwater Intercept and Treatment System as shownbelow: 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 showthe DRVIP concentrations in the Of1post OU from 1994through 1996. Thefigures showthat theDEMP plume has significantly smaller concentrations, particularly in theFirst CreekPathway. Table 2 liststhe chernical-specific ARAR standards forthe offpost treatment system. The annual reports for fiscal years 1994 to 1996 show that the average organic and inorganic

U:\WPDOCS\OFFGWTS.WPD 6analyte concentrations oftheplant effluent have successfully been below ARAR standards for each analyte.
5.0 Construction and O&M Costs
Theestimated cost ofthe Offpost Pump and Treat System IRAwas $18,800,000 'with estimated annual O&M costs of$375,000. Construction ofthe Ofl[post Groundwater Intercept and Treatment Systembegan 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 ofoperating 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 IRAwas intended to intercept and treat RMA contaminated groundwater migrating northward from the RMAboundary. Based onthe treatment standards set in theDecision Document (BLA, 1989), this IRA has achieved itsintended purpose by effectively intercepting the north plume group and treating it to below ARAR standards. Average concentrations ofDEVIP in the Oflpost OU have significantly reduced during each year ofoperation. Dueto its successful operation, the Offpost Groundwater Intercept and Treatment System hasbeen incorporated into both the Onpost and Oflpost Records ofDecision (ROD) forcontinued operation until the northplume concentrations meet ROD remediation requirements. Sampling continues to ensure that levels ofDMP do not exceed CSRGs listed in Table 3. The system isoperated and maintained to ensure the groundwater is adequately treated.

U.\WPDOCS\OFFGWTS.WPD 77.0 References Department ofthe Army. Final Draft Assessment ofMechanisms Impacting NNitrosodimenthylanmine (NDMA) Fate Within the North Boundary Containment System, Rocky Mountain Arsenal; June 13, 1996. Document Control Number: 3102-036-RT-BJLW. Department ofthe Army. Rocky Mountain Arsenal Oflpost Treatment System Operational Assessment, FY94 Final Report; June 1996. Document Control Number: 3102-036-RT-BJKL. Department ofthe Army. Final Startup Report ForThe Groundwater Intercept and Treatment System North ofRocky 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, FinalReport; December 1988. Harding Lawson Associates. Rocky Mountain Arsenal Offpost Operable Unit Final Record ofDecision, Rocky Mountain Arsenal, Commerce City, Colorado; December, 1995. Document Control Number: 3102-036-RT-BHTN. Harding Lawson Associates. FinalWork 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 forthe Ground-Water Intercept and Treatment System North ofRocky Mountain Arsenal Interim Response Action; January 1991. Document Control Number: 7760-009-RTBPMW. Harding Lawson Associates. Final Decision Document forthe Ground-Water Intercept and Treatment System North ofRockyMountain Arsenal Interim Response Action; July 1989. Document Control Number: TZ4-CO8001-FR-AO504. Harding Lawson Associates. Oftpost Interim Response Action Alternatives Assessment DraftFinal Report; December 1988. Document Control Number 198-RII-RTGUCD.

UAWPD0CS\0FFGWTS.WPD 8R.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 ofRocky Mountain Arsenal, Denver, Colorado- Report 1,Hydrogeological Definition, Vicksburg, Miss. U. S.Department ofJustice. Federal Facility Agreement and Settlement Agreement to Ensure the Continued Cleanup ofRocky Mountain Arsenal; February 17, 1989. Document Control Number: TES.

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=ýNw Pipre 6cTable 1: Offpost Operable Unit Groundwater Chemicals of Concern

Exposure Point Concentration (,vgA)' Chemicals ofConcern '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 ofconcern 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 R02Table 2 Chemical-Sp,ecific 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 Ifmultiple samples were taken during aquarter. 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 ID2Table 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 thatwere adjusted downward to reduce overal.1 risk. arsenic, benzene, chlorobenzene, 1,3-dichlorobenzene, trichloroethylene, andxylene. ARAR Applicable orrelevant and appropriate requirement CBSG Colorado Basic Standards forGroundwater CSRG Containment system remediation goal ENB Estimated Natural Background at thenorthboundary ofthe 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. PQLpresented onlywhen the PQLis greater thanthe CSRG. b. PQLattainable bythe U.S. Army. c. 1,3-Dichlorobenzene has not traditionally beenlisted as atarget analyte in the Rocky Mountain Arsenal target analyte suite; therefore, the analytical method used to analyze forvolatile halogenated organic compounds willneed tobe reviewed to ensure that 1,3-Dichlorobenzene can bedetected. d. PQLlisted in the CBSG standards. e. Theremediation goal forNNDMEA was established at 7parts pertriUion (ppt)in the Conceptual Remedy Agreement. The current PQLreadily available is 20ppt. f. Inorganic standard for chloride willbe met by natural attenuation consistent with the onpost remedial action. g. Inorganic standard for sulfate may be the naturalbackground concentration, which hasbeen established as 540,000 Ig/1 at the North Boundary Containment System, and will.be metbynatural attenuation consistent with onpostremedial action. The natural ýackground concentration of sulfate at the OQITS has not beenestimated dueto thelimited historical data available; however, the sulfate concentrations tend toincrease naturally as grbundwater migrates northward in this area.

2 of 2 Harding Lawson Associates 21914 102091 0311091396 ID2