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To determine whether nearby residents or people who work or recreate at RMA are exposed toon-site contaminants or to contaminants migrating from the site, ATSDR has evaluated theenvironmental and human components that lead to human exposure. This pathways analysisconsists of five elements: 1) a source of contamination, 2) transport through anenvironmental medium, 3) a point of exposure, 4) a route of human exposure, and 5) anexposed population. If exposure has occurred and sufficient data exists, it is possible toevaluate whether the exposure was or is at a level and of sufficient duration to be a threatto public health. The evaluation of the public health implications of exposure to RMAcontaminants follows our evaluation of exposure pathways.

ATSDR categorizes an exposure pathway as a completed or a potential exposure pathway if theexposure pathway cannot be eliminated. Pathways are considered to be completed when all fivepathway elements exist. Potential pathways are missing at least one of the five elements. It is possible that data could be developed now or in the future to establish the existence of themissing pathway element(s). Potential pathways indicate that exposure to a contaminant couldhave occurred in the past, could be occurring now, or could occur in the future. An exposurepathway can be eliminated if at least one of the five elements is missing and will never bepresent.

Table 15 identifies the completed exposure pathways not eliminated by land use controls, theIRAs, or BCSs. Table 18 identifies the potential exposure pathways. The discussion thataccompanies those tables incorporates only those pathways that are important and relevant to the site.

Completed Exposure Pathways

Contaminants have historically migrated from production, transport, and disposal areas along (unsaturated zone), surface water, groundwater, and eolian (windblown) pathways. In the past,human and other biological receptors, both on and off post, have potentially been exposed tocontaminants via these pathways. The surface water pathway has eliminated by discontinuinguse of the liquid waste disposal and process water networks. IRAs have been designed to reduceand control the threats to off-post receptors, and land-use restrictions have minimized risks to humans on post. IRAs have also been designed to isolate biota from the most toxic sources (eg. Basin F liquids and Basin A dust IRAs).

Although on-post sources have historically generated the most concern, contamination fromunidentified off-post sources located to the south and southwest of RMA are known to havemigrated on post in groundwater and surface water (see CDPHE, 1992a; ATSDR, 1983; EPA,1990; and CDM, 1993).

Because decisions have been made and IRAs implemented to reduce or eliminate manypathways of human exposure to contaminants, the pathways analyzed in this section of thepublic health Assessment are those present or future pathways not eliminated by previousactions. Past pathways of human exposure are not evaluated in detail in this Assessmentbecause, in many instances, the contaminant and environmental data are insufficient topermit a meaningful, retrospective evaluation and the possible health effects resultingfrom those past, unquantified exposures are the subject of ongoing, ATSDR-funded healthstudies conducted by the Colorado Department of Public Health & Environment and byColorado State University, Department of Environmental Health. The objectives and status of those studies are discussed in the Health Outcome Data Evaluation section of this Assessment.

Groundwater Pathways - Offpost

Alluvial Aquifer - Unconfined Flow System

The groundwater of the Alluvial Aquifer underlying RMA is a major migration pathway capableof transporting observable contamination over long distances. Within the surficial alluvialdeposits, fairly continuous paleochannels carved in the underlying bedrock are very efficientroutes of migration of contaminated groundwater. These alluvial deposits are generally severalorders of magnitude more permeable than the underlying Denver and Arapahoe Formations. Unconfined groundwater follows the major flowpaths across RMA towards the Irondale,Northwest, and North Boundary Containment Systems. Contaminant plumes aid in identifyingthese flowpaths and depict the confinement of contaminants within these pathways.

In a few cases, poor well construction practices or damaged well casings have apparentlyallowed mixing of Alluvial Aquifer contaminants with water pumped from the underlyingDenver and Arapahoe Formations confined aquifers. mixing of those contaminants with thedeeper aquifer waters during pumping or testing. These wells do not indicate contamination ofthe confined aquifers.

Many RMA contamination sources, as well as non-RMA sources to the south and west of theArsenal, have contributed to the plumes of contaminated groundwater extending to the north andwest of RMA. Sandy soils and topographic depressions allow infiltration of contaminatedsurface water that may result in recharge to the unconfined aquifer. In addition, wastewaterdischarges, sewer leaks, and chemical spills are capable of infiltrating directly to the watertable. Direct introduction to groundwater may also occur at sources such as underground storage tanks,transfer pipelines sewers, sumps, basins, ditches, and disposal pits that are in direct contact with the watertable.

Contamination has entered the Alluvial Aquifer primarily from sources in the waste disposalbasins and manufacturing areas as well as other off site sources, and has migrated north andnorthwest of RMA. The most contaminated portions of the plume are below or downgradient ofmajor sources in the RMA interior. The overall plume concentrations and configuration suggestthat the greatest contaminant releases to the unconfined flow system have occurred from Basin Aand the lime settling basins, the South Plants chemical sewer, South Plants tank farm andproduction area, and the Army and Shell trenches in Section 36. Subsidiary contributions fromformer Basin F to the contaminant plume migrating beneath Section 26 are indicated. The motorpool area and rail classification yard in the Western Study Area, and sources in North Plantssuch as the tank farm, are also major sources of contaminant releases to the unconfined flowsystem (EBASCO, 1992, p. 2-30).

The Alluvial Aquifer has carried contaminants off post along paleochannels for an unknownperiod of time during RMA's 40-year history. Although specific dates have not been establishedfor the arrival of groundwater contaminants at the north, northwestern, or western boundaries ofRMA, DIMP and other organic chemicals were detected in a well north of RMA in 1974 (ESE,1988, p. 3-49), and TRCLE (TCE) and other organic solvents were found in South AdamsCounty municipal wells on the west side of RMA in 1981 (HLA, 1992b, p. I-9). Plumes ofcontaminated groundwater extended beyond the western RMA boundary in 1981 when theIrondale BCS was completed; beyond the northwest boundary in 1984 when the Northwest BCSbecame operational; and beyond the north boundary in 1978 when the North BCS becameoperational (ESE, 1988).

The concentration of contaminants in the Alluvial Aquifer in the Offpost Study Area to thenorthwest and north of the arsenal is affected not only by the presence of paleochannels but alsoby the presence of irrigation ditches. Specifically, about 0.25 to 2.5 miles north of RMA, twolarge irrigation ditches, the O'Brian Canal and the Burlington Ditch traverse the Offpost StudyArea from southwest to northeast (see Figure 2). Infiltration of surface water from those ditchesis a significant source of recharge to the Alluvial Aquifer in the study area. Estimates containedin the Offpost Remedial Investigation (ESE, 1988, p. 3-33, 3-58) suggest that aquifer rechargedirectly from the ditches may be about 180 gallons/minute/mile or about 2,400 gpm ofgroundwater recharge for the Offpost Study Area. Downslope from the ditches, 82 percent ofthe irrigation waters applied to fields are from those ditches. This surface water irrigation isestimated to represent an additional Alluvial Aquifer recharge of about 23 to 25 inches/year orabout 2,335 gpm for the area.

Together, the leakage from the ditches and the infiltration from ditch irrigation are estimated torepresent about 67 percent of the total Alluvial Aquifer recharge in this area, and the rechargefrom the NWBCS and NBCS account for about 6 percent (ESE, 1988, p. 3-146). Because of thediluting effects of the infiltration from the irrigation ditches and the downslope application ofthose irrigation waters, the Alluvial Aquifer contaminant concentrations measured downgradientfrom those ditches are generally an order of magnitude less than the contaminant concentrationsmeasured just upgradient from the O'Brian Canal and closer to the RMA boundaries (ESE, 1988,p. 3-58).

In order to fully describe the location and character of the present Offpost groundwater pathwaysof contamination it is first necessary to briefly describe the "historical" contamination detectedand the effects of the groundwater intercept and treatment systems (BCSs and IRA-A). Byexamining this "historical perspective" and trends of change in contaminant level it is possible tomore fully describe the nature of the present completed groundwater pathways.

North Plume Group Pathway

Historical Perspective

Two of RMA's most significant groundwater contaminant plume complexes extenddowngradient from the North Boundary Containment System (NBCS). An estimated 250 to 325gpm are intercepted by the NBCS and reinjected to recharge Offpost groundwater flow (ESE,1988, p. 3-31). As groundwater flows northward from the NBCS area (estimated linear flowrates of 1.5 to 10 ft/day) into the Offpost Study Area, it divides into two major, distinctflowpaths separated by a groundwater divide located about one third of a mile north of the RMAboundary (see Fig. 5). This division of groundwater flow is caused, at least in part, by a DenverFormation (impermeable bedrock) high located in the subsurface just north of the MaulReservoir along this section of the First Creek channel. Contaminated groundwater passing tothe east of this divide comprises the Northern plume which extends north through the central andwestern portions of section 13 and then northwesterly through sections 12 and 11 (T2S, R67W)towards O'Brian Canal. Contaminated groundwater passing to the west of the divide continuesto the northwest forming the First Creek plume. Both plumes are bounded by large areas ofunsaturated alluvium in sections 13 and 14 (ESE, 1988, p. 3-28-30).

Research done for the Offpost Operable Unit Remedial Investigation Addendum (HLA, 1992a,p. 61) has delineated additional minor paleochannels that lie in this same area and were part of the same ancient drainage system. Those minor paleochannels also carry RMA contaminantsand are: (1) a channel located to the south of and parallel to the First Creek paleochannel,leading from the NBCS through the southwest corner of section 14; (2) a channel that flowsnorth of the First Creek paleochannel, also in section 14: and (3) a channel that is an easterntributary to the Northern paleochannel in section 12.

Beyond the O'Brian Canal both the First Creek and Northern plume complexes extendnorthwestward for about 2 - 2.25 miles before reaching the present course of the northeast-flowing South Platte River (see Fig. 5).

ATSDR determined the contaminants of potential concern in the Offpost groundwater on theNorth and Northwest Plume Groups and those contaminants are listed in Table 5A. Within theNorth Plume Group pathway the distribution of those specific contaminants differs in detailbetween the Northern and the First Creek plumes complexes. However, the most widespreadcontaminant detected in the groundwater of the Offpost Study Area is theorganophosphorus compound diisopropyl methylphosphonate (DIMP), a highly mobile,semivolatile organic chemical associated with the manufacture of Army nerve-agent. Because ofits mobility and persistence in the Offpost Study Area and the fact that it is solely related to RMA, DIMP is a good indicator of migration pathways (ESE, 1988, p. 1-3 & 3-124).

DIMP, as illustrated by the 1990 plume map given in Figure 6, is distributed in a near-continuous plume extending from the north and the northwest boundaries of RMA to the SouthPlatte River. In general, the highest concentrations of DIMP occur between the north boundaryof RMA and the O'Brian Canal. The highest concentrations of DIMP detected in the Offpostwere 5,800 ppb in the First Creek Plume and 860 ppb in the Northern Plume. The 1985-1993"historical data" recorded in the RMAED database indicates that a maximum of 1620 ppb (in anirrigation well) and an average of 72.6 ppb DIMP were found in private tap water wells in theplume area between the Arsenal and the South Platte River. The Offpost concentration of DIMPhas, since 1985, declined considerably under the influence of the groundwater treatmentfacilities.

It must be noted that, beginning in September and completed in October, 1990 domestic wellusers with wells contaminated by a trace or more of DIMP were offered and most provided withbottled water by the state of Colorado or, if more than 600 ppb DIMP was detected, areplacement well drilled to a confined aquifer system by RMA (Dan Collins, Tri-County HealthDept., December 8, 1993, and October 18, 1994, personal communication; B. O'Grady, CDPHE,December 12, 1993, personal communication; Tim Kilgannon, RMA, December 08, 1993,personal communication). Thus, because DIMP is one of the most wide-spread, RMA-related contaminants in the Alluvial Aquifer of the Offpost Study Area, by greatlyreducing the number of residents exposed to DIMP, the exposure by ingestion of wellwater contaminated by other, less extensively disseminated semivolatile, volatile, andinorganic compounds was largely eliminated as well.

Other semivolatile organic compounds of potential concern that have been detected in theOffpost include dicyclopentadiene (DCPD); the organochlorine pesticides dieldrin and endrinand much less frequently aldrin and chlordane; atrazine; and the organosulfur compoundchlorophenylmethyl sulfoxide (CPMSO) and less frequently chlorophenylmethyl sulfone(CPMSO2) and sulfide (CPMS; see Table 5A).

The relative distribution of those contaminants was assessed in the Offpost RI and Addendum(ESE, 1988; HLA, 1992a) and incorporated in the Offpost Operable Unit EndangermentAssessment/Feasibility Study (HLA, 1992b, p.1 - 12-14). From those documents and the 1985-1993 "historical" contaminant data recorded in RMAED, several generalizations can be drawn. First, it should be noted that the highest contaminant levels for semivolatile compounds in theOffpost occur downgradient from the NBCS and upgradient from the O'Brian Canal. Compounds such as dieldrin and other organochlorine pesticides have been found, almostexclusively, upgradient from the canals. The following additional generalizations can be madeabout the historical distribution and concentration of those contaminants:

  • The detections of DCPD have been generally limited to the First Creek Plume but isolated occurrences of DCPD have been detected in the Northern paleochannel (HLA, 1992a, p. 62). The maximum concentration of DCPD in the Offpost 1490 ppb, but it has not been detected in tap water in concentrations greater that 37.9 ppb.
  • The highest tap water concentrations of dieldrin and endrin were detected in the First Creek plume (0.87 and 0.78 ppb respectively) and in much lower concentrations in the Northern Plume.
  • Overall, the distribution of atrazine is similar to the organochlorine pesticides, but the maximum concentration (72.9 ppb) was found in the Northern Plume while a lesser concentration (46.0 ppb) was found in the First Creek Plume. Atrazine has not been found in concentrations greater than 5.8 ppb in tap water samples collected from this plume group.
  • The organosulfur compounds CPMSO and CPMSO2 have been found inconcentrations as high as 380.2 ppb and 60.9 ppb respectively in this plume group. Theconcentration of CPMSO was higher in the Northern paleochannel and the CPMSO2 wasfound in greater concentrations in the First Creek paleochannel. Tap waterconcentrations of CPMSO have not exceeded 17 ppb and CPMSO2 concentrations havenot exceeded 10.1 ppb.

Based upon the same data sources used to assess the semivolatiles, the volatile organiccompounds of potential concern most frequently detected in the Northern Plume Group includechloroform, chlorobenzene, dibromochloropropane (DBCP), tetrachloroethylene (TCLEE), 1,2-dichloroethane (12DCLE), and benzene. The highest concentrations of those volatilecompounds was detected upgradient of the O'Brian Canal and only isolated detection of lowconcentrations of those compounds were found downgradient from the canals. The followingadditional generalizations can be made about the historical distribution and concentration ofthose contaminants:

  • During the 1985-1993 interval the distribution of chloroform in the Offpost Study area was almost as wide-spread as that of DIMP. In the Northern Plume Group, chloroform occurred primarily in the Northern plume in concentrations that were as high as 1660 ppb. Tap water concentrations of chloroform have not exceeded 100 ppb.
  • Chlorobenzene contamination was confined to localized portions of the First Creek and Northern plumes near and upgradient to the O'Brian Canal. As much as 260 ppb chlorobenzene have been detected in this plume group, but tap water concentrations of chlorobenzene did not exceed 9.9 ppb.
  • DBCP was found in the Northern plume in concentrations as high as 14.0 ppb and in tap water as much as 0.46 ppb was found.
  • TCLEE and 12DCLEE were found in both the Northern and First Creek plumes. TCLEE has been found in greater concentrations in the Northern plume, and 12DCLEE occurred in greater concentrations in the First Creek plume. The maximum concentration of TCLEE and 12DCLEE detected were 1200 ppb and 2210 ppb respectively. Tap water samples contained no more than 81.8 ppb of TCLEE and 25 ppb of 12DCLEE.
  • Benzene was found in both plumes of the Northern Plume Group at a maximumconcentration of 109 ppb. Tap water concentrations of the compound did not exceed 12ppb.

Using the data recorded in the Offpost RI and Addendum (ESE, 1988; HLA, 1992), OffpostOperable Unit Endangerment Assessment/Feasibility Assessment (HLA, 1992b, p. 1-12-14), andRMAED, the inorganic contaminants of potential concern have been identified include arsenic,lead, mercury, nitrate, and selenium. RMA is not the source of these contaminants that mayresult from several sources, including natural sources of these elements. The followinggeneralizations can be made about the historical distribution and concentration of inorganiccontaminants detected in the North Plume Group:

  • Arsenic has been detected at levels greater than natural background levels for the RMA area (<2.5 ppb; ESE, 1988, p. 3-69) upgradient from the O'Brian Canal in the First Creek plume and in scattered occurrences in the Northern plume. The maximum value detected in Offpost monitoring wells was 37.4 ppb, but arsenic levels averaged 6.8 ppb (max. 14.4) in tap water samples collected.
  • Lead is naturally present in the Alluvial Aquifer south of RMA at a level of less than 18.6 ppb (ESE, 1988, p.3-69, 3-74). Lead is not considered to be a RMA-related contaminant. Lead has been detected at level greater than this background level in the First Creek plume and at some sampling locations downgradient from the O'Brian Canal. The maximum concentration of lead detected in the Northern Plume Group was 119 ppb in tap water.
  • Mercury background concentrations measured upgradient from RMA is less than 0.36 ppb (ESE, 1988, 3-69). Mercury has been detected in scattered, sporadic locations at levels greater than the background value in 108 of 1338 samples collected (1985 - 1993). The maximum concentration of mercury detected in the Offpost Study Area was 3.66 ppb in tap water. Mercury has been detected in 23 of 312 tap water wells sampled in the Offpost and those detections average 1.1 ppb. The FY90 Comprehensive Monitoring Program (CMP) reported possible field or laboratory contamination of samples and thus, some of the mercury data may be questionable (HLA, 1992b, p. I-16).
  • Nitrate background levels measured upgradient (south and southeast) from RMArange from 700 - 22,900 ppb (ESE, 1988, p. 3-69). Historically, Onpost nitrates havealso been detected upgradient from the BCSs in a similar range of concentrations. Thus,nitrates detected in the Offpost area north of the Arsenal cannot be considered to be aRMA-related contaminant. Total nitrate-nitrite, undifferentiated (NIT) levels in excessof the 10,000 ppb MCL have been detected in widely-separated drinking water wells inthe Offpost Study Area. Of the 325 tap water samples collected in the 1985-93 interval,303 samples contained an average of 5,485 ppb nitrate.

    Drinking water wells that have been found, at one time or another, to contain elevatednitrate levels are generally located two or more miles north of RMA, in the more denselypopulated areas of the Offpost. Since the January 30, 1991, 85 wells located north(downgradient) of the Burlington Ditch and O'Brian Canal have been found to containnitrate levels greater than 10,000 ppb. Continued monitoring by TCHD hasdemonstrated that those nitrate levels fluctuate but show a generally sustained decliningtrend. See Figure 8 for examples of this trend. As of August, 1996, no drinking waterwells in the Offpost Study area have been found to contain nitrate level in excess ofthe 10,000 ppb MCL.

  • Selenium was detected in 6 of 167 samples collected in the Offpost Study Area inconcentrations averaging 281 ppb (max. 679 ppb) in monitoring wells and in 3 of 3 tapwater samples collected at concentration averaging 20 ppb (max. 24.7 ppb).

Effects of the North Boundary Containment System (NBCS) and IRA-A the Groundwater
Treatment System North of RMA

To fully describe the present character of the Northern Plume Group pathway, it is important to summarize the effects of the North Boundary Containment System, operational since 1978, andthe planned effects of IRA-A, the groundwater intercept and treatment system installed in 1993,astride the Northern and First Creek paleochannels, just upgradient of the O'Brian Canal (seeFig. 5).

Evaluation and monitoring of the effectiveness of the NBCS has documented the changes incontaminant concentration and distribution over time. The Offpost Operating Unit RemedialInvestigation (ESE, 1988, p. 3 - 123-133) summarizes a ten-year record (1978-88) of decline inthe downgradient concentration of key-contaminants as well a pattern of systematic changes inthe downgradient distribution of those contaminants.

That data shows, for highly mobile contaminants such as DIMP, DCPD, and DBCP, theconcentration generally exhibits exponential decline with time (see Fig. 9). Dieldrin, a lessmobile contaminant, exhibits only a slight decrease over the decade evaluated.

The areal distribution of those contaminants also exhibits a progressive or systematic change. Over the fifteen-year period of operation of the NBCS, the effects of intercept, treatment, andreinjection have produced, for highly mobile contaminants, greater than a one-half mile hiatus orinterruption in the continuity of the plume concentrations. For example, DIMP at aconcentration of about 1000 ppb is found immediately upgradient to NBCS, but in 1988, anequal concentration of DIMP could not be found within about one-half mile downgradient fromNBCS. Thus, the "trailing edge" of the more-mobile contaminants plumes is moving away fromRMA and migrating towards additional sources of dilution (eg. the irrigation canals and theinduced recharge of flood irrigation). As before, less mobile contaminants such as dieldrin donot show as large or as systematic a reduction in apparent plume size.

The results of the Offpost RI (ESE, 1988; HLA, 1992a) have shown: 1) the First Creek andNorthern paleochannels are the primary pathways for migration of groundwater contaminants inthe Offpost; 2) the highest concentration of contaminants in the Offpost alluvial groundwateroccurs along these two pathways upgradient of the Burlington Ditch and the O'Brian Canal; and3) the greatest flux of contaminants to downgradient areas is along those paleochannels wheresaturated alluvium thicknesses are greatest and contaminant concentrations highest. Also,because of the demonstrated effectiveness of the NBCS in reducing the size and concentration ofcontaminants in the First Creek and Northern plumes in the Offpost area, the decision was made,consistent with the provisions of the Federal Facility Agreement (EPA, 1989) to undertake IRA-A and construct an additional groundwater intercept and treatment system in both the First Creekand Northern paleochannel/plume areas, just upgradient from the irrigation canals. Implementedin 1993, this IRA groundwater intercept and treatment facility located partially in sections 11, 12, and 14 (T2S, R67W) will, over time, further reduce the concentration of contaminantsdowngradient from this facility. To permit construction of the IRA and to ensure elimination of present and possible future human exposure from ingestion of Alluvial Aquifer well water in the area above the greatest concentration of contaminants in the First Creek plume, Shell Chemical purchased approximately 360 acres lying in the SW1/4 section 13 and the S1/2 S1/2 and the SE1/4 section 14 (T2S, R67W). This acreage has been converted to non-residential use.

Northwest Plume Group Pathway

Historical Perspective

As groundwater flows from the Northwest Boundary Containment System (NWBCS) into theOffpost Study Area, much of the flow is channeled into a deeply incised, north-northwest-trending paleochannel. Groundwater flow not channeled down this pathway spills into a secondshallow, north-trending paleochannel beginning in the southern portion of section 14, lying tothe east of the main paleochannel about one quarter mile. (see Fig. 5). The total groundwaterflow being intercepted by the NWBCS has been estimated to be about 185 gpm or about 55-75percent of the groundwater flow intercepted by the NBCS. In the Offpost area, lineargroundwater velocities in the Alluvial Aquifer deposits in this paleochannel are estimated torange from 5.5 to 22.5 ft/day (ESE, 1988, p. 3-31).

The NWBCS, completed in 1984, intercepts a contaminated plume of groundwater in theAlluvial Aquifer, the Northwest Plume. This plume, is characterized in the Onpost by thepresence of DIMP, chloroform, DBCP, endrin, chlorobenzene, and dieldrin at concentrationabout an order of magnitude less than the contaminant concentrations approaching the NBCS(ESE, 1988, p. 3-58).

Contamination downgradient from the NWBCS has consisted mainly of chlorobenzene,chloroform, DIMP, and dieldrin. In general, those contaminants and other minor contaminantsoccur in relatively low concentrations compared to contaminant concentrations both up- anddowngradient to the NBCS (ESE, 1988, p.3-58,; HLA, 1992a, p. 63). Unlike the North PlumeGroup, the concentration of contaminants in the Alluvial Aquifer does not seem to be diluted byrecharge from the irrigation ditches traversing the plume's path (HLA, 1992a, p.63).

The contaminants of potential concern in the Northwest Plume Group have included in thosecontaminants listed in Table 5A. The distribution and concentration of the specific contaminantsin the Offpost area along the course of the Northwest Plume Group are summarized in thefollowing discussion.

The semivolatile organic compounds of potential concern detected in the Northwest PlumeGroup are DIMP and dieldrin. The historical distribution of those contaminants was assessed inthe Offpost RI and Addendum (ESE, 1988; HLA, 1992a) and incorporated in the OffpostOperable Unit Endangerment Assessment/Feasibility Assessment (HLA, 1992b, p.1-12-14). From those documents and the 1985-1993 contaminant data recorded in the RMAED, severalgeneralizations can be drawn.

  • DIMP was detected in the northwest paleochannel at a maximum of 80 ppb in a domestic tap water well about 2 miles northwest of the RMA boundary and just upgradient to the Fulton Ditch. The plume of DIMP contamination has extended from the RMA northwest boundary to the South Platte River, a distance of about 3 miles. The concentrations of DIMP measured in the Northwest plume have generally been below 10 ppb.
  • Dieldrin was detected downgradient from the NWBCs in two apparently distinctplumes. Concentrations of dieldrin in those plumes ranges from about 0.05 to 0.1 ppb,with the greatest concentrations detected closest to the RMA boundary. Dieldrin has notbeen detected in tap water samples collected from the Northwest Plume Group.

The volatile organic compounds of potential concern detected in the Offpost Study Areadowngradient to the NWBCS in the Northwest Plume group have been chloroform andchlorobenzene. The relative distribution of those contaminants was assessed in the Offpost RIand Addendum (ESE, 1988; HLA, 1992) and incorporated in the Offpost Operable UnitEndangerment Assessment/Feasibility Assessment (HLA, 1992b, p.1-12-14). From thosedocuments and the 1985-1993 historical contaminant data recorded in the RMAED, severalgeneralizations can be drawn.

  • Chloroform concentrations in the Northwest Plume Group have been considerablylower than those detected in the North Plume and based upon the data collected for theOffpost RI and RI Addendum (ESE, 1988, p. 3-64; HLA, 1992, p. 44) range from amaximum of 25.8 ppb near the northwest RMA boundary to a minimum of about 0.05ppb (the CRL) at the measured extent of the plume near the Fulton Ditch and about 2-miles beyond the NWBCS.

    Chloroform, like DIMP, is highly mobile in the groundwater flow system, and thus theextent of chloroform contamination in the Northwest Plume Group described in the RIAddendum (HLA, 1992a, Fig. 3-6) may be approximately the maximum area ofhistorical contamination. Within this plume area, chloroform has been detected in twotap water wells at levels ranging from >0.05 to <5.0 ppb.

  • Chlorobenzene was, in the samples collected for the RI (ESE, 1988, p. 3-66),consistently observed in Offpost samples collected downgradient to the NWBCS atconcentrations from 1.0 to 7.0 ppb. The samples collected in 1989-1990 for the RIAddendum (HLA, 1992, p. 44) recorded sporadic detections in wells downgradient to theNWBCS and in wells downgradient to the irrigation canals. Chlorobenzene was found ina maximum concentration of 4.53 ppb in an isolated detection in a monitoring well, butwas not detected in tap water samples collected.

Using the data recorded in the Offpost RI and Addendum (ESE, 1988; HLA, 1992a), OffpostOperable Unit Endangerment Assessment/Feasibility Assessment (HLA, 1992b, p.1-12-14), andthe RMAED, the following information is know about the non-RMA sourced inorganiccontaminants of potential concern in the Northwest Plume Group pathway:

  • Arsenic has been detected at levels greater than natural background levels for the RMA area (<2.5 ppb) in isolated occurrences downgradient to the NWBCS. A maximum of 5.8 ppb arsenic was found in a monitoring well near the northwest RMA boundary.
  • Mercury has been detected in four monitoring well samples collected in the OffpostStudy Area at distances ranging from 2,000 to 7,000 feet from the RMA northwestboundary. Mercury concentrations in those wells ranged from 0.210 to 1.64 ppb (HLA,1992, p. 50).

Effects of the Northwest Boundary Containment System (NWBCS)

The NWBCS, located in section 22, T2S, R67W (see Figure 2 and 5), became operational in1984. Contaminant bypass was noted at the northeast end of the system in 1988 and a series ofactions have been undertake to upgrade the system to eliminate contaminant bypass. Operationof the NWBCS was continued during the Short-Term Improvements IRA as series ofimprovements were made to the system between 1988 and August, 1991. Under the provisionsof a NWBCS Long-Term Improvements IRA and both the Onpost and Offpost RODs (FosterWheeler, 1996; HLA, 1995), the performance of the system will continue to be monitored toassess the possible need for additional modifications to this system.

Both the RI and the Addendum (ESE, 1988; HLA, 1992a) noted that the highest concentrationsof chloroform were still found immediately downgradient to the NWBCS. If the NWBCS waseffectively removing this highly mobile contaminant its distribution by 1991 would be expectedto be similar to that described above for DIMP downgradient to the NBCS. Less mobilecontaminants such as dieldrin were found in the highest concentrations immediately below theNWBCS and decreased further downgradient of the system; an expectable pattern of distributionand analogous to that of the NBCS. At that time, no pattern of dilution downgradient to theirrigation ditches could be detected in this area.

Data on the principal contaminants downgradient to the NWBCS continues to be gathered by theRMA Comprehensive Monitoring Program (CMP). NWBCS-specific data was also gathered toevaluate the Short-Term Improvements IRA. That investigation determined that systemmodifications have greatly improved and probably eliminated offpost migration of the targetcontaminants. By January, 1993 the downgradient concentrations of dieldrin in monitoringwells had decreased below the detection limit (0.05 ppb) and the measured concentrationchloroform had decreased at both the system influent and effluent wells to about 6 ppb (MKES,1993).

Past Pathway Summary - Northern and Northwestern Plume Group Pathways

A past pathway of human exposure to groundwater contaminants from drinking water (tapwater) wells producing water from the Alluvial Aquifer existed from an indeterminate time afterthe start of operations at RMA in 1942 until the replacement of most of those contaminated wellsin 1990 by bottled water and/or the construction of replacement wells drilled to underlyingconfined and uncontaminated aquifers. Prior to October 1990, a completed pathway existed forhuman exposure by ingestion, inhalation, and dermal exposure to groundwater contaminatedwith those chemicals listed as historical detections in Table 5A.

ATSDR-funded health studies on the possible effects of human exposure to RMA contaminantsare being conducted by the Colorado Department of Public Health & Environment (CDPHE)and, by an ATSDR grant through CDPHE, by the Department of Environmental Health,Colorado State University. Those site-specific, focused health studies eliminate the need toqualitatively assess the possible health effects of past (pre-1990) exposure to groundwater-borncontaminants north and northwest of RMA. However, to date, those studies have notdemonstrated human health effects that can be attributed directly to exposure to RMAcontaminants.

Present and Possible Future Alluvial Aquifer Pathways

Northern and Northwestern Plume Group

Present and possible future pathways of human exposure to those contaminants listed as the1990-1993 detections in Table 5A exists for those individuals who have continued (sinceOctober, 1990) to use private wells (tap water) located within the Northern and NorthwesternPlume complexes that produce water from the Alluvial Aquifer. The duration and degree ofexposure to RMA-related contaminants is, in large part, a function of the distance from RMA;the relative position of the producing wells to the center of mass of the contaminated plume; and the location of the wells relative to the O'Brian Canal and Burlington Ditch.

Exposure to DIMP and other contaminants (RMA or non-RMA-related) at some level continuesfor those Alluvial Aquifer well users residing within the Offpost Study Area that: 1) Did notaccept or is not aware of the bottled water; 2) Accepted bottled water for drinking (ingestion)uses, but continue using contaminated well water for other domestic purposes; or 3) Wereprovided replacement water (bottled water or a new well to the confined aquifer system) butcontinued to use contaminated Alluvial Aquifer water.

The number of Alluvial Aquifer drinking water wells or irrigation wells, in use within or nearthe Offpost Study area has been determined by a review of the extensive inventory of those wellsincorporated in Tri-County Health Department the Offpost Study Area. An October 1, 1994query of that database, updated last on September 28, 1994, shows there are a total 517 alluvialwells present in the Offpost Study Area. Of those, 163 are not used and may be put back intosome type of service at a later date. Domestic and drinking water wells total 127, of which 56are drinking and domestic use and 71 are domestic use only. The remaining 227 wells are usedfor irrigation or other types of use. By April 1994, 119 of the 127 domestic and drinking waterwells have been sampled by the Tri-County Health Department. Some of those domestic anddrinking water wells have never shown any contamination and some are beyond the limits of theOffpost Study area (TCHD, 1994; Dan Collins, Tri-County, April 25, 1994, personalcommunications).

On January 5, 1994 the Colorado Department of Public Health & Environment providedinformation and a database printout describing the bottled water recipients in the Offpost StudyArea. Listed were "active accounts" for individuals or businesses receiving bottled water fromthe state. The 119 "active accounts" represent substitution of bottled water in lieu of well waterfor the drinking water (ingestion) use in that home or establishment.

Those bottled water accounts replace the use of about 85 wells for drinking water use, but thoseresidences supplied with bottled water presumably continue to use well water for cooking,washing, and irrigation purposes. CDPHE records indicate that of the 85 wells "replaced" withbottled water, about 45 of those wells are developed in the Alluvial Aquifer and 40 areproducing water from the Arapahoe Aquifer.

Data derived from several queries of the RMAED (10/14/93, 03/31/94, 04/26/94, 05/03/94), theCDPHE Bottled Water Database (01/05/94, 05/09/94) and the Tri-County Offpost Study Areaprivate well database (12/20/93, 05/11/94), coupled with review and database audits performedby CDPHE and Tri-County, indicates that there were a total of about 31 Alluvial Aquiferdrinking water (tap water) domestic or commercial facility wells that, as of the April 1994sampling event, remained in use and that produced water containing one or more of thecontaminants of potential concern.

Table 13 identifies the specific wells and contaminants that comprise a present and possiblefuture pathway of human exposure to contaminants that may result in adverse health effects. Please note that of the 31 individual wells that contain one or more contaminants abovecomparison values (CVs), only 6 wells (456A, 565A, 579A, 739A, 973A, and 1276A) were notserved by the CDPHE bottled water program in 1994. Using the demographic data presented inTable 3 of this Assessment and data derived from the TCHD (1994) database, it is estimated thata total of about 20 persons were using those affected wells in 1994. Ingestion, inhalation, and dermal exposure to contaminant(s) currently exists from use of those 6 wells.

Residents, supplied with bottled water, used the remaining 25 wells listed in Table 13 mayexperience inhalation and dermal exposure to contaminants at the levels listed in the Table and,based upon Tri-County Health Department estimates, ingestion of about 5 percent of their totaldaily ingestion through incidental ingestion and such uses as cooking and brushing

Table 13.

Recent (1991-1996) detections of drinking water well contaminants at levels of potential concern: Alluvial/Unknown Aquifer, Offpost Study Area (a)
Well # Detected Value >
CV (b)
Sampling Date
Well Use
Bottled Water
Provided(e) & (g)
Number of
Volatile Halogenated Organics - (VHOs)
1,2-Dichloroethane (12DCLE) (CV = 0.4 ppb CREG; 5 ppb MCL)
967A1.57 92197DY / NQ?6
Chloroform (CHCL3) (CV = 6 ppb CREG; 100 ppb MCL Total Trihalomethane)
540A8.3491078D - Trailer Ct.Y72*
967A18.992197DY / NQ?6
Tetrachloroethylene (TCLEE) (CV = 0.7 ppb CREG; 5 ppb MCL)
540A2.1691322D - Trailer Ct.Y72*
Atrazine (ATZ) (CV = 3 ppb LTHA & MCL)
ICP Metals / Anions & Cations
Arsenic (AS) (CV = 0.02 ppb CREG; 50 ppb MCL;
Natural Background Level <2.5 ppb)
Lead (PB) (CV = 15 ppb EPA Action Level; Natural Background Level <18.6 - <37.2 ppb)
540A92.691322D - Trailer Ct.Y72*
550A65.591317DY / NQ?16
Nitrate (NIT) (CV = 10,000 ppb MCL) NOTE: All of the wells listed below are contain less than 10,000ppb Nitrate as of August 1, 1996.
540A26,000/10,00091322/93168D - Trailer Ct.Y72*
547A12,700/6,60094277/95213DDY / NQ?3*
Nitrate (NIT) - continued
    (a) RMAED database runs of 04/26/94 and 05/04/94; CDPHE database queries of 01/05/94 and 05/09/94; andTri-County Health Department database queries of 12/20/93 through 08/01/96.
    (b) > CV = greater than or equal to the Comparison Value (see the Environmental Contaminants and OtherConcerns section of this Assessment for further discussion).
    (c) yr/day = the year followed by the chronologic, numerical date (eg. "94109" is April 19, 1994).
    (d) The uses of wells in the RMA Offpost Study Area were inventoried by the Tri-County Health Departmentand incorporated in their database of Offpost wells. The symbols used are "D" for domestic use (cooking,washing, and bathing) and "DD" for drinking and domestic use. Wells coded "D" are assumed to involve 5%ingestion from brushing teeth and incidental ingestion. Wells originally coded "DD" that currently receivebottled water are not currently used for drinking use are the equivalent of "D" wells at this time.
    (e) Source - Colorado Department of Public Health & Environment, Bottled Water database, 12/20/93 andsubsequent queries through 05/10/94.
    (f) Source - Colorado Department of Public Health & Environment, Bottled Water and Tri-County HealthDepartment databases. Entries marked with an "*" are estimates based upon the demographic data compiled forthe Offpost Study Area (see Table 4).
    (g) Wells that may not qualify (NQ) for replacement under the provisions of the Conceptual Agreement(Colorado, 1995) and the Offpost ROD (HLA, 1995).

teeth. Using the data on number of residents supplied with bottled water compiled by CDPHEand supplementing that data, where necessary, with estimates based on the demographic datapresented in this Assessment (see Table 3) and data from the TCHD database, it is estimated that a total of about 169 persons used those affected wells in 1994.

Table 13 also identifies 6 wells where, in 1994, residents were being exposed to multiplecontaminants. Those 6 wells are 540A, 578A, 579A, 613A, 616A, and 967A. Of those 7 wells,only well 579A was not on the bottled water program as April 1994.

Despite the thorough inventory and active monitoring of Offpost well water use conducted byTri-County Health Department, both Tri-County and ATSDR recognize that there may be someAlluvial Aquifer well users in this large area that may have been missed by the inventory, areunaware of the bottled water program, or have refused to cooperate. Thus, there may be a verysmall number of additional domestic wells in use that may contain contaminant levels similar to1990-93 levels given in Table 5A.

The decision (Offpost ROD, HLA, 1995; Onpost ROD, Foster Wheeler, 1996) to supplymunicipal water or an alternative water source to domestic well users within the "DIMPfootprint area" (DIMP >0.392 ppb) will result in the availability of replacement water for manyof the wells that, at one time or another, have been found to contain detectable level ofcontaminants. Therefore, the duration of any current pathway of exposure will be brief. In thefuture, owners of any additional domestic wells, new or existing, found to have DIMPconcentrations of 8 ppb (or other relevant Colorado Basic Groundwater Standard at the time) orgreater will be connected to a water distribution system or provided with an alternativepermanent solution (Foster Wheeler, 1996). For this reason, any future pathway of exposure to contaminants exceeding relevant standards should be brief.

The CDPHE, TCHD, and Army have completed an initial determination of the specific wellsthat will be replaced by municipal or alternative water supplies (CDPHE, 1996). This reviewindicated that about 32 wells previously replaced by bottled water fall outside the area ofcontamination described by the "DIMP footprint area". Those wells will not will apparently notqualify for the continuation of the bottled water program.

Only three wells (547A, 550A, and 967A) listed in Table 13 will apparently not qualify for areplacement water supply. Sampling conducted in 1994 found 12,700 ppb nitrate in well 547A. However, by July 1995 nitrate levels in that well had dropped to 6,600 ppb; below the 10,000ppb MCL. Therefore, this well is not now considered to be of public health concern. Samplingof well 550A in 1991 found 65.5 ppb lead. That well, used by 16 people, has not been retested. When the bottled water program is discontinued, users of this well may be exposed to elevatedlevels of lead in their drinking water. Sampling of well 967A in 1992 found 1.57 ppb 12DCLEand 18.9 ppb CHCL3. Both those sampling results are below the MCLs for those compounds. Evaluation of the contaminant levels detected in this well indicate that no adverse health effects are expected from the continued use of this well.

Arapahoe Aquifer - Confined Flow System

Historical Perspective

The top of the Arapahoe Formation is approximately 250-300 ft below the ground surface at theRMA north and northwest boundaries and subcrops (is the bedrock unit which immediatelyunderlies the surficial alluvial deposits comprising the Alluvial Aquifer) about 3 milesdowngradient of those boundaries in the Offpost Study Area. The sandstone aquifers of theArapahoe Formation are artesian (ie. in all wells, the water level rises above the bottom of theupper confining bed). The aquifer is confined by the "buffer zone" which consists of 30-100 ftof clay shale and claystone. Where present (not truncated by erosion), this clay-rich bedprovides hydraulic separation from the Denver Formation and any contaminants that may bepresent in shallower aquifers. Regional groundwater flow in the Arapahoe is generally fromsouth to north, but is affected by local pumpage (ESE, 1988, p. 3 - 46-47).

As part of the Offpost RI Addendum activities, the nature and possible extent of RMA-relatedcontamination in the confined flow system of the Arapahoe Formation was initiated (HLA,1992a, p. 54-58). The data compiled for the Offpost Endangerment Assessment reflectssampling of a total of 30 Arapahoe Formation wells (HLA, 1992b, p. I-17). Two isolated andquestionable detections of DIMP (.5 to 3.9 ppb) and chloroform (1.2 to 24.9 ppb) weremeasured. On the basis of these data, the organic contamination in the Arapahoe Formationappears sporadic and localized, possibly as a result of well construction of subsequent damage to the well casing (HLA, 1992a, p. 56). One of the recorded detections of chloroform in theArapahoe Formation exceeds the health based CREG comparison value of 6.0 ppb.

Tri-County Health Department established typical ranges of specific electric conductivity(reflective of inorganic water chemistry) of water produced from Arapahoe Formation wells(Tri-County, 1989; HLA, 1992a, p.57). Using those ranges of conductivity, evaluation ofArapahoe well water in the Offpost identified only one well with greatly elevated conductivitythat was apparently being affected by Alluvial Aquifer contamination (HLA, 1992a, p. 57).

Subsequent investigation by the staff of Tri-County Health Department identified a total of 17 deep wells, completed in the confined flow system of the Denver or Arapahoe Formations thatdisplay anomalous, high conductivity measurements, detections of DIMP, or that are not in usebut have not been sealed. Those wells, which may represent a real or potential conduit ofcontamination from the overlying Alluvial Aquifer to the confined aquifer system, weresubmitted as candidates for closure. Through the dispute resolution process specified in the FFA(EPA, 1989) review of those candidate wells, additional wells suggested by CDPHE, and otherdata was conducted by the Army, EPA, and Shell Oil Company. Nine Arapahoe or unknownaquifer wells were selected for closure under the provisions of the IRA for closure of abandonedRMA wells (see IRA-D, Table 3), extended by mutual agreement to the Offpost Study Area. Those wells are: 299A, 395A, 486C, 588A, 914A, 985A, 1065A, 1144A, and 1171A. The finalOffpost ROD (HLA, 1995, p. C-1) specifies the criteria to be used to ensure that poorlyconstructed wells are identified and closed to prevent future contamination of lower aquifers.

Present and Possible Future Arapahoe Aquifer Pathway

April 21 and May 4, 1994 queries of RMAED have disclosed detections of arsenic and lead atlevels above health or regulatory based comparison values (CVs). Those wells are listed inTable 14. Using this April 1994 contaminant data, coupled with the Tri-County and CDPHEdata on use and bottled water, resulted in the identification of one residence with a domestic wellcontaining contaminants of potential concern that received bottled water and 6 residences withdomestic wells that did not receive bottled water from CDPHE at the time this data wascompiled. Those wells listed in Table 14 appear to comprise, at the time the data was compiled,a present pathway of human exposure to contaminants that may result in adverse health effects. It must be noted that past sampling of Arapahoe wells has yielded inconsistent results fromsampling event to sampling event (HLA, 1992b, p. I- 18). Future sampling of those wells maynot yield results consistent with the levels recorded in Table 14.

Of the 6 affected wells not served with bottled water in 1994, about 18 people may experienceingestion, inhalation, and dermal exposure from the use of those wells. Three people aresupplied bottled water to replace the ingestion use of their contaminated Arapahoe wells.

The decision to provide replacement water to well users within the "DIMP footprint area"(DIMP >0.392 ppb; HLA, 1995) will apparently not result in replacement water for wells 864Aand 1354A. Users of those wells have been and will continue to be exposed to low levels ofarsenic in their drinking water; levels well below the 50 ppb MCL.

Pathway Summary - Offpost Alluvial and Arapahoe Aquifers

A present and potential future pathway of human exposure to contaminants at levels that mayresult in adverse health effects exists in the Offpost Study Area north of RMA for users of someAlluvial and Arapahoe Aquifer wells.

The Alluvial Aquifer wells listed in Table 13 represented in April 1994 a completed presentpathway for a total of about 189 people. The concentration of many of Offpost contaminants,identified in the preceding tables and text, is declining in time due to the combined effects ofBCSs, dilution by infiltration of surface waters for large irrigation ditches that traverse the area(eg. DIMP), and the degradation, over time, of the organochlorine pesticides (OCPs). Permissive evidence associates some of the contaminants detected with a source at RMA, butmultiple off-post sources are indicated for the VHOs, atrazine, arsenic, lead, and nitrate. Since April 1994, the concentration of nitrate in the drinking water wells of the Offpost Study

Table 14.

Recent detections of drinking water well contaminants at levels of potentialconcern: Arapahoe Aquifer, Offpost Study Area, as of April 1994 (a)
Well # Detected Value
> CV (b)
Sampling Date
Well Use
Bottled Water
Provided(e) (g)
Number of
ICP Metals/ Anions & Cations
Arsenic (AS) (CV = 0.02 ppb CREG; 50 ppb MCL)
Lead (PB) (CV = 15 ppb EPA Action Level)
    (a) RMAED database runs of 04/21/94 and 05/04/94; CDPHE database queries of 01/05/94 and 05/09/94; andTri-County Health Department database queries of 12/20/93 through 05/11/94).
    (b) CV = greater than or equal to the Comparison Value (see the Environmental Contaminants and OtherConcerns section of this Assessment for further discussion).
    (c) yr/day = the year followed by the chronologic, numerical date (eg. "94109" is April 19, 1994).
    (d) The uses of wells in the RMA Offpost Study Area were inventoried by the Tri-County Health Departmentand incorporated in their database of Offpost wells. The symbols used are "D" for domestic use (cooking,washing, and bathing) and "DD" for drinking and domestic use. Wells coded "D" are assumed to involve 5%ingestion from brushing teeth and incidental ingestion. Wells originally coded "DD" that currently receivebottled water are not currently used for drinking use are the equivalent of "D" wells at this time.
    (e) Source - Colorado Department of Public Health & Environment, Bottled Water database, 12/20/93 andsubsequent queries through 05/10/94.
    (f) Source - Colorado Department of Public Health & Environment, Bottled Water database. Entries markedwith an "*" are estimates based upon the demographic data compiled for the Offpost Study Area (see Table 4).
    (g) Wells that may not qualify (NQ) for replacement under the provisions of the Conceptual Agreement and theOffpost ROD (HLA, 1995).

Area has continued to decline. Sampling completed in 1996 did not find nitrate contaminationabove the MCL in any drink water well in the area.

Of the wells listed in Table 13, only three (547A, 550A, and 967A) may not qualify forreplacement water under the provisions of the Conceptual Agreement (Colorado, 1995) and theOffpost ROD (HLA, 1995). Approximately 25 people use those wells.

The available data also indicates that the present and, to a lesser extent, possible futurecontamination of Offpost Arapahoe wells is likely due now, and in the future, to interminglingof Alluvial Aquifer-borne contaminants with confined flow system aquifer horizons alongpathways created by poorly constructed or damaged wells. However, the Offpost ROD (HLA,1995) decision to close those poorly constructed wells will eliminate future contamination of theaquifer. Given the artesian nature (upward groundwater gradient) of the Arapahoe Aquifer,some of the affected wells listed here may not represent contamination of the aquifer, but rather,intermingling of Alluvial and Arapahoe Aquifer waters within the casing or well annulus beforesampling or use. Nevertheless, the Arapahoe wells listed in Table 14 represent locallycompleted pathways of exposures for about 18 persons. Of those wells listed in Table 14, onlytwo wells (864A, 1354A) may not qualify for replacement water under the provisions of theConceptual Agreement (Colorado, 1995) and the Offpost ROD (HLA, 1995). Approximatelysix people use those wells.

Throughout the Offpost Study Area ingestion exposure to contaminants has been reduced by theCDPHE bottled water program and the replacement of some wells by the Arsenal. TheConceptual Agreement (Colorado, 1995) and the subsequent Offpost and Onpost RODs (HLA,1995; Foster Wheeler, 1996) specify the U. S. Army and Shell Oil will continue to supplybottled water for cooking and drinking purposes until a permanent alternative water supply isprovided. The remedy decision documented in the RODs ensures that a future potential pathwayof exposure to contaminants in the Offpost Study Area is minimized or eliminated.

Commerce City -Irondale/Western Groundwater Plume Pathway

Historical Perspective

Along the western side of RMA a network of north- to northwest-trending paleochannels areincised in the Denver Formation. Those paleochannels are filled with relatively thick deposits ofsaturated alluvium which localizes the flow of what is termed the Western Plume Group in theRemedial Investigation Summary Report (EBASCO, 1992, p. A3-113). The hydraulicconductivity of those deposits is high (400-900 ft/day; CDPHE, 1992a, p. 38) and the rates ofgroundwater and contaminant migration are higher for this plume group than for the North andNorthwest Plume groups. Much of the contamination in the Western Plume Group originated inthe areas south and southwest of RMA, and, in the process of migrating across RMA, hasmerged with plumes that originated near the RMA railyards and motor pool (EBASCO, 1992, p.A3-113; EPA, May 1990, S. Adams Co. Fact Sheet). The paleochannel network concentratesthe multi-sourced contaminants in a plume that exits the RMA boundary in the northwest cornerof section 33 (T2S, R67W) in the vicinity to the intersection of Quebec Street and Highway 2.

Because the Western Plume Group originates from a number of on- and off-post sources thatdiffers markedly from the other plume groups, the contaminants which characterize this plumeare substantially different from those plumes as well. ATSDR has reviewed and evaluated thesampling data compiled by both RMA and the EPA and has identified the contaminants ofpotential concern in this plume group (see CDPHE, 1992a, p. 9-38; EBASCO, 1992, p. A3 -114-118). Those contaminants are given in Tables 6 and 7 and are the 1,1-Dichloroethylene,1,2-Dichloroethene, Chloroform, Tetrachloroethylene, Trichlorethylene, Benzene, andDibromochloropropane.

The history of contamination in the adjacent offpost, Commerce City area is complex and issummarized in the Chemical Sales Company Interim Public Health Assessment (CDPHE,1992a) and fact sheet prepared by the EPA (1990 - April & May). In 1981, during a randomnational survey of drinking water systems, the EPA detected several organic contaminants in theAlluvial Aquifer municipal wells supplying the South Adams County Water and SanitationDistrict (SACWSD). Sampling in 1982 and 1985 confirmed those results. Those affectedSACWSD municipal wells are located within about one mile west and northwest of the RMAboundary.

Because of the proximity to RMA, it was assumed initially that the Arsenal was the source of thedetected contamination in those municipal wells. Beginning in 1985, the EPA studied the areawest of the Arsenal and south of E. 80th Avenue (initially called RMA First Operable Unit andlater described as the Chemical Sales Company, Operable Unit 2) and the Army studied the areanorth of E. 80th Avenue. A ROD for the first operable unit was signed June 4, 1987 that calledfor permanent granular activated carbon (GAC) treatment of the Alluvial Aquifer water usedwithin the off-post study area; the Klein Water Treatment Plant, funded by EPA and the Army,which began operating in November 1989 (EPA, 1990).

The required treatment facility, the SACWSD Klein Water Treatment Plant was designed,constructed, and began operation in November 1989. The Klein Water Treatment Plant is astate-of-the-art facility, with its own sophisticated water quality laboratory. Continuouscomputer monitoring and weekly lab analyses performed for 1,1-Dichloroethene, 1,1-Dichloroethane, Cis-1,2-Dichloroethene, 1,1,1- Trichloroethane, Trichloroethene, andTetrachloroethene are designed to assure a safe water supply for the South Adams County areaserved (Jim Jones, SACWSD, December 17, 1993, personal communication).

Prior to the completion of the Klein Treatment Plan and to assure that the spread ofcontaminated groundwater beyond the RMA boundaries was controlled as rapidly as possible,Shell Oil Company constructed the Irondale Boundary Containment System (IBCS), locatedastride the main paleochannel in Section 33 (T2S, R67W). This system of groundwaterextraction wells and a carbon treatment system, installed to intercept and treat the WesternPlume Group contaminants, became operational in 1981. Because the flow of Alluvial Aquifergroundwater is strongly controlled by the north-trending paleochannel, it unlikely thatappreciable quantities of RMA-sourced groundwater contaminants migrated into off post afterthe completion of the IBCS.

Also prior to the completion of the Klein facility, under direction of the EPA, two emergencyresponse actions were also undertaken at this time to reduce exposure to alluvial contaminants. Those actions were:

  • A temporary GAC system was installed May 1986 to treat Alluvial Aquifer water produced by SACWSD municipal wells.
  • During the interval from the summer of 1986 through fall 1987, free municipal watertaps were installed at more than 400 residences in the Commerce City area adjacent toRMA that were previously served by private Alluvial Aquifer wells. Those water systemtaps were installed for residences located within an area bounded by Sand Creek, QuebecStreet, E. 88th Ave., and Holly Street (the "EPA Study Area").

After the municipal tap installation program, a follow-up investigation by the ColoradoDepartment of Public Health & Environment determined that 21 residences and businesses maystill using water from private Alluvial Aquifer wells for domestic, tap water purposes. Watersource information was unavailable for six to seven additional residences (CDPHE, 1992a). Theopportunity for free municipal tap was offered again in 1992 and 13 additional residences wereconnected then (Jim Berkley, EPA, January 6, 1994, personal communication).

It is estimated that no more than 10 residences or businesses located in the area south of 88thAvenue may be still be unconnected to the SACWSD municipal system . A long-timeCommerce City resident noted during a November 4, 1993 ATSDR Public Availability Sessionthat many Commerce City residents still use their private Alluvial Aquifer wells for lawn andgarden irrigation.

Pathway Summary - Irondale/ Western Plume

A past pathway of human exposure to the contaminants of potential concern given in Table 7(contaminants of the Chemical Sales Co. OU2 area) existed for individuals that worked orresided in the "EPA Study Area" of Commerce City and that were supplied with AlluvialAquifer well water. RMA is not the source of the contaminants in this pathway. The duration of that past pathway varied as follows:

  • Residents and workers that used Alluvial Aquifer water supplied by SACWSD municipal wells were exposed to the contaminants of concern listed in Table 7, at concentrations approximated by those given in the Table for an unknown period ending May 1986.
  • Residents and workers that used private Alluvial Aquifer well water and were hooked up to the SACWSD system in 1986-1987 were exposed to the contaminants of concern listed in Table 7 for an unknown period of time ending with the date of the municipal tap connection. The concentration of the contaminants involved in that exposure varied by proximity to contamination sources and the axis of the plume complex flowpaths.
  • Residents and workers in facilities that did not accept SACWSD municipal taps until1992 were exposed to the contaminants of Table 7 until the date of tap hook-up. Theconcentration of those contaminants varied by location as with other private well users.

The "EPA Study Area" is part of the study area included in the ongoing ATSDR-funded healthstudies. No further analysis of this past pathway is included in this Assessment.

A potential present and future pathway of human exposure may exist to the non-RMAgroundwater contaminants in this "EPA Study Area" (see Table 7). This potential pathway ofexposure will be more fully discussed in the Potential Pathways section of this section of thisAssessment.

Table 15.

Completed Exposure Pathways.
Pathway NameCompoundsExposure Pathway ElementsTime
SourceMediaPoint of
Route of
Exposed Population
Offpost Study Area
Alluvial Aquifer wells

- North Plume Group

- Northwest Plume



Alluvial Aquifer
contamination from
RMA that is beyond
the NBCS and the
- Alluvial
Domestic (tap
water) Alluvial
Aquifer wells.
Ingestion (A)


Dermal contact

Estimated total of189 people.

Potential future totalof <25 people?


Future (?)

Offpost Study Area
Arapahoe Aquifer wells

Alluvial Aquifer
intermingled with
Arapahoe Aquifer
- Arapahoe
Domestic (tap
water) Arapahoe
Aquifer wells
Ingestion (B)


Dermal contact

Estimated total of 18people.

Potential future totalof <6 people?



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