PUBLIC HEALTH ASSESSMENT
ROCKY MOUNTAIN ARSENAL
ADAMS COUNTY, COLORADO
To determine whether nearby residents or people who work or recreate at RMA are exposed to on-site contaminants or to contaminants migrating from the site, ATSDR has evaluated the environmental and human components that lead to human exposure. This pathways analysis consists of five elements: 1) a source of contamination, 2) transport through an environmental medium, 3) a point of exposure, 4) a route of human exposure, and 5) an exposed population. If exposure has occurred and sufficient data exists, it is possible to evaluate whether the exposure was or is at a level and of sufficient duration to be a threat to public health. The evaluation of the public health implications of exposure to RMA contaminants follows our evaluation of exposure pathways.
ATSDR categorizes an exposure pathway as a completed or a potential exposure pathway if the exposure pathway cannot be eliminated. Pathways are considered to be completed when all five pathway 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 the missing pathway element(s). Potential pathways indicate that exposure to a contaminant could have occurred in the past, could be occurring now, or could occur in the future. An exposure pathway can be eliminated if at least one of the five elements is missing and will never be present.
Table 15 identifies the completed exposure pathways not eliminated by land use controls, the IRAs, or BCSs. Table 18 identifies the potential exposure pathways. The discussion that accompanies those tables incorporates only those pathways that are important and relevant to the site.
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 to contaminants via these pathways. The surface water pathway has eliminated by discontinuing use of the liquid waste disposal and process water networks. IRAs have been designed to reduce and 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 from unidentified off-post sources located to the south and southwest of RMA are known to have migrated 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 many pathways of human exposure to contaminants, the pathways analyzed in this section of the public health Assessment are those present or future pathways not eliminated by previous actions. Past pathways of human exposure are not evaluated in detail in this Assessment because, in many instances, the contaminant and environmental data are insufficient to permit a meaningful, retrospective evaluation and the possible health effects resulting from those past, unquantified exposures are the subject of ongoing, ATSDR-funded health studies conducted by the Colorado Department of Public Health & Environment and by Colorado 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 capable of transporting observable contamination over long distances. Within the surficial alluvial deposits, fairly continuous paleochannels carved in the underlying bedrock are very efficient routes of migration of contaminated groundwater. These alluvial deposits are generally several orders 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 identifying these flowpaths and depict the confinement of contaminants within these pathways.
In a few cases, poor well construction practices or damaged well casings have apparently allowed mixing of Alluvial Aquifer contaminants with water pumped from the underlying Denver and Arapahoe Formations confined aquifers. mixing of those contaminants with the deeper aquifer waters during pumping or testing. These wells do not indicate contamination of the confined aquifers.
Many RMA contamination sources, as well as non-RMA sources to the south and west of the Arsenal, have contributed to the plumes of contaminated groundwater extending to the north and west of RMA. Sandy soils and topographic depressions allow infiltration of contaminated surface water that may result in recharge to the unconfined aquifer. In addition, wastewater discharges, 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 disposal basins and manufacturing areas as well as other off site sources, and has migrated north and northwest of RMA. The most contaminated portions of the plume are below or downgradient of major sources in the RMA interior. The overall plume concentrations and configuration suggest that the greatest contaminant releases to the unconfined flow system have occurred from Basin A and the lime settling basins, the South Plants chemical sewer, South Plants tank farm and production area, and the Army and Shell trenches in Section 36. Subsidiary contributions from former Basin F to the contaminant plume migrating beneath Section 26 are indicated. The motor pool area and rail classification yard in the Western Study Area, and sources in North Plants such as the tank farm, are also major sources of contaminant releases to the unconfined flow system (EBASCO, 1992, p. 2-30).
The Alluvial Aquifer has carried contaminants off post along paleochannels for an unknown period of time during RMA's 40-year history. Although specific dates have not been established for the arrival of groundwater contaminants at the north, northwestern, or western boundaries of RMA, 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 Adams County municipal wells on the west side of RMA in 1981 (HLA, 1992b, p. I-9). Plumes of contaminated groundwater extended beyond the western RMA boundary in 1981 when the Irondale BCS was completed; beyond the northwest boundary in 1984 when the Northwest BCS became operational; and beyond the north boundary in 1978 when the North BCS became operational (ESE, 1988).
The concentration of contaminants in the Alluvial Aquifer in the Offpost Study Area to the northwest and north of the arsenal is affected not only by the presence of paleochannels but also by the presence of irrigation ditches. Specifically, about 0.25 to 2.5 miles north of RMA, two large irrigation ditches, the O'Brian Canal and the Burlington Ditch traverse the Offpost Study Area from southwest to northeast (see Figure 2). Infiltration of surface water from those ditches is a significant source of recharge to the Alluvial Aquifer in the study area. Estimates contained in the Offpost Remedial Investigation (ESE, 1988, p. 3-33, 3-58) suggest that aquifer recharge directly from the ditches may be about 180 gallons/minute/mile or about 2,400 gpm of groundwater recharge for the Offpost Study Area. Downslope from the ditches, 82 percent of the irrigation waters applied to fields are from those ditches. This surface water irrigation is estimated to represent an additional Alluvial Aquifer recharge of about 23 to 25 inches/year or about 2,335 gpm for the area.
Together, the leakage from the ditches and the infiltration from ditch irrigation are estimated to represent about 67 percent of the total Alluvial Aquifer recharge in this area, and the recharge from the NWBCS and NBCS account for about 6 percent (ESE, 1988, p. 3-146). Because of the diluting effects of the infiltration from the irrigation ditches and the downslope application of those irrigation waters, the Alluvial Aquifer contaminant concentrations measured downgradient from those ditches are generally an order of magnitude less than the contaminant concentrations measured 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 pathways of contamination it is first necessary to briefly describe the "historical" contamination detected and the effects of the groundwater intercept and treatment systems (BCSs and IRA-A). By examining this "historical perspective" and trends of change in contaminant level it is possible to more fully describe the nature of the present completed groundwater pathways.
Historical Perspective
Two of RMA's most significant groundwater contaminant plume complexes extend downgradient from the North Boundary Containment System (NBCS). An estimated 250 to 325 gpm 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 flow rates of 1.5 to 10 ft/day) into the Offpost Study Area, it divides into two major, distinct flowpaths separated by a groundwater divide located about one third of a mile north of the RMA boundary (see Fig. 5). This division of groundwater flow is caused, at least in part, by a Denver Formation (impermeable bedrock) high located in the subsurface just north of the Maul Reservoir along this section of the First Creek channel. Contaminated groundwater passing to the east of this divide comprises the Northern plume which extends north through the central and western 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 continues to the northwest forming the First Creek plume. Both plumes are bounded by large areas of unsaturated 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 contaminants and 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 flows north of the First Creek paleochannel, also in section 14: and (3) a channel that is an eastern tributary to the Northern paleochannel in section 12.
Beyond the O'Brian Canal both the First Creek and Northern plume complexes extend northwestward 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 the North and Northwest Plume Groups and those contaminants are listed in Table 5A. Within the North Plume Group pathway the distribution of those specific contaminants differs in detail between the Northern and the First Creek plumes complexes. However, the most widespread contaminant detected in the groundwater of the Offpost Study Area is the organophosphorus compound diisopropyl methylphosphonate (DIMP), a highly mobile, semivolatile organic chemical associated with the manufacture of Army nerve-agent. Because of its 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 South Platte River. In general, the highest concentrations of DIMP occur between the north boundary of RMA and the O'Brian Canal. The highest concentrations of DIMP detected in the Offpost were 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 an irrigation well) and an average of 72.6 ppb DIMP were found in private tap water wells in the plume area between the Arsenal and the South Platte River. The Offpost concentration of DIMP has, since 1985, declined considerably under the influence of the groundwater treatment facilities.
It must be noted that, beginning in September and completed in October, 1990 domestic well users with wells contaminated by a trace or more of DIMP were offered and most provided with bottled water by the state of Colorado or, if more than 600 ppb DIMP was detected, a replacement well drilled to a confined aquifer system by RMA (Dan Collins, Tri-County Health Dept., 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 greatly reducing the number of residents exposed to DIMP, the exposure by ingestion of well water contaminated by other, less extensively disseminated semivolatile, volatile, and inorganic compounds was largely eliminated as well.
Other semivolatile organic compounds of potential concern that have been detected in the Offpost include dicyclopentadiene (DCPD); the organochlorine pesticides dieldrin and endrin and much less frequently aldrin and chlordane; atrazine; and the organosulfur compound chlorophenylmethyl 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 Endangerment Assessment/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 the Offpost occur downgradient from the NBCS and upgradient from the O'Brian Canal. Compounds such as dieldrin and other organochlorine pesticides have been found, almost exclusively, upgradient from the canals. The following additional generalizations can be made about the historical distribution and concentration of those contaminants:
Based upon the same data sources used to assess the semivolatiles, the volatile organic compounds of potential concern most frequently detected in the Northern Plume Group include chloroform, chlorobenzene, dibromochloropropane (DBCP), tetrachloroethylene (TCLEE), 1,2-dichloroethane (12DCLE), and benzene. The highest concentrations of those volatile compounds was detected upgradient of the O'Brian Canal and only isolated detection of low concentrations of those compounds were found downgradient from the canals. The following additional generalizations can be made about the historical distribution and concentration of those contaminants:
Using the data recorded in the Offpost RI and Addendum (ESE, 1988; HLA, 1992), Offpost Operable Unit Endangerment Assessment/Feasibility Assessment (HLA, 1992b, p. 1-12-14), and RMAED, 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 may result from several sources, including natural sources of these elements. The following generalizations can be made about the historical distribution and concentration of inorganic contaminants detected in the North Plume Group:
Drinking water wells that have been found, at one time or another, to contain elevated nitrate levels are generally located two or more miles north of RMA, in the more densely populated 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 contain nitrate levels greater than 10,000 ppb. Continued monitoring by TCHD has demonstrated that those nitrate levels fluctuate but show a generally sustained declining trend. See Figure 8 for examples of this trend. As of August, 1996, no drinking water wells in the Offpost Study area have been found to contain nitrate level in excess of the 10,000 ppb MCL.
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, and the 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 (see Fig. 5).
Evaluation and monitoring of the effectiveness of the NBCS has documented the changes in contaminant concentration and distribution over time. The Offpost Operating Unit Remedial Investigation (ESE, 1988, p. 3 - 123-133) summarizes a ten-year record (1978-88) of decline in the downgradient concentration of key-contaminants as well a pattern of systematic changes in the downgradient distribution of those contaminants.
That data shows, for highly mobile contaminants such as DIMP, DCPD, and DBCP, the concentration generally exhibits exponential decline with time (see Fig. 9). Dieldrin, a less mobile 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, and reinjection have produced, for highly mobile contaminants, greater than a one-half mile hiatus or interruption in the continuity of the plume concentrations. For example, DIMP at a concentration of about 1000 ppb is found immediately upgradient to NBCS, but in 1988, an equal concentration of DIMP could not be found within about one-half mile downgradient from NBCS. Thus, the "trailing edge" of the more-mobile contaminants plumes is moving away from RMA and migrating towards additional sources of dilution (eg. the irrigation canals and the induced recharge of flood irrigation). As before, less mobile contaminants such as dieldrin do not 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 and Northern paleochannels are the primary pathways for migration of groundwater contaminants in the Offpost; 2) the highest concentration of contaminants in the Offpost alluvial groundwater occurs along these two pathways upgradient of the Burlington Ditch and the O'Brian Canal; and 3) the greatest flux of contaminants to downgradient areas is along those paleochannels where saturated alluvium thicknesses are greatest and contaminant concentrations highest. Also, because of the demonstrated effectiveness of the NBCS in reducing the size and concentration of contaminants 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 Creek and Northern paleochannel/plume areas, just upgradient from the irrigation canals. Implemented in 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 contaminants downgradient 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.
Historical Perspective
As groundwater flows from the Northwest Boundary Containment System (NWBCS) into the Offpost 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 second shallow, north-trending paleochannel beginning in the southern portion of section 14, lying to the east of the main paleochannel about one quarter mile. (see Fig. 5). The total groundwater flow being intercepted by the NWBCS has been estimated to be about 185 gpm or about 55-75 percent of the groundwater flow intercepted by the NBCS. In the Offpost area, linear groundwater velocities in the Alluvial Aquifer deposits in this paleochannel are estimated to range 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 the Alluvial Aquifer, the Northwest Plume. This plume, is characterized in the Onpost by the presence of DIMP, chloroform, DBCP, endrin, chlorobenzene, and dieldrin at concentration about 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 contaminants occur in relatively low concentrations compared to contaminant concentrations both up- and downgradient to the NBCS (ESE, 1988, p.3-58,; HLA, 1992a, p. 63). Unlike the North Plume Group, the concentration of contaminants in the Alluvial Aquifer does not seem to be diluted by recharge 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 those contaminants listed in Table 5A. The distribution and concentration of the specific contaminants in the Offpost area along the course of the Northwest Plume Group are summarized in the following discussion.
The semivolatile organic compounds of potential concern detected in the Northwest Plume Group are DIMP and dieldrin. The historical distribution of those contaminants was assessed in the Offpost RI and Addendum (ESE, 1988; HLA, 1992a) and incorporated in the Offpost Operable Unit Endangerment Assessment/Feasibility Assessment (HLA, 1992b, p.1-12-14). From those documents and the 1985-1993 contaminant data recorded in the RMAED, several generalizations can be drawn.
The volatile organic compounds of potential concern detected in the Offpost Study Area downgradient to the NWBCS in the Northwest Plume group have been chloroform and chlorobenzene. The relative distribution of those contaminants was assessed in the Offpost RI and Addendum (ESE, 1988; HLA, 1992) and incorporated in the Offpost Operable Unit Endangerment Assessment/Feasibility Assessment (HLA, 1992b, p.1-12-14). From those documents and the 1985-1993 historical contaminant data recorded in the RMAED, several generalizations can be drawn.
Chloroform, like DIMP, is highly mobile in the groundwater flow system, and thus the extent of chloroform contamination in the Northwest Plume Group described in the RI Addendum (HLA, 1992a, Fig. 3-6) may be approximately the maximum area of historical contamination. Within this plume area, chloroform has been detected in two tap water wells at levels ranging from >0.05 to <5.0 ppb.
Using the data recorded in the Offpost RI and Addendum (ESE, 1988; HLA, 1992a), Offpost Operable Unit Endangerment Assessment/Feasibility Assessment (HLA, 1992b, p.1-12-14), and the RMAED, the following information is know about the non-RMA sourced inorganic contaminants of potential concern in the Northwest Plume Group pathway:
Effects of the Northwest Boundary Containment System (NWBCS)
The NWBCS, located in section 22, T2S, R67W (see Figure 2 and 5), became operational in 1984. Contaminant bypass was noted at the northeast end of the system in 1988 and a series of actions have been undertake to upgrade the system to eliminate contaminant bypass. Operation of the NWBCS was continued during the Short-Term Improvements IRA as series of improvements were made to the system between 1988 and August, 1991. Under the provisions of a NWBCS Long-Term Improvements IRA and both the Onpost and Offpost RODs (Foster Wheeler, 1996; HLA, 1995), the performance of the system will continue to be monitored to assess the possible need for additional modifications to this system.
Both the RI and the Addendum (ESE, 1988; HLA, 1992a) noted that the highest concentrations of chloroform were still found immediately downgradient to the NWBCS. If the NWBCS was effectively removing this highly mobile contaminant its distribution by 1991 would be expected to be similar to that described above for DIMP downgradient to the NBCS. Less mobile contaminants such as dieldrin were found in the highest concentrations immediately below the NWBCS and decreased further downgradient of the system; an expectable pattern of distribution and analogous to that of the NBCS. At that time, no pattern of dilution downgradient to the irrigation ditches could be detected in this area.
Data on the principal contaminants downgradient to the NWBCS continues to be gathered by the RMA Comprehensive Monitoring Program (CMP). NWBCS-specific data was also gathered to evaluate the Short-Term Improvements IRA. That investigation determined that system modifications have greatly improved and probably eliminated offpost migration of the target contaminants. By January, 1993 the downgradient concentrations of dieldrin in monitoring wells had decreased below the detection limit (0.05 ppb) and the measured concentration chloroform 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 (tap water) wells producing water from the Alluvial Aquifer existed from an indeterminate time after the start of operations at RMA in 1942 until the replacement of most of those contaminated wells in 1990 by bottled water and/or the construction of replacement wells drilled to underlying confined and uncontaminated aquifers. Prior to October 1990, a completed pathway existed for human exposure by ingestion, inhalation, and dermal exposure to groundwater contaminated with those chemicals listed as historical detections in Table 5A.
ATSDR-funded health studies on the possible effects of human exposure to RMA contaminants are 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 to qualitatively assess the possible health effects of past (pre-1990) exposure to groundwater-born contaminants north and northwest of RMA. However, to date, those studies have not demonstrated human health effects that can be attributed directly to exposure to RMA contaminants.
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 the 1990-1993 detections in Table 5A exists for those individuals who have continued (since October, 1990) to use private wells (tap water) located within the Northern and Northwestern Plume complexes that produce water from the Alluvial Aquifer. The duration and degree of exposure 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 continues for those Alluvial Aquifer well users residing within the Offpost Study Area that: 1) Did not accept 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) Were provided replacement water (bottled water or a new well to the confined aquifer system) but continued to use contaminated Alluvial Aquifer water.
The number of Alluvial Aquifer drinking water wells or irrigation wells, in use within or near the Offpost Study area has been determined by a review of the extensive inventory of those wells incorporated in Tri-County Health Department the Offpost Study Area. An October 1, 1994 query of that database, updated last on September 28, 1994, shows there are a total 517 alluvial wells present in the Offpost Study Area. Of those, 163 are not used and may be put back into some type of service at a later date. Domestic and drinking water wells total 127, of which 56 are drinking and domestic use and 71 are domestic use only. The remaining 227 wells are used for irrigation or other types of use. By April 1994, 119 of the 127 domestic and drinking water wells have been sampled by the Tri-County Health Department. Some of those domestic and drinking water wells have never shown any contamination and some are beyond the limits of the Offpost Study area (TCHD, 1994; Dan Collins, Tri-County, April 25, 1994, personal communications).
On January 5, 1994 the Colorado Department of Public Health & Environment provided information and a database printout describing the bottled water recipients in the Offpost Study Area. Listed were "active accounts" for individuals or businesses receiving bottled water from the state. The 119 "active accounts" represent substitution of bottled water in lieu of well water for 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 those residences 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" with bottled water, about 45 of those wells are developed in the Alluvial Aquifer and 40 are producing 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), the CDPHE Bottled Water Database (01/05/94, 05/09/94) and the Tri-County Offpost Study Area private well database (12/20/93, 05/11/94), coupled with review and database audits performed by CDPHE and Tri-County, indicates that there were a total of about 31 Alluvial Aquifer drinking water (tap water) domestic or commercial facility wells that, as of the April 1994 sampling event, remained in use and that produced water containing one or more of the contaminants of potential concern.
Table 13 identifies the specific wells and contaminants that comprise a present and possible future 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 above comparison values (CVs), only 6 wells (456A, 565A, 579A, 739A, 973A, and 1276A) were not served by the CDPHE bottled water program in 1994. Using the demographic data presented in Table 3 of this Assessment and data derived from the TCHD (1994) database, it is estimated that a 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 may
experience 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 total
daily ingestion through incidental ingestion and such uses as cooking and brushing
CV (b) (ppb) |
(yr/day)(c) |
(DD/D)(d) |
Bottled Water Provided(e) & (g) (Y/N) |
Number of Residents(f) | |
| Volatile Halogenated Organics - (VHOs) | |||||
| 1,2-Dichloroethane (12DCLE) (CV = 0.4 ppb CREG; 5 ppb MCL) | |||||
| 967A | 1.57 | 92197 | D | Y / NQ? | 6 |
| Chloroform (CHCL3) (CV = 6 ppb CREG; 100 ppb MCL Total Trihalomethane) | |||||
| 540A | 8.34 | 91078 | D - Trailer Ct. | Y | 72* |
| 608A | 25.0 | 91291 | DD | Y | 3* |
| 616A | 8.33 | 92120 | D | Y | 6 |
| 967A | 18.9 | 92197 | D | Y / NQ? | 6 |
| Tetrachloroethylene (TCLEE) (CV = 0.7 ppb CREG; 5 ppb MCL) | |||||
| 338A | 2.76 | 93272 | DD | Y | 2 |
| 540A | 2.16 | 91322 | D - Trailer Ct. | Y | 72* |
| 578A | 1.03 | 91324 | D | Y | 5 |
| 579A | 1.43 | 91324 | DD | N | 3* |
| 613A | 1.25 | 92028 | DD | Y | 2 |
| 614A | 1.12 | 91283 | DD | Y | 4 |
| 616A | 1.26 | 92120 | D | Y | 6 |
| Triazines | |||||
| Atrazine (ATZ) (CV = 3 ppb LTHA & MCL) | |||||
| 373D | 4.53 | 92031 | D | Y | 5 |
| 613A | 5.81 | 92028 | DD | Y | 2 |
| ICP Metals / Anions & Cations | |||||
| Arsenic (AS) (CV = 0.02 ppb CREG; 50 ppb MCL; Natural Background Level <2.5 ppb) | |||||
| 565A | 5.69 | 92118 | DD | N | 3* |
| 583A | 14.0 | 92120 | D | Y | 5 |
| 594A | 7.42 | 92119 | D | Y | 8 |
| 616A | 10.9 | 92120 | D | Y | 6 |
| 940A | 8.29 | 92118 | D | Y | 2 |
| Lead (PB) (CV = 15 ppb EPA Action Level; Natural Background Level <18.6 - <37.2 ppb) | |||||
| 372A | 97.0 | 91318 | D | Y | 1 |
| 540A | 92.6 | 91322 | D - Trailer Ct. | Y | 72* |
| 550A | 65.5 | 91317 | D | Y / NQ? | 16 |
| 551A | 100.0 | 91317 | D | Y | 5 |
| 578A | 119.0 | 91324 | D | Y | 5 |
| 579A | 98.8 | 91324 | DD | N | 3* |
| 602A | 77.1 | 91322 | D | Y | 1 |
| 603A | 65.2 | 91322 | D | Y | 4 |
| Nitrate (NIT) (CV = 10,000 ppb MCL) NOTE: All of the wells listed below are contain less than 10,000 ppb Nitrate as of August 1, 1996. | |||||
| 331A | 11,300 | 94112 | D | Y | 3 |
| 332B | 13,000 | 92141 | D | Y | 2 |
| 332D | 10,500 | 91296 | D | Y | 2 |
| 456A | 13,000 | 92121 | DD | N | 2 |
| 540A | 26,000/10,000 | 91322/93168 | D - Trailer Ct. | Y | 72* |
| 541A | 12,000 | 92035 | DD | Y | 3* |
| 547A | 12,700/6,600 | 94277/95213 | DD | Y / NQ? | 3* |
| 551A | 12,000/10,400 | 91317/93320 | D | Y | 5 |
| 578A | 14,000 | 91324 | D | Y | 5 |
| Nitrate (NIT) - continued | |||||
| 579A | 14,000 | 91324 | DD | N | 3* |
| 613A | 11,000 | 92028 | DD | Y | 2 |
| 616A | 16,000 | 92120 | D | Y | 6 |
| 739A | 13,000 | 93084 | D | N | 3* |
| 969A | 11,000 | 91030 | D | Y | 3* |
| 973A | 10,000 | 92126 | DD | N | 3* |
| 981A | 12,000\10,200 | 91290\92176 | D | Y | 4 |
| 1276A | 24,000 | 92126 | DD | N | 6 |
| 1305A | 12,000/13,900 | 91324/93321 | D | Y | 2 |
teeth. Using the data on number of residents supplied with bottled water compiled by CDPHE and supplementing that data, where necessary, with estimates based on the demographic data presented 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 multiple contaminants. 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 by Tri-County Health Department, both Tri-County and ATSDR recognize that there may be some Alluvial Aquifer well users in this large area that may have been missed by the inventory, are unaware of the bottled water program, or have refused to cooperate. Thus, there may be a very small number of additional domestic wells in use that may contain contaminant levels similar to 1990-93 levels given in Table 5A.
The decision (Offpost ROD, HLA, 1995; Onpost ROD, Foster Wheeler, 1996) to supply municipal water or an alternative water source to domestic well users within the "DIMP footprint area" (DIMP >0.392 ppb) will result in the availability of replacement water for many of the wells that, at one time or another, have been found to contain detectable level of contaminants. Therefore, the duration of any current pathway of exposure will be brief. In the future, owners of any additional domestic wells, new or existing, found to have DIMP concentrations of 8 ppb (or other relevant Colorado Basic Groundwater Standard at the time) or greater will be connected to a water distribution system or provided with an alternative permanent 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 wells that will be replaced by municipal or alternative water supplies (CDPHE, 1996). This review indicated that about 32 wells previously replaced by bottled water fall outside the area of contamination described by the "DIMP footprint area". Those wells will not will apparently not qualify for the continuation of the bottled water program.
Only three wells (547A, 550A, and 967A) listed in Table 13 will apparently not qualify for a replacement 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 10,000 ppb MCL. Therefore, this well is not now considered to be of public health concern. Sampling of 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 elevated levels of lead in their drinking water. Sampling of well 967A in 1992 found 1.57 ppb 12DCLE and 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 the RMA north and northwest boundaries and subcrops (is the bedrock unit which immediately underlies the surficial alluvial deposits comprising the Alluvial Aquifer) about 3 miles downgradient of those boundaries in the Offpost Study Area. The sandstone aquifers of the Arapahoe Formation are artesian (ie. in all wells, the water level rises above the bottom of the upper confining bed). The aquifer is confined by the "buffer zone" which consists of 30-100 ft of clay shale and claystone. Where present (not truncated by erosion), this clay-rich bed provides hydraulic separation from the Denver Formation and any contaminants that may be present in shallower aquifers. Regional groundwater flow in the Arapahoe is generally from south 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-related contamination in the confined flow system of the Arapahoe Formation was initiated (HLA, 1992a, p. 54-58). The data compiled for the Offpost Endangerment Assessment reflects sampling of a total of 30 Arapahoe Formation wells (HLA, 1992b, p. I-17). Two isolated and questionable detections of DIMP (.5 to 3.9 ppb) and chloroform (1.2 to 24.9 ppb) were measured. On the basis of these data, the organic contamination in the Arapahoe Formation appears 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 the Arapahoe 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 of Arapahoe well water in the Offpost identified only one well with greatly elevated conductivity that 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 that display anomalous, high conductivity measurements, detections of DIMP, or that are not in use but have not been sealed. Those wells, which may represent a real or potential conduit of contamination from the overlying Alluvial Aquifer to the confined aquifer system, were submitted 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 other data was conducted by the Army, EPA, and Shell Oil Company. Nine Arapahoe or unknown aquifer wells were selected for closure under the provisions of the IRA for closure of abandoned RMA 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 final Offpost ROD (HLA, 1995, p. C-1) specifies the criteria to be used to ensure that poorly constructed 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 at levels above health or regulatory based comparison values (CVs). Those wells are listed in Table 14. Using this April 1994 contaminant data, coupled with the Tri-County and CDPHE data on use and bottled water, resulted in the identification of one residence with a domestic well containing contaminants of potential concern that received bottled water and 6 residences with domestic wells that did not receive bottled water from CDPHE at the time this data was compiled. 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 from sampling event to sampling event (HLA, 1992b, p. I- 18). Future sampling of those wells may not 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 experience ingestion, inhalation, and dermal exposure from the use of those wells. Three people are supplied 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 864A and 1354A. Users of those wells have been and will continue to be exposed to low levels of arsenic 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 may result in adverse health effects exists in the Offpost Study Area north of RMA for users of some Alluvial and Arapahoe Aquifer wells.
The Alluvial Aquifer wells listed in Table 13 represented in April 1994 a completed present
pathway 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 of
BCSs, 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, but
multiple 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
> CV (b) (ppb) |
(yr/day)(c) |
(DD/D)(d) |
Bottled Water Provided(e) (g) (Y/N) |
Number of Residents(*)(e) | |
| ICP Metals/ Anions & Cations | |||||
| Arsenic (AS) (CV = 0.02 ppb CREG; 50 ppb MCL) | |||||
| 549A | 3.09 | 92119 | DD | N | 3* |
| 595A | 9.59 | 92119 | D | Y | 3 |
| 601A | 14.40 | 92120 | DD | N | 3* |
| 864A | 7.42 | 92120 | DD | N/NQ | 3* |
| 1354A | 6.12 | 92119 | DD | N/NQ | 3* |
| 1380A | 2.73 | 92121 | DD | N | 3* |
| Lead (PB) (CV = 15 ppb EPA Action Level) | |||||
| 931A | 18.6 | 92205 | DD | N | 3* |
Area has continued to decline. Sampling completed in 1996 did not find nitrate contamination above 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 for replacement water under the provisions of the Conceptual Agreement (Colorado, 1995) and the Offpost ROD (HLA, 1995). Approximately 25 people use those wells.
The available data also indicates that the present and, to a lesser extent, possible future contamination of Offpost Arapahoe wells is likely due now, and in the future, to intermingling of Alluvial Aquifer-borne contaminants with confined flow system aquifer horizons along pathways 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 the aquifer. 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 before sampling or use. Nevertheless, the Arapahoe wells listed in Table 14 represent locally completed pathways of exposures for about 18 persons. Of those wells listed in Table 14, only two wells (864A, 1354A) may not qualify for replacement water under the provisions of the Conceptual Agreement (Colorado, 1995) and the Offpost ROD (HLA, 1995). Approximately six people use those wells.
Throughout the Offpost Study Area ingestion exposure to contaminants has been reduced by the CDPHE bottled water program and the replacement of some wells by the Arsenal. The Conceptual 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 supply bottled water for cooking and drinking purposes until a permanent alternative water supply is provided. The remedy decision documented in the RODs ensures that a future potential pathway of 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 are incised in the Denver Formation. Those paleochannels are filled with relatively thick deposits of saturated alluvium which localizes the flow of what is termed the Western Plume Group in the Remedial Investigation Summary Report (EBASCO, 1992, p. A3-113). The hydraulic conductivity of those deposits is high (400-900 ft/day; CDPHE, 1992a, p. 38) and the rates of groundwater and contaminant migration are higher for this plume group than for the North and Northwest Plume groups. Much of the contamination in the Western Plume Group originated in the areas south and southwest of RMA, and, in the process of migrating across RMA, has merged 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 concentrates the multi-sourced contaminants in a plume that exits the RMA boundary in the northwest corner of 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 that differs markedly from the other plume groups, the contaminants which characterize this plume are substantially different from those plumes as well. ATSDR has reviewed and evaluated the sampling data compiled by both RMA and the EPA and has identified the contaminants of potential 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, and Dibromochloropropane.
The history of contamination in the adjacent offpost, Commerce City area is complex and is summarized 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 random national survey of drinking water systems, the EPA detected several organic contaminants in the Alluvial Aquifer municipal wells supplying the South Adams County Water and Sanitation District (SACWSD). Sampling in 1982 and 1985 confirmed those results. Those affected SACWSD municipal wells are located within about one mile west and northwest of the RMA boundary.
Because of the proximity to RMA, it was assumed initially that the Arsenal was the source of the detected contamination in those municipal wells. Beginning in 1985, the EPA studied the area west of the Arsenal and south of E. 80th Avenue (initially called RMA First Operable Unit and later described as the Chemical Sales Company, Operable Unit 2) and the Army studied the area north of E. 80th Avenue. A ROD for the first operable unit was signed June 4, 1987 that called for permanent granular activated carbon (GAC) treatment of the Alluvial Aquifer water used within 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 a state-of-the-art facility, with its own sophisticated water quality laboratory. Continuous computer monitoring and weekly lab analyses performed for 1,1-Dichloroethene, 1,1-Dichloroethane, Cis-1,2-Dichloroethene, 1,1,1- Trichloroethane, Trichloroethene, and Tetrachloroethene are designed to assure a safe water supply for the South Adams County area served (Jim Jones, SACWSD, December 17, 1993, personal communication).
Prior to the completion of the Klein Treatment Plan and to assure that the spread of contaminated groundwater beyond the RMA boundaries was controlled as rapidly as possible, Shell Oil Company constructed the Irondale Boundary Containment System (IBCS), located astride the main paleochannel in Section 33 (T2S, R67W). This system of groundwater extraction wells and a carbon treatment system, installed to intercept and treat the Western Plume Group contaminants, became operational in 1981. Because the flow of Alluvial Aquifer groundwater is strongly controlled by the north-trending paleochannel, it unlikely that appreciable quantities of RMA-sourced groundwater contaminants migrated into off post after the completion of the IBCS.
Also prior to the completion of the Klein facility, under direction of the EPA, two emergency response actions were also undertaken at this time to reduce exposure to alluvial contaminants. Those actions were:
After the municipal tap installation program, a follow-up investigation by the Colorado Department of Public Health & Environment determined that 21 residences and businesses may still using water from private Alluvial Aquifer wells for domestic, tap water purposes. Water source information was unavailable for six to seven additional residences (CDPHE, 1992a). The opportunity for free municipal tap was offered again in 1992 and 13 additional residences were connected 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 88th Avenue may be still be unconnected to the SACWSD municipal system . A long-time Commerce City resident noted during a November 4, 1993 ATSDR Public Availability Session that many Commerce City residents still use their private Alluvial Aquifer wells for lawn and garden 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 or resided in the "EPA Study Area" of Commerce City and that were supplied with Alluvial Aquifer well water. RMA is not the source of the contaminants in this pathway. The duration of that past pathway varied as follows:
The "EPA Study Area" is part of the study area included in the ongoing ATSDR-funded health studies. 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-RMA
groundwater contaminants in this "EPA Study Area" (see Table 7). This potential pathway of
exposure will be more fully discussed in the Potential Pathways section of this section of this
Assessment.
Table 15 - Completed Exposure Pathways.
| Pathway Name | Compounds | Exposure Pathway Elements | Time | ||||
| Source | Media | Point of Exposure |
Route of Exposure |
Exposed Population | |||
| GROUNDWATER - Offpost Study Area Alluvial Aquifer wells - North Plume Group
- Northwest Plume |
12DCLE CHCL3 TCLEE ATZ AS PB NIT AS |
Alluvial Aquifer contamination from RMA that is beyond the NBCS and the NWBCS. |
Groundwater - Alluvial Aquifer |
Domestic (tap water) Alluvial Aquifer wells. |
Ingestion (A)
Inhalation Dermal contact |
Estimated total of
189 people.
Potential future total of <25 people? |
Past Present Future (?) |
| GROUNDWATER - Offpost Study Area Arapahoe Aquifer wells |
AS PB |
Alluvial Aquifer contamination intermingled with Arapahoe Aquifer water |
Groundwater - Arapahoe Aquifer |
Domestic (tap water) Arapahoe Aquifer wells |
Ingestion (B)
Inhalation Dermal contact |
Estimated total of 18
people.
Potential future total of <6 people? |
Past Present Future(?) |
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