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To determine whether on-base employees and on- and off-base residents are exposed to contaminants migrating from the site, ATSDR evaluated the environmental and human components leading to human exposure. That pathways analysis consists of five elements: a source of contamination; transport through an environmental medium; a point of exposure; a route of human exposure; and an exposed population.

ATSDR classifies exposure pathways as completed, potential, or eliminated. For a completed pathway to exist, five elements must be present, and there must be evidence exposure to a contaminant has occurred, is occurring, or will occur. In the case of a potential pathway at least one of the five elements is missing, but could exist. Potential pathways suggest that exposure to a contaminant could have occurred, could be occurring, or could occur. A pathway is eliminated when at least one of the five elements is missing and will never be present. Table 18 identifies completed exposure pathways at McAFB; Table 19 identifies potential exposure pathways. Table 20 estimates the number of persons exposed by way of completed exposure pathways and the number of persons potentially exposed by way of potential exposure pathways. The discussion following the tables addresses only the pathways important and relevant to the site.

Table 18. Completed Exposure Pathways

Private Wells OU D Groundwater
(Private Wells)
Residences Ingestion, Inhalation,
Skin Contact
Residents west of base Past
Base Supply Wells Basewide Groundwater
(Supply Wells)
Base residences and public water sources Ingestion, Inhalation,
Skin Contact
On-base residents and workers Past
Surface Soil OU B, SA 12 Surface Soil SA 12, DRMO lot Ingestion, Skin Contact
dust inhalation
Workers and visitors to the salvage yard, remedial workers Past
Sediment (Magpie Creek) Basewide Sediment Magpie creekbed
old creek channel
Ingestion Children Past
Ambient Air, on base Industrial activities and hazardous waste sources, on base Air On-base residential and worker areas Inhalation Residents and workers in defined risk areas Past
Ambient Air, off base OU D, and other sources, on base
(industrial areas)
Air Residences and nearby businesses Inhalation Residents near OU D and OU B Past


Private Wells OU D Groundwater Residences Ingestion, Inhalation,
Skin Contact
Residents west of base Present
Municipal Wells OUs B, A, and C Groundwater
(Municipal Supply)
Residences and businesses Ingestion, Inhalation,
Skin Contact
Residents Past
Private Wells OUs B, C, and A Groundwater
(Private Wells)
Residences Ingestion, Inhalation,
Skin Contact
Residents Past
Subsurface Soil Basewide Subsurface Soil Construction and remedial areas Ingestion, Skin Contact, Inhalation of dust Workers Past
Surface Soil Basewide Surface Soil Open, unpaved areas on base Ingestion, Skin Contact Residents, workers, and visitors Past
Ambient Air OU D, and other on-base sources Air Residences and nearby businesses Inhalation Residents off base and in base housing in define risk areas Past
Surface Water and Sediment Basewide Surface Water, Sediment Magpie Creek Ingestion, Skin Contact Residents off base, especially children Past
Biota OU D, and other areas Crops and Livestock Residential yards Ingestion Residents Past

Table 20. Estimated Populations For Complete and Potential Exposure Pathways

Location Estimated Number Pathway Type Contaminants in the Pathway
Residents west of base (OU D) 60-240 Private Well TCE, DCE, MC, VC, PCE, DCA, lead, As, Cd
unknown Sediment (Magpie Creek) cadmium, lead
unknown Ambient Air TCE, PCE, MC, DCE, benzene, TCA
unknown Biota unknown
unknown Surface water and sediment, Magpie Creek unknown
Residents on base 300 Base Supply Well TCE at a minimum (other VOCs also present)
Ambient Air unknown
Surface Soil unknown
Workers on base 400 Ambient Air mercury
16,000 Base Supply Well TCE at a minimum (other VOCs also present)
Ambient Air PCE, TCE, DCE, MC, benzene
Surface Soil unknown
Workers and visitors (SA 12/OU B1) unknown Surface Soil PCBs, dioxins, furans
Residents southwest of base (OU B) unknown Municipal Well unknown
Private Well unknown
Biota unknown
Construction workers on base unknown Subsurface Soil TCE, DCE, TCA, PCE, VC, benzene, PCBs, Cr, Pb, Cd, As, Tl


A. Completed Exposure Pathways

Completed exposure pathways discussed in this section are listed in Table 18. Estimates of populations exposed to contaminants by way of these pathways are listed in Table 20.

Groundwater Pathways

McAFB is on an alluvial plain that slopes west from the foothills of the Sierra Nevada Mountains. Elevations range from 80 feet above mean sea level (msl) at the eastern base boundary to about 50 feet above msl at the western boundary (east-west distance about 2 miles). The sedimentary deposits beneath the base are complex; individual lithologic units undergo abrupt lateral and vertical changes or pinch out over short distances. Within the top 220 feet, soils include relatively permeable sandy deposits interspersed with less permeable deposits of silt and silty clay. The water table is between 80-110 feet below ground surface and changes seasonally. Below 120 feet, the water-bearing strata are generally semi-confined and believed to be interconnected with the unconfined zone. Vertical groundwater movement between the various water-bearing zones occurs due to the changes and lateral discontinuity of the semi-confining layers.

Municipal and private well pumping near Sacramento have created two cones of depressions in the groundwater system; one is just south of McAFB, the other is about 15 miles from the base and south of downtown Sacramento. These depression areas cause changes to the area groundwater flow patterns. Base production wells draw groundwater from depths of approximately 145-400 feet below ground surface. When base production wells are inactive, groundwater flow at McAFB is south or southwest toward the city depression. When the base wells are in use, however, there are localized shifts in flow direction. The principal on-base supply well (BW 18) pumps about 1000 gallons per minute which affects groundwater flow at all monitored depths in OU B.

To evaluate groundwater movement and contamination at McAFB, a contractor has divided the groundwater system (from the surface to a depth of 430 feet below surface) into six geohydrologic zones. The zones are designated from shallowest to deepest as A, B, C, D, E, and F. Monitoring well data are being studied continually to define the hydrology and evaluate contamination. Little information is available on the deeper zones. The monitoring well system at McAFB is extensive; more than 300 wells are on and off base. Groundwater contamination has been identified in OUs A, B, C, and D. Off-site MWs are strategically positioned to assess movements of contaminant plumes toward municipal production wells and residential neighborhoods. Groundwater plumes contaminated with VOCs (primarily TCE, PCE, and DCE) have migrated off site from OUs D and B. The OU D plume is believed to be contained in the shallower zones, but the OU B and OU C plumes have detectable levels of VOC contaminants in zones A, B, C, and D (5). An OU C plume is responsible for the contamination of city well 150, which has since been abandoned.

Private Well Pathway

Groundwater was the first medium characterized at McAFB that was contaminated because of past waste disposal practices on base. Because TCE and other contaminants of concern were detected in 1979 in residential wells west of OU D, the Groundwater Task Force, and later the IRP, concentrated their efforts on that exposure pathway.

McAFB provided an alternate water supply (bottled water) to residences in the identified impact area (Fig. 4) with documented VOC contamination in their potable water. Alternative water was provided to each individual household after contaminants were detected in that particular well. If residents given the bottled water stopped drinking the private well water, exposure by ingestion was eliminated. Residents were still exposed to the VOC-contaminated well water (by inhaling volatilized chemicals and by absorption following dermal contact) if they used it for bathing and showering. Some residences used bottled water for several years, until 1986 or 1987, when service connections to the municipal water supply were completed.

During the first phase of the Groundwater Task Force study, 30 wells were sampled quarterly. Before the final remedial action, the number of wells tested had increased to 240. Of those, approximately 80 were contaminated with VOCs that ATSDR considered contaminants of concern; about 20 private wells were contaminated with VOCs and metals at concentrations above health comparison values (35).

During 1986-87, the base connected 550 homes in a defined groundwater contamination impact area (Fig. 4) to the municipal water supply system. The defined impact area included residences with contaminated private wells and properties that could be in the plume pathway. Property owners were given the choice of abandoning their wells, or continuing to use them for irrigation; in those cases, the wells were disconnected from the homes. Backflow valves (to prevent any possible contamination of the municipal water) were attached to the municipal water system at the homes where occupants elected to continue to use their wells for irrigation. Those valves are checked and maintained annually. The northern portion of the residential area receives water from the Rio Linda Water District, the southern portion from the City of Sacramento Water District. After connections to the municipal supply were completed, McAFB discontinued groundwater sampling at the private wells. McAFB now samples off-site groundwater monitoring wells; municipal production wells are sampled by other agencies.

ATSDR has not found complete, updated records on current use of private wells by residences provided with alternative municipal water. In discussions with citizens and area water district personnel, ATSDR staff were informed that some wells are no longer usable because of the drought in the area (lowering of the watertable has left some shallow wells dry). It is also possible that some residents have reconnected their private wells for potable use because of ongoing water restrictions imposed during the drought. During the ATSDR public availability sessions, no one reported using their private wells for potable purposes.

Groundwater contaminant data on private wells is not current, because sampling of those wells ended when the residences were connected to public water systems. Residents using contaminated water from their private wells were exposed to contaminants of concern by way of water use, including drinking, showering, bathing, and irrigating. The routes of past exposures include ingestion of, inhalation of, and dermal contact with the contaminants. Anyone currently using a private well for potable purposes in the contaminated groundwater plumes areas has similar routes of exposure. People using the groundwater for irrigation could be exposed by inhaling contaminants from the droplets of water spray in the air and by ingesting biota that have bioaccumulated contaminants.

The groundwater treatment plant (GWTP) for OU D is operating as projected. The OU D plume is reported to be moving back toward the base (48). Graphic presentations of the OU D plume indicate its current position is within the boundaries of the operable unit (10). The groundwater plumes migrating from OU B have affected groundwater in all tested zones. Use of water from any private wells in the OU B plume area would result in exposure to VOCs by ingestion, inhalation, and dermal contact.

Base Supply Wells Pathway

In 1979, groundwater samples from base supply wells contained TCE and other VOCs. Affected wells included BWs 1, 2, 12, and 18. TCE levels were above the MCL (5 ppb). The major production well (BW 18) showed TCE contamination of 140 ppb before it was taken off line. Water samples from various distribution areas around the base provide information on TCE concentrations at the point of public consumption. Concentrations at the point of consumption in 1980, when TCE levels peaked, ranged from 1.7 to 22.5 ppb (Table 1-B). No information is available to determine the size of the potentially exposed populations using water from the different distribution points. Other VOCs were detected in the base potable water system at that time, but data at the distribution points are insufficient to discuss those exposures.

People could have been exposed to contaminated groundwater in the base water supply wells by way of VOC-ingestion, inhalation, or dermal contact. The exposed population included residents of base housing and workers on base. Military personnel and their dependents, a transient population, would have had shorter exposure times. Civilian employees would have been exposed to groundwater for longer periods. Any exposure from these wells ended when they were taken off line in 1979 through 1980. Table 1-A lists the specific date each well was taken off line.

Surface Soil Pathways

On-site Surface Soil Pathway

On-base surface soils at each of the OUs will be analyzed during the RI/FS. OU B was the first area for sampling of surface soil. Results have detected high concentrations of PCBs in the DRMO lot (site SA 12 or OU B1). The DRMO yard has been in operation since the 1950s; no information exists to determine how long the PCBs have been present in the surface soils. Although the site is within base boundaries, visitors often go there to purchase salvage materials; base workers also use the area. Historically, the area surface soils were covered by a layer of gravel and perforated steel sheeting, which provided partial cover. Weather conditions at McAFB are usually dry; dust from the surface soil may be transported by the wind.

PCB compounds are not likely to volatilize and would be expected to adhere to soil particles. Site 12A air monitoring in 1992 did not detect PCB; human exposure to PCBs through the inhalation route is highly unlikely. Workers and visitors to the area have been and are being exposed to the PCB-contaminated surface soil by direct dermal contact and by incidental soil ingestion.

PCBs were detected in dust removed by a swipe sample from a furniture surface in the recreation area inside building 700. PCBs were also detected on the two bootsoles sampled. In addition to wind transport, contaminated soils may be transported from the DRMO yard on the workers/visitors and clothing articles. Surface water samples collected during rain events from drainage ditches adjacent to the yard detected levels of PCBs and dioxins. Sediment samples from the drainage system also contained PCBs. This surface runoff migration of contaminants from OU B1 is discussed in the ATSDR health consultation (Appendix D).

Since the PCB contamination was determined, McAFB has taken precautions to limit transport of the soils. First, overboots were worn by workers or visitors entering the yard, then areas with the highest contamination were covered with solid steel planking. When migration by surface water runoff was discovered, the areas with highest PCB levels were covered or capped. A cap will be constructed over the entire site in 1994 to prevent any further human exposures.

Past transport of contaminated soils, by the wind and surface water, may have caused migration of the PCBs from the boundaries of the DRMO site. Residential properties are located next to the site.

Dioxins and furans have been detected in OU B surface soils. Adequate information on other contaminants are not yet available to define exposures; those exposures will be discussed in the Potential Exposure Pathway section of this public health assessment.

Sediment Pathway (Magpie Creek)

ATSDR has received reports that large fuel spills were released from McAFB into Magpie Creek in the past. In addition, citizens described odors associated with color changes in the creek water because of chemical releases. Those past releases could have contributed to accumulation of contaminants in sediment. Preliminary results for sampling sediments in the creekbed (sampling performed in the spring, 1992) indicate cadmium, a heavy metal, above health comparison levels for pica children. Metals would adhere to the soils and are not expected to be in the surface water at any great extent. No other contaminants were reported at levels above background or at levels of health concern.

There are reports that children play in Magpie Creek and along the creekbed. Exposure would occur through dermal contact with and ingestion of contaminants in the sediments. The contaminants were consistently detected along the creekbed; sampling locations included two sampling points west of Dry Creek Road. Further sampling is required to fully characterize the extent of the contamination in this pathway. Since metals, such as cadmium, can bioaccumulate into crops, residential use (for gardening, etc.) of sediments, or soils, from the creek area would potentially expose persons through ingestion of contaminants in food.

Ambient Air Pathways

On-site Air Emissions Pathway

On-base workers have been exposed to various contaminants of concern in ambient air. Data, such as mercury levels detected in Building #252, support the occurrence of those exposures. The mercury levels were reported throughout building 252 in the breathing zone; air sampling occurred after most of the workers were removed from the building for a remodeling project. Workers were exposed by inhaling contaminated ambient air (30). A maximum of 400 long-term (over one year) workers in the building, plus remedial workers involved in remodeling of the building, inhaled mercury vapors in the air. The base clinic occupational health staff measured mercury levels in the urine of approximately 30 of the 400 personnel potentially exposed in Building 252. That health study and its results will be discussed in the Public Health Implications section of this report.

In addition to the documented mercury exposure, many on-base personnel have been exposed to VOCs in ambient air. Those contaminants are released during industrial activities from the shops using solvents, fuels, and other VOC-containing compounds. Workers are exposed by way of inhalation and dermal contact. Base occupational health personnel stated that McAFB has documented OSHA violations because of contaminant emissions in air.

Air samples from air monitoring station #5, in the base industrial area, detected VOCs at concentrations above health comparison values. Air monitoring data for all portions of McAFB were not available for ATSDR evaluation. However, a simulated model of ambient air exposure for McAFB and surrounding areas was provided in the air toxics report (26). Using the amounts of contaminants released to the air, the report discusses various exposure scenarios to identify potential exposures (both on and off site). The investigation simulated the degree of exposure that would result from inhaling contaminants emitted from McAFB into the air. On-base activities responsible for the emissions include metal plating, fuel storage and distribution, painting, degreasing, and solvent wipe-down. The simulation indicated an increased risk of exposure on base in OU B, OU H, residential housing north of OU H, and off base in an elliptical area bounded by Marconi Avenue to the south, Elverta Road to the north, 100 feet east of Watt Avenue to the east, and the Rio Linda area to the west.

For certain chemicals, ATSDR cannot determine if people are being exposed to concentrations of health concern because the technically achievable detection limit is above the health comparison value. For example, the detection limit for methylene chloride is 1.8 ppbv and the health comparison value 0.61 ppbv. Thus, a "non detect" level for methylene chloride could actually be of health concern. Therefore, additional inhalation exposures to contaminants released from hazardous waste locations that may be of health concern could be occurring.

Off-site Ambient Air Pathway

In addition to the simulated air emission isopleths discussed in this section, contaminants have been detected in air by monitoring residential crawl spaces, ambient air in yards, private well heads, and off-base soil gas monitoring wells adjacent to OU D. Ambient air sampling on private property and the air monitoring stations on the perimeter of OU D have detected contaminants of concern in the air. The prevailing northwest wind direction (information provided by McAFB, along with a wind rose - see Figure 8) transmits contaminated ambient air into the Rio Linda neighborhoods. Residents are and have been exposed to the contaminants by inhaling the air. Children playing around the crawl spaces of homes would be particularly susceptible to inhalation of the volatile compounds.

The sources for this pathway are combined. Some VOCs, such as benzene and MC, are present in the Sacramento background ambient air. However, certain high results for MC and 1,1,1-TCA in residential areas near the base may be elevated due to industrial activities at McAFB or residential activities. Other VOCs measured in crawl spaces and yard air samples correspond to high levels of soil gases near OU D (i.e., TCE, and 1,1-DCE).

To further define this pathway, crawl space samples should also be analyzed for methane to provide necessary information on the potential for explosive conditions.

Completed Exposure Pathways Summary

Completed exposure pathways have been identified on and off site. On-site workers have been and may be exposed currently to VOCs (TCE, 1,1-DCE, PCE, benzene, and MC) and mercury by inhaling contaminated ambient air. Long-term workers (military and civilian) and residents had past exposure by way of ingestion and inhalation of, and skin contact with, TCE in the groundwater (base supply well pathway). The third exposure pathway for on-site workers and base visitors is ingestion of or direct contact with PCB-contaminated surface soil.

Off-site residents west of OU D have been exposed to contaminants in ambient air, sediments, and groundwater (private residential well). Specific exposures include ingestion of cadmium and lead in sediments (Magpie Creek); inhalation of MC, 1,1-DCE, benzene, TCE, 1,1,1-TCA and PCE in the ambient air; and ingestion and inhalation of, and skin contact with TCE, 1,1-DCE, PCE, 1,2-DCA, MC, VC, mercury, arsenic, lead, and cadmium in private well water. Table 20 describes those exposures.

B. Potential Exposure Pathways

Municipal Well Pathway

City well 150 has been abandoned because of TCE and other VOC contaminants from the OU B groundwater plume. City well 132 is in the pathway of the same plume; monitoring wells are being used to track the plume migration. Well 132 is now used for emergencies only. If it, or other city supply wells, were used after contaminant concentrations were above levels of concern, people using the water would be exposed by ingestion and inhalation of, and dermal contact with the contaminants. A carefully planned and executed sampling and analytic protocol is necessary to prevent the possibility of more exposures. The current tracking and monitoring of the plumes emanating from McAFB (combined with the on-site groundwater treatment systems) appears adequate.

Private Well Pathway

Municipal water connections to residences in the groundwater plume impact area (west, and southwest to the base, Fig. 4) provided an alternative source of potable water to 550 homes. Private wells were no longer analyzed when municipal water service was provided. The groundwater plume of concern during the early 1980s was near OU D. Today, several plumes are migrating from OU B and C (Fig. 6). City and county ordinances prevent the construction of new wells in the identified impact area. Because the private well data are not current, previously uncontaminated wells could now have VOCs and other contaminants at levels of health concern.

Past sampling procedures used field-filtered samples, which do not provide accurate data on heavy metal contamination. Results of the recent ATSDR health consultation indicated that the previous concentrations of organic and inorganic contaminants in groundwater have decreased. However, those results are from a small sample of private wells (37). A complete well-use survey is needed to determine how private wells are used (abandoned, inoperable, used for irrigation) and to identify residents who could be exposed to contaminants of concern by ingestion and inhalation of, and dermal contact with, contaminated groundwater.

Subsurface Soil Pathway

Elevated concentrations of contaminants of concern have been found in soil samples collected from OUs A, B, C, and D and other areas of the base. ATSDR has no information indicating that there has been human exposure to contaminants in on-base soils. Site workers involved in activities that disturb the subsurface soil, such as excavation and remediation, would be potentially exposed to the contaminated medium by inhaling contaminants released to the air, and ingestion of and skin contact with the contaminated soil. Future exposures during remedial actions could be minimized if workers are aware of the risks, wear appropriate personal protective equipment, and comply with applicable health and safety guidelines. Currently, the site has SAs and PRLs on-base that are not yet environmentally characterized. Also, additional areas may be identified during the remedial process in OUs that have not yet been studied. Because of the lack of final contaminant characterization of the soils and ongoing construction at McAFB, it is feasible that construction workers moving or removing soils could be exposed to contaminants by ingestion, inhalation and skin contact with the medium. Subsurface soil is considered a potential pathway because no data confirm such exposure. The potential for exposure existed in the past, and may yet during activities that disturb on-base subsurface soils.

Surface Soil Pathway

Sampling and analysis of surface soils at McAFB has just begun. The only data available are results for OU B1. Other OUs will be sampled later in the RI/FS for each OU. The current lack of sampling information prevents ATSDR from fully assessing the surface soil exposure pathway. Residents living on base, workers in areas with contaminated soil, or visitors could be exposed by way of inhalation and ingestion of, or dermal contact with, contaminated surface soil. Exposure could have occurred in the past, may be occurring now, or may in the future. When surface soil sampling data are available, ATSDR will be able to assess the pathway.

Off site surface soils may be contaminated if migration has occurred onto nearby residential properties. Such a potential exists for properties near the DRMO yard; the site is on the base boundary and homes are located along Bell Avenue within 500 feet of the PCB contaminated soils. Residents may be exposed by ingestion of or dermal contact with contaminants in the surface soils. This pathway is considered potential since data are not yet available to evaluate contamination levels. ATSDR evaluation of residential surface soils adjacent to OU D resulted in the elimination of this pathway for those residents.

Surface Water (Magpie Creek) Pathway

Past McAFB activities may have resulted in contamination of the surface waters in Magpie Creek. Residents, especially children, may have been exposed through dermal contact with, ingestion of, or inhalation of contaminants in the water. Present testing of the discharge waters leaving the base into the creek indicate no contaminants from McAFB entering the surface water. Metal contaminants, above background levels, were recently detected in the creek sediments. Depths of the metal-contaminated sediments indicate some type of releases of the contaminants into the creek have occurred over time. ATSDR has no data on historical surface water sampling and analysis to evaluate this pathway.

Biota Pathway

Contamination of the food chain is possible if certain contaminants, such as cadmium, bioaccumulate into fruits and vegetables grown in residential gardens (49). ATSDR has no sampling data with which to assess that pathway. The population at risk is residents who ingest contaminants from produce grown in contaminated soil or irrigated with contaminated groundwater. The exposed population is limited to the McAFB area because there are no commercial farms near the base.

C. Eliminated Pathways

Base Well Pathway

Base production wells are a past completed exposure pathway. TCE and other VOC contamination was reported in the base supply system during 1979-1980. Exposure to VOCs could have occurred by way of ingestion, inhalation, or direct skin contact. The past completed base well pathway has been eliminated by abandoning the wells and treating the contaminated groundwater.Base production wells are sampled every two weeks. Effluent from the treatment system for the major base production well (BW 18) is sampled weekly. Groundwater treatment systems are operating in OUs D, C, and B. Because of the current testing schedule and the active role the base is taking in groundwater cleanup, additional exposure by way of the base well pathway is highly unlikely.

Off-site Surface Soil (OU D) Pathway

Disposal waste sites in the area designated OU D at McAFB included open pits. Past practices of burning waste or leaving sludge uncovered provided opportunities for the site contaminants to be transported off site by wind, flooding, surface runoff, and/or excavation and construction. Residences on 20th Street, with yards adjoining OU D at McAFB (estimated population: 30-40) were the most likely to have incurred surface soil contamination as a result of such transport. Because of the inherent volatility of VOCs detected at OU D, and the dry, hot climate in Sacramento, ATSDR did not expect to find high levels in surface soils. Analysis of surface soils at those residences have not detected any contaminants emanating from McAFB that are at levels of public health concern. Therefore this pathway is eliminated.

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