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ATSDR evaluates exposure pathways to determine whether people accessing or living near MCAS Yuma could have been (past scenario), are (current scenario), or will be (future scenario) exposed to site-related contaminants. In evaluating exposure pathways, ATSDR determines whether exposure to contaminated media has occurred, is occurring, or will occur through ingestion, dermal (skin) contact, or inhalation. When exposure to contaminated media occurs, the exposure pathway is regarded as "complete." To determine whether completed pathways pose a potential public health hazard, ATSDR compares contaminant concentrations to health-based comparison values (CVs). Comparison values are calculated from scientific literature available on exposure and health effects. These values, which are derived for each of the media, reflect the estimated contaminant concentration for a given chemical that is not likely to cause adverse health effects, given a standard daily ingestion rate and standard body weight. If contaminant concentrations are above CVs, ATSDR further analyzes exposure variables (e.g., duration and frequency) and the toxicology of the contaminant. This exposure evaluation process is summarized in Figure 3.

ATSDR evaluated a number of sites, both on-station and off-station, to determine whether potential exposure to contaminated media would result in past, current, or future public health hazards. Based on extensive review of available data and compiled information, ATSDR concluded that there were only three potential exposure pathways of concern, all of which were on-station: exposure to contaminated groundwater; exposure to ACM at the Radar Hill Disposal Area; and exposure to organic lead in surface soil at the Flight Line, Shops Area, and Fire School Area. ATSDR determined that the rest of the investigated sites do not represent public health hazards, based on one or more of the following reasons: 1) no contaminants were detected, 2) contaminants were detected at concentrations that are too low to pose a health hazard, 3) access to the area was (past scenario), is (current scenario), and will be (future scenario) highly restricted, and/or 4) impacted areas have been or will be remediated (ADEQ, 1997; Earth Technology, 1991 ; JEG, 1996; Southwest Division, 1997; Stearns, 1985; Uribe, 1996, 1997a,b; Yuma, 1997a,b, 1998a,b,c,e,f,g,h,i,j,k).

Evaluation of Groundwater Exposure Pathway

Contaminants from OU2 Site 7 (Fire School Area), underground storage tanks, and perhaps from other unknown sources, have leached into the groundwater. The primary groundwater contaminants of concern at MCAS Yuma are TCE, tetrachloroethene (PCE), 1,1-dichloroethene (DCE), 1,1-dichloroethane (DCA), JP-5 jet fuel, and fuel constituents (primarily BTEX). There are eight primary contaminated groundwater plumes of concern (see Figure 4 and Appendix A for details) (JEG, 1996; Yuma, 1998a; OHM, 1997, 1998a,b).

Other groundwater contaminants were present above CVs at MCAS Yuma. These contaminants are not of concern, however, because most of them are not associated with any known sources of contamination at MCAS Yuma, and most of the metals are naturally occurring and within background concentrations. Others contaminants, such as chloroform, are likely the result of off-site contamination (e.g., chloroform is a byproduct of water treatment processes using chlorine). The remaining contaminants occur only in isolated spots where exposures are unlikely, or the contaminants are not present at levels that could pose a public health hazard. Moreover, ongoing remedial activities will continue to remove groundwater contaminants (JEG, 1996; OHM, 1998c).

The Navy is actively remediating Area 4 (Fuel Farm Area plume) under the UST program using air sparging and vapor extraction. Also under the UST program, the Navy is remediating Area 8 (plume in vicinity of Building 310) using vapor extraction and pumping. The Navy plans to complete remediation of Area 8 by the end of 1998 (OHM, 1998a).

Remediation and containment has begun for contaminated groundwater plumes not covered by the UST program, and full remedial alternatives are being evaluated for the FS. The proposed plan calls for the Navy to remediate the contaminated plumes by active measures and monitored natural attenuation. In addition, since Area 1 (plume in vicinity of Building 230) has reached the station perimeter, the current plan calls for the Navy to contain this plume through the use of vertical recirculation wells. The Navy also plans to remediate hot spots in Area 1 through mass removal (AS/SVE). After a five-year review, if natural attenuation has not been successful, the current plan calls for the Navy to remediate contamination sources using pump-and-treat techniques (Yuma, 1998a; JEG, 1998a).

Groundwater generally flows under unconfined conditions beneath the station. The direction of flow is principally from the southeast to the northwest. Regularly performed monitoring has revealed that none of the contaminated plumes have migrated beyond the station perimeter (JEG, 1996 ; OHM, 1997, 1998b; Yuma, 1997a, 1998a,c,d).

Although the groundwater at MCAS Yuma is contaminated, no one has ever been exposed to the contaminated water. The groundwater is not used as a water supply source. Drinking water is supplied to the station directly from the Colorado River, via a canal system. Berms protect the canals' banks, so contamination from runoff is not a concern. Every year, for about two weeks, the canals are closed for cleaning, and an alternate source of water must be used. Since 1996, the station has relied on water from the city of Yuma during this period; the city gets its water from the Colorado River via its own canal system. Until 1996, drinking water at MCAS Yuma was supplied from an on-site well during the canal-cleaning period. The on-site well is located in the southeast corner of the station, upgradient of the major contaminant plumes. All drinking water on the station has always met federal drinking water standards (JEG, 1996; Yuma, 1997a, 1998a,c).

There are 14 active off-site wells located within 3 miles to the northwest (downgradient) of MCAS Yuma. None of these wells is located within a half mile of the station. Of the 14 downgradient wells, eight are dewatering wells, three are irrigation wells, one is an industrial well, one is a domestic well, and one is a municipal well. Dewatering wells are generally installed to lower the groundwater table and are not used for drinking water. Within a 1-mile radius of MCAS Yuma, all water used for irrigation purposes is obtained exclusively from the Colorado River via a system of canals. Figure 4 shows the closest downgradient wells. (JEG, 1996, 1998a,b; Stearns, 1985; Yuma, 1998a).

    Current Exposure

No public health hazards are associated with current exposure to contaminated groundwater at MCAS Yuma. No on-site wells are used for drinking water, and the contaminated plumes have not moved beyond the station perimeter (OHM, 1998b; Yuma, 1997a, 1998a,c,d).

    Future Exposure

No public health hazards are associated with future exposure to contaminated groundwater at MCAS Yuma. The contaminated plumes are being remediated and monitored (see Appendix A and the Public Health Action Plan below for details). The Navy will also implement a containment policy to prevent future contaminant migration beyond the station's perimeter. All off-site wells are sufficiently distant to eliminate any likelihood of future exposure; many of the wells also have dewatering wells between them and the station. The on-site drinking well is upgradient of contaminated groundwater plumes. Should new wells be drilled at MCAS Yuma, ATSDR will reevaluate the potential for exposure to contaminated groundwater (JEG, 1996, 1998a,b; OHM, 1997a, 1998a,b; Stearns, 1985; Yuma 1997a, 1998a,c,d).

    Past Exposure

No public health hazards are associated with past exposure to contaminated groundwater at MCAS Yuma. The on-site well that was used prior to 1996 for drinking water is located upgradient of the contaminated groundwater plumes. In addition, exposure to water from this well would have been minimal because the well was only used for about 2 weeks each year, and residents of the station typically live on the station for only about 3 years. Drinking water at the station has always met all federal drinking water standards (Yuma, 1998c,e).

Evaluation of Asbestos-Containing Material (ACM) at the Radar Hill Disposal Area

The Radar Hill Disposal Area (see Figure 5) covers approximately 14 acres and is located to the south and west of Radar Hill within the central portion of MCAS Yuma. This area was used in the 1940s and 1950s for burning or burying station trash, and, more recently, for the disposal of construction debris, including broken concrete slabs. The Radar Hill Disposal Area was covered and graded in the early 1950s (JEG, 1996; Stearns, 1985; Uribe, 1997a).

There are two discrete areas within the Radar Hill Disposal Area that contain ACM:

  • Site 4A is north of Building 38 and east of Building 40. This area contains small pieces of asbestos-containing transite, cement pipe, and roofing materials, mixed with soil and other construction debris. ACM was found over an area of approximately 56,400 square feet (Uribe, 1996, 1997a).

  • Site 4B is west of Radar Hill. It has one debris pile containing approximately 3 cubic yards of ACM fiberboard. The area of contamination was estimated to be approximately 350 square feet (Uribe, 1996, 1997a).

Under the ROD, which was signed by all parties at the end of 1997, surface ACM and ACM-contaminated soil will be remediated. Workers will clean the surface ACM manually and will remove the upper inch of soil beneath the ACM. ACM-contaminated soil will be excavated with conventional construction equipment. The Navy will dispose of all ACM from the sites at a permitted facility (Uribe, 1997a).

    Past and Current Exposures

Access to the Radar Hill Disposal Area is restricted by partial fencing and warning signs are posted at Site 4A, but not at Site 4B. The area is fairly isolated. ACM is not present in very large quantities, and most of it is buried underground. ATSDR therefore concludes that no public health hazards are associated with current and past exposures to ACM at the Radar Hill Disposal Area. Because the Radar Hill Disposal Area has not yet been remediated, ATSDR recommends that warning signs be posted at Site 4B as a precautionary measure (Uribe, 1996, 1997a; Yuma, 1997b, 1998a).

    Future Exposure

The Navy has planned and budgeted removals in 1998 for CAOCs 4A and 4B in the Radar Hill Disposal Area. In addition, although ATSDR found no potential exposure pathways at these sites, the Navy has also planned and budgeted removals in 1998 for CAOCs 7A and 7B in the Fire School Area and debris piles south of the combat Aircraft Loading Apron and CAOC 9, the Horse Stable Area. These planned remedial activities should eliminate any potential exposures, therefore, no public health hazards are associated with future exposures to ACM at the Radar Hill Disposal Area.

Evaluation of Organic Lead in Surface Soil

The predominant source of organic lead in the environment is the use of tetralkyl lead compounds (primarily tetraethyl lead) as anti-knock additives in gasoline. In 1995, the Navy sampled for total organic lead (of which tetraethyl lead is only one possible component) at 11 sites that were suspected of having vehicle-related waste streams (JEG, 1996).

Organic lead was detected in six soil samples at MCAS Yuma:

  • Flight Line: Two nearly adjacent surface soil samples had organic lead levels of 0.88 mg/kg and 0.83 mg/kg, respectively. Both sample areas are paved, so there is no access to the contaminated surface soil. Access to the Flight Line has always been restricted (Yuma, 1998b).

  • Shops Area: Two samples, the second a field duplicate of the first, had organic lead levels of 12.80 mg/kg and 9.20 mg/kg, respectively. Other nearby samples did not contain organic lead. The sample area is separated and fenced off from the nearby single enlisted personnel barracks and dining facilities (which were built in the early 1980s), and is covered with dirt. The area is currently used by station maintenance contractors, primarily for storage. The area has been used historically for vehicle maintenance and public works (Yuma, 1998b,i,j,k).

  • Fire School Area: Two samples, one at the surface and one 2 to 3.5 feet below the surface had organic lead levels of 2.9 mg/kg and 0.53 mg/kg, respectively. The surface sample was taken from former Fire Training Pit 15, which was apparently used during the 1970s for fire and crash training. Former Fire Training Pit 15 is located in an area between two runways that has been covered with sealant since the early 1980s in order to prevent loose debris from being kicked up by planes. Exposure is therefore very limited. The subsurface sample was collected from the middle of an ACM site located within the broader Fire School Area (see Appendix A). Because of the presence of ACM, this area is fenced and warning signs are posted, so exposure is very unlikely (Yuma, 1998j).

Tetraethyl lead is quite toxic, with an adult reference dose media evaluation guide (RMEG) CV of 0.07 mg/kg, which is orders of magnitude below the levels of organic lead at MCAS Yuma. Tetraethyl lead degrades in a matter of months, however, and MCAS Yuma has not purchased leaded gasoline since 1987. The organic lead currently present in the soil is likely in the form of mineralized ionic ethyl lead breakdown products of tetraethyl lead. There are no CVs available for these breakdown products (JEG, 1996).

While the absorption, distribution, metabolism, and toxicity of inorganic lead have been extensively studied, only limited information is available on the toxicity of organic lead components. The limited data available on alkyl lead compounds indicate that the toxicokinetic profiles and toxicological effects of these compounds are qualitatively and quantitatively different from those of inorganic lead. Some of the toxicologic effects of alkyl lead, however, appear to be mediated through metabolism to inorganic lead (ATSDR, 1997; JEG, 1996).

Organic lead compounds are rapidly absorbed through the skin and by inhalation and ingestion. In the body, tetraethyl lead may be converted to triethyl lead, which is a more severe neurotoxin than inorganic lead. One study of subchronic exposure to tetraethyl lead in laboratory animals showed histopathologic effects in the liver and thymus. Very little information is available about the toxicity of the breakdown products of tetraethyl lead (ACGIH, 1993; ATSDR, 1997; CSIRO, 1998; IRIS, 1998).

Exposure to inorganic lead at sufficiently high levels has been shown to cause neurological damage and kidney damage in adults and children. It has also been shown to cause complications during pregnancy and retarded physical and mental development in exposed children. Exposure to inorganic lead at sufficiently high levels may cause increased blood pressure in middle-aged men and can damage the organs responsible for sperm production. No comparison value exists for environmental levels of inorganic lead because no thresholds have been demonstrated for the most sensitive effects in humans. Exposure is generally measured by blood lead levels (ATSDR, 1997). The Centers for Disease Control and Prevention determined that blood lead levels of 10 µg/dL in children were considered to be elevated (CDC, 1991).

Current and Future Exposure

The Flight Line and Fire School Area sample locations are in isolated areas and covered by pavement and a sealant, respectively. These areas do not pose a current or future public health hazard because of the protective barriers and infrequent exposures. Since use of the Shops Area sample location is restricted and primarily only for storage, and the organic lead detected appears isolated, this area also does not pose a current or future public health hazard. No public health hazards are associated with current or future exposures to organic lead at MCAS Yuma.

Past Exposure

The very high levels of organic lead found at some locations, while not a current or future concern, do raise questions about potential exposure in the past. The current high levels of tetraethyl lead degradation products suggest the possibility that high levels of tetraethyl lead may have been present in the past. Past exposure at the Flight Line was quite limited. Although past exposure at the Fire School Area and Shops Area may have also been quite limited, ATSDR does not have sufficient information to evaluate potential past exposures to organic lead at these areas. ATSDR, therefore, concludes that past exposures to organic lead at the Fire School Area and Shops Area pose an indeterminate public health hazard.


ATSDR recognizes that infants and children may be more sensitive to environmental exposure than adults in communities faced with contamination of their water, soil, air, or food. This sensitivity is a result of the following factors: 1) children are more likely to be exposed to certain media (e.g., soil or surface water) because they play and eat outdoors; 2) children are shorter than adults, which means that they can breathe dust, soil, and vapors close to the ground; and 3) children are smaller, therefore, childhood exposure results in higher doses of chemical exposure per body weight. Children can sustain permanent damage if these factors lead to toxic exposure during critical growth stages. ATSDR is committed to evaluating their special interests at sites such as MCAS Yuma, as part of the ATSDR Child Health Initiative.

ATSDR evaluated the likelihood that children living on MCAS Yuma may have been or may be exposed to contaminants at levels of health concern. ATSDR did not identify any situations in which children were likely to be or have been exposed to chemical contaminants at levels which pose a health concern. Although children playing in the fenced portion of the Southeast Station Landfill (south of North Ordnance Road) may be exposed to low levels of a variety of contaminants (see Appendix A), the contaminant levels do not pose a public health hazard because the area is fairly restricted and exposures would be limited and of short duration. Children may also be exposed to beryllium at the section of family housing built over the First Sewage Lagoon (see Appendix A). These potential exposures do not pose a public health hazard, however, because the beryllium levels are in the range of background concentrations. It is unlikely that children will come in contact with other on-site contaminated media because these sites are not in close proximity to the residential housing on the station and generally have restricted access.

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