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In this section, exposure pathways are evaluated to determine whether people accessing or living near Mountain Home AFB could have been (past scenario), are (current scenario), or will be (future scenario) exposed to site-related contaminants. When evaluating exposure pathways, ATSDR identifies whether exposure to contaminated media has occurred, is occurring, or will occur through ingestion or dermal (skin) contact. If exposure was or is possible, ATSDR then considers whether contamination is present at levels that might affect public health.

ATSDR uses comparison values in selecting contaminants for further evaluation within an exposure pathway. Comparison values are calculated from scientific literature available on exposure and health effects. These values, which are derived for each of the different 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. Because comparison values represent thresholds of toxicity, exposure to chemical concentrations above comparison values does not necessarily produce health effects. In fact, ATSDR comparison values are designed to be many times lower than levels at which no effects were observed in experimental animals or epidemiologic studies. If contaminant concentrations are above comparison values, ATSDR further analyzes exposure variables (for example, duration and frequency) and the toxicology of the contaminant. Figure 4 summarizes this exposure evaluation process.

Comparison values used in this document include EPA's maximum contaminant levels (MCL) and ATSDR's environmental media evaluation guides (EMEG), reference dose media guides (RMEG), and cancer risk evaluation guides (CREG). MCLs are enforceable drinking water regulations developed to protect public health, but they also consider economic and technological factors. CREGs, EMEGs, and RMEGs are strictly health-based comparison values developed by ATSDR that are not enforceable. A glossary of environmental and health terms and a description of the comparison values used in this public health assessment are provided in Appendix A and Appendix B, respectively.

ATSDR analyzed all 33 IRP sites at Mountain Home AFB to determine if there are any associated past, current, or future public health hazards. ATSDR reviewed the available data and information compiled on all recognized or designated IRP sites at Mountain Home AFB to ensure a thorough evaluation of all potential or completed pathways of human exposure to contaminants. ATSDR identified exposure to contaminated groundwater as the only completed pathway that could potentially be a health concern (Table 1). An analysis of available information (Table 2) for the other IRP sites indicates that these sites are not associated with any public health hazard because (1) no site-related contaminants were present, (2) contaminant concentrations detected are too low to pose a health hazard, (3) past and current exposures have been prevented by strict security measures, and/or (4) affected areas have been or will be remediated.

Evaluation of Soil Exposure Pathway

Soil surveys conducted at Mountain Home AFB included soil gas sampling, test pits, and soil core samples. Soil gas sampling was used to identify volatile organic compound (VOC) contamination (Fire Protection Training areas only) and to determine if additional soil sampling was necessary. Slightly elevated levels of metals (arsenic, lead) and polycyclic aromatic hydrocarbons (PAH) were detected most frequently in the soils (often at unspecified depths) at various locations throughout Mountain Home AFB.

Slightly elevated levels of metals (arsenic, beryllium, and lead) and PAHs were found in the surface soil (0.5 to 1 foot deep) at the B Street Landfill. The ash disposal area of the B Street Landfill was capped in 1995 and warning signs are posted around the landfill to prevent trespassing. Slightly elevated levels of arsenic (5.6 ppm), lead (32.7 ppm) and pesticides were found concentrated in a small area of the surface soil at the Entomology ShopYard (SD-12). The site is currently covered with pavement and past exposures are believed to be unlikely because SD-12 was an industrial facility with limited access. Although elevated levels of barium (4,270 ppm), cadmium (748 ppm), lead (369 ppm), and PAHs such as benzo(a)pyrene (5.5 ppm) were detected at unspecified depths within the Accumulation Pad (SS-29), unauthorized access is limited due to the perimeter fence surrounding the site. Slightly elevated levels of arsenic (3.9 ppm), beryllium (1.2 ppm), lead (522 ppm), and PAHs (benzo(a)pyrene, 0.33 ppm) were detected in the surface soil at the Auto Hobby Shop/Munitions Trailer (SD-24); however, this site is fenced, so unauthorized access is believed to have been very limited.

At the Flight Line Spill (ST-11), fuel constituents (benzene, toluene, ethylbenzene, and xylenes [BTEX]), metals (arsenic, beryllium, and lead) and PAHs were found at slightly elevated concentrations at the release point of the spill and in the soils surrounding the pipeline. The contaminated soils are inaccessible to the public as the spill occurred underneath an aircraft parking lot.

The potential for exposure to contaminated soil at Mountain Home AFB is believed to be minimal because the public has limited access to the base. The base perimeter is fenced and there are guards positioned at the base entrance to prevent unauthorized access to the base. Several of the sites are not accessible as they are paved over and located near or on the flight line. Exposure to contaminated soils by base personnel is expected to be intermittent and of a short duration as the average tour of duty is about 3 years at Mountain Home AFB.

    Past, Present, and Potential Future Exposures

ATSDR evaluated available data and information from on-site investigations to assess the potential exposures to soil. ATSDR determined that human exposure to contaminated soil at Mountain Home AFB does not present a past, current or potential future pathway because contamination was either (1) detected at levels that did or do not pose a public health hazard; (2) detected at depths below the ground's surface that are inaccessible to the public; or (3) detected in areas where site access was restricted, unlikely, or infrequent. Remediation and cleanup actions have been completed. Deed restrictions were imposed at ST-11 to prevent future residential development and access to potential chemicals of concern. For these reasons, ATSDR concludes that no public health hazards are associated with exposure to on-site soils.

Evaluation of Groundwater Exposure Pathway

Hydrogeology. The groundwater table is located approximately 360 feet below ground surface (bgs) and is protected from above by a thick layer of basalt bedrock (340 to 350 feet). There is a slight gradient to the water table (10 feet per mile northeast of the base and 0.5 feet per mile on the base) (Bigsby, 1995). The groundwater flows to the southwest but is influenced somewhat by the base production and nearby irrigation wells from May through September due to seasonal variation (Bigsby, 1995). The groundwater level drops between 1 to 2 feet a year as the groundwater discharge exceeds the recharge at the base.

Groundwater Use. Mountain Home AFB obtains its drinking water from 11 on-base production wells ranging from 375 to 610 feet bgs. There are 35 private wells used for drinking water and irrigation within a 2-mile radius of the base; most of these wells are used for agricultural purposes. The closest private residence and well is about 0.5 miles east of the base (Figure 5).

Groundwater Quality (Base Wells). Base production wells used for drinking water are monitored quarterly in compliance with the Safe Drinking Water Act. In addition, during the base-wide groundwater investigation, four rounds of groundwater monitoring (including the base production wells, the base monitoring wells, and the off-site irrigation wells) were conducted. Low levels of VOCs, semi-volatile organic compounds (SVOC), and metals were detected during the basewide groundwater investigation.

Trichloroethylene (TCE) was detected at low concentrations in some samples collected during routine monitoring of the drinking water supply and during the basewide groundwater investigation. One time only, TCE was detected at a level slightly above ATSDR's CREG, but below EPA's MCL (Woodward-Clyde, 1993). In multiple other samples, TCE was detected below the CREG and EPA's MCL. Comparison of modeled historic releases of hazardous substances with actual groundwater concentrations indicates that the source of the TCE is probably the former fire protection areas located on the south-central part of the base (Woodward-Clyde, 1995). TCE distribution in the base production and monitoring wells is influenced by seasonal variations in the direction of groundwater flow (Woodward-Clyde, 1996). TCE has been transported both north toward base production wells and southwest in the direction of regional groundwater flow (Woodward-Clyde, 1996).

Benzene, pentachlorophenol, arsenic and vanadium were sporadically detected above ATSDR comparison values. Several trihalomethanes have been detected in base production wells but not in base monitoring wells (Woodward-Clyde, 1992, 1996). Trihalomethanes may be associated with the chlorination process used in the base water supply system (Woodward-Clyde, 1995). Bromoform (53.8 ppb), bromodichloromethane (2.2 ppb), and chlorodibromomethane (7.9 ppb) were detected at levels above CREGs but below EPA MCLs.

Elevated nitrate levels detected in the Mountain Home AFB water supply may be attributed to fertilizers applied to the golf course and runway, aircraft de-icing operations, and waste water infiltration from the sewage lagoons (Air Force, 1996). Base production wells with nitrate levels exceeding the MCL (10 ppm) were either shut down or the water supply was mixed with water from another production well to reduce nitrate concentrations to levels below the MCL. Base production wells # 1 and # 4 were recently used only in emergency situations (e.g., fire fighting) and in times of high seasonal demand (e.g., lawn watering and golf course maintenance), respectively, due to elevated nitrate levels (Air Force, 1997a). Well # 1 was taken out of service in 1996 and well # 4 was closed in 1997 (Air Force, 1998). Residential and industrial facilities at Mountain Home AFB generally receive drinking water from the well nearest to them. There is no base-wide central mixing reservoir prior to treating the drinking water; although base production wells # 2 and # 4 were mixed in order to reduce nitrate levels (EA Engineering, 1993). The nitrate levels are carefully monitored and the base's "nitrate action level" is one-half of the MCL (Air Force, 1997a).

One area of perched groundwater is located at Mountain Home AFB under the Flight Line Fuel Spill (ST-11). Benzene was detected above ATSDR's comparison value in the perched groundwater. Fuel constituents are no longer migrating and are not believed to have migrated more than 169 feet below the surface (Woodward-Clyde, 1995). The perched groundwater has not been used as a drinking water supply and deed restrictions have been placed on property at ST-11, to prohibit the perched water supply from being used as a potable water source.

Groundwater Quality (Off-base Wells). Low levels of TCE (up to 1 ppb) have been detected at two off-base wells. The source of the TCE has not been established. These concentrations are below ATSDR's comparison values.

    Past, Present, and Potential Future Exposures

The maximum TCE concentration detected in the base's drinking water supply is slightly above ATSDR's CREG. All other samples had concentrations measured below the CREG. Because the CREG is a screening tool, exposure to TCE concentrations above the CREG are not necessarily associated with health effects. ATSDR evaluated the potential for adverse health effects from exposure to TCE-contaminated groundwater. Accounting for sensitive subpopulations (e.g., children) identified in the demographics section of this report, ATSDR estimated the potential exposure dose for both on-site residents and on-site workers. In deriving the human exposure doses, ATSDR incorporated information about the frequency and duration of contaminant exposure and contaminant concentrations. ATSDR assumed that adults drank 2 liters of well water each day and weighed 70 kilograms (kg) and children drank 1 liter of water each day and weighed 10 kg. To calculate a conservative exposure dose, ATSDR used the exposure period of 30 years and the maximum detected contaminant concentration. As a result of this exposure assessment, ATSDR concludes that consumption of TCE in on-site drinking water is not likely to result in health problems for the on-site residents or on-site workers from past, current, and future exposures.

ATSDR evaluated the health significance of exposure to bromoform, bromodichloromethane and chlorodibromomethane to on-site residents and to on-site workers as outlined above (see Appendix C). Based on this exposure assessment, ATSDR concludes that consumption of bromoform, bromodichloromethane, and chlorodibromomethane in on-site well water is not likely to result in health problems for the on-site residents or on-site workers from past, current, and future exposures. For these reasons, ATSDR concludes that no public health hazards are associated with exposure to groundwater at Mountain Home AFB.

Evaluation of Surface Water and Sediment Pathway

There are no permanent streams on or near the base. The nearest surface water bodies are the Snake River (2.5 miles south), Canyon Creek (4 miles west), and Rattlesnake Creek (> 4 miles east). Canyon and Rattlesnake Creeks are usually dry, and convey water resulting from heavy precipitation to the Snake River.

The only surface water features on the base are the flight line storm drain (SD-25) and the wastewater lagoons of the lagoon landfill (LF-1). Potential exposure to these surface water bodies is unlikely as SD-25 is adjacent to the flight line and LF-1 is part of an active wastewater treatment facility. Slightly elevated levels of lead (up to 47.5 ppm) and total recoverable petroleum hydrocarbons (TRPH) have been detected in the sediment at LF-1. Benzene (0.004 ppm) and bis(2-ethylhexyl)phthalate (0.025 ppm) were detected in the surface water at LF-1 above ATSDR comparison values. Elevated levels of arsenic (8.3 ppm), lead (282.5 ppm), and PAHs were detected in the sediment at SD-25. The surface water at SD-25 was found to contain bromodichloromethane, pentachlorophenol, and phenol at concentrations slightly above ATSDR comparison values.

    Past, Present, or Potential Future Exposures

The potential for significant surface water and sediment exposure is low at Mountain Home AFB because (1) there is a limited amount of surface water and sediment (SD-25 and LF-1) and (2) both surface water bodies are relatively inaccessible to the public. For these reasons, ATSDR concludes that no public health hazards are associated with exposure to surface water and sediment at Mountain Home AFB.


No community health concerns have been identified during ATSDR's site visits, Restoration Advisory Board meetings, or reported by the IDHW.


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 Mountain Home AFB, as part of the ATSDR Child Health Initiative.

ATSDR evaluated the likelihood that children living on Mountain Home AFB 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 the children may have been exposed to elevated levels of TCE, bromoform, chlorodibromomethane or bromodichloromethane in the drinking water supply, conservative exposure estimates indicate that the maximum detected concentrations do not pose a health concern. It is unlikely that the children would have come in contact with other on-site contaminated media because these sites are not in close proximity to the residential housing on the base and have restricted access due to their industrial nature or proximity to the flight line.

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