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Overview of Exposure Evaluation Process

ATSDR evaluated exposure pathways at Luke AFB to determine whether people accessing or living near the base could have been (past scenario), are (current scenario), or will be (future scenario) exposed to site-related contaminants through ingestion, inhalation, or dermal (skin) contact. When exposure to contaminated media occurs, the exposure pathway is regarded as "complete."

To determine whether completed pathways pose a potential health hazard, ATSDR compares contaminant concentrations to health-based comparison values. Comparison values are calculated using scientific literature available on exposure and health effects. These values reflect maximum concentrations of contaminants in a specific medium (e.g., soil, sediment, surface water, groundwater, air) that are not expected to produce adverse health effects, given a standard daily ingestion rate and standard body weight. If contaminant concentrations are above comparison values, ATSDR further analyzes exposure variables (for example, duration and frequency) and the toxicology of the contaminant.

ATSDR uses several media-specific comparison values to select environmental contaminants for further evaluation, including Cancer Risk Evaluation Guides (CREGs) and EPA's Maximum Contaminant Levels (MCLs). Appendix B provides a description of the comparison values used in this evaluation to assess environmental contamination. Appendix A provides a glossary of environmental and health terms presented in this PHA.

ATSDR reviewed the available data and information compiled on all recognized or designated IRP Sites (see Table 1) at Luke AFB to ensure a thorough evaluation of all potential or completed pathways of human exposure to contaminants. ATSDR did not identify any completed exposure pathways, nor did the Agency identify any community concerns (see Table 4).

Evaluation of Soil Exposure Pathway

Soil investigations at Luke AFB consisted of : 1) geophysical surveys, 2) soil gas surveys, 3) a soil gas scan (of the bulk fuel storage area [IRP Site SS-42] only), 4) test pit excavations, and 5) soil sampling. The primary site contaminants detected in surface and subsurface soils and sediments at the base were semivolatile organic compounds (SVOCs), petroleum hydrocarbons, polychlorinated biphenyls (PCBs), dioxin, and several metals. Of these, petroleum hydrocarbons were the most frequently detected contaminant (found at a total of 20 IRP sites).

Some contaminants detected during soil investigations were found at levels slightly above ATSDR's comparison values. However, unauthorized public access to the base is minimal because of the limited residential and industrial development in the area surrounding Luke AFB. The base boundary is fenced and the entrance is guarded. The contaminated sites are in non-residential areas, therefore exposure to contaminated soils is expected to be infrequent and of short duration. Consequently, ATSDR believes that any human exposure to soil contamination at Luke AFB will be limited.

    Past, Current, and Potential Future Exposures

ATSDR evaluated the data and information gathered during on-site investigations to assess potential exposure pathways via soil (see Table 1). ATSDR determined that human exposure to contaminated soil at Luke AFB does not present a past, current, or potential future pathway because contamination was either 1) detected at levels that did not or do not pose a public health hazard; 2) detected at depths below the ground's surface that are inaccessible to the general public; or 3) detected in areas where site access is restricted, unlikely, or infrequent. In addition, most remediation and cleanup of contamination at the base has begun. Deed restrictions have also been recommended at some IRP sites to prevent future residential redevelopment in areas where contaminant levels could be of potential concern if individuals had access to these sites (see Table 1--IRP Sites RW-02, LF-03, FT-07E, DP-13, LF-14, DP-23, LF-25, and SD-38) (Rothrock, 1997). Currently, there is legal review within the Air Force on actually placing deed restrictions on active installations. The Air Force feels there should be no need to place deed restrictions as there is no public access to active facilities. Deed restrictions should be required only for closing/transferring bases. For these reasons, ATSDR concludes that no public health hazards are associated with exposure to on-site soils.

Evaluation of Groundwater Exposure Pathway

Hydrogeology: Groundwater in the Luke AFB area exists under unconfined conditions. The depth to groundwater on base is approximately 350 feet. Groundwater in the base area generally moves in a southwesterly direction. High levels of naturally occurring arsenic, fluoride, dissolved solids, and minerals (various kinds) have been identified in the Luke AFB area, particularly in the area south of the base (Geraghty & Miller, 1997c).

Groundwater Use: Luke AFB currently receives its drinking water from seven drinking water production wells (PWs) located on the base (see Figure 3). This water supply system is composed of two parts: the high pressure (south base) system and the low pressure (north base) system. The low pressure zone is supplied primarily by Well 13. During high demand, Wells 9, 10, and 12 are used in addition to water from Well 13 to supply water to this system. Water from the low pressure zone is pumped into the system and the elevated water tower at the Water Plant. The depth of these drinking water wells ranges from 598 feet to 1,200 feet below ground surface.

A number of off-site private wells visible from the perimeter road were identified during ATSDR site visits. Most of these wells are used only for irrigation purposes (USAF, 1997). The Air Force has periodically sampled one private drinking water well, located on the southeast side of the base, at the owner's request (July 1990, December 1994, March 1995, and November 1997). No other private drinking water wells exist in the vicinity of Luke AFB. The nearest housing area, located 100 yards from the base, receives water from the local public water district (ADHS, 1997).

Groundwater Quality (Base Wells): Groundwater from base drinking water wells is sampled regularly to ensure that it meets all federal and state safe drinking water requirements. Fluoride and xylenes have been detected at concentrations below MCLs in wells of the base's low pressure water system. (Again, this system is used only for supplemental water during periods of high water demand [USAF, 1997].) Naturally occurring high concentrations of arsenic, fluoride, dissolved solids, and various minerals have been identified in the Luke AFB area, particularly in the area south of the base.

Groundwater samples were collected from monitoring wells installed on base during routine quarterly or semiannual sampling conducted between December 1991 and March 1996. Samples were analyzed for volatile organic compounds (VOCs), semivolatile organic compounds (SVOCs), and total and dissolved metals. The samples were also analyzed for boron and ethylene dibromide/1,2-dibromo-3-chloropropane (DBCP) during the fourth quarter of 1992. A few substances were detected infrequently and at concentrations below its respective MCL (Geraghty & Miller, 1997c; Luke AFB, 1997).

Groundwater Quality (Off-Site): The Air Force has periodically sampled one private drinking water well to the southeast of the base for metals, herbicides, pesticides, nitrates, and VOCs. No contaminants have been detected in the well water at concentrations above drinking water standards. Groundwater in the area of the well typically flows to the southwest. Groundwater is monitored upgradient to this well. No contaminants have been detected at concentrations above backgrounds levels or drinking water standards. The Air Force plans to stop testing this well in 1998 (USAF, 1997).

    Past, Current, and Potential Future Exposures

As part of the RI, a model was used to assess whether constituents detected in soils and sediments could eventually migrate to the groundwater. The modeling results demonstrate that it is highly unlikely that contaminants found in most soil and sediments on Luke AFB will leach to the groundwater in the future (Geraghty & Miller, 1997c). The model took into account: a) the area's climate and evaporation rate, b) the presence of only moderately permeable soils, c) the thickness of the zone above the water table, d) the low observed soil contaminant concentrations, and e) the relatively short half-lives of any detected contaminants. There was only one exception to these findings; the model concluded that contaminants in the soils and sediments of the bulk fuels storage area (SS-42) could likely reach the groundwater. To treat the contaminated soil and prevent future migration of contamination to the groundwater, a soil vapor extraction (SVE) system operates at this site. No contaminants have been detected in groundwater at levels that pose health hazards to date.

All drinking water at the base is supplied by base wells that are regularly tested to meet federal and state drinking water standards. During routine testing, no contaminants have been detected in these drinking water wells at concentrations above MCLs. Likewise, additional groundwater monitoring on base has detected no contaminants at levels that pose a public health hazard. For these reasons, ATSDR concludes that no public health hazards are associated with exposure to groundwater on site.

Evaluation of Surface Water Exposure Pathway

Surface streams and rivers near Luke AFB, including the Agua Fria, Salt, and Gila Rivers, are dry most of the time. These rivers typically convey water only during and immediately following storms, which do not occur often in this desert environment. All three rivers are dammed for irrigation and flood control.

There are only four areas on base that convey surface water: a) the wastewater treatment plant (WWTP) effluent canal (IRP Site SD-21), b and c) two drainage ditches (IRP Sites SD-20 and SD-26), and d) an irrigation canal that passes through the skeet range (IRP Site OT-41). The four sites are briefly described below. For more detailed information on these sites, see Table 1.

  • IRP Site SD-21. A canal on this site receives the treated effluent discharged from the base WWTP. The effluent is primarily composed of domestic wastewater and is routinely monitored in accordance with EPA regulations under the National Pollutant Discharge Elimination System (NPDES).

    No VOCs or SVOCs were detected in any surface water samples collected from this site during remedial investigations.

  • IRP Site SD-20. A drainage ditch on this site was used to receive periodic overflows, which consisted mainly of petroleum-based materials, from an oil/water separator. This oil/water separator was fixed in 1992 to prevent further overflows.

    Although no surface water samples were taken from this site, no VOCs or SVOCs were detected in surface and subsurface soil, sediment, or groundwater samples at levels that could result in adverse health effects. Groundwater monitoring at this site is scheduled to continue through 1998.

  • IRP Site SD-26. A surface drainage ditch (Hush House Canal) on this site received discharged oil/water separator waste. Drainage from other facilities, the runways, and taxiways to the west of the site are also channeled to this ditch. The primary source of potential contamination on this site (oil/water separator discharge) was eliminated in 1993 when the oil/water separators were connected to the base WWTP.

    Although no surface water samples were collected during remedial investigations, no VOCs or SVOCs were detected in surface and subsurface soil or sediment samples at levels that could result in adverse health effects.

  • IRP Site OT-41. An unlined irrigation canal passes through the active base skeet range on this site. This site was listed as a potential source of contamination because of the potential for lead shot entering the canal and being transported off site.

    No lead was detected in surface water samples collected from this site. Sediment samples were collected from the irrigation canal on this site and further downstream. The data indicate low levels of lead on site (up to 33 ppm), which slightly exceed background concentrations. Sediment samples collected downstream of the site did not contain elevated lead levels.

    Past, Current, and Future Exposures

Because a limited amount of surface water exists in the Luke AFB area, the potential for significant surface water exposures (past, current, and future) is low. No contaminants were detected during remedial investigations at levels that could result in adverse health effects. In addition, the sources of potential contamination at these sites have been removed or eliminated. No community concerns have been expressed about exposure to surface waters at or near Luke AFB. For these reasons, ATSDR concludes that no public health hazards are associated with surface water exposures at Luke AFB.

Evaluation of Air Exposure Pathway

In May 1992, air quality monitoring was performed to determine air quality upwind and downwind of the identified IRP sites at Luke AFB. Air samples were analyzed for total suspended particulate (TSP) matter, metals, VOCs, and SVOCs.

Ambient air data indicate minimal difference in contaminant concentrations upwind and downwind of the IRP sites. Only a small percentage of air samples contained detectable concentrations of contaminants; most samples were below the analytical detection limit. According to the results of this ambient air study, the IRP sites have a negligible impact on the surrounding ambient air quality at Luke AFB, even in close proximity downwind to the boundaries of the IRP sites (Geraghty & Miller, 1997c).

    Past, Current, and Potential Future Exposures

Ambient air sampling data indicated that the IRP sites do not adversely impact the surrounding ambient air quality. Because of the limited residential and industrial development in the area surrounding Luke AFB, public access to the base IRP sites is believed to be minimal. For these reasons, ATSDR believes that any human exposures to air contamination at Luke AFB do not pose a public health problem.


No community health concerns have been identified during ATSDR's site visits, Restoration Advisory Board meetings, and other public forums.

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