USAF BROOKS AIR FORCE BASE
SAN ANTONIO, BEXAR COUNTY, TEXAS
Although groundwater is not being used for domestic purposes, some community members in the neighborhood located to the southwest of BAFB have expressed concern about hazardous materials detected in groundwater in the vicinity of BAFB (6,7). As a part of the ongoing base wide environmental program, groundwater samples are collected every six months. These samples are analyzed for a wide spectrum of organic and inorganic contaminants (1,8). A number of VOCs have been detected in groundwater in the vicinity of the FPTA2.
ATSDR evaluated the quality assurance and quality control procedures required by regulation for the sampling and analysis performed by the Air Force. Based on this evaluation, it is ATSDR's conclusion that the Air Force conducted a thorough sampling and analysis of the groundwater. These data are an accurate representation of the existing conditions at the FPTA2 and area in question (shown in Figure 1).
Of the 11 off-base wells sampled, only one monitoring well, MW-39, contained trichlorethene (TCE) in an amount that exceeded the national regulatory value for drinking water(1). In the most recent sampling, the level of TCE found in this well is around nine to ten parts per billion (ppb), slightly above the EPA Maximum Contaminant Level (MCL) of 5.0 ppb(2) (8). The only other contaminant detected, 1,2,-dichloroethene-cis found in off-base monitoring wells (MWs) was detected at 12.0 ppb in MW-52. This amount is below the EPA MCL of 70 ppb (8).
Figure 1 shows the location of the wells used for the sampling of groundwater in this area, and the present extent of the TCE contamination.
Evaluation of Exposure Potential for VOC-Contaminated Groundwater
Domestic Use of Contaminated Groundwater
Information collected by BAFB has revealed that the contaminated aquifer has not been used for residential domestic purposes since the provision of municipal water lines to the community in 1959 (9). FPTA2 was used from 1945 to 1960. Previous work by ATSDR, (1,2,3, 4) examined the possibility that contamination may have reached any offsite domestic wells prior to the provision of water lines. It was the conclusion of this previous evaluation that, based on the expected rate of migration of contaminated groundwater from FPTA2, the contaminants would not have extended offsite until after the alternate water system was in place. It is therefore not likely that, in the past, people were exposed to contaminants in domestic water supplies.
The 1994 well survey conducted by BAFB did not find any private water wells in use in the area. It is therefore not possible for people to currently be exposed to contaminated drinking water or to contaminants in the groundwater by bathing with the water, inhaling steam during showering, or other incidental contact with the water during domestic use.
Based on scientific information available on the effects of TCE on people, the levels of TCE detected in off base wells is not sufficient to cause adverse health effects if this water were to be used for domestic purposes such as drinking or bathing (10). Therefore, even in the unlikely event that domestic wells were installed in the area in the future, and people were to come in contact with contaminants in this groundwater, by drinking, inhaling it or by the water coming in contact with the skin, contaminant levels in this groundwater are not high enough to present a public health hazard.
Inhalation of Contaminants Migrating Up from Groundwater to Soil Gas
An ongoing public health assessment (PHA) being conducted by ATSDR at Kelly Air Force Base (KAFB) has studied the exposure potential for VOCs in soil gas (11). The environmental conditions studied in the KAFB evaluation are similar to those found in the BAFB area. As a part of the fate and transport evaluation of the likelihood of adverse public health impact from VOCs in soil gas, the KAFB PHA has determined that specific levels of these VOCs have to be present in groundwater before a release to the soil as soil gas can occur.
The KAFB PHA recommends that soil gas contamination might be considered for evaluation of potential public health hazard when the level of VOCs in groundwater exceeds one percent of the solubility in water of that VOC. The solubility of TCE in water is 1,366,000 ppb. One percent of that amount is 13,660 ppb (11). The maximum amount of TCE detected in the recent off-base sampling was 10.3 ppb. Based on this evaluation, VOCs in soil gas do not present a public health hazard.
Dermal Contact and Incidental Ingestion of Contaminants in Surface Water
In conversations with the US Geological Survey (12), it was pointed out that the shallow aquifers in the area can discharge water into local water bodies, such as streams, ponds or lakes. The contaminant plume map (Figure 1) clearly shows that the extent of contamination is limited and does not appear to have reached any surface water bodies in the area. However, if surface water became contaminated, exposure would be through infrequent and incidental contact, such as exposure of the skin or ingestion of small amounts during recreational activities (swimming, wading, fishing). This type of exposure to the levels that would be likely to reach surface water would not present a public health hazard. Furthermore, the BAFB groundwater withdrawal program will reduce the amount of contaminants in the aquifer and will further decrease the likelihood that contaminants will reach any surface water bodies. Even in the event that some discharge did occur to surface water, in the future, the levels detected in the plume are not sufficient to pose a public health hazard if it migrated into surface water.
Evaluation of Exposure Potential for the Treated Discharges
Concern has also been expressed that exposure to the air and water discharges from the treatment plant could present a public health hazard.
The treatment system extracts VOC-contaminated groundwater and removes the VOCs by burning in a direct-flame combustion device. The resulting vapors are routed through a carbon absorption system to further decrease remaining VOCs (5). The resulting by-product is vented to the air. EPA and TNRCC regulations require that the air emissions be monitored and that levels of VOCs remaining in the gases discharged must be 50 to 10,000 time less than levels than have been associated with any adverse effects and therefore are not considered to represent a public health hazard (1).
This treated water, from which the VOCs have been largely removed, are discharged into No Name Creek. The contaminant levels leaving the treatment plant are also monitored as required by state and federal law. Treated water will meet federal and state drinking water standards (1, 13, 14). The levels of VOCs remaining in the treated water are also well below amounts that might cause a possible public health hazard.