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INITIAL RELEASE DDJC Tracy Off-Site Wells Municipal Water Supply: Past Current and Future Exposures The city of Tracy municipal water is unlikely to be affected by site related contaminants. The city supplies municipal water to area residents from 10 municipal supply wells. The nearest municipal well to the DDJC Tracy site is situated approximately 1.5 miles crossgradient and away from the predominant direction of groundwater flow from the DDJC Tracy site. The municipal wells also draw from the Lower Tulare Aquifer, where only low level site-related contamination has been detected during RI activities. The city of Tracy monitors the drinking water regularly for chemicals, including TCE and PCE, to ensure that chemical concentrations do not exceed their respective MCLS. (MCLS are enforceable drinking water regulations that are protective of public health.) To date, the drinking water has met these safe drinking water standards (Tracy Water Department, 1997). Private Wells Past Exposures Contaminated groundwater has migrated off site in a north-northeasterly direction where private wells are located. Through a 1991 survey, the California Department of Health Services (CDHS) identified 18 private, domestic-use wells serving private residences within a 1-mile radius of the site. Of these 18 wells, i l wells are located upgradient or crossgradient to the site, while seven wells are located downgradient of the site and in or near the VOC plume. DDJC Tracy sampled 12 of the 18 wells for VOCs and metals in February 1987, and two of these wells were sampled again in September or November 1991. (Six additional upgradient private wells were sampled in 1991 for dissolved and total metals to provide a one-time estimate of background levels in groundwater, rather than to characterize potential contamination associated with the plume.) Contaminant concentrations in the crossgradient and upgradient wells from the site were below the MCLS (and CREGs) for all VOCs and metals except chromiurn, which was detected in one well at a concentration of 56 ppb—above the CA MCL of 50 ppb but below the EPA MCL of 100 ppb for chromium. TCE, but no PCE, was detected in two downgradient wells at concentrations greater than ATSDR's CREG and the EPA and CA MCL. The highest TCE (6.7 ppb) concentration was detected in a well that draws water from the Upper to Middle Horizons of the Upper Tulare Aquifer in relatively close proximity to the northern boundary of the site. Carbon tetrachloride was also detected in the two downgradient wells at levels (0.6 to 1.8 ppb) above ATSDR's CREG of 0.3 ppb, but below the EPA MCL of 5 ppb. Historically, carbon tetrachloride has been infrequently found at isolated monitoring wells on and off site, but at levels below the MCL. Although the source of the carbon tetrachloride has not been determined and 10