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INITIAL RELEASE DDJC Tracy approximately 2,500 feet from the depot boundary. PCE concentrations of 5 ppb or greater extend 1,600 feet (see Figure 5). Of the other VOCs detected at concentrations exceeding comparison values, none is distributed as widely as TCE or PCE, or at such high concentrations. For example, 1,1-DCE was detected in a limited number of groundwater samples at maximum on-site concentrations of 11.9 ppb—above the ATSDR CREG, but only slightly above CA MCL of 6 ppb. Similarly, pesticides, primarily dieldrin, were detected at levels greater than comparison values in the north-northeast comer of the site near the VOC plume, but not with the same frequency or high concentration as TCE and PCE (Woodward-Clyde Consultants, 1992b). Waste management practices and accidental spills have contributed to pesticide contamination in the soil, which subsequently leached into the Above Upper and Upper Horizons groundwater(Montgomery Watson, 1995). A dieldrin plume appears to emanate from the sanitary sewage lagoons and is entirely contained within the 5 ppb contour of the VOC plume associated with OU 1, whereas other pesticides show no discernable pattern. Dieldrin has been detected off site in groundwater at levels above ATSDR's CREG of 0.002 ppb, but below the EMEG and RMEG of 0.5 ppb. Metals and other inorganic compounds were also found in groundwater below the site. Sampling in 1991 found arsenic, chromium, and lead widely distributed in the groundwater below the site. However, these metals had not migrated off site at levels greater than their respective comparison values. Because of their frequency, high on-site and off-site concentrations, mobility, and potential health concerns, TCE and PCE are the principal compounds of concern in the groundwater. DDJC Tracy Water Supply Wells Past Exposures DDJC Tracy drinking water comes from on-site water supply wells that draw water from the Lower Tulare Aquifer. During the late 1980s and early 1990s (when contamination was first detected on site), drinking water was supplied by water supply well (WSW) 4, located in the path of the VOC plume, and WSW 7, located approximately 2,000 feet upgradient and away from the plume. DDJC Tracy used WSW 4 until 1992, when they took it out of service because of low water yield and signs of rusting (Woodward-Clyde Consultants, 1992b). By the early 1990s, DDJC Tracy installed two additional upgradient wells—WSW 8 and 9—that were used in addition to WSW 7. (WSWs 1, 2, 3, 5, and 6 had been abandoned.)Water from these wells was pumped into a 250-kilogram water tank, blended, and chlorinated prior to use by the depot. 8