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6.1.4. Miscellaneous VOCs <br /> • 6.1.4.1. While TCE and PCE have been detected in the shallow groundwater underlying <br /> and downgradient of DDRW-Tracy, other VOCs have been detected in private wells and <br /> Upper and Middle Horizon monitoring wells at concentrations close to method detection <br /> limits. Additional VOCs detected during the January 1993 sampling round include <br /> chloroform, 1,1,1-TCA, vinyl chloride, 1,1,2,2-tetrachloroethane, and 1,2- <br /> dichloropropane. Of these five additional contaminants, vinyl chloride, carbon <br /> tetrachloride, 1,2-dichloropropane, and 1,1,2,2-tetrachloroethane were not detected in the <br /> September 1992 sampling round. Samples from monitoring wells LM36 and LM62 both <br /> contained chloromethane at levels just above the detection limit (1.23 and 1.51 µg/L, <br /> respectively) in September 1992, but chloromethane was not detected in samples from <br /> these wells in January 1993. <br /> 6.1.4.2. Although four VOCs (TCE, chloroform, 1,1,2,2-tetrachloroethane, and carbon <br /> tetrachloride) were detected in private wells, none were detected at concentrations above <br /> their respective Federal MCLS, although two were detected at concentrations above their <br /> California MCLS (carbon tetrachloride and 1,1,2,2-tetrachloroethane). None of these <br /> • compounds were detected at concentrations exceeding four times the detection limit, so <br /> concentrations are quite low. The solvents TCE, 1,1,2,2-tetrachloroethane, and carbon <br /> tetrachloride were detected in the Rose well at concentrations of 3.78 µg/1.., 1.07 µg/L, <br /> and 1.3 µg/L., respectively. The presence of TCE in this well may indicate the leading <br /> edge of the TCE plume in the Middle Horizon because this well is assumed to be <br /> screened in that horizon. Chloroform was detected in the Pombo well at 1.59 µg/L. The <br /> MCL for total trihalomethanes covers chloroform, among other compounds, and is set at <br /> 100 µg/L. Because this well is cross-gradient of DDRW-Tracy, the occurrence of <br /> chloroform is not likely to be related to DDRW-Tracy operations. <br /> 6.1.5. Background VOC Concentrations <br /> 6.5.1.1. Monitoring wells LM11 and LM12 are upgradient background wells relative to <br /> the known TCE and PCE plumes. No volatile organic contaminants were detected in <br /> either of these background wells during the January 1993 sampling round. <br /> 6-3 <br />