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' 3. Background Geochemistry Previous studies at the site have demonstrated that subsurface conditions have supported ' biodegradation of carbon tetrachloride (Haley & Aldrich, 2001a and 2003). The presence of chloroform (a daughter product formed by the partial dechlorination of carbon tetrachloride) in quarterly monitoring results suggests that natural attenuation is taking place at this site at an unquantified rate. Background conditions in the aquifer have been characterized by samples collected in August ' 2001 and October 2003. Table 4 and Table 5 summarize the available information regarding the groundwater geochemistry and metal concentrations. Dissolved Oxygen. The majority of the dissolved oxygen levels measured onsite fall between the ranges of 1.0 to 8.0 mg/L. Nitrate. The detection of nitrate (as nitrogen) at concentrations up to 230 mg/L suggests that the presence of nitrate in groundwater may be limiting the rate of natural attenuation of carbon tetrachloride. This finding also shows that the aquifer is contaminated with nitrate, probably ' indicative of regional impact from the use of fertilizers on surrounding agricultural land. Sulfate. Concentrations of sulfate greater than 20 mg/L may cause competitive exclusion of dechlorination. However, in many plumes, with high concentrations of sulfate, reductive dechlorination still occurs. Sulfate concentrations at the French Camp site range from 30 to 130 mg/L. Methane. Outside of oil and gas fields, methane is usually indicative of strongly reducing conditions. Methane was not detected in groundwater samples from three of the four wells in ' which it was analyzed (in August 2001 only). In the fourth well (MW-313), the presence of methane in a groundwater sample was reported by the laboratory at a concentration of 52 gg/L. Given the results discussed above, the low methane values are reasonable because ' methanogenesis (production of methane from organic matter and carbon dioxide) is only common in groundwater when oxygen,nitrate, and sulfate have all been depleted. Chloride. The range of chloride concentrations across the site (from 7 to 160 mg/L). Although the breakdown of carbon tetrachloride would release chloride, the background concentrations of chloride are high enough to mask any addition of chloride to the groundwater system, considering the relatively low concentration of carbon tetrachloride present. Rather, dechlorination and natural compositional differences between water in the different depth zones of the aquifer probably both contribute to the chloride variability in groundwater. ' Alkalinity. Alkalinity is important in the maintenance of groundwater pH because it buffers the groundwater system against acids generated during both aerobic and anaerobic biodegradation. ' Increases in alkalinity result from the dissolution of minerals in the aquifer matrix driven by the production of metabolic carbon dioxide from microorganisms. Like chloride, bicarbonate alkalinity varies across the French Camp site, with a high value in samples from well MW-3B. ' This is also historically the well with the highest carbon tetrachloride concentration. This high value at MW-3B may represent an alkalinity halo due to enhanced microbial activity in this contaminated zone, although natural compositional differences in this parameter probably also contributes to the alkalinity variability across the site. R:\C0NTGRAN\CC14Cargi11\French Camp\Waste Discharge Application\supp info 6-29-04.doc 8