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Former Discovery Chevrolet June 6, 1997 <br /> Tracy,Califomia Report of Additional Subsurface Investigation <br /> ;,s f <br /> _. Groundwater Samples <br /> Groundwater samples collected from monitoring wells MW-1 through MW-5 were <br /> submitted to Excelchem under COC for Bio-parmeters analyses. Groundwater samples from <br /> MW-4 and MW-5 were also analyzed for TPHg, and BTEX, by modified EPA methods <br /> 35501$015/602. These laboratory results are summarized in Table 3. Additionally, water <br /> samples from all five wells were analyzed for bio-parameters: alkalinity, nitrate, nitrite, <br /> sulfate, total dissolved solids (TDS) and dissolved iron (Dfe). A discussion of the bio- <br /> parameter analyses is included in the next section. TPHg and benzene were detected in the <br /> water sample collected from well MW-4 with reported concentrations of 415 ppb and 1.1 <br /> ppb, respectively. The sample from well MW-5 had no detectable concentrations of TPHg <br /> and BTEX at or above the laboratory detection limits. <br /> EVALUATION OF INTRINSIC BIOREMEDIATION <br /> Almost all petroleum hydrocarbons are biodegradable under aerobic or anaerobic conditions <br /> (Borden, et al., 1995). A discussion of both aerobic and anaerobic conditions and how they <br /> apply to the site arc included below. Table 2 lists the results of selected bioparameters <br /> (Alkalinity, Nitrate, Nitrite, Sulfate, TDS, and Dfe) from laboratory analyses and dissolved <br /> oxygen(DO) readings from field measurements for each of the five groundwater monitoring <br /> wells on this site. <br /> Aerobic Bioremediation <br /> The viability of intrinsic bioremediation has been demonstrated in repeated field and <br /> laboratory experiments (McAllister and Chiang, 1994; Davis, et al., 1994; Borden, et al., <br /> 1995). Studies have established that petroleum hydrocarbon consuming microbes occur in <br /> fresh and salt water, soil, and groundwater at ambient levels up to 1 million cells per <br /> milliliter of groundwater (Zobell, 1946; Litchfield and Clark, 1973). The presence of <br /> >= petroleum hydrocarbons in groundwater has been found to increase the numbers of <br /> 3001-11.rpt 7 HORIZON ENVIRONMENTAL INC. <br />