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Former Discovery Chevrolet June 6, 1997 <br /> Tracy,California Report of Additional Subsurface Investigation <br /> • All monitoring wells were measured on April 4, 1997, and a groundwater <br /> gradient for the site was calculated at 0.004 to the north-northwest. This is <br /> consistent with historical gradients. <br /> • All wells were sampled and analyzed for bio-parameters. The geochemical data <br /> indicates that aerobic and anaerobic biodegradation is occurring or has occurred <br /> and is likely remediating the dissolved petroleum hydrocarbon plume, gasoline <br /> hydrocarbon compounds desorbing from the soil and possibly hydrocarbons in <br /> the capillary fringe. <br /> • The overall historical trend has been a reduction in dissolved hydrocarbon <br /> concentrations through natural attenuation in the groundwater beneath the site. <br /> Based on the data collected to date, natural attenuation through intrinsic <br /> t bioremediation will continue to decrease the levels of gasoline hydrocarbon <br /> compounds dissolved in the groundwater beneath the site. <br /> RECOMMENDATIONS <br /> Based on the data collected to date it is the opinion of Horizon that the gasoline <br /> hydrocarbons have been sufficiently delineated in the groundwater at this time and no <br /> additional groundwater monitoring wells required. Based on the distance and depth of <br /> public and private wells from the site and the use or lack of use of the shallow groundwater <br /> in the area it is our recommendation that no active groundwater remediation be required. <br /> Based on the relatively high concentrations of gasoline hydrocarbons detected around the <br /> former product lines it is possible that if construction work occurs in this area that exposure <br /> .,; to the environment would be possible. If the site use remains as it is and the site remains <br /> 3001-1 l.rpt 12 HORIZON ENVIRONMENTAL INC. <br />