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Former Discovery Chevrolet June 6, 1997 <br /> Tracy,California Report of Additionai Subsurface Investigation <br /> �f <br /> Based on the data in Table 2, the DO levels are high enough to make it likely that aerobic <br /> degradation is occurring at the edge and inside of the dissolved petroleum hydrocarbon <br /> plume. Groundwater monitoring wells that have or recently have had detectable <br /> concentrations of petroleum hydrocarbons (MW-1 through MW-4) are depleted in nitrate, <br /> indicating that ancrobic degradation is likely occurring or has recently has occured within <br /> the dissolved area of the plume. <br /> CONCLUSIONS <br /> Based on the data collected during this investigation Horizon concludes the following: <br /> • Hand augered soil borings in the vicinity of the former product lines and gasoline <br /> UST detected gasoline hydrocarbon at 5 and 8 feet bsg, indicating that <br /> hydrocarbon impacted soil remains in and around the former UST site. The <br /> highest hydrocarbon concentrations are along the former product lines that <br /> extend to the west from the southwest corner of the former UST cavity and the <br /> west side of the UST Cavity. The product lines apparently run under the concrete <br /> building apron to the former dispenser that was reportedly just inside the service <br /> bay area. <br /> • Soil samples from the down-gradient groundwater monitoring well MW-4 boring <br /> detected gasoline hydrocarbon in the soil sample collected at 6% feet bsg., <br /> possibly the capillary fringe. -00,f/ <br /> • Groundwater samples collected from monitoring well MW-4 indicated relatively <br /> low concentrations of benzene. A 6_ � 3S"S~ `//! <br /> 7_� T Ll G- <br /> • Both soil and groundwater samples collected from the cross-gradient monitoring <br /> well MW-5 were below detection limits for gasoline hydrocarbons. <br /> 3001-11,rpt 11 HORIZON ENVIRONMENTAL INC. <br />