Laserfiche WebLink
,%W <br />4.2 Groundwater <br />TPHG was detected as high as 12,000 ppb in the heart of the dissolved contaminant plume in <br />September 1995. TPHG was recently detected at a maximum concentration of 5,500 ppb. <br />Benzene was detected at a maximum concentration of 120 ppb. Downgradient monitoring wells <br />MW 1 and MW2 were non -detect for petroleum hydrocarbon constituents. The lateral extent of <br />the dissolved hydrocarbon plume has been essentially defined. However, the presence of <br />overhead and underground utilities on the western and southern boundaries of the property <br />prevented establishing a "zero line" for the dissolved contaminant plume. TPHG and benzene <br />were detected at 400 and 2.4 ppb, respectively, in boring SB7 near the western property <br />boundary, and at 2000 and 20 ppb, respectively in boring SB6 near the southern property <br />boundary. Nevertheless, based upon the relatively impermeable soil and the evidence of intrinsic <br />bioremediation occurring beneath the site, we would expect the lateral extent of petroleum <br />hydrocarbons beneath the site to be severely limited. The estimated extent of dissolved <br />petroleum hydrocarbons beneath the site is shown on Figure 3. <br />5.0 SPECIFIC INFORMATION FOR CLOSURE REQUEST <br />—' <br />5.1 Introduction <br />This section provides information specifically required for closure review as per Appendix B of <br />the Tri -Regional Board Staff Recommendations. <br />5.2 Rationale for Closure with Residual Petroleum Hydrocarbon Impacted Groundwater <br />The source of the hydrocarbon release(s) has been removed with the excavation of the former <br />underground fuel storage tanks and the impacted soil. Approximately 1,530 cubic yards of <br />contaminated soil was removed and remediated by passive aeration. The treated soil was <br />verified clean by laboratory analysis and was subsequently used as backfill <br />The total volume of affected material removed was approximately 1,530 cubic yards. Due to <br />the minimal regulatory oversight of the project, insufficient historical data is available to provide <br />groundm\nnuioklosure 10 <br />