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Q r Request for Site Closure Page 3 Chevron U.S.A. Products Company,3246 East Fremont Street, Stockton, CA January 3, 1995 Detailed discussions of the site setting and surroundings are presented in reports by EA (1987, 1990) and Groundwater Technology (1993), Figure 1 shows the site location, topography, and surrounding features. Figure 2 is a site vicinity map showing adjacent properties and nearby wells. Figure 3 is a detailed site map showing current and former locations of groundwater monitoring wells and soil samples. RATIONALE FOR SITE CLOSURE Current site conditions indicate that environmental concerns at the site are of no immediate or future threat to current and potential beneficial uses of groundwater beneath the site. Other environmental (human health via exposure) risks associated with in-place hydrocarbon-impacted soil have been shown to be minimal (EA, 1987). As stated in the Addendum to Appendix A of the Tri-Regional Guidelines (CRWQCB-NCR, SFB, CVR, 1990), a rationale for site closure should be included with the request for closure. Based on a review of the results of past investigations and risk assessment, two compelling reasons exist as a basis for no further environmental action, including: Non-extensive hydrocarbon impact and small mass of hydrocarbons remaining in the subsurface Lack of significant leachate potential Non-Extensive Hydrocarbon Impact Based on the data provided by the numerous investigations conducted at the site between 1987 and 1994 (Attachment 2, Table 1), conclusions can be drawn as to the extent of hydrocarbon-impacted material currently in place at the site. The extent of hydrocarbon-impacted soil to be left in place encompasses an area of approximately 500 square feet in vicinity of the former UST pit (Figure 4). Vertically the extent of hydrocarbon-impacted soil is confined to depths between 15 to 40 feet BGS (Figures 5 and 6, and Table 1). Since laboratory analytical data does not exist from each sampling point, certain areas of the subsurface are better defined than others. To accurately depict the extent of hydrocarbons remaining in the subsurface, lateral boundaries were estimated for the 15-30 feet BGS zone and the 30-40 BGS zone. Lateral boundaries are extrapolated from the laboratory analytical data of soil samples collected from 30 - 40 feet BGS in applicable sampling points. The lateral boundaries of hydrocarbon-impacted soil are defined by benzene and TPH-G concentrations below laboratory detection limits in soil samples collected from MW-2 to the south, MW-3 to the west, EA-4 and pit 0072dos.rpt ❑1 GROUNDWATER ❑❑❑ TECHNOLOGY