ARCHIVED REPORTS_CONCEPTUAL SITE MODEL

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3. Nature and Extent of Subsurface Impacts 3.1 Extent of Hydrocarbons in Soil Historic soil data tables and concentration figures, presenting hydrocarbon distribution in soil are presented in Appendices E and F, respectively. The highest petroleum hydrocarbon concentrations (TPHd, TPHg, and MtBE) detected in soil were 14,000, 2,700 and 2.2 milligrams per kilograms (mg/kg), respectively. These hydrocarbon compounds were detected in shallow soil located around the loading rack area and near tank 137. The location of these maximum concentrations appears to be consistent with the areas where previous spills have occurred. Previous investigations indicate that there are three main areas at the site where soil has been impacted by petroleum hydrocarbons: 1) the area northeast of the loading racks associated with the onsite fueling station; 2) the area surrounding AST #137; and 3) the area around the former Keylock facility. The lateral extent of soil impact appears to be limited to these general areas and does not appear to extend offsite. Concentrations of petroleum hydrocarbons in these areas appear to be the highest around the capillary fringe (between 3 to 8 feet bgs). However, detections of petroleum hydrocarbons have been reported as deep as approximately 29 feet bgs. The lateral and vertical extent of hydrocarbons in soil has been adequately defined. 3.1.1 Former Transmix UST Based on soil samples collected during g well installation, concentrations of petroleum hydrocarbons were reported in soil surrounding the transmix UST (Figures 3 and 4). Analytical results of petroleum hydrocarbons in soil in MW-65B, MW-67UA, MW-68A, and MW-69UA indicate vertical definition, and lateral definition east of the former transmix UST. Petroleum hydrocarbon concentrations summarized in tables provided in Appendix E of soil samples collected and analyzed during previous investigations from soil borings PL-5, PL-6, NSB-14 and NSB-16 indicate vertical delineation further south of the office building, as illustrated on the soil concentration figures provided in Appendix F. Results of previous investigations suggest that soil impacts to the west of the former transmix UST are commingled with the petroleum hydrocarbon impacts associated with the area northeast of the loading racks, as summarized in Appendices E and F. 3.2 Extent of Hydrocarbons in Groundwater Groundwater monitoring and sampling has been ongoing since August 1985. Groundwater elevations have fluctuated by approximately 5 feet since monitoring and sampling began. Although the groundwater flow direction has been shown to vary, the principal historical groundwater flow direction has been reported to the north and northeast. Historical groundwater elevation and concentration summary tables are presented in Table 2. Chemical concentrations versus time graphs for numerous onsite wells are presented in Appendix G. The highest concentrations of TPHg (230,000 µg/L) and TPHd (2,100,000 µg/L) were for asam rab-water le g P collected in September 2003 from boring USB-2 located between AST #131 and AST #139. The highest concentrations of BTEX collected from water sample USB-2 were 19,000 µg/L, 15,000 µg/L, 4,900 gg/L, and 19,000 µg/L, respectively. Additionally, the greatest concentration of MtBE detected was 5,300 µg/L. As described previously, monitoring well EW-1 was subsequently installed near this location. BLASLAND, BOUCK &LEE, INC. 3124106 _ engineers,scientists,economists 3-1 J:\D0006\44634 00161022 Conceptual Site Model.doc