ARCHIVED REPORTS_FEASIBILITY STUDY AND CORRECTIVE ACTION PLAN

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4.2 HYDROCARBON DISTRIBUTION IN GROUNDWATER There are 38 groundwater monitoring wells at the site; 37 of these monitor shallow groundwater conditions (5 to 39 fbg) and 1 well (DW-2) monitors deeper groundwater between 55 and 60 fbg. The wells have been gauged and sampled since 1990, and samples are routinely analyzed for petroleum constituents. Historical groundwater analytical data from the monitoring wells are presented in Table 3, and available grab groundwater data are presented in Table S. SPHs have historically been measured in wells MW-1, MW-10, MW-11,MW-13, MW-15, MW-16, and MW-17, with the most recent SPH detection during May 2006 (0.05 feet). No SPHs have been detected in any wells since then, indicating that SPH is fully depleted across the site. During the fourth quarter of 2011, wells contained up to 17,500 micrograms per liter (gg/L)TPHd, 20,000 µg/L TPHg, 9,190 µg/L benzene, and 1,470µg/L methyl tertiary-butyl ether (MTBE), with maximum concentrations in source area well MW-10. Trend graphs,including the time to reach water quality objectives (WQOs), are included in Appendix C. The trend analyses shows that COC concentrations are stable to decreasing in the site wells and further indicate that the plumes are not migrating and are collapsing to the cores. This is further illustrated in the following section (4.3) presenting COC plume models. Current TPHd,TPHg,and benzene concentrations in down-gradient perimeter wells are shown in the following table. Well MW-20 is located on site, and wells MW-34 and MW-36 are off site. These data demonstrate that there is no significant risk to potential down-gradient receptors. 241729(33) 8 CONESTOGA-ROVERS&ASSOCIATES