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S ECOR `- - 5.0 DETERMINATION OF COPCs, EPCs, AND SLs This section presents the selection of COPCs, estimation of EPCs, and selection of relevant SLs for soil and ground water at the Site. After EPCs have been established for each COPC, it is compared to the applicable SL, as presented later in Section 6.0(Tier 1 RBCA. 5.1 SELECTION OF CHEMICALS OF POTENTIAL CONCERN COPCs are defined as chemicals that are potentially site-related and for which data are of sufficient quality for use in a risk assessment (USEPA, 1989). For conservative purposes, all detected analytes in soils and groundwater. Thus, COPCs in soils are TPHg, ethylbenzene, toluene, xylenes, and MTBE. COPCs in groundwater are benzene, MTBE and DIPE. 5.2 ESTIMATION OF EXPOSURE POINT CONCENTRATIONS EPCs are representative chemical concentrations at the point of exposure. To arrive at EPCs in soil at the Site, the 95% upper confidence limit (UCL) of the arithmetic mean for soils in the 0-10 feet bgs zone, which is potentially available for direct contact (Cal-EPA, 1992), was used in the Tier 1 RBCA. The relevant soil dataset is presented in Table C-4 of Appendix C. Since soil samples collected from MW-1 to MW-3 are at depths of greater than 10 feet bgs, none of these samples were included in the relevant soil dataset. Due to natural biodegradation at the Site,these COPC levels in soil could be ND at this point. According to the USEPA, to account for the impact of seasonal variations when calculating EPCs for COPCs in groundwater, analytical data from at least two quarters is required, and data from four quarters is preferred (USEPA, 1993). The USEPA also recommends that the groundwater EPCs are either estimated by(1)the 95% upper confidence limit(UCL)of the arithmetic mean for wells across the Site; or (2) the arithmetic mean of data obtained from a well or wells in the center of the plume (USEPA, 1993). Since different constituents were detected in on-site wells, the USEPA's first recommendation regarding groundwater EPCs was used. Thus, data from all on-site wells were selected to be included in the RBCA: MW-1, MW-2, and MW-3. Groundwater data from Second Quarter 2006 (6115106) through First Quarter 2007 (03108/07) for these wells (representing one full year of monitoring data to account for seasonal variation), as presented in Table C-5 of Appendix C, were used in the RBCA. Statistical analysis was performed for the relevant soil and groundwater datasets, using the USEPA Pro- UCL software program (USEPA, 2004c). The following parameters were estimated and presented in Tables 1 and 2 for soil and groundwater, respectively: • Arithmetic mean (for the central tendency exposure [CTE]case); • Standard deviation; • Minimum sample quantitation limit(SQL); • Maximum SQL; 10