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Stantec No Further Action Request Report Chevron and ARCO Commingled Plume No. CP0034 April 10, 2012 Stantec calculated the mass of hydrocarbons remaining in the soil using historical soil analytical data. Per the soil mass calculations, 2,658.26 lbs of GRO, 101.47 lbs of DRO, 17.30 lbs of benzene, and 7.31 lbs of MTBE remained at the site prior to remedial activities. The mass calculation estimates are rough estimates and are based on soil analytical data that is up to 25 years old. The mass calculations do not take into consideration bio-remediation, due to naturally occurring micro-organisms, and natural attenuation. Per the past SVE activities at the site, approximately 4,967 lbs of GRO, 61.3 lbs of benzene, and 4.5 lbs of MTBE have been recovered from site. Mass calculations for petroleum hydrocarbons in soil are summarized in Table 4. Historical remedial system data is included Attachment D. Residual concentrations of petroleum hydrocarbons that may remain in the soil beneath the site do not pose a significant health or environmental risk, due to the lack of sensitive receptors near the site and because the site is comprised of two operating gasoline service stations on asphalt/concrete paved lots. 2.5.2. Distribution of Hydrocarbons and Oxygenates In Groundwater Groundwater monitoring and sampling has bee conducted at the Chevron facility since March 1989"and at the ARCO facility since August 1988. Current and historical groundwater analytical data is included as Table 5. GRO concentrations have decreased to below laboratory MRLs in all wells with the exception of Chevron wells-MW-9R (IIn Ng/L) and U8L&R(140 pg/L) and ARCO well E-10 (2,300 pg/L). However, trend analysis shows that concentrations in these wells are decreasing and will meet water quality objectives (100 pg/L) within approximately nine years. GRO trend analysis graphs for wells MW-21R, MW-5R, and E-10 are included in Appendix E. A groundwater hydrocarbon concentration map for the first quarter 2012 is included as Figure 21. Benzene and MTBE and concentrations have decreased to below or near to laboratory MRLs in all wells with the exception of ARCO well E-10. However, trend analysis shows that benzene concentrations are expected to meet water quality objectives (1 pg/L) in aRproximatel years. MTBE concentrations are expected to meet water quality objectives (13 pg/L) in one year_ rnzene and MTBE trend analysis graphs for well E-10 are included in Appendix E•/ 94275 NFAR 2012.doc 10