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ARCO Station 548 C O R <br /> 1612 West Hammer Lane,Stockton <br /> Remedial Action Plan <br /> May 31,2006 <br /> southern end of the UST complex (6,800 pg/L). Non_-detectable concentrations of TPHg at <br /> MW-4 and MW-5 suggest that significant downgradient migration of impacted groundwater has <br /> not occurred (Appendix D). �,,�, --� � <br /> Benzene-impacted groundwater is confined to the UST complex and extends downgradient. <br /> The maximum benzene concentration (89 pg/L) reported in groundwater for March 2006 was in <br /> VW-1. <br /> Groundwater impacted by MtBE is confined to the site in the area of the former UST complex; it <br /> extends east and northeast from the UST complex. The maximum reported concentration of <br /> MtBE reported during March 2006 was 99 pg/L in VW-1. <br /> Figures in Appendix D show that petroleum-impacted groundwater is located primarily around <br /> the former UST complex and downgradient from the complex for approximately 50 feet. The <br /> northward extension of the plumes was caused by the release of product into soil during the <br /> drilling of boring B-18 which punctured a product line in 1993. This release resulted in <br /> groundwater contamination around VW4, which has now become commingled with the larger <br /> plume around the former UST complex. <br /> Based on the minimal amount of plume migration since groundwater monitoring and SVE/AS <br /> mitigation of the plume began, which is shown by the decreasing concentrations over time, <br /> groundwater impacts at the site do not present a threat to the municipal supply wells identified in <br /> the sensitive receptor survey. <br /> The site conceptual model (URS, 2004) estimated remaining drocarbon mass at the site as <br /> 12.6 pounds of TPHg, 0.2 pounds of benzene, and 0.4 pounds of MtBE. <br /> 4.3 Dissolved Petroleum Hydrocarbon and MtBE Trends <br /> The highest concentrations of petroleum h arocarbons, specifically TPHg and MtBE, have <br /> historically been detected in on-site wel - Fluctuating TPHg and MtBE trends have been <br /> observed in this well. In off-site well M - , P g concentrations have generally increased over <br /> time; however concentrations are relatively low at 77 pg/L. TPHg and MtBE concentrations in <br /> all of the wells are included in Appendix D. <br /> Vw- S <br /> 1:0P-ARC0104 BP Valley Porttollo15481Reports\RAP1548 RAP.doc 7 <br />