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Ms Lori Duncan Page 3 San Joaquin County Environmental Health Department ARCO 6080 February 20,2006 Mass Remaining in Soil Three separate areas of significant petroleum hydrocarbon impacts to soil were identified during the September 1999 upgrade activities Based on the calculations shown on Table E-1 (Attachment E), the total masses of TPHg and MtBE in soil are approximately 237 pounds and 0 04 pounds,respectively Mass Remaining in Groundwater The area of impacted groundwater is assumed to be approximately 64 square feet and the MtBE plume is assumed to be an average thickness of 10 feet Assuming a porosity of 0 32, the volume of impacted groundwater is 192 cubic feet Assurrung an average MtBE concentration of 0 9µg(L and multiplying by the two unit conversion factors yields an estimated 0 0000116 pounds of MtBE dissolved in groundwater SENSITIVE RECEPTORS In February 2006, URS performed a well records review of the Department of Water Resources (DWR) and City of Lathrop Public Works Department records The information requested was as follows general well information, well construction information, and well locations within a radius of 2,000 feet from the site No existing water supply wells were identified within the search area URS also conducted a 500- foot site walk to identify any sensitive receptors No sensitive receptors were identified with the exception of five wells noted at the 76 gas station located directly south of the site The 76 gas station site is also listed on the GeoTracker web site as containing numerous monitoring wells CONCLUSIONS AND RECOMMENDATIONS The following information fulfills the RWQCB Central Valley Region's outstanding requirements for site closure The RWQCB checklist for No Further Action at UST sites is included as Attachment F • All USTs and appurtenances have been upgraded at the site • The extent of soil and groundwater impacts has been ascertained from the investigations conducted at the site • Total mass-of TPHg and MtBE in soil was approximately 237 pounds and 0 04 pounds, respectively,in 1999 • Total mass of MtBE in groundwater is 0 0000116 pounds in 2005 • Existing MtBE concentrations in MW-2 is below the maximum contammant level • TPHg and BTEX are no longer identified at or above the detection lurut • The declining groundwater concentrations indicate that the remaining concentrations of petroleum hydrocarbons in soil do not present a threat to groundwater � r K IWprocessli5 BP ARCO\060801Closure Rpts106080C]osure Report doe