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7r.r� ti,/' ATC Associates Inc. 1117 Lone Palm Avenue,Suite 201 Modesto,California 95351 209-579-2221 fax: 209-579-2225 ASS O C I ATE S INC. Three groundwater monitoring wells (MW-1, MW-2, and MW-3) were installed at the site on March 29 and 30, 1988. Initial groundwater samples collected from these wells did not contain detectable concentrations of petroleum hydrocarbons. A sensitive receptor survey was conducted within a 2,000-foot radius of the site. Groundwater monitoring well(MW-4)was installed north of the former USTs on April 19, 1988. In late April 1988, the open excavation associated with the former USTs was expanded by excavating petroleum-impacted soils above the groundwater. An additional 350 to 400 cubic yards of soil were removed from the former UST excavation between April 25 and 28, 1988. Laboratory analyses of soil samples collected from the sidewalls of the enlarged excavation indicated low concentrations of petroleum hydrocarbons remaining at the south and southeastern portion of the excavation. Free floating product was measured in well MW-4 by Parker Environmental Services (PES) during September 1992. A passive, selective membrane skimmer to recover the floating product was installed in well MW-4 on February 11, 1993. Soil borings BH-1 through BH-7 were advanced at the site to depths of approximately six-tior'seven+,s p feet bgs on February 26, 1993. Soil borings B-8 through B-21 were advanced and four additional groundwater monitoring wells (MW-5 through MW-8) were installed at the site to evaluate the lateral extent of impacted groundwater between July and October 1995. In a letter dated April 11, 2002, the San Joaquin County Environmental Health Department (SJCEHD) indicated that upgradient groundwater monitoring wells MW-2 and MW-3 could be reduced to annual groundwater monitoring and that the lateral and vertical extent of soil contamination remained undefined. On September 1.1 and 12, 2003, a 500-gallon UST that had contained used oil was excavated and removed from the site by Walton Engineering, Inc. Petroleum hydrocarbons and metals were detected in a soil sample collected at the base of the UST from a depth of approximately seven feet bgs. Refer to Grayland Environmental's Underground Storage Tank and Fuel Dispenser Removal Report, dated October 16, 2003, for more information. Two cone penetration testing (CPT) borings were advanced at the site to depths of approximately 80 feet bgs on October 14, 2004. The CPT borings were advanced to evaluate the vertical extent of groundwater beneath the site. Petroleum hydrocarbons and methyl tertiary butyl ether(MTBE) were detected in the sail and groundwater samples collected from the CPT boring locations. QCPT.2-7 ') g rsr,+IGels fG -Pcrn. Cela . A sensitive receptor report titled Sensitive Receptor Survey Report for Former Cheaper Store #37, 15 East Grant Line Road, Tracy, California was submitted in August 2008. In the sensitive receptor report, ATC concluded that the contaminant plume has not yet been fully defined to the north in the shallow water-bearing zone. Sensitive receptors located east and south of the site are not generally at S:\Environmental\25847\38\Workplans\2010 May SGS.doc 2