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(YVATC v A S S O C I A T E S I N C V INTRODUCTION 6, On behalf of The Customer Company, ATC Associates Inc. (ATC) has developed a site conceptual model (SCM) for the former Cheaper Store #37 located at 15 East Grant Line Road in Tracy, California (Figures land 2). This SCM was prepared in response to a request from Ms. Vicki McCartney and Mr. Nuel Henderson of the San Joaquin County Environmental Health Department (SJCEHD). The corrective actions proposed in this SCM were prepared to address the lack of data regarding the vertical extent of petroleum impacted soil and groundwater at the site. BACKGROUND INFORMATION 6w On March 3, 1988, one 10,000-gallon capacity underground storage tank (UST) used to store leaded gasoline and one 10,000-gallon capacity UST used to store unleaded gasoline were excavated and removed from the site. Groundwater was encountered in the UST excavation at approximately seven feet below I✓ ground surface (bgs). Laboratory analyses of two samples of groundwater collected from within the excavation indicated the groundwater had been impacted by petroleum hydrocarbons. On March 29 and 30, 1988,three monitoring wells (MW-1,MW-2, and MW-3)were installed at the site and `+ a sensitive receptor survey within a 2,000-foot radius of the site was conducted by Kleinfelder. According to Kleinfelder's Groundwater Assessment Report dated April 12, 1988, petroleum hydrocarbons were not detected at or above laboratory reported detection limits in the groundwater samples collected from MW-1, MW-2, and MW-3 on April 1 and 4, 1988. The groundwater flow direction was reported to be toward the north. On April 19, 1988, an additional monitoring well (MW-4) was installed north of the former location of the USTs. According to Kleinfelder's Additional Monitoring Well report dated April 22, 1998, petroleum hydrocarbons were detected in the groundwater sample collected from MW-4 on April 19, 1988. The r, groundwater flow direction was reported to be toward the northwest on April 20, 1988. In late April 1988, the open excavation associated with the former USTs was expanded by excavating petroleum impacted vadose zone soils. Kleinfelder's Soil.Removal Report dated May 3, 1988, approximately 350 to 400 cubic yards of soil were excavated from April 25-28, 1988. Laboratory analyses of soil samples collected from the sidewalls of the enlarged excavation indicated low concentrations of petroleum i hydrocarbons remained at the south and southeastern portion of the excavation. On November 20, 1990, groundwater monitoring was conducted at the site by Dames & Moore. According Dames & Moore's Groundwater Sampling and Analyses report dated December 12, 1990, petroleum hydrocarbons were not detected at or above the laboratoryreported detection limits in p groundwater samples collected from MW-2 and MW-3. Petroleum hydrocarbons were detected in the groundwater sample collected from MW-1. Approximately 0.35 feet of free product was measured in MW-4. The groundwater flow direction was reported to be to the north, northeast. On September 21, 1992, groundwater monitoring was conducted at the site by Parker Environmental Services (Parker). According to Parker's letter dated September 29, 1992, petroleum hydrocarbons were detected in the groundwater samples collected from MW-1 and MW-3. MW-2 was filled with sand to a Former Cheaper#37 January 29 2004 ATC Project Number 5425847.0037 Page 1