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ADDITIONAL ASSESSMENT REPORT <br /> . JAY'S MINI MART <br /> STOCKTON, CALIFORNIA <br /> TUNE 2003 <br /> MTBE and tertiary Butanol (TBA)were detected in the soil at concentrations of up to 210 mg/kg <br /> and 1,800 mg/kg,respectively, at depths of between 30 and 40 feet bgs TPH G, toluene, and <br /> MTBE were reported m groundwater at 800 µgll, 18 µg/1, and 10 �ig/l, respectively <br /> On December 23, 1999, the County directed that an additional site investigation be conducted to <br /> assess the impact of petroleum constituents, including MTBE, to groundwater at the site <br /> GHH assumed environmental consulting responsibilities for the site in March 2002, and prepared <br /> an Additional Assessment Workplan in May 2002 This workplan was approved by the County <br /> Geology and Hydrogeology <br /> Geologically, the property is underlain by the Quaternary Modesto Formation and the deeper <br /> Quaternary Riverbank formation, both of which consist mainly of arkosic alluvium derived <br /> mainly from erosion of the Sierra Nevada Mountains to the east Both the Modesto and <br /> Riverbank formations are part of the Great Valley sedimentary sequence of rocks which fill the <br /> large Great Valley sedimentary basin which was originally derived as a forearc basin between the <br /> Sierran are to the east and the subduction zone to the west Stockton is located within the San <br /> Joaquin Valley which consists of the southern two thirds of the Great Valley province The <br /> northwesterly flowing San Joaquin River and its tnbutanes are the mainn drainage feature of the <br /> San Joaquin Valley Groundwater is present at the site at approximately 50 feet below ground <br /> surface(bgs) <br /> Dunng the drilling of three bonngs (SB-2 through SB-4), the dominant material encountered <br /> included silt, silty sand, and fine-grained sand Groundwater was encountered at approximately , <br /> 54-feet bgs in each of the three borings Although no site specific gradient information has been rpLir�- - <br /> gathered at the site, groundwater flow direction is most likely to the west, towards the confluence <br /> of the Calaveras and San Joaquin Rivers <br /> SCOPE OF WORK <br /> The following tasks were completed in accordance with GHH's Additional Assessment <br /> Workplan, dated May 2002, which was approved by the County and implemented by GHH <br /> • Prepare site specific health and safety plan (HSP) <br /> • Obtain Underground Service Alert (USA) clearance <br /> • Obtain necessary boring permits from the County <br /> • Dnll three borings (SB-2 through SB-4) in the areas near the former UST, dispenser <br /> islands, and fuel lines <br /> 2 <br />