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APPENDIX A- HISTORIC SUMMARY <br /> LANGSTON'S MARKET <br /> I15615 South 7" Street, Lathrop, California <br /> June 1990, A 4,000-gallon UST failed at Langston's Market following a helium pressure test and <br /> 1,200 gallons of unleaded gasoline was released <br /> April 1992 Murray and Associates manually drilled eight 2-inch diameter soil borings to a depth <br /> of 17 feet (groundwater between 17-18 feet) Found that soil contamination in the <br /> vicinity of the former UST extended down to capillary fringe zone and water table, <br /> with little lateral spreading SB-4 about 2 5 feet west of UST pit detected 14 ppm <br /> petroleum hydrocarbons at 14 feet and 7 ppm at 17 feet SB-8 about 33 feet NW was <br /> non-detect <br /> April 1993 Installation of four groundwater monitoring wells at the site Soil contamination was <br /> verified to exist from 15 feet below bsg to below ground water Benzene <br /> concentrations in ground water samples up to 17,000 ppb <br /> Sept 1994 Benzene concentrations up to 31,000 ppb in groundwater <br /> Dec 1994 San Joaquin County Environmental Health Department (EHD) approved as an <br /> interim remediation activity a Remedial Excavation Work Plan to excavate 3,000 <br /> tons of soil <br /> IJuly 1995 After an asbestos survey, removal of surface structures and the four ground water <br /> monitonng wells by SEMCO <br /> Aug 1995 Pothole trenching by WHF Environmental to delineate the extent of contamination <br /> failed due to sandy soil, WHF was given approval to proceed with excavation of the <br /> former tank pit area <br /> Sept 1995 Sidewalls and bottom of the excavated area sampled by WHF Soil samples to north <br />' and west show small amounts of contamination remaining under 50 ppm Excavation <br /> was limited to the east and south by roadways and utilities,remaining contamination <br />' in those directions were less than 100 ppm <br /> Sept 1995 WHF and Frontier Drilling conducted six soil borings and completed four as ground <br />' water monitoring wells Borings terminated between 34-35 5 feet after sampling <br /> below a clay interval between 27-33 feet EW-1 (MW-1)and MW-2 are 4-inch wells <br /> that can be used for groundwater extraction The wells were surveyed by a State- <br /> licensed surveyor <br /> • <br />