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i HISTORIC SUMMARY OF LANGSTON'S MARKET Based on the San Joaquin County Flood Control District and Water Conservation District map titled Lines of Equal Elevation of Groundwater Sprang 1999,ground water elevation at the site is less than 10 feet and flows generally towards the northwest 1990,June A 4,000-gallon UST failed at Langston's Market following a helium pressure test and 1,200 gallons of unleaded gasoline was released 1992 April Murray and Associates manually drilled eight 2-inch diameter soil bonngs to a depth ' of 17 feet (groundwater between 17-18 feet) Found that soil contamination in the vicinity of the former UST extended down to capillary fringe zone and water table, with little lateral spreading SB-4 about 2 5 feet west of UST pit detected 14 ppm petroleum hydrocarbons at 14 feet and 7 ppm at 17 feet SB-8 about 33 feet NW was non-detect 1993,April Installation of four ground water monitoring wells at the site Soil contamination was verified to exist from 15 feet below bsg to below ground water Benzene concentrations in ground water samples up to 17,000 ppb 1994, Sept Benzene concentrations up to 31,000 ppb in groundwater • 1994 Dec PHS-EHD approved as an interim remediation activity a Remedial Excavation Work Plan to excavate 3,000 tons of soil 1995 July After an asbestos survey, removal of surface structures and the four ground water monitoring wells by SEMCO I1995 Aug Pothole trenching by WHF Environmental to delineate the extent of contamination failed due to sandy soil, WHF was given approval to proceed with excavation of the Iformer tank pit area 1995 Sept Sidewalls and bottom of the excavated area sampled by WHF Soil samples to north ' and west show small amounts of contamination remaining under 50 ppm Excavation was limited to the east and south by roadways and utilities, remaining contamination ' in those directions were less than 100 ppm 1995 Sept WHF and Frontier Drilling conducted six soil borings and completed four as ground water monitoring wells Borings terminated between 34-35 5 feet after sampling below a clay interval between 27-33 feet EW-1 (MW-1)and MW-2 are 4-inch wells that can be used for groundwater extraction The wells were surveyed by a State- licensed surveyor a1999 April 16 AGE advanced 4 Geoprobe soil borings at the site to collect soil and grab ground I