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D Crownover/28 June 1991 2 corner of their site, the tanks have been in place since the site was a retail gasoline station The Stockton Taxi Company has also been placed upon the San Joaquin underground fuel tank leak list In August 1988, a 1,000-gallon gasoline UST was removed from the north side of the API building by Precision Industries of Stockton, California A soil sample collected from the native soil immediately beneath the tank backfill material, at the time the tank was removed, contained 3,400 parts-per-million (ppm) total petroleum hydrocarbons (TPH) and 61 ppm benzene WGR has conducted two subsurface investigations at the site In August and September 1989, WGR supervised the drilling of four soil borings,B-1 through B-4,and completed them as monitor wells MW-1 through MW-4, respectively Total purgeable petroleum hydrocarbons (TPPH) were detected only in soil samples from boring B-1, at concentrations up to 5,200 ppm at a depth of 65 5 feet below ground surface TPPH was detected in groundwater samples from monitor wells MW-1 and MW-3 at concentrations of 510,000 parts-per-billion (ppb) and 300 ppb, respectively Benzene was detected in the groundwater samples from monitor wells MW-1, MW-2 and MW-3 at concentrations of 3,200 ppb, 0 1 ppb and 3 0 ppb, respectively Groundwater samples from monitor wells MW-1, MW-2 and MW-3 also contained 1,2-dichloroethane (1,2-DCA) at concentrations of 2,100 ppb, 20 ppb and 8 ppb, respectively (WGR Subsurface Investigation Report, November 1989) In May and June 1990, WGR supervised the drilling of three more soil borings, B-5 through B-7, which were completed as monitor wells MW-5 through MW-7 20 ppm of TPH as diesel was detected in four soil samples from boring B-7 at depths between 15 5 and 66 0 ft Groundwater samples collected during this investigation were found to contain as follows separate- phase liquid hydrocarbons characterized as gasoline were detected floating on top of the water column in well MW-1 TPPH was detected in samples from wells MW-5 and MW-7 at 1,500 ppb and 460 ppb,respectively Benzene was detected in samples from MW-6 and MW-7 at 0 6 ppb and 5 7 ppb,respectively Toluene,ethylbenzene, and total xylenes were detected in MW-5,MW-6,and MW-7 Samples from wells MW-2, MW-5, and MW-7 contained 1,2-DCA at concentrations of 27 ppb, 120 ppb, and 9 ppb, respectively GROUNDWATER SAMPLING On 13 June 1991,WGR environmental technicians measured depth to water in monitor wells MW-1 through MW-7 according to WGR standard operating procedures (SOP-8 included in • 066L1JE1 es