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r <br /> RZAAMA <br /> . Working To Restore Nature <br /> I May 4, 1993 <br /> Unocal Station No 6981, Stockton, California <br /> 3.0 SCOPE OF PREVIOUS INVESTIGATIONS <br /> 1 <br /> In May and June 1988, RESNA collected nine soil samples from test pits excavated near the <br /> dispenser islands and observed removal of three 12,000-gallon underground storage tanks <br /> at the site (Applied GeoSystems, July 8, 1988) The tanks were replaced with two gasoline <br /> 1 tanks, which are presently at the site Soil samples collected from the test pits and beneath <br /> the tanks contained gasoline hydrocarbons Approximately 1,600 cubic yards of soil were <br /> excavated and aerated onsite and transported to Forward Landfill in Stockton, California <br /> The excavation was terminated due to the proximity to the road and existing structures <br /> Hydrocarbon-cont arrung soil was left in-place In December 1988, a soil vapor survey was <br /> performed to assess the extent of hydrocarbon containing soil Results of the test did not <br /> provide conclusive data on hydrocarbons below 13 feet below grade (Applied GeoSystems, <br /> February 19, 1991) <br /> Nineteen soil borings have been drilled at or near the site Thirteen of these borings were <br /> converted into ground-water monitoring wells, four were converted into vapor-extraction <br /> wells, one into a pumping test well, and one soil boring was backfilled The initial five <br /> monitoring wells were installed in late January and early February 1990 and have been <br /> predominantly dry since October 1990 Three additional wells and the four vapor-extraction <br /> wells were installed in March 1992 The final five monitoring wells were installed in August <br /> 1992 and are used to delineate hydrocarbons in ground water The details of the borings <br /> and well constructions are presented in Tables 1 and 2 <br /> In March 1992, a vapor-extraction test was performed at the site Results of this test <br /> indicated that using a wellhead flowrate of 80 cubic feet per minute, an effective radius of <br /> influence of approximately 50 feet would be achieved per well (RESNA, June 15, 1992) as <br /> 38032-13 3 <br />