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R10/71-51-1C 3.0 SITE DESCRIPTION 31 SITE LOCATION The subject site is located south of the District 10 Headquarters, at 1603 South B Street, Stockton, California Figure A presents a location map of the site 32 SITE HISTORY In 1978, one of the four(4) underground fuel tanks belonging to the facility was removed as it developed a leak Soil contamination investigations around the tank complex were undertaken during August 1987 Hydrocarbon contaminants were detected in all the four (4) soil boreholes which were converted to monitoring wells The groundwater was also found to be contaminated In January 1988, the remaining three (3) tanks were also removed The sosl samples from the tank cavity showed low levels of hydrocarbons being present Subsequently, five (5) additional monitoring wells in 1990 and three (3) monitoring wells in 1992 were placed on the site The groundwater levels from 1987 to 1992 dropped more than 20 feet In 1992, groundwater from only five out of twelve (12) monitoring wells could be sampled 3 3 PREVIOUS INVESTIGATIONS A Soil and Groundwater Evaluation Report (Woodward Clyde Consultants, November 1987) was prepared summarizing the field investigation of underground tanks in Shop 10 A Problem Assessment Report (Woodward Clyde Consultants, July 1990) was prepared on July 1990 on Regional Water Quality Control Board's request A Supplemental Site Investigation (Alton Geoscience, July 1992) was undertaken to determine the type, quantity and extent of contaminants detected in the soil and groundwater The site is underlain by predominantly silty sand and poorly-graded sand with some silty clay units The groundwater average depth beneath the site was 66 feet below grade (fbg) on April 7, 1992 The soil contaminant concentration ranges were 270 ppm to 4 ppm of total petroleum hydrocarbons as gasoline (TPH-G), 4 4 ppm to 0 3 ppm benzene at 46 to 56 fbg beneath the former tank cavity The groundwater is also contaminated with dissolved phase hydrocarbons The contaminant concentration ranges in the groundwater samples were from 1 9 ppm to 231 ppm for TPH-G, from 0 001 ppm to 25 7 ppm, from 0 001 ppm to 38 8 ppm, from 0 0004 ppm to 4 ppm and from 0 0004 ppm to 15 9 ppm of benzene, toluene, ethylbenzene, and xylene (BTEX) respectively 7