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N.001 June 14, 1991 20548, 001.03 Harding Lawson Associates Ms. Margaret Lagorio San Joaquin County Page 2 Subsurface conditions on the property consist of fluvial sediments that include interbedded layers of clays, silts and fine sands. An on-site water well, apparently 70 feet deep, produces drinking water for facility employees. BACKGROUND Petroleum fuels are apparently no longer dispensed at the site. When the 1,000-gallon gasoline tank was removed in April 1989, a soil sample taken at the north end of the tank was found to have 200 parts per million (ppm) of total petroleum hydrocarbons (TPH) as gasoline. The excavation in the area of the gasoline tank was deepened by approximately one foot, and subsequent sampling and analyses performed; results apparently showed that the TPH as gasoline concentrations were reduced to 124 ppm. In June 1989, the 5, 000- and 10, 000-gallon diesel tanks were removed; no detectable TPH was found in samples from that area (Covenant Environmental Consulting, February 8, 1990) . We understand that in November 1990, a consultant prepared a work plan for submission to the SJC/PHS/EHD, which outlined an investigation to evaluate the presence of petroleum hydrocarbons within a very short distance of the gasoline tank. This work plan was apparently approved by Ms. Darlene Reuss of the SJC/PHS/EHD and subsequently implemented, with six soil samples taken (from 0 to 50 feet deep) and tested for TPH concentrations. None of the six samples showed detectable concentrations of TPH, although the sample from a depth of 44.5 to 45 feet had 5 parts per billion (ppb) of benzene (Gemini Petrographic Investigations, Initial Soil Investigation Report dated November 1990) . PROPOSED WORK On May 10, 1991, a representative of HLA attended a meeting with Ms. Margaret Lagorio of the SJC/PHS/EHD and Ms. Carmelita Cozad of Cozad Trailers. Ms. Lagorio stated that although the SJC/PHS/EHD had approved the previous work plan, additional subsurface investigation would be required to determine the scope of additional work that would be necessary for closure of the UST investigation. Specifically, Ms. Lagorio indicated that the following investigation tasks be performed: • Install three groundwater monitoring wells, one of which must be within 10 feet of the gasoline tank, downgradient from that tank.