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Due to the presence of elevated concentrations of petroleum hydrocarbons in the tank pit soil samples, further investigation was required by San Joaquin County Public Health Services, Environmental Health Division (PHS/EHD). In a two -phased approach beginning on November 1, 1990, RESNA Industries, Inc. personnel advanced six soil borings (MW1 through MW6) to depths ranging from 27 to 30 feet below ground surface (bgs) at the project site to accomplish the following: 1) define the lateral and vertical extent of hydrocarbon impacted soils; 2) determine soil stratigraphy; and 3) install groundwater monitoring wells within the uppermost aquifer in order to assess groundwater quality. The boring/well locations are plotted on Figure 2. Field methodology and analytical results are discussed in detail in RESNA's Interim Problem Assessment Report, dated January 13, 1992. The site's station building and garage were demolished in December 1994 to facilitate corrective action activities including the removal of hydrocarbon contaminated soil which was present beneath the station building. The demolition of the station building was requested in PHS/EHD correspondence dated July 7, 1994. A required demolition asbestos survey conducted previously on the station building determined the presence of asbestos containing materials. E. T. Abatement, of Modesto, California, properly removed and disposed of the site's suspected asbestos containing materials on November 11, 1994. In February 1995, Smith Technology personnel supervised the excavation of approximately 1,400 cubic yards of soil from the vicinity of the former underground storage tanks. Approximately 10,000 gallons of groundwater was pumped from the open excavation into an on-site Baker Tank. Following the collection of confirmation soil samples, the excavation pit was backfilled with imported self compacting material. The excavation was limited to the northwest by a municipal well compound and associated underground utilities (including a high volume six-inch water main) and to the northeast by a postal building. Excavation activities are discussed in detail in Smith Technology's Excavation Summary Report, dated October 24, 1995. On November 26 and 27, 1996, a total of eight soil borings (SB 1 through SB 8) were advanced to depths ranging from approximately 20.5 to 45.5 feet bgs under the supervision of Smith Technology. The boring locations are shown on Figures 3 and 4. The distribution of residual TPHg and benzene in soil beneath the site is generally between 11 and 30 feet bgs and extends approximately 70 feet from north of the former UST excavation near the Thornton Municipal Supply Well No. 2 compound and associated underground water lines. Benzene was generally detected between 20 and 26 feet bgs in the vicinity of borings SB2, SB4, SB6 and SB8 and at 11 feet bgs in MW4. The lateral extent of benzene in soil is shown on Figure 3. The estimated lateral extents of benzene concentrations in groundwater during this subsurface investigation data are presented on Figure 4. The results of the November 1996 investigation is summarized in Smith Technology's aforementioned Problem Assessment Report - Interim Remedial Plan dated September 4, 1997. • WA94326\REPORTS\WRKPLN.EM 2 � �