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L Regional groundwater within the shallow water-bearing zone occurs from less than 10 feet below grade to as deep as 50 feet below grade. Regional groundwater gradient is to the northeast. The flood basin deposits of the shallow water bearing zone have well developed soil profiles consisting of interbedded layers of clays, silts, and sands, and consequently poor permeability. During the period of investigation the depth to groundwater at the subject site has ranged from 38 feet �• to 47 feet below grade. When groundwater levels were last measured in March 1994 the depth to water was 43.04 feet below grade. r 3.0 BACKGROUND The past and present use of the site is the City of Stockton Police facility. Three underground storage tanks were formerly located on the site(Figure 2). These tanks were used to fuel facility vehicles. 33.1 History-of Soil and Groundwater InvestiLation During precision testing at the facility (May 1988), leakage was detected from the product line of a 10,000 gallon unleaded gasoline tank. Elevated levels of gasoline hydrocarbons were recorded in two `~ of four soil samples collected June 9, 1988 from a trench located adjacent to the product lines (Figure 2). Although individual testing of the tanks indicated that they were mathematically tight, a small hole was found in the unleaded product line in the area of highest contamination. Based upon these leak indicators PHS/EHD required further investigation to assess the extent of contamination. �.� On October 19, 1989, a soil boring was drilled to a total depth of 55 feet below grade to determine the vertical extent of hydrocarbon impacted soils and to facilitate the installation of a groundwater monitoring well, MW 1 (Figure 2). The boring/monitoring well was located as close as possible to the area of suspected soil contamination near the product line. Laboratory analyses of soil samples collected at five foot intervals from 10.0 to 40.0 feet in the well boring did not detect the presence of f gasoline constituents above the method detection limits(Table 1). Historical analyses of groundwater samples collected from monitoring well MW1 indicate that groundwater beneath the site has not been impacted by petroleum hydrocarbons(Table 2). 3.2 Remedial Actions 3.2.1 July 1992 Excavation On July 23, 1992, under the direction of RESNA Industries, Inc. (RESNA), now Smith Environmental, approximately 300 cubic yards of native and backfilled soil were excavated by Falcon Energy Associates (Falcon Energy, license #584524) using a backhoe tractor. Visible hydrocarbon impaction of soil was evident to a depth of approximately 6 to 6.5 feet below grade. Photoionization detector (PID) measurements detected organic vapors intermittently to approximately 8 feet below grade. A layer of clay from approximately 7 to 10 feet below grade apparently limited the vertical 4 w:194368Veports\sesoi1.dm 2 SMTH