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PROBLEM ASSESSMENT REPORT AND CORRECTIVE ACTION PLAN 1.0 BACKGROUND 11 Site Description and History The subject site is located at 1603 B Street in Stockton, California (see Vicinity Map, Figure 1) The site is owned by Caltrans and is occupied by Caltrans Shop 10 It is understood that four 5,000- gallon gasoline/diesel fuel underground storage tanks (USTs) and one 1,000-gallon waste-oil UST were present at the site until their removal in February and March of 1989 Current improvements at the site consist of a shop building and an office building situated at the ' northern portion of the site, and a storage building at the southern portion The current site layout is depicted on the Site Plan, Figure 2 Future development plans call for the construction of a building at the central portion of the site 12 Previous Environmental Studies ' initial subsurface investigation at the site was performed by Woodward Clyde Consultants (WCC) in 1987 The work performed by WCC included the installation and sampling of four groundwater monitoring wells (MW-1 through MW-4) in the vicinity of the four 5,000-gallon gasoline/diesel USTs Sample results indicated that groundwater beneath the USTs was impacted with total petroleum hydrocarbons as gasoline (TPHg) and benzene at concentrations ranging from 154 to 231 milligrams per liter (mg/1) for TPHg and ranging from 14 7 to 25 7 mg/I for benzene In February and March of 1989, the four gasoline/diesel fuel USTs and the waste-oil UST were removed by ' Placer Tractor Service Soil samples collected from the native soil beneath the former gasoline/diesel fuel USTs indicated the presence of residual TPHg at concentrations ranging from 2 2 to 3,700 ' milligrams per kilogram (mg/kg) TPHd was present beneath the diesel and waste oil USTs at concentrations ranging from 100 - 1,700 mg/kg, and I I-12 mg/kg, respectively Soil analytical data relative to the UST removals is summarized on Table 1 Site assessment performed by WCC between April 1989 and March 1990 consisted of the ' installation of five additional groundwater monitoring wells (MW-5 through MW-9) and the collection and chemical analysis of both soil and groundwater samples Soil analytical data indicated that TPHg was not detected in the soil samples collected from these five borings Analytical data ' indicated the presence of TPHg in groundwater at concentrations ranging from 1 9 to 3 4 mg/l In April 1992, Alton Geoscience (Alton) installed three groundwater monitoring wells (MW-10 through MW-12) and drilled two soil borings (AB-1 and A13-2) Alton also performed a limited ' vapor extraction pilot test in September 1992 to evaluate the feasibility of using soil vapor extraction as a remediation alternative In October 1993, Environmental Assessors, Inc (EA) installed six monitoring wells (MW-13, MW-14, MW-16, MW-19, MW-20 and MW-24) and drilled four soil borings (SB-15, SB-17, SB-18 and S13-22) ' Project No 58100-06-18A -3- December 9, 1996