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the total depth of the borings A silty sand to sand unit was encountered at approximately 50 feet bgs in the western and southern portion of the site Cross-sections were prepared as part of the hydrogeologic assessment and are presented as Figures 4a, 4b, and 5 The cross-section location lanes are shown on Figure 2 Depth to groundwater in monitoring wells has varied from approximately 23 to 48 feet bgs Quarterly groundwater sampling at the site began in July 1991 Table 1 presents the historical groundwater elevations and laboratory analytical results The groundwater gradient and direction as submitted in the quarterly groundwater monitoring reports were reviewed To provide consistency in interpreting groundwater gradients and directions over time, the commercially available software Surfer Version 6 04 by Golden Software Inc was used to assess groundwater elevations across the site Both summer and winter quarterly events were selected ' to assess any seasonal variation at the site Data from two quarters per year from April 1992 to July 2000 were assessed to determine an average gradient direction and magnitude The results of this assessment are summarized in the rose diagram presented in Figure 2 The average groundwater flow direction at the site over time was to the south 80 degrees east with a magnitude of 0 003 ' The groundwater elevations for each well over time are graphically presented along with concentrations of petroleum hydrocarbons and well construction details in Appendix B ' 2.4 SITE HISTORY AND PREVIOUS INVESTIGATIONS 2.4.1 Chronological Summary of Investigations A chronological summary of investigations performed at the site is presented below ' • February 1991 Two inclined soil borings (SB 1 and SB2) were drilled adjacent to and beneath the three 10,000-gallon USTs on 22 February 1991 The locations of these borings are shown in Figure 2 These 61 5-foot borings were drilled at a 30 degree angle from vertical to assess soil beneath the USTs to an actual vertical depth of approximately 53 feet bgs A total of 12 soil samples were collected for analysis from depths of 17 to 44 feet bgs A soil sample collected at approximately 35 feet bgs beneath the central tank location contained 16,000 milligrams per kilogram (mg/kg) Total Petroleum Hydrocarbons as gasoline (TPH-g) and benzene at 63 mg/kg Of the remaining I l soil samples, benzene was detected in eight of the samples at concentrations ranging from 0 003 to 0 33 mg/kg, and TPH-g was detected in five of the samples at concentrations ranging from 0 19 to 0 8 mg/kg (AEGIS 1991a) Analytical results for soil samples are presented in Table 2 and graphically on Figure 6 • July 1991 Groundwater monitoring wells MWl—MW3 (Figure 2) were installed onsite to 55 feet bgs Petroleum hydrocarbons were not detected in the soil samples at or above the laboratory method detection limits, with the exception of the sample collected from MW3 at 20 5 feet bgs, which contained toluene and total xylenes at concentrations of 0 010 mg/kg TPH-g and benzene were detected in groundwater samples collected from wells MW2 and F 1Pi ojects170137U1asterUVP\CI osure 110010 i 37enext doe 4 i