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respectively The groundwater samples from borings SB-1 through SB-4 also P Y contained detectable concentrations of TPHd and TPHg, whereas SB-5 and SB-6 did not contain detectable concentrations of TPHd or TPHg In August 1998, ATC advanced three additional soil borings, which were completed as groundwater monitoring wells designated MW-1 through MW-3 The monitoring wells were completed to a depth of approximately 28 ft bgs and were located in the vicinity of the former USTs Analytical data from the soil and groundwater samples contained detectable concentrations of petroleum-hydrocar-bo �tbree In April and June 2000, ATC advancedsLen add itional-soil_borings t the site,r of which were completed as groundwater monitoring wells The borings were designated SB-8, SB-9, SB-10, SB-12, and monitoring wells MW-4 through MW-6 (Figure 2) A soil sample collected from the boring for monitoring well MW-4 contained detectable concentrations of TPHg and TPHd at a depth of 20 5 ft bgs Soil samples from SB-8 through SB-10, SB-12 and monitoring wells MW-5 and MW-6 did not contain detectable concentrations of TPHg, TPHd, benzene, toluene, ethylbenzene, xylenes (BTEX), or oxygenate fuel additives Grab samples of groundwater from borings SB-8 through SB-10 did not contain detectable concentrations of TPHg, TPHd, BTEX, or oxygenates fuel additives, however the groundwater sample collected from SB-12 did contain trace amounts of toluene In June and July 2000, groundwater samples were collected from monitoring wells MW- 1 through MW-6 The groundwater sample collected from MW-3 contained concentrations of TPHd Groundwater samples collected from monitoring wells MW-2 and MW-4 exhibited detectable concentrations of xylenes and TPHg, and elevated concentrations of TPHd Groundwater samples collected from MW-1, MW- 5, and MW-6 did not contain detectable concentrations of TPHd or TPHg ATC conducted quarterly groundwater monitoring at the site from August 1998 through June 2002 Groundwater samples were analyzed for TPHg, TPHd, and BTEX 1.2.3. Site Geology In general, soil encountered in shallow borings advanced at the site consists predominantly of clay and silt to a depth of approximately 10 to 15 ft bgs Silty sand ' and sand are encountered below the silt/clay layer to the maximum depths explored (generally approximately 30 to 35 ft bgs) One boring, SB-12, was advanced to a depth of 51 5 ft bgs, and a sandy clay lens was encountered at a depth of 41 to 46 5 ft bgs with a layer of sand beneath the clay A gravel layer which was encountered during the installation of MW-3 and MW-4 at approximately 20 ft bgs during previous investigations was not encountered during this investigation Backfill material encountered in the former tank excavation area consists primarily of silty sand with gravel Soil boring logs with soil descriptions and types encountered during this investigation are presented in Appendix A 1.2.4 Site Hydrogeology ' On 15 April 2004, water levels were measured in monitoring wells MW-1 through MW-6 Groundwater was encountered at depths ranging from 6 20 to 11 65 ft below 2