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i 3.0 PROBLEM ASSESSMENT 3.1 Geology and Hydrogeology The soil stratigraphy beneath the site consists of sdiciclastic silt and clay mixtures interbedded with thin, laterally discontinuous sand and silty sand lenses less than five feet thick Only one silty sand interval, encountered between 40 and 50 feet bgs, can be correlated between more than one borehole (Figures 3 and 4) This subsurface sand interval is approximately five feet thick, extends approximately 60 feet north and east (downgradient) of the former USTs and currently coincides with the depth of the groundwater table During the drilling of MW-6 and MW-7, saturated sod was first encountered between 43 and 44 feet bgs. On May 1, 1998, prior to well purging and sampling, the static depth-to-groundwater was measured in both wells at approximately 40 feet bgs On May 1, 1998, the groundwater elevation beneath the site ranged from a high of 23 74 feet below mean sea level (msl) in MW-1 to a low of 23 89 feet below msl in MW-4 Groundwater levels have risen an average of 6 18 feet since they were last monitored on February 12, 1998 The groundwater gradient was calculated at approximately 0 0014 foot/foot in a northeast direction The groundwater flow direction and gradient are illustrated on the groundwater surface map (Figure 5) The historical groundwater monitoring data are summanzed in Table 1 3.2 Nature and Extent of Subsurface Impacts Soil and groundwater beneath the site are impacted by gasoline hydrocarbons originating from the former UST system 3 2 1 Sod Impact To date, a total of 35 soil samples have been collected for analysis from beneath the two former USTs and from 15 sod borings drilled at the site The histoncal analytical data from these soil samples are summanzed in Table 1 In soil, the greatest concentrations of gasoline hydrocarbons (12,000 mg/kg of TPH-as-Gasoline and 26 mg/kg of benzene)were detected at a depth of 16 feet bgs from MW-1, directly beneath the former USTs Vertically, the majority of the gasoline hydrocarbon mass is concentrated from 10 to 30 feet bgs beneath and immediately north of the former USTs As shown on the two cross-sections