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Bank of Stockton Page 13 BofS (Cuter) -Preliminary Soil and Groundwater Investigation Project No 102 2 April 8, 1997 5.0 CONCLUSIONS & RECOMMENDATIONS ' 5.1 Conclusions ' Based on information collected in to date, Geo-Phase Environmental Inc has reached the following conclusions pertaining to the soil and groundwater conditions at the Cutter Lumber site 1 A cone of gasoline impacted soil in the vadose zone has been reasonably well defined ' by the soil borings described above The released hydrocarbons consist almost entirely of gasoline with a small component of diesel Gasoline was detected in water saturated soil and/or groundwater samples from all 9 borings 2 It seems likely that the small amount of diesel range hydrocarbons detected in the soil and groundwater from a small area in the immediate vicinity of the tank pit and former ' dispenser island would not have reached groundwater at all, if not for the much larger volume of gasoline having entrained the diesel and carred it down to depth 3 The distribution of petroleum hydrocarbons suggests that the groundwater gradient lies in a northeasterly direction This is consistent with the direction of flow which has been documented at the Western Lift site near southerly end of the Property The flow of groundwater has transported gasoline range hydrocarbons at least 185 feet north of the release point The extreme limits of the groundwater plume have not yet been ' established however Figure 4 shows the minimum probable lateral extent of the plume, and it could extend well beyond the area shown in the figure ' 4 The concentration of gasoline range hydrocarbons in the groundwater sample directly beneath the tank pit (B6W) is about '/2 percent of the sample volume This strongly suggest that free product may exist there The area of free product is suspected to encompass several hundred square feet directly beneath the tank pit 5 The soils in the area of the fuel release to a depth of at least 40 feet consist almost ' entirely of clay and clayey silt Based on field observations, the soil has very low permeability The vadose zone extends to a depth of about 35 feet below the surface It is expected that the very fine grained nature of the soil may render vapor extraction ' as a remedial method very inefficient at best, and impossible at worst 6 It is unlikely the groundwater plume will begin to recede naturally until the free product is removed and the very highly contaminated soil in the immediate area of the release is remediated Unfortunately non-intrusive remedial methods seem unlikely to 2937 Veneman Ave ,#B240 Geo-Phase Environmental Inc Phone (209) 569-0293 Modesto, CA 95356 Fax (209) 569-0293