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I Bank of Stockton Cutter Lumber Site Page 13 Groundwater Monitoring Well Construction ' Project No 1012 . April 24, 1998 I ' The gasoline contaminant plume in the groundwater appears to trend to the g P g'Y' PP ' northeast which is consistent with what is known of the historical gradient in this area (see Figures 5 & 6) It is possible that the past year's abnormally high precipitation has temporarily altered the direction of ground water flow as well ' as its elevation A mathematic calculation of gradient and bearing of flow based on MW-6, MW-10 and MW-11 yields a direction of flow as N65E and a gradient of 0004 feet per foot 1 Western Lift Plume No detectable levels of gasoline range hydrocarbons were found in groundwater ' water from MW-1 and MW-4, in the vicinity of the original release, the Western Lift site Benzene was detected in concentrations dust above the method detect limit The fact that the screened interval in both of these wells is now ' considerably below the groundwater surface may have produced samples which are not representative of conditions closer to the groundwater surface ' Cutter Lumber Plume The groundwater in the area of MW-7, directly beneath the location of the former gasoline UST, while severely impacted by gasoline range hydrocarbons I. did not have measurable free phase product floating on the surface of the groundwater 1 The analytical results from water samples of MW-10 and MW-11 contain low, but detectable levels of petroleum hydrocarbons suggesting the extreme down- gradient limits of the groundwater plume have not yet been established but the linit is more than 180 feet from the point of release The monitoring well MW-6 lies about 30 feet south of the point of release and it clearly shows that the up- gradient limit of the contaminant plume has not yet been defined Low, but detectable levels of gasoline range hydrocarbons and BTEX compounds ' were found in the water sample from the deep well, MW-8A This suggests that a vertical gradient may have transported gasoline range hydrocarbons to a depth ' of 80 feet or more On the other hand, the conductor casing in this well extends to a depth of 49 feet, while detectable contamination exists in the soil to depths of at least 55 feet It is possible that the drilling equipment may have carried ' low levels of hydrocarbon contamination to the deeper zone and cross- contaminated it Future groundwater monitoring events should demonstrate which of these possibilities is actually the case ' No diesel range petroleum hydrocarbons were detected in any of the soil or • groundwater samples It appears that the diesel detected in previous ' 2937 Veneman Ave ,#B240 Geo-Phase Environmental Inc Phone (209) 569-0293 Modesto, CA 95356 Fax (209) 569-0293