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tBank of Stockton Cutter Lumber Site Page 6 Spring Quarter Groundwater Monitoring Report ' Project No 102 2 July 13, 1998 ' 3.0 DISCUSSION 3.1 Discussion of Data Field and analytical data results are summarized in Tables 1 and 2 and ' illustrated in Figures 3, 4 & 5 The maps (figures 3,4 & 5) of groundwater gradient suggest that the flow of ' groundwater is to the east by northeast During two of the depth to water measurement events the measurable groundwater level in MW-8A was less than that measured in the shallow groundwater monitoring well This suggests that ' the deep well may be semi-confined, since the head in the deep well has not equilibrated with that of the shallow Alternatively, the deeper well which is completed opposite a permeable sand layer, may simply be responding more ' quickly to water recharge from an off-site recharge area The most recent measurement showing nearly identical depth to water in MW-8A and MW-813 ' would tend to support this interpretation The gasoline contaminant plume in the groundwater appears to trend to the I northeast which is consistent with what is known of the historical gradient in this area (see Figures 5 & 6) Mathematical calculation of gradient and bearing of flow based on MW-6, MW-10 and MW-11 for the months of April, May and ' June indicate gradient slopes between 0 0008 and 0 0021 ft/ft with a consistent flow direction ranging between N590E and N710E The slope of the groundwater gradient has increased steadily over the past three months along with the ' groundwater elevation 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 this limit is near to those particular wells 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 The samples collected from the shallow groundwater monitoring wells in the vicinity of the plume still have relatively high levels of gasoline range contaminants There has, however been a decrease in the overall TPH and volatile organic hydrocarbon concentration by 30-50% in wells MW-6, 7 & 813 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 2937 Veneman Ave ,#B240 Geo-Phase Environmental Inc Phone (209) 569-0293 Modesto, CA 95356 Fax (209) 569-0293