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Gealogica17eC"CS lnc Page 5 ' Groundwater Monitoring Report Project No 723 2 • May 15, 2002 • Figure 6 illustrates TPH-G and Benzene concentrations versus elevation in deep well ' MW-101, which lies near the core of the plume The concentrations are approaching stability The steep decline in the initial concentrations suggests that the contaminants detected in the well were initially a drilling artifact The low benzene concentration now ' detected (1 3 gg/1) is probably representative of the lower extent of the groundwater plume • Figure 7 illustrates the TPH-G concentrations in the shallow wells The high ' concentration in well MW-3 creates a localized node Additional investigation is needed to gain a more accurate site conceptual model 3.0 CONCLUSIONS & RECOMMENDATIONS Conclusions I The groundwater contaminant plume is not defined vertically or laterally 2 Groundwater flow trends to the northeast to east The southerly flow direction recorded ' on December 4, 2000, appears to be anomalous 3 High levels of BTEX and TPH-G are present in the groundwater of all six water table monitoring wells It is noted that the groundwater elevation exceeded the level of the well screens Low concentrations of BTEX and TPH-G are present in both of the deeper wells 4 Oxidation-Reduction Potential measurement (Table 4) shows that seven of the eight wells are screened in areas of the groundwater plume that are affected by the biodegradation reactions occurring within the plume Negative ORP values indicate that the wells are within the halo of reaction depleted water caused by biodegradation of the contaminants This corroborates well with the laboratory data Recommendations 1 Additionalroundwater monitoring wells are needed to define the lateral extent of the g g plume off site GTI proposes installing two well pairs (shallow and intermediate wells) MW-7/MW-107 & MW-8/MW-108 to the north and northeast of the site respectively — see Figure 2 for proposed well locations A deep well is also needed in the location of well MW-4 GTI proposes to install well MW-104 adjacent to well MW-4 to define further the up gradient plume Mr Jeff Wong of the San Joaquin County Department of Public Health/Environmental Health Division has indicated they soon will be issuing a directive for a work plan for further investigation 2 Continue quarterly groundwater monitoring, with the next monitoring event scheduled in June 2002 ' 3 Due to the infrequent detection of the oxygenated fuel compounds at this site, future quarterly monitoring events will not include analysis by EPA method 8260 However, testing for MTBE by 8020/602 is justified