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Creologicaf Technics Ince Page 8 Groundwater Morutonng Report Project No 723 2 February 28,2005 ' between the water table and the TPH-G concentrations Benzene concentrations have stabilized in the well ' ❑ Figure 11 illustrates TPH-G and Benzene concentrations versus elevation in well MW-4, which lies up gradient of the core of the plume The contaminant concentrations exhibit a decreasing trend in this well ' ❑ Figure 12 illustrates TPH-G and Benzene concentrations versus elevation in deep well MW-101, which lies near the core of the plume The TPH-G and benzene concentrations display a decreasing trend The steep decline in the initial concentrations suggests that ' the contaminants detected in the well were initially a drilling artifact ❑ Oxidation-Reduction Potential (ORP) and Dissolved Oxygen (DO) measurements (Table 4) show that all fifteen wells monitored are screened in areas of the groundwater plume ' that are affected by the biodegradation reactions occurring within the plume The low to highly negative ORP values during the October 2004 event indicate that the wells are ' within the halo of reaction depleted water caused by biodegradation of the contaminants The low DO values of the wells also reflected this condition ' 3.0 CONCLUSIONS & RECOMMENDATIONS Conclusions 1 Groundwater flow trends northeast to east The southerly flow direction recorded on December 4, 2000, appears to be anomalous ' 2 High levels of BTEX and TPH-G are present in the groundwater of all six on site shallow and intermediate monitoring wells (MW-1 through MW-6) 3 The main mass of contaminants has migrated to a depth of 45 to 70 ft bgs, indicating that ' the contaminants are mobile 4 Cross gradient deep well MW-207 contained low levels of TPH-G and BTEX for the first time This probably represents the edge of the advancing plume ' 5 Down gradient wells MW-8/108/208 contain low levels of TPH-G and BTEX 6 The groundwater contaminant plume is not defined laterally down gradient of MW-108 7 Well set MW-9/109/209 defines the lateral extent of the plume in an up gradient ' direction The intermediate and deep levels (MW-109 & MW-209) both contained trace BTEX for the first time 8 The low levels of contaminants in wells MW-101 and MW-201 suggests that these wells ' define the vertical limit of the plume beneath the site ' Recommendations 1 Continue quarterly groundwater monitoring 2 The SJC/EHD records for the adjacent Shell gas station should be reviewed to determine ' the degree of co-mingling of the plumes beneath the subject and adjacent Shell Oil sites Mr Jeff Wong of the San Joaquin County Environmental Health Department (SJC/EHD) supplied GTI staff with a copy of a July, 2003 groundwater monitoring report for the ' Shell Service Station facility across N El Dorado Street The report indicates that the