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cwfogkal Tec/r a Mc Page 8 Groundwater Monitoring Report Project No 723 2 August 31,2004 ' 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 seventeen wells are screened in areas of the groundwater plume that are affected by the biodegradation reactions occurring within the plume The highly negative ORP values indicate that the wells are within the halo of reaction depleted water caused by biodegradation of the contarrunants The low DO values of the wells also reflect 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 have migrated to a depth of 45 to 70 ft bgs, indicating ' that the contaminants are mobile. 4 Trace levels of BTEX are present in shallow/intermediate wells MW-7/107 5 Down gradient wells MW-8 and MW-208 contain low levels of TPH-G and BTEX ' Intermediate well MW-108 contains high levels of BTEX and TPH-G 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 8 Wells MW-7/107/207 define the lateral extent of the plume in a cross gradient direction 9 The low levels of contaminants in MW-101 and lack of contaminants in well MW-201 ' suggests that these wells define the vertical limit of the plume beneath the site 10 The highly negative ORP and low DO values suggests that the contaminant plume is ' stable and that natural attenuation processes are dormant, which corroborates well with the laboratory data t