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',""Logical T"chw"inc Page 5 Groundwater Monitoring Report Project No 723 2 July 17,2003 0 Figure 5 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 6 illustrates TPH-G and Benzene concentrations versus elevation in deep well MW-101, which lies near the core of the plume The TPH-G concentrations are approaching stability while the benzene concentrations have fallen to levels below the laboratory reporting limits The steep decline in the initial concentrations suggests that 1 the contaminants detected in the well were initially a drilling artifact The continued presence of the TPH-G suggests that this well is within the plume • Figure 7 illustrates the TPH-G concentrations contours in the shallow wells The high concentration in well MW-3 creates a localized node Additional site investigation will allow a more accurate site conceptual model 3.0 CONCLUSIONS & RECOMMENDATIONS ' Conclusions 1 Groundwater flow trends to the northeast-to-east---,The soherly flow direction recorded on December 4 20Q0, appears o be anomalous 2 It is noted that the groundwater eleva ion exceeded the level of the well screens 3 High levels of BTEX and TPH-G are present in the groundwater of all six water table ' monitoring wells Low concentrations of BTEX and TPH-G are present in intermediate well MW-101 Generally, most of the wells show stable levels of contaminant concentrations over time ' 4 The groundwater contaminant plume is not defined laterally However, the decreasing concentration of contaminants in MW-201 suggests that this well may define the vertical edge of the plume in this location ' ` 5 Oxidation-Reduction Potential and Dissolved Oxygen measurements (Table 4) show that all eight wells are scieened in areas of the groundwater plume that are affected by the biodegradation reactions occurring within the plume The highly negative ORP and ' low DO values indicate that the wells are within the halo of reaction depleted water caused by biodegradation of the contaminants This suggests that the contaminant plume is stable and that natural attenuation processes are dormant, which corroborates well with the laboratory data Recommendations i 1 In Jul 27 2002 we submitted a work plan for additional site characterization with Y � p , subsequent addendums The San Joaquin County PHS/EHD has approved the installation of three multi-chamber wells and these will be installed pending receipt of encroachment permits from the City of Stockton