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Cieologkal Technia t ac Page 5 Groundwater Monitoring Report Project No 425 2 July 26, 2002 - • All three of the domestic wells were non-detect for the event ' • Oxidation-Reduction Potential (ORP) was measured (Table 5) The results suggest that well MW-2 is in the core of the contamination plume or is immediately down gradient of the plume Negative ORP values indicate that the wells are within the halo of reaction depleted water caused by biodegradation of the contaminants For the June 2002, event, five water table wells had negative ORP values 3.0 CONCLUSIONS AND RECOMMENDATIONS The following conclusions have been drawn regarding the status of the groundwater at this site and recommendations for future work ' Conclusions 1 The water table groundwater gradient historically flowed in a northerly direction at I about 0 002 ft/ft prior to May 1998 However, since that time, the gradient has fluctuated from north to northeast 2 The intermediate depth groundwater has flowed easterly, with two shifts to southeast during September 2000 & September 2001 The northerly flow direction during December 3 event was anomalous, as the December 21 event was again consistent with the historical data showing an easterly flow 3 Vertical groundwater gradients have been found to be flowing upward during winter and early spring and downward during late spring and summer This may be due to the annual cycle of the groundwater pumping needs of the community ' 4 The dominance of upward vertical flow of groundwater may be the primary reason the contaminants has not impacted the drinking water aquifer 5 The shallow water table groundwater plume is laterally defined except to the west tHowever, the data indicate that the contamination does attenuate toward the west 6 Petroleum hydrocarbon concentrations in the groundwater at the site fluctuate with each ' monitoring event The residual contamination in soil up-gradient of MW-2 remains a significant source of impact to the groundwater plume 7 The deeper groundwater plume is defined vertically since MW-102 and MW-202 remain non-detect for contaminants 8 The three domestic wells sampled for the event remain non-detect ' Reconimcndations The following recommendations are made 1 The current quarterly monitoring schedule should be maintained to determine ' contaminant trends 2 San Joaquin County PHS/EHD issued a letter on July 10, 2002 denying GTI's work plans for bio-venting In the letter they directed that additional characterization needs to