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CTealogctaI Tecluefes lru Page 6 Groundwater Monitoring Report Project No 425 2 January 25,2002 ' . Wells MW-4 and MW-5 contaminant concentrations exhibit a stable trend • Wells MW-102 and MW-202 did not contain contaminant concentrations above the ' laboratory reporting limits for the fifth consecutive event • All three of the domestic wells were non-detect for the event r Oxidation-Reduction Potential (ORP) was measured (Table 6 & Figure 9)- 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 December 3, 2001 event, seven 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 northern direction at 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 direction of north for the December 3 event was anomalous and the December 21 event was consistent with the historical data 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 groundwater pumping needs of the community The positive gradients recorded for the December 2001 events are consistent with the December 2000 event ' 4 The dominance of upward vertical flow of groundwater may be the primary reason the contaminants has not impacted the drinking water aquifer During heavy pumping, the contaminants would be moved into deeper portions of the aquifer and a slower rate then they would be moved back to a shallower depth in low pumping periods of the year 5 The shallow water table groundwater plume is laterally defined except to the west 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 • 1