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'f GeoCogtcaf?echnus Inc Page 5 ' Groundwater Monitoring Report Protect No 425 2 • April 23,2001 • Oxidation-Reduction Potential (ORP) was measured (Table 6) and the results suggest that several of the wells are in the core of the contamination plume or are immediately down gradient of the plume The negative ORP values indicate that the wells are within the halo of reaction depleted water caused by biodegradation of the contaminants MWs -2, -5, -102, -105 - - 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 shifted towards the northeast The reason for the anomalous southeasterly direction is not known at this time 2 The intermediate depth groundwater flows more easterly than the water table, which is consistent with previous monitoring events The cause for this shift is not known at this time 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 A comparison of monitoring events conducted in April 1999 & March 2000 reveal positive gradients The March 2001 event gradient was also positive 4 The dominance of upward vertical flow of groundwater may be the primary reason the containinants 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 ' 6 Petroleum hydrocarbon concentrations in the groundwater at the site fluctuate with each monitoring event 7 The deeper groundwater plume is defined vertically since MW-102 and MW-202 remain non-detect for contaminants 8 Two domestic wells associated with this site remain non-detect A third domestic well detected trace amounts of toluene and xylene contamination for one event in Sept 2000, which was retested and found to be non-detect The detection is determined to be an anomaly possibly caused by cross contamination