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- -------- � u �eolaaI°I"ecFimrs Inc - -_ - - --- - - - - - - - - - - - _ - -- V —r Page S— - - --- -- µ- g Groundwater Momtonng Report Project No 425 2 February 1,2001 3.0 CONCLUSIONS AND RECOMMENDATIONS f, 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 1 the northeast The cause for this shift is not known at this time 2 The intermediate depth groundwater flows more easterly then 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 the December 1999 and December 2000 monitoring events reveal opposite- negative and 1 positive gradients, respectively The reason for this anomaly is not conclusive, however it could be due to fall 1999 being so dry that the community continued pumping at high rates late into the year 4 The dominance of upward vertical flow of groundwater may be the primary reason the contaminantsl&Aot 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 intermittently contains trace toluene and xylene contamination 9 GTI has previously stated that the groundwater plume appears to pose a significant threat to public health and the environment because of the domestic wells associated with the residences surrounding the property But as wells MW-7 and MW-107 remain free of contaminants with each passing quarter, the apparent low mobility of the plume reduces the risk associated with off site groundwater receptors 1 Recommendations The following recommendations are made I The 12 monitoring wells and 3 domestic wells should be sampled for natural attenuation parameters during future monitoring events so it may be determined if remediation by natural attenuation is occurring