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i 1 Ms Margaret Lagono Page 7 of 8 Based upon the screened intervals in the onsite monitoring wells along with the deeper lithology and { contaminant profiles encountered in bonngs CB 1/CB I A and CB5, it would appear that the sandy interval from approximately 65 to 80 feet is the interval of greatest concern This unit has high levels of groundwater contaminants, relatively high permeability, and relatively Iow sorption potential This would be expected to ' be a preferential flow pathway Although the highest concentrations of contaminants in groundwater were found at the depth of 88 feet, the ' lithology at this depth consists of a very thin clayey sand encased in fine-grained materials and does not represent a preferential flow pathway of concern The sandy unit encountered from approximately 105 to 115 feet(Unit D)could provide a preferential#towpath This unit was present both in CBl/lA and in CB5 Contaminant concentrations in groundwater,however,were relatively low Encroachment of Offsite Groundwater Contamination ' The high levels of MTBE, TPHg and BTEX found in the groundwater of upgradient well MW 14 clearly illustrates that encroachment from an offsite plume has occurred The hydraulically upgradient position of ' well MW 14 relative to MTBE impacted wells MW4 and MW3 suggests that this contamination most likely has its origin at the Top Filling Station (101 Wilson Way) It is possible that a release at the ARCO Station (130 Wilson Way)may have contributed to the plume of contamination �s RECOMMENDATIONS ' Ground Zero recommends removing the vapor extraction unit from the site and destroying extraneous groundwater monitoring and vapor extraction wells Based on the abundant historical quarterly data we also recommend a reduction in the frequency of sampling We do not recommend installing any wells to monitor the sandy unit found at a depth of approximately 105- 115 feet because the levels of contaminants in that zone are relatively minor compared to those in the shallower wets We do not recommend installing any wells screened across the 80-90 foot interval despite high contaminant concentrations in soil and groundwater because of the low mobility of contaminant in this ' zone We recommend the following disposition of the wells at the site ' Retain Well Rationale MW 1 monitor upgradient edge of plume—sample semi-annually MW2 monitor downgradient/cross gradient southeastern boundary of plume—sample semi-annually MW7 monitor boundary between onsite plume and Top/ARCO plume—sample semi-annually MW8 monitor downgradient/cross gradient northem boundary of plume—sample semi-annually MW9 monitor north of plume—sample annually MW 10 monitor northeast of plume—sample annually ' R11t1110WreC 9bmn&dit