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Geological Tec)uIks t1w Page 9 Groundwater Monitoring Report Project No 723 2 October 25,2005 ' ❑ Figure 12 illustrates TPH-G and Benzene concentrations versus elevation in deep well MW-101, which hes near the core of the plume The TPH-G and benzene concentrations ' have displayed a decreasing trend, although a slight increasing trend is evident for the past year The steep decline in the initial concentrations suggests that the contaminants detected in the well were initially a drilling artifact ' ❑ Figures 10 — 12 also show the seasonal decline in groundwater elevation following the large spring increase in groundwater elevation ❑ Oxidation-Reduction Potential (ORP) and Dissolved Oxygen (DO) measurements (Table ' 7) show that most of the wells monitored are screened in areas of the groundwater plume that are affected by the biodegradation reactions occurring within the plume The low to highly negative ORP values indicate that the wells are within the halo of reaction ' depleted water caused by biodegradation of the contaminants The low DO values of the wells also reflected this condition 3.4 CONCLUSIONS & RECOMMENDATIONS Conclusions 1 Groundwater flow trends northeast to east The southerly flow direction recorded on M December 4, 2000, appears to be anomalous The deepest set of CMTFM wells displayed an anomalous groundwater depression in the vicinity of MW-101 Since this is only the third time the interval has been monitored additional data is needed before reaching any ' conclusions 2 High levels of BTEX and TPH-G are present in the groundwater of all six on-site shallow and intermediate monitoring wells (MW-1 through MW-6) ' 3 The main mass of contaminants has migrated to a depth of 45 to 70 ft bgs, indicating that the contaminants are mobile 4 Cross gradient deep well MW-207 contained low levels of TPH-G for the fourth ' consecutive event This probably represents the edge of the advancing plume 5 Down gradient wells MW-8/108/208 contain low levels of TPH--G and BTEX as compared to the on-site wells ' 6 The groundwater contaminant plume is not defined laterally down gradient of the MW- 8/108/208 well set 7 Well set MW-9/109/209 historically defines the lateral extent of the plume in an up ' gradient direction However, all three aquifer levels shallow, intermediate and deep (MW-9, MW-109 & MW-209) contained TPH-G for the first time in the investigation The mechanism by which the contamination has mobilized up gradient is unknown but may be a result of preferential pathways or dispersion down concentration gradient lines, and/or the low groundwater gradient slope value could lend to ineffective gradient control ' parameters 8 The low levels of contamination in well MW-101 and lack of contaminants in MW-201 suggests that these wells define the vertical limit of the plume beneath the site