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N <br /> t <br /> . Ms Linda Turkatte <br /> December 11, 1995 <br /> Page 2 <br /> A total of six groundwater monitoring wells (MW 1 through MW6) were installed at the site to explore <br /> the lateral and vertical extent of groundwater contamination Groundwater beneath the site generally <br /> flows to the southeast The Iocations of the wells and a recent potentiometric surface map of the <br /> shallow aquifer beneath the site are shown on Figure 2 <br /> Groundwater sampling indicated that dissolved contaminants associated with the former xylene tank had <br /> impacted groundwater beneath the site Due to the volatile nature of xylene and its amenability to <br /> biodegradation by naturally occurring bacteria, it was concluded that "passive remediation" would <br /> effectively reduce residual contamination in the saturated zone over time Passive remediation is the <br /> term given natural processes such as dispersion and biodegradation to reduce contaminants in the <br /> subsurface Naturally occurring microbes in most soils degrade gasoline hydrocarbons and its <br /> components, including xylenes <br /> A quarterly groundwater monitoring and sampling program was established to monitor trends in <br /> groundwater elevations and contaminant concentrations Groundwater monitoring and analytical data <br /> are summarized in Tables 1 and 2, respectively <br /> Evaluation of Passive Remediation <br /> A review of analytical results indicates that dissolved contaminants at the site have decreased <br /> significantly over time This decreasing trend of contaminant concentrations is illustrated on Figures <br /> 3 through 7 The figures plot contaminant concentrations versus time for all site wells No dissolved <br /> contaminants were detected in samples collected during the most recent sampling event, performed on <br /> September 28, 1995 Figure 8 plots the estimated mass of contaminants in groundwater over time The <br /> figure depicts a reduction from approximately 7 65 pounds of dissolved contaminants (as TPHG) on <br /> December 5, 1991 to zero pounds on September 28, 1995 Mass calculations and estimated areal extent <br /> for each monitoring event are included in Attachment A <br /> The continued absence of contaminants in downgradient wells MW5 and MW6 indicates that the <br /> reduction in contaminants across the site is not due to lateral migration of the contaminant plume <br /> Further, groundwater elevations measured beneath the site on September 28, 1995 approximate the <br /> highest groundwater elevations recorded at the site since the initial monitoring wells were installed in <br /> April 1991 (Figure 9) We would expect any residual contamination remaining in the capillary fringe <br /> zone to manifest as dissolved contaminants as it is resaturated by the rising water table Thus, the non- <br /> detect analytical results of groundwater samples collected on September 28, 1995 indicates that no <br /> significant residual contamination remains at the site <br />• <br /> groundzeUaidlawlphs 1295 Cpl <br />