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Geologicalrecluttcslnc. Page 4 Groundwater Monitoring Report Project No.750.2 -. August 30,2004 2.0 FINDINGS & RECOMMENDATIONS Findings The results of the groundwater sample analyses show the following: • With the exception of MW-4, none of the site's six monitoring wells detected any of the analyzed constituents. • MW-4 showed the presence of trace amounts of toluene (0.5µg/L) and total xylenes (0.7µg/L). This is the only time any constituent has been detected over the past 41 months and is likely a sampling error. Future monitoring events should provide a better picture of site conditions. • Chart 1 illustrates the gradual decrease in groundwater elevation over time and the exponential decrease in TPH-G concentrations in MW-1. • Vertical groundwater gradient was calculated for the event of May 7, 2004, and was found to be insignificant. The magnitude of the vertical gradient has been getting progressively smaller during the last several monitoring events. Twelve of the fifteen past monitoring events show a downward trend to groundwater flow. • Oxidation-Reduction Potential measurements (Table 4) are positive for all the wells. This suggests that the volume of contaminants leaching into groundwater has been stopped and that biodegradation of the contaminants is at a rate sufficient to mitigate the residual BTEX/TPH-G plume in the area around the source without depleting the oxygen. • The dissolved oxygen measured at the site is between 4.3 and 7.3 ppm and is conducive to natural attenuation of BTEX/TPH-G. • It appears that soil contamination has been reduced significantly by the vapor extraction �. process, that oxygen has been added to the subsurface and that only trace amounts of groundwater contamination is present in one monitoring well. • Excellent progress has been made at this site. Clean-up goals have been met and the effort has exceeded MCL's for the known constituents at the site. • Biodegradation indicators suggest that the trace remnants of soil contamination will degrade by natural attenuation. `r • Groundwater monitoring data shows that groundwater has returned to background conditions. Recommendations: • Continue quarterly groundwater monitoring at the site. _ • Perform vapor extraction as directed by the county in an effort to remove the trace amount of remaining contamination (pending authority to construct). • After the vapor extraction system has operated for 6 months, if not sooner, the site should be considered for closure.