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Geological rechda/". Page 10 Groundwater Monitoring Report Project No.750.2 February 16,2005 r 1 Influent and Effluent readings recorded in the field using a Thermo Environmental Instruments Model 580B Organic Vapor Meter 2 Efficiency percentage calculated as Effluent/Influent 3 Efficiency percentage calculated as(Influent-Effluent)/Influent 4.2 Mass Removal r Mass removal calculations are completed utilizing the results of weekly OVM readings and monthly laboratory analyses of the SVE system influent and effluent. These calculations are presented in Table 5 in Appendix A. As of November 30, 2004, the system has removed an estimated cumulative total of 68 gallons or 300 pounds of TPH-G. 5.0 FINDINGS & RECOMMENDATIONS Findings The results of the groundwater sample analyses show the following: • All groundwater-monitoring wells were non-detect above laboratory reporting limits for all analyzed constituents. • Chart 1 illustrates the gradual decrease in groundwater elevation over time and the exponential decrease in TPH-G concentrations in MW-1. 0 Vertical groundwater gradient was calculated for the event of August 12, 2004, and was found to be insignificant. o The magnitude of the vertical gradient has decreased significantly from the initial values measured in 2001. o Fourteen of the past sixteen monitoring events show a downward trend to groundwater flow. o The downward vertical gradients are typically equal too or less than the horizontal groundwater gradients. • Oxidation-Reduction Potential measurements 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 2.4 and 5.0 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 no groundwater contamination is present in the monitoring wells. • Excellent progress has been made at this site. Clean-up goals are being met and the groundwater is free of known constituents and has returned to background conditions. 0 Biodegradation indicators suggest that the trace remnants of soil contamination will degrade by natural attenuation.