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in March 1996. The results of this monitoring event are reported in the First Semester 1996, Semi Annual Groundwater Monitoring Report, May 1996. The monitoring program indicated that groundwater levels fluctuate seasonally with groundwater elevations ranging from 2 to 13 feet bgs. Results of each groundwater monitoring event are presented in groundwater -monitoring reports L. submitted to the SJCPHS and RWQCB. A comprehensive table listing all of the analytical results from the groundwater monitoring sampling at the site from 1989 to L, 1996 is presented in Appendix C, Table C-2. The groundwater sampling results indicated that a petroleum hydrocarbon groundwater plume is present at the site. Initially the plume did not reach MW-9. In the fourth quarter of 1994, and periodically since then, low levels of petroleum hydrocarbons have been detected in MW-9. MW-9 is believed at be at the leading edge of the plume. LO The extent of the petroleum hydrocarbon plume at the Stockton Service Center has remained very consistent throughout the groundwater monitoring program from September 1993 to March 1996. Figure 6 taken from the first monitoring report, followed by Figure 7 copied from the most recent monitoring report, show the initial and final estimates of the contaminant plume, respectively. Notice that after 2-1/2 years of monitoring, the plume has expanded only slightly, primarily in the southwest direction. Not only is the hydrocarbon plume stable, but concentrations of hydrocarbon compounds are decreasing with time. Charts 1 through 5 depict this reduction in concentration with time for total petroleum hydrocarbons as gasoline (TPH-g), benzene, toluene, ethylbenzene, and xylenes. The charts display trends in chemical concentrations for groundwater monitoring wells MW-1, MW-8, and MW-9. When evaluating the charts, it is apparent chemical concentrations in groundwater are undergoing a natural dispersion and degradation with time. Furthermore, evaluation '" of Figures 6 and 7 reveals that the extent of the contaminant plume is stable with time. L,. 2.4 Remedial Actions LO In January 1994, an active in situ bioremediation was selected for the site based on the feasibility study. The bioremediation consisted of extracting groundwater from a converted monitoring well, mixing the extracted water with enzymes, biosurfactants, and amino acids, and reinjecting the enzyme-laden water into the aquifer. Prior to designing and implementing the remediation, several tests were Iwo 96R7 East Say Municipal Utility District 40429700.fcr 7 Final Closure Report