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i S A� Discussion The contaminants are localized in the area north of the former UST location and to the east of the mobile homes Excavation in the tank area has removed the contaminated soil, and excavation wall samples laboratory data indicate that only residual concentrations remain Previous subsurface investigations have shown that there are TPHG contaminants and they have extended slightly onto the Tracy High School property, but the BTEX components were very low However, groundwater sampled from well MW-4 showed that contaminants were not detected, indicating that dissolved contaminants are not mobile The microbial counts show a decrease in the plume area compared to upgradient well MW-6 This could indicate that the biologic degradation of the more volatile components has been occurring for some time The ferrous iron is also low, suggesting that anaerobic reactions may have operated upon the plume, degrading the TPH component This indicates that volatiles are highly depleted hydrocarbon plume (as compared to a "fresh" fuel spill) and that biodegradation processes have degraded the volatiles The Benzene concentrations are extremely low and are approaching the 1 ppb level The aquifer is of relatively low permeability and so groundwater velocities are interpreted as low, causing the plume spread slowly to the north (see Wright report dated July 29, 1996) Comparing this data to the previous study, it suggests that contaminants have migrated slowly along the capillary fringe under the parking lot and possibly along the agricultural pipeline backfill to a limited extent Conclusions and Recommendations The data collected in this and previous investigations shows that the plume has migrated north about 300-feet The plume has slightly impinged upon Tracy High School property along the capillary fringe, however, dissolved components were not observed in Well MW-4 The BTEX is very degraded when compared to a"fresh" fuel, with low concentrations observed Hydrocarbon concentrations are anticipated to continue to decrease with time since the tank source and some contaminated soil have been removed Since the plume has spread on the capillary fringe and is depleted in volatile components, neither a groundwater pumping nor soil vapor extraction program is considered cost effective to remove these contaminants Wright recommends that the groundwater monitoring wells be monitored at quarterly intervals to meet the requirements of the PHS-EHD for underground tank petroleum releases The monitoring program will be used to track contaminants and parameters to estimate the biodegradation activity in the plume area Wright will also review the possibility of augmenting the biodegradation process by the addition of oxygen releasing compounds to selected on site wells The addition of oxygen releasing compounds has been effective in augmenting biodegradation and should degrade remaining volatiles If concentrations continue to decline over the next four quarters, Wright will request that this site be considered for closure Limitations This report has been prepared specifically for the Mr Ed's site at 595 East 11th Street in Tracy, CA,and was done according to the State and local agency suggested guidance documents for these investigations The interpretations, conclusions and recommendations made herein are based on the data and analysis for the soil and water samples collected on-site and should be reviewed in the context of the whole report and previous site work Please note that reports of contamination must be submitted to the agencies in a timely manner Wight Environmental Services, Inc is not responsible for errors in laboratory analysis and reporting, or for information withheld during the course of the study, and no warranty or guarantee is expressed or implied therein Page 6