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3 <br /> chemical and flow data support a stable plume configuration and low flow velocity. <br /> The aquifer is stratigraphTally thin and contains dispersed clay in sand which results in relatively low hydraulic <br /> conductivity and groundwater velocities are interpreted as low, causing the plume spread slowly to the north (see <br /> Wright report dated July 29, 1996). Comparing this data to the previous studies, it suggests that contaminants have <br /> migrated slowly along the capillary fringe under the parking lot and possibly along the agricultural pipeline porous <br /> aggregate backfill to a limited extent. Investigation data and monitoring well contaminant data trends support this <br /> interpretation. <br /> Interpreted Microbial Degradation Affects noon the Plume <br /> The microbial counts show a decrease in the plume area compared to upgradient well MW-6. This data indicates <br /> that the biologic degradation of the more volatile components has been occurring for some time. The ferrous iron is <br /> also low, suggesting that anaerobic reactions may have operated upon the plume, degrading the TPH component. <br /> This indicates that volatiles are highly depleted hydrocarbon plume (as compared to a "fresh" fuel spill) and that <br /> biodegradation processes have degraded the volatiles. This results in a `stabilized plume wherein residual <br /> contaminants at the capillary fringe are not mobile even when changing water levels causes desorbtion and `spikes" <br /> in concentrations. Contaminants have not been observed at the most downgradient well MW-4 which is positioned <br /> on a nearly straight flowline path from the tank pit and M W-9. <br /> .Outcome <br /> The quantity and observed concentrations of the contaminants cannot be removed cost effectively by soil vapor <br /> extraction or groundwater pumping.The potential excavation limitations and denial for encrochment by the West <br /> Side Irrigation District were presented to Wright in their written response. While residual contaminants are present, <br /> excavation(deemed most cost effective by Wright) is prohibited by the WID encroachment denial. <br /> Conclusions and Recommendations <br /> The data collected in this and previous investigations shows that the plume edge has migrated north about 300-feet. <br /> The plume has slightly impinged upon Tracy High School property along the capillary fringe, however, dissolved <br /> constituents were not observed in Well MW-4 indicating little net plume movement. BTEX concentrations are very <br /> degraded when compared to a "fresh" fuel. Groundwater fluctuations may cause short duration "spikes" of <br /> contaminants, however, these appear very limited in mobility in the historic data trends, and the long term trend is <br /> declining dissolved volatile constituents. Hydrocarbon concentrations are anticipated to continue to decrease with <br /> time since the tank source and some contaminated soil have been removed. Since the plume has spread upon the thin <br /> capillary fringe and is depleted in volatile components, neither a groundwater pumping nor soil vapor extraction <br /> program is considered cost effective (given limited water yields and depleted volatile content) to remove these <br /> contaminants. Excavation of limited capillary fringe contaminants along the area of MW-9 is not feasible since <br /> encrochment by WID will not be granted to Wright. <br /> In our opinion, on the basis of the work discussed herein and previous report submittals, further workis not <br /> warranted. Wright recommends that this site be considered for closure by PHS EHD at this time.The PHS-EHD <br /> representative requested that WRIGHT continue with site monitoring pending a possible closure of the site following <br /> PHS-EHD review of this letter. <br /> Mr. Ed's Muffler Project No. 4090-P Page 4 <br />