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Benzene has occasionally been detected in MW-1 and a few other wells within the p)ume, but concentrations are low (Table 4) Wells that are marginal to the plume, such as IAW4 and MW-5, have either not been impacted by benzene or have shown a steady deck ne in benzene concentrations, indicating that the fringes of the plume have been actively deip-aded and that the plume has shrunk in extent(Figure 7) 4.0 SUMMARY AND RECOMMENDATIONS Upgradzent Environmental has now completed the scope of work authorized by the State UST Cleanup Fund under proposal no 96006 This scope included the drilling of a to Al of 200 feet of Geoprobes distributed among seven borings and infecting approximatc,ly 70 pounds of Oxygen Release Compound into each boring The ORC was pumpec as a powder/water slurry over a 10-foot depth interval in each boring As explained in the Site Work Plan, calculations of the volume of slurry indicate that approximately 4 gallon:. were infected into each linear foot of borehole, resulting in the infection of slurp, into approximately 3 cubic feet of soil per linear foot The slurry should therefore have liad a radius of immediate influence of 1 foot from each borehole With an average spacing of 8 feet between bonngs, this indicates that most of the impacted soil and groundwater could not have been directly affected by the ORC slurry However, over time, the ORC mould presumably dissolve into the groundwater and release oxygen that could increase the potential for biodegradation of contaminants over a larger area At some point, the DRC would be exhausted, and no more oxygen would be released Monitonng of dissolved oxygen and hydrocarbon concentrations for the nine-month period between April and December 1998 indicates that there was an initial release of oxygen to the groundwater, increasing dissolved oxygen concentrations to between 1 and 3 mg/1 This increase was associated with decreases in the concentrations of most hydrocarbons d unng the second quarter of the year In the third quarter, dissolved oxygen concentrations decreased to zero, and hydrocarbon concentrations decreased in some wells while increasing in others. Results from the fourth quarter demonstrate rather conclusively th.it the ORC has been depleted and is no longer releasing oxygen to the groundwater Hydrocarbon concentrations appear to have stabilized in some wells(e g MW-2),risen in others (SIN-1), and remained below or decreased below detection limits in most other wells After discussing these results with Mr Michael Infurna of PHS/EHD,Upgradient Environmental recommends preparing a Site Closure Report for submission to PHS/E FID and the UST Fund The report would include summaries of the work that has been performed at the site since investigation began in 1990, compilations of all laboratorydata in tabular and graphic formats, and a completed No Further Action checklist and related items pursuant to Appendix B of the Tri-Regional Board Recommendationsfor Prelint.nary Investigation and Evaluation of Underground Tank Sites If authorized by the client in early January 1999, the Closure Report could be submitted by the end of January 4