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Dissolved TPHg concentration in M",',/1 has been below the detcr`ton 'irn,t of 50 ag/1 over `li'_ past four quarters Concentrations of TPHg in MWZ have shown an �rreguiar unstable groundwater trend since The October 200a vapor extraction program began ranging from 2,300 ug/1 to less then 50 ug/1 The results during the third quarter were below the detection limit All remaining wells have consistently been less than the detection limit MTBE concentrations from well MW1 and MW2 have decreased by one order of magnitude since the wells wvere .nstalled in 1995 Stnce the e•.tr action tests began, MTBE concentrations have been irregular with periods of increases and decreases most likely the result of rebound effects from the extraction events Benzene concentrations have been non-detected in all wells with the exception of MWI and MW2 Since October 2003, benzene concentrations have been non-detect in _'.VIW2 since that time however, concentrations have fluctuated from 60 ugl to less then 0 50 ug/1 The most slgiificant increase was observed from October 2004 through January 2005, a direct result of the initial soil vapor extraction performed in October 2004 6 0 RECOMINIENDATIONS ATC recommends performing soil confirmation sampling in the vicinity of the former underground storage tank to fill in data gaps discussed in EHD's March 17, 2004, correspondence If results show remedial efforts to date are successful, the remediation activities will be discontinued and a closure plan will be prepared Upon concurrence, ATC will prepare a } �} work plan and permit applications Based on the successful vapor extraction events summanzed above, ATC recommends installing and operating a full time vapor extraction and treatment system We evaluated two alternative systems, both capable of removing hydrocarbon-impacted vapor and treating the recovered vapor prior to discharge Positive displacement vacuum blower with vapor phase carbon polish prior to discharge to atmosphere, and • Electnc catalytic oxidizer package including a vacuum blower An evaluation of the data presented in Table 6 showed that the hydrocarbon and oxygenate removal rate was somewhat proportional to flow up to approximately 70 scftn Above this flow rate, hydrocarbon and oxygenate concentrations declined sharply i At a flow rate near optimum, we estimate that a significant proportion of the contaminants remaining in the soil vadose zone can be removed by either of the systems proposed in 4 to 6 y - - -- monthsFor this analysis, we have assumed that contaminant concentrations will approach non- detect in six months at a declining rate approximated by a second degree parabola (see table below) )1 �- 6 s �enviromenml162577-Diesel Perflreportslmcenm_rem42 dm