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CLEA.RWATER C, a 0 L P Ic fnv:rnnmenta[ Servrres Conclusions and Recommendations At the time of UST removal in 1988, gasoline concentrations soil beneath the USTs were reported to be >10,000 mg/kg Following investigation activities, it was estunated that approximately 5,050 lbs of TPHg resided in the vadose prior to remediation Additionally, SPH had been observed intermittently in the areas of MW-1 and MW-2 Maximum pre-remediation dissolved-phase TPHg, benzene, and MTBE concentrations were 150,000 pg/L, 9,400 ug/L, and 3,800 pg/L, respectively Operation of SVE/AS system from November 1998 to September 1999 resulted in a dramatic reduction of contaminants beneath the site Based on system operation data, it was estimated that 7,377 lbs of TPHg were removed from the subsurface during remedial efforts Confirmation borings in March 2000 indicated that only a limited residual volume of soil contamination remains in the area of MW-2 It was estimated that approximately 37 lbs of sorbed-phase TPHg are now present beneath the site The maximum TPHg concentration in this area was 240 mg/kg Based on these data, it appears the SVE/AS system removed over 99% of the TPHg present before remediation It is likely that the limited mass of residual gasoline in the soil will naturally degrade over time, and will not pose an ongoing threat to groundwater, as evidenced by ongoing groundwater monitoring results Groundwater contaminants have also decreased in dramatic fashion since site remediation occurred Maximum post-remediation TPHg, benzene and MTBE concentrations are currently 220 Pg/L, 0 55 gg/L, and 150 pg/L, respectively Residual dissolved-phase contaminants are restricted to the uppermost 10 feet of water column, based on non-detect results for the sparge wells, and do not extend laterally much beyond their well locations Additionally, no other Fuel Oxygenates (TBA, ETBE, DIPE, TAME) or Lead Scavengers (1,2-DCA, EDB) were detected in the groundwater beneath the site It is likely that these low residual dissolved-phase contaminant concentrations will naturally attenuate below detectable limits within short period of time Samples from the on-site domestic well continue to be free of any petroleum contaminants Considering the well was not impacted by elevated contaminants levels prior to remediation, it seems appears that the well is not threatened by the minimal post-remediation contaminant levels now present in the soil and shallow groundwater beneath the site Based on all historical site information, it appears that soil and groundwater beneath the site have been effectively remediated to the extent practicable, and residual contaminants do not appear to pose a threat to any nearby receptors Accordingly, Clearwater recommends that the site receive regulatory closure Following closure, all investigation/remediation wells should be properly abandoned and all subsurface remediation conduits should be removed and properly backfilled/compacted ZB107C GWMR 3Q00 5 August 3 2000