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• Remedial Status Report-Tosco(Former BP)Station No 11193 November 13,2001 At this time the purging was transferred back to MW-1 to recover sufficient groundwater to make up the 5,000 gallon target volume The results of this performance test indicated that purging the MW-1 well produced a far greater volume of water than MW-2, which dewatered Based on the volumes purged, MW-I appears to have good hydraulic communication with the UST pit Purging MW-1 also appeared to result in a greater effect on the static groundwater levels in observation wells than did purging monitoring well MW-2 Purging MW-1 appears to pull groundwater inwards from the all parts of the site of the site toward the presumed source For these reasons, groundwater purging is currently being conducted from well MW-1 MTBE Mass Removal MTBE concentrations from the grab samples were used to estimate the mass of MTBE removed in groundwater purged from the UST pit at the site An estimated MTBE concentration for each purging event was derived from averaging MTBE concentrations from groundwater samples collected on dates prior to and following the corresponding purging event The estimated concentrations were used to calculate the MTBE mass removed during each event Based on this method, the estimated total MTBE mass removed in groundwater purged from the site between August 3, 2000 and October 2 2001, is 11 39 pounds The estimated MtBE removed for each event, the cumulative MTBE removed, and the calculation methods are presented in Table 1 • Conclusions and Recommendations The results of this investigation indicate that MW-1 is the most effective well to use for purging groundwater and controlling migration The performance test results demonstrate that purging MW-1 produces drawdown in nearby wells and therefore is pulling groundwater towards the presumed source Based on MTBE concentrations in nearby wells (Blaine Tech, Event of July 27, 2001 MW-3, 1190 ppb, MW-5, 1810 ppb, MW-8, 123 ppb), the weekly purging program has been successful in preventing the migration of elevated concentrations of MTBE from the immediate vicinity of the UST pit (MW-2 and VW-l) The weekly purging program has resulted in the removal of 11 39 pounds of MTBE from the site However, the purging program has not successfully reduced elevated MTBE concentrations in the vicinity of the UST pit (MW-2 and VW-1) Therefore, to reduce this MTBE impact, GR recommends that a Dual-Phase Vapor Extraction Pilot Test (DPE Test) be performed at the site The purpose of the test is to provide interim removal of MTBE from the subsurface and to provide feasibility test data for the site A work plan describing the DPE Test has been submitted under separate cover In addition, GR is recommending the continuation of this groundwater purging and monitoring program for migration control purposes Onyx vacuum trucks will continue to use MW-1 for purging groundwater Additional grab groundwater samples will be collected on a periodic basis to determine the effectiveness of the purging program The recommended program is ongoing at this time • 14020504-2 3