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Remedial Status Report-Tosco(Former BP)Station No 11194 <br /> January 3,2001 Y <br /> r' <br /> Groundwater Analytical Data and MtBE Mass Removal <br /> MTBE concentrations from the grab samples were used to estimate the mass of MTBE removed in <br /> groundwater purged from the UST pit at the site An estimated MTBE concentration for each purging <br /> event was derived from averaging MTBE concentrations from groundwater samples collected on <br /> dates prior to and following the corresponding purging event The estimated concentrations were <br /> used to calculate the MTBE mass removed during each event Based on this method, the estimated <br /> total MTBE mass removed in groundwater purged from the UST pit between May 3, 2000 and <br /> September 12, 2000 is 52 41 pounds The estimated MtBE removed for each event, the cumulative <br /> MTBE removed, and the calculation methods are in Table 1 <br /> Performance Test <br /> In conjunction with the May 3, 2000 groundwater purging and sampling, GR conducted a <br /> performance test to qualitatively analyze of the radius of influence generated during removal of <br /> groundwater from monitoring well AMW-5 and UST pit observation well OBS-W Both wells have <br /> 4-inch diameter PVC casings During purging, GR personnel collected depth to water (DTW) <br /> readings from UST pit observation well OBS-E and on-site monitoring wells MW-2, MW-3, MW-5, <br /> and AMW-4,AMW-5,and AMW-6 which were used as observation wells(Figure 1) DTW readings <br /> were collected continuously, beginning prior to initiation of groundwater purging until 90 minutes <br /> after completion of purging activities, for a cumulative total of 180 minutes <br /> • Purging began in well AMW-5, which dewatered in 5 minutes The well was purged for 90 minutes, <br /> generating approximately 250 gallons of groundwater No drawdown was noted in any of the <br /> observation wells The distances from AMW-5 to the observation wells ranged from 6 feet from <br /> MW-2 to 89 feet from AMW-4 Well AMW-5 had an initial DTW reading of 5 68 feet bgs and had <br /> recharged to 5 75 feet bgs within 20 minutes after purging ceased(I 10 minutes total) <br /> Purging was restarted from UST pit observation well OBS-W at 110 minutes after start of the test <br /> The well was dewatered within 20 minutes of initiating purging(130 minutes into test) OBS-W was <br /> purged for a total of 70 minutes (180 minutes into test), generating approximately 4,000 gallons of <br /> groundwater Drawdown in the observation wells after 70 minutes ranged from 0 29 feet in MW-2 <br /> (5 5 feet from UST slab), 018 feet in AMW-5 (6 5 feet to UST slab), 0 12 feet in MW-3 and MW-5 <br /> (7 5 and 14 feet from UST slab, respectively), 0 09 feet in AMW-4 (36 feet to UST slab) to no <br /> drawdown for AMW-6(20 feet to UST slab) Groundwater drawdown data are summarized in Table <br /> 2 <br /> The results of this performance test indicated that purging OBS-W produced groundwater at a higher <br /> rate than did purging AMW-5, which dewatered Purging OBS-W also produced a significantly <br /> greater drawdown in MW-2 than purging monitoring well AMW-5 Purging OBS-W appears to pull <br /> groundwater inwards from the northern portion of the site toward the presumed source (UST pit) For <br /> these reasons,groundwater purging was and is being conducted primarily from OBS-W <br /> 14018704-2 <br /> 2 <br />