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Mr. Paul Supple • • <br /> Arco Station No. 6100 <br /> 25775 Patterson Pass Road <br /> Page 2 of 6 <br /> in monitoring wells MW-1 and MW-2 and in vapor wells VW-1, VW-2, VW-3, <br /> VW-4, VW-5 and VW-6 during one or both quarters reported. Selected wells and <br /> the on-site domestic well were sampled during the reported quarters for <br /> laboratory analysis. Some wells contained a few tenths of a foot of water, which <br /> may just be water sitting in the bottom caps of the wells, other wells were dry. <br /> The ground water elevations in the wells noted above were erratic, with <br /> intervening wells screened at similar or greater depths that had no measurable <br /> water, suggesting that perched water was encountered in the wells, not the <br /> regional first ground water. All of the ground water samples analyzed contained <br /> detectable concentrations of one or more analytes, except for the first quarter <br /> results for MW-2. Free product has been encountered and periodically removed <br /> from MW-1; a sheen of free product was observed in VW-1 and VW-4 during the <br /> first quarter 2002 monitoring event. <br /> Delta noted that the domestic well onsite has a gravel filter pack from 35 feet bsg <br /> to depth and that the water level has been measured at 40 feet bsg, high above <br /> the water levels in the monitoring and vapor wells and the deeper reaches of the <br /> vadose zone. Delta suggested that some of the water in the wells may have <br /> originated from seepage from the filter pack into the surrounding soil and noted <br /> some inconsistencies with that scenario. <br /> Delta concluded: <br /> • The vertical extent of hydrocarbon impact has been adequately assessed <br /> both on and offsite; <br /> • The major portion of the hydrocarbon mass was encountered between 35 <br /> and 65 feet bsg; <br /> • Soil borings installed down-dip from the USTs and dispenser islands did <br /> not encounter significant hydrocarbons except in the boring for MW-2; <br /> • Ground water in MW-2, the only newly installed monitoring well to receive <br /> ground water, was impacted by TPH-g and benzene; <br /> • A definitive source for the ground water encountered in MW-1 has not <br /> been identified; <br /> • Fuel oxygenates in the subsurface appear to have been adequately <br /> characterized. <br /> Delta recommended that dissolved TPHg, benzene and floating product <br /> encountered in MW-1 should be investigated in the down-dip direction from the <br /> site. <br /> Based on review of the above cited reports, and portions of other reports, EHD <br /> comments as follows: <br />