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105-0&,2002 0".Wif, PUC o cera a®Llogfc Investiaflon for Nunan Page 9 0,11 DO I Where K = 0.0002 cm/sec X 2,830 ft/day/cm/sec = 0.566 ft/day b = thickness of the pumped aquifer = 210 - 60 feet = 150 feet thick I4 = V& = 0.566 X 150 = 84.9 W/day o = 0.015 IQ = 35 gal/min X 0.134 cubic feet/min/gal/min X 1,440 min/1 day = 6,753.6 WIday I X = Q/2TW5 = the distance of the down-gradient rind of the capture zone from the stagnation point therefore X = 6,753.6/2 (84.9) (3.14) 0.015 = 844'/2feet IIn summary, since the site is located 1,100 feet down-gradient from this well, it is greater than the distance of 844'/2 feet which is the furthest down-gradient extent I of the capture zone to be reached and sustained by pumping. Since the zone of affective drawdown of the pumping well cannot reach the site, potential dissolved contaminants cannot become entrained by the pumping State Water Supply well No. 39055. ICONCLUSIONS No gasoline oxygenates or lead scavengers were identified in soil or groundwater beneath the site. The primary source of gasoline discharge was removed a decade ago. The benzene plume in soil has been defined by six(6) shallow soil borings, I one deep soil boring and three (3) deep groundwater monitoring wells. Benzene was identified in soil no deeper than 60 feet bgs and is not leachable to groundwater which is isolated in a perched aquifer at a depth of approximately 80 to 86 feet bgs. The only gasoline constituent identified during the sampling of three (3) properly developed and constructed groundwater monitoring wells during this recent subsurface investigation was 5 ppb toluene in MW-2. The likelihood of dissolved benzene or toluene emanating from the site and reaching any of the drinking water supply wells identified during the GeoSoly, LLC investigation of this site is very remote. The residual benzene contamination left in soil is not a threat to the drinking water beneficial uses of groundwater. RECOMMENDATIONS This case should be regulated as a "soils only" case and should be formally closed with respect to the issue of the site's threat to the beneficial uses of groundwater for drinking water. One additional groundwater sampling event should be performed to verify that the groundwater has not been impacted by hydrocarbons at levels in excess of drinking water standards. If no significant contaminant trends are identified which would suggest that the groundwater would be further degraded, then the site should be closed and the groundwater monitoring wells should be properly abandoned. 05-03-2008 GeoSofv, LLC-lfydrogeologfc lnvesagatlon for!Hunan page 9 of 10