SITE INFORMATION AND CORRESPONDENCE

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Mr. Paul Supple Arco Station No. 6100 25775 Patterson Pass Road Page 3 of The relatively lower contaminant concentrations at shallower depth encountered in the UST area and the relatively higher contaminant concentration previously encountered in soil in the dispenser area seem to show the dispenser area was the scene of the more significant fuel release(s). The recently obtained shallow soil data from the dispenser area presented in the SSR tends to confirm this hypothesis. Deep soil borings B-34, B-36, B-37 and B-38, advanced immediately around the dispenser area, show contaminants extending to as much as 106 feet bsg with less than detectable concentrations below the 106-foot bsg sample to total depth investigated (as much as 139 feet bsg). EHD agrees that the vertical extent of impacted soil onsite appears to have been adequately delineated in the source area. The bulk of the hydrocarbon mass has been encountered at depths between 10 and 45 feet bsg, with localized high concentrations encountered at depths ranging from 45 to 80 feet bsg. A definitive source of the ground water encountered may not have been identified, but Delta's hypothesis that the domestic well is contributing water to the local shallower subsurface may have some merit. Mapping the encountered ground water elevations does not initially seem to support the idea as some more distal wells had higher ground water elevations than some of the more proximal wells, relative to the domestic water well. However, after EHD's review of the cross sections and selected well logs it appears that wells with measurable ground water commonly were screened through a sandy interval and bottomed in a fine-grained unit. With the exception of VW-6, the top of the water in the wet wells nearly coincides with the sand/silt contact. Dry wells appear to bottom in a sand unit or to not encounter a sand unit in the screened interval. This would seem to suggest that meteoric water and/or water seeping from the domestic well is migrating as a thin saturated zone along the contacts between overlying sand units and the underlying fine-grained units, predominantly silt. If this is so, then one of the controlling factors for encountering ground water in the wells would be completing the well with the well screen passing from a sand into an underlying fine-grained unit that will retard the vertical and lateral migration of the water long enough to accumulate in the portion of the well below the sand. A well with a screen interval bottoming in a sand may not yield water as the whole sand interval is not likely to be saturated, only a thin interval along the bottom of the sand. Of course, the attitude of the sand/silt contacts and location and recharge rate of the water source would also be controlling factors. The high water level in the domestic well, reported at 40 feet bsg, in contrast to the commonly deep reaches of the vadose zone, locally at least to 95 feet bsg, suggests that the domestic well is completed in a confined aquifer and that water rising up its filter pack seeps into the coarse-grained units it encounters and migrates away from the well. If this scenario is correct, rather than being a