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most of the water samples Only the two samples near the dispenser island (HP-1 and DM-1) contained detectable concentrations (Table 3) 2.3 Excavation Backfill After drilling was completed, clean fill dirt was imported to backfill the excavation, and the area was resurfaced Later, a new above-ground fueling facility was constructed 2.4 Groundwater Monitoring With the concurrence of PHS/EHD, the damaged domestic well (DM-1) was destroyed after a second round of sampling, but the monitor wells were placed on a program of quarterly groundwater monitoring They were sampled in April, August, and December of 1996 (Table 3) Toluene was reported in TT-2 at a very low concentration in one quarter, and diesel was reported in TT-3 in another quarter Groundwater gradient maps were constructed for each quarter, and indicated that the groundwater flow direction vaned from east-southeast to southwest The depth to groundwater varied only slightly, remaining between 55 and 60 feet throughout the year(Table 4) When the fourth monitoring event was completed, Upgradient recommended that the site be closed with no further action No groundwater monitoring took place in 1997 or 1998, but monitoring resumed in June 1999 and continued quarterly through the end of the year The samples were analyzed by Sequoia Analytical Laboratory, and diesel, most BTEX compounds, and MTBE were reported in TT-2 in June Except for diesel, these results `• were not confirmed in August, and when TT-2 was sampled in December, even diesel was absent(Table 3) Therefore Upgradtent concluded that the June 1999 results were erroneous and again recommended site closure I 3.0 SITE GEOLOGY The site is underlain by the Modesto and Riverbank formations, which consist regionally of clastic sediment that ranges from clay to gravel and was deposited as stream-channel and overbank sediment on glacially-fed alluvial fans ) In boring TT-1, which was continuously cored, these formations comprise eight lithologic units that form a series of upward-fining sand beds separated by silt beds Several of the thicker beds can be traced in cross section to one or more of the other borings (Figures 3 and 4) Most beds are shades of yellow, orange, brown, or red, indicating varying degrees of weathering and oxidation Most of the thick sand beds are coarse to very coarse grained at the base, indicating deposition by relatively strong currents in relatively large channels One such channel appears to be oriented in a general east-west direction,because the channel deposit thins northward and southward from HP-1 and was not recognized in HP-2 or TT- 3 (Figure 4) The uppermost unit (Unit 1) is an upward-fining fine to coarse-grained sand bed that is approximately 20 feet thick The lower part of unit 1 is moderately sorted to well sorted and appears to be quite permeable The underground tanks were set in this unit 3