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• EXECUTIVE SUMMARY Four borings were drilled dunng the fourth quarter of 2004, and seven new monitor wells were installed in the 151, 2"d, and 3rd aquifers in the Riverbank Formation These sand bodies are fluvial channel deposits that are oriented generally east-west, although all are somewhat sinuous They are 10-30 feet thick in the central part of the site, but thin and die out to the north and south The thickness axis of all three trends beneath or close to the former UST cavity The top of this formation is at approximately 60 feet at the site, and the borings penetrated it to a depth of 105 to 127 feet Three of the four borings were located beyond the known extent of soil contamination, and no samples from these borings exhibited visual or olfactory evidence of contamination One or two samples from each aquifer in each boring were analyzed for gasoline and diesel fuel, and none contained detectable concentrations of hydrocarbons The fourth boring was located within 20 feet of the former UST cavity, and samples from this boring were analyzed at 10-foot intervals from 30 to 120 feet Four of the ten samples, one each from the three aquifer zones in the Riverbank Formation and one from the overlying Modesto Formation, contained detectable concentrations of diesel fuel Concentrations were low, reaching a maximum of 4400 parts per million dust below the base of Modesto aquifer Thirteen water samples were collected and analyzed, and diesel was detected in all but two of these In the three wells that sampled the 151 Riverbank aquifer, concentrations ranged from lust above the 50-parts-per-billion detection limit to 620 parts per billion in the easternmost well This well appears to be located cross-gradient relative to the present direction of groundwater flow in this aquifer, but with only three sampling points, neither the gradient nor the diesel plume are well defined Five additional wells are planned in this aquifer in the next phase of drilling Five of the seven wells that sample the 2"d Riverbank aquifer yielded diesel concentrations ranging from 59 to 600 parts per billion Diesel in this aquifer appears to be centered north of the former UST cavity The three water samples from the deepest (3rd) Riverbank aquifer had diesel concentrations ranging from 56 to 390 parts per billion, and the plume in this aquifer also appears to be located north of the UST cavity This aquifer is only modestly impacted, and with furthei monitoring and purging, concentrations might decline even lower I i 1