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when the static water level uses above the Riverbank Formation into the basal Modesto Formation, and then gradually decline as the water level recedes below the Modesto-Riverbank contact during dry seasons or years Concentrations do not increase when the water level drops below the Modesto Formation into the Riverbank Formation, because soil within the Riverbank is largely uncontaminated 6.0 CONCLUSIONS From the data that have been collected at this site since 1994, we can draw several conclusions regarding the scope of the contaminant problem, as listed below 1) The principal contaminant of concern is diesel fuel In soil samples, maximum diesel concentrations have ranged from 10,000 to 20,000 parts per million, which is approximately 20 tunes greater than maximum gasoline concentrations In groundwater samples (excluding hydropunch samples, which are notoriously inaccurate), maximum diesel concentrations have ranged from 50,000 to 200,000 ppb (in GT-10 only), which is between 100 and 400 times greater than gasoline concentrations in some cases Further, the maximum methyl-tertiary-butyl-ether(MTBE) concentration in any sample has been 8 parts per billion, and the maximum benzene concentration has never exceeded 20 parts per billion 2) A large proportion of the diesel fuel was removed by overexcavation of the former UST cavity in 1989 However, approximately 7,000 cubic yards of contaminated soil are estimated to still be present beneath the former tank cavity at depths that are beyond the reach of excavation equipment Contamination starts at a depth of 35 feet (base of the excavation) and extends to a depth of 70 feet directly below the former UST cavity Laterally, the contamination dissipates at shallower depths, depending on the depth to the base of the Modesto channel In most places, diesel fuel migrated downward through this permeable channel until it encountered the impermeable silt and clay that underlies the channel, and the hydrocarbons became bound up in the soil above and below the contact between these two deposits Throughout the impacted area,this contact occurs at a depth of less than 55 feet, and contamination is therefore largely confined to the interval 40-60 feet 3) Few options for remediation of this soil are available Diesel fuel is not amenable to soil vapor extraction because of its low volatility Further, a significant proportion(30-50%?) of the contaminants are locked up within impermeable silt and clay beds, which are not amenable to any type of vapor treatment, regardless of the nature of the contaminant Limited investigation of the abundance of soil microbes was conducted in 1996 to assess the potential for natural attenuation by biological activity, and the results were not encouraging Hence, the relatively low health risks posed by diesel fuel at this depth may not warrant the expenditure of considerable sums of public money to reduce contaminant concentrations 7