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• 3. 3. 2 Ground-water Exposure The major pathway for potentially harmful human exposures to contaminated ground water is contact with water from domestic wells downgradient of the site. Use of potable waters can result in inhalation, ingestion, and dermal intake of contaminants. For the Stockton site, total daily intakes from these various routes have not been estimated because the drinking water criteria were designed to cover all potential routes. The criteria are based on a person ingesting 2 liters of untreated tap water per day (EPA 1985) . This level greatly overestimates the typical daily intakes of tap water, 0 . 1-0.2 L per day (ICRP 1978) . Thus the assumption of ingesting 2 liters is very conservative and does not suggest a need to calculate intakes more specifically. 3.4 RISK CHARACTERIZATION During the excavation of underground storage tanks at former Chevron Service Station 9-7780, soil and ground-water hydrocarbon contamination were discovered. The extent of soil contamination cannot be wide, since soil samples taken from the area surround- ing the tank pit did not have detectable concentrations of petro- leum hydrocarbons (Table 3) . In addition, the highest measured soil concentrations of toluene and xylenes were 2 orders of magnitude below STLC values calculated using the method developed by the Central Valley Regional Water Quality Control Board. The level of benzene in the contaminated tank pit was above the STLC, through volatilization, as indicated by the soil pile contaminant analysis done by GTI (Bane and Taggart 1987: Table 3) . Most of the contaminated soil was probably excavated and placed on the soil surface to aerate. Five months after excavation, the levels of hydrocarbons in the aerated soil were below detection limits. This diminution of concentrations is probably the result -- - of volatilization. The levels of contaminants in any unexcavated soils have also probably decreased, since the excavation pit has i 22