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INITIAL RELEASE DDJC Tracy Conc. = Maximum concentration in the water(ppb) CF = Conversion factor: 10-3 IR = Ingestion rate: adult= 2 liters per day; child= 1 liter per day EF = Exposure frequency, or number of exposure events per year of exposure (7 days per week x 50 weeks or 350 days per year) ED = Exposure duration, or the duration over which exposure occurs: adult= 30 years; child = 6 years BW = Body weight. adult= 70 kilograms(kg) (154 pounds); child= 16 kg (35.2 pounds) AT = Averaging time, or the period over which cumulative exposures are averaged: 30 years or 6 years x 365 days The estimated exposure doses calculated are conservative estimates and may overestimate actual doses received by private well users. Actual doses associated with exposure to water in the in private wells is expected to be less than estimates presented above, based on the following reasons: • The dose is estimated using the maximum contaminant concentration present in private well. The actual concentration present in private wells, if any, over the possible exposure duration is likely to be much lower. • The exposure frequency of 350 days per year is extremely conservative, particularly when assuming that residents used drinking water containing the highest concentrations on a daily basis. • Similarly, the exposure duration is extremely conservative. The exposure duration used to estimate the exposure dose was 30 years for adults and 6 years for children. While 30 years has been determined as the upper-bound estimate for the length of time a resident could have occupied a house in this area, it may overestimate the length of exposure. Likelihood of Health Effects From Ingestion of Private Well Water Noncancer Carbon tetrachloride was detected at 1.8 ppb and above ATSDR's comparison value. Although the source of carbon tetrachloride is not known with certainty, it is believed to be unrelated to the DDJC Tracy site. As a prudent health measure, however, ATSDR evaluated the health significance of drinking water containing carbon tetrachloride. In evaluating exposure, ATSDR assumed that a 70-kg adult drinks 2 liters and a 16-kg child drinks 1 liter of private well water per day. Because it is also not known with certainty for how long or to what levels people might have C-3