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INITIAL RELEASE DDJC Tracy may not unrelated to the DDJC Tracy, the contamination does not appear to be widespread (Montgomery Watson, 1995). When evaluating health significance of an exposure pathway, ATSDR estimates exposure doses and compares the values to standard health guidelines, such as ATSDR's minimal risk levels (MRLs) or EPA's reference doses (RfDs). These health guidelines provide a conservative estimate of daily exposure to noncarcinogens that are not likely to result in noncancer adverse health effects. A long-term or chronic exposure(365 days or more)MRL or Rfl) has not been established for TCE. Therefore, when evaluating chronic exposure to TCE in the private wells, ATSDR focused on cancer. In evaluating cancer risk for either TCE or carbon tetrachloride, ATSDR used EPA's cancer potency factors (CPFs) to define the relationship between the exposure dose and the potential for cancer to occur. In deriving human exposure doses, ATSDR incorporated information about the frequency and duration of exposure. Because ATSDR does not know with certainty when migrating contaminants first reached the private wells or how much was present during the exposure period, ATSDR made several conservative assumptions. Although one of the two wells was installed as recently in 1988 and residents now use an alternate water supply, ATSDR conservatively assumed exposure occurred over a 30-year period for an adult (EPA's national upper bound limit at one residence) and over 6 years for a child. ATSDR assumed that these individuals drank only water containing the maximum detected level of TCE over the exposure period. ATSDR believes that these assumptions overestimate actual exposure because no one is thought to have been consistently using water with the highest TCE level over a 30-year period. ATSDR also evaluated exposure to carbon tetrachloride in the private wells, although the contamination appears to be unrelated to DDJC Tracy (DDJC Tracy, 19976). ATSDR also evaluated potential health hazards from dermal contact with or inhalation of vapors that might occur during domestic uses. The methods used and assumptions applied in deriving the estimated exposure doses and cancer risk are described in greater detail in Appendix C. The estimated exposure doses and excess cancer risk for ingestion of private well water are provided in Appendix C, Tables C-1 and C-2. The estimated exposure doses for an adult and a child exposed to carbon tetrachloride in private well water are less than the RfD. In addition, the estimated exposure levels for TCE or carbon tetrachloride are not expected to cause excess cancer in the population. Therefore, drinking private well water in the past, or using it in the home (such as for bathing or washing), did not cause harmful noncancer effects for residents, or increase their risk of cancer. I1