Abstract
The U.S. Environmental Protection Agency (EPA) administers the Safe Drinking Water Act (SDWA) to ensure that the water obtained from a public water supply is safe to drink. Under the SDWA, EPA establishes enforceable maximum contaminant levels (MCLs) for contaminants that may have an adverse health effect and are known or anticipated to occur in water. Occasionally, public water supplies are unable to meet the MCL at the time it becomes enforceable. The SDWA allows public water supplies to apply to the State for a temporary variance or exemption from an MCL(s) as long as the concentration of the contaminant(s) exceeding the MCL(s) does not result in an unreasonable risk to human health. EPA has developed guidance to assist States in determining what level above the MCL presents an unreasonable health risk. In developing this guidance, the toxicity exhibited by each regulated contaminant is evaluated individually. Consideration is also given to the available risk assessments (short-term and long-term) for each contaminant, comparing carcinogenic risks with noncarcinogenic assessments. Other factors that may be considered on a contaminant by contaminant basis include for example, past exposure and expected duration of the variance or exemption period, population sensitivity, and volatilization of the contaminant from drinking water.