This is not currently possible, for two reasons. First, the national data systems for drinking water do not track concentrations of contaminants in drinking water, but rather the frequency with which standards are exceeded. Second, the information on violations is incomplete because not all public water systems fully monitor contaminants or report their monitoring results. We can use information about violations as a surrogate for exposure to unacceptably high levels of drinking water contaminants. We also need to consider information about water systems that do not monitor or report results, because we do not know with certainty whether populations served by these systems are at risk.
Data are available only for public water systems.
Approximately 42 million people are served by private drinking water systems, which are not required to monitor and report the quality of drinking water. We do not have information to indicate whether these people are at risk. The percentage of children served by public water systems that exceeded a Maximum Contaminant Level or violated a treatment standard decreased from 19 percent in 1993 to 8 percent in 1998. Every category of violation decreased between 1993 and 1998. The largest decline was for violations of the microbial contaminants standards.