Community Drinking Water Indicators
On this website you will find data and information about the levels of four contaminants in drinking water: nitrate, arsenic, disinfection by-products, and lead. These contaminants were selected for the Tracking Network because they occur more frequently in drinking water at levels that may be of public health significance. The contaminants also have a range of natural and man-made sources.
The drinking water data used in the Tracking Network are gathered as part of the water quality monitoring requirements set out by the U.S. Environmental Protection Agency (EPA) and state agencies. They are not gathered specifically to assess the level of exposure or to track changes in water quality over time. However, these data are the only set of consistent drinking water quality data nation-wide. The Tracking Network has adapted these data for public health uses.
The main issues considered in the development of the contaminant-specific measures presented on this Web site included the following:
Exposure patterns: Summary measures of concentration (for example, the annual average vs. the annual maximum) were chosen for each contaminant based on the type of exposure and health-effect that typically occurs. For example, the average concentration each quarter (3-months) made the most sense for disinfection byproducts because its concentration can change quickly and it can have short-term effects during pregnancy. In contrast, the maximum annual concentration for nitrates was considered an appropriate level because a high concentration exposure can have acute effects on infants.
Monitoring requirements / sampling frequency: Each of the four contaminants on this website has different sampling requirements in terms of frequency and location of sampling. Monitoring requirements are based on the potential for contamination. A community water systems is sampled more frequently if the possibility of a contaminant occurring in the water at unhealthy levels is high. Sampling frequency also depends on the source water type, treatment type, historical levels, and sometimes, the size of the population served. Routine monitoring requirements can vary from quarterly (high possibility of contamination) to once every nine years (low possibility of contamination). Sometimes it is important to understand the monitoring requirements when interpreting the summary measures presented on this Web site.
Censored values: Censored values refer to contaminant concentrations that are below limits of detection for a laboratory analysis method, or below a certain reporting level. The amount of sample concentration results censored varies by contaminant and laboratory methods, and can range from 10-90%. Methods for handling censored values were carefully considered and defined for each measure.
Reference or benchmark levels: Several measures use comparisons to commonly-used benchmarks, such as the Maximum Contaminant Level (MCL). However, the public health significance of reference levels varies by contaminant. For example, MCLs are developed to take into consideration both a risk based value, as well as how feasible and practical it is to treat water so that the contaminant concentration is lowered.