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Oregon EPHT: Water Indicator

EPHT Drinking Water Indicator: Maximum contaminants level (MCL) violations and mean concentrations in Community Water Systems.

To see explanations, tables, graphs and maps for each measure, click on the measure name below.

  • Arsenic – MCL violations and mean concentrations by CWS and number of people served. 
  • Disinfection byproducts (DBPs) – MCL violations and mean concentrations of TTHM and HAA5 by CWS and number of people served. 
  • Nitrates – MCL violations and mean concentrations by CWS and number of people served.

Or, view the full EPHT report on Drinking Water Contaminants (pdf).


Information about Indicators: Indicators provide information about a population’s health status with respect to environmental factors when clear measurable links are not available. As part of the nationwide EPHT network implementation, work groups developed indicators and measures. Guidelines were established for the collection of data and the calculation of measures in a nationally-consistent way (called Nationally Consistent Data and Measures, or NCDMs).

Previous indicator initiatives have focused on public water system compliance at the national level, but few have examined using state-specific contaminant data to track trends and to integrate environmental information with health effect data for potential relationships between drinking water quality and human health. EPHT surveillance of drinking water aims to improve the availability of quality data, identify priority issues and contaminants, and develop surveillance measures that are consistent with national goals. Ultimately, tracking drinking water contaminants will help practitioners improve decisions that protect public health.

The primary factors considered in developing drinking water indicators were the epidemiologic and toxicological evidence supporting an environmental exposure/health link; the uniformity of data collection within states and across the country; uniqueness of the contaminants’ chemical and physical properties; and the occurrence in drinking water more frequently at levels which may be of public health significance.   Considering all the factors, it was determined that valid and reliable measures could be generated for disinfection byproducts (DBPs), arsenic and nitrates in community water systems (CWS).

The measures used to estimate the potential populations exposed to these contaminants examine size and proportion of populations served, and the contaminants concentrations in the water systems. These measures together indicate the potential for public health impacts from contaminant levels of concern.

For more information about environmental health indicators from the CDC:

Calculations:  The drinking water data used in the Tracking Network are gathered as part of the water quality monitoring requirements set out by the EPA and state agencies.   The level of the contaminant is measured in two ways: in comparison to a benchmark level, such as the maximum contaminant level (MCL), and in terms of average or maximum contaminant concentrations. Community water systems represent non-transient public water systems that serve at least 15 connections or provide year round service for at least 25 people. 

Time period:  Calculation of these measures includes data collected as early as 2002, the first year during which representative data were collected for all of the contaminants of interest. 

Data sources:  Data were provided through the Safe Drinking Water Information System (SDWIS) of the Oregon Health Authority (OHA) Drinking Water Program: