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Evaluating Contaminant Occurrence and Exposure Data for Six-Year Review of Drinking Water Standards

The term "contaminant occurrence" means the locations where a contaminant has been found in drinking water and the levels at which the contaminant is found. Pertaining to Six-Year Review, EPA’s goal in evaluating contaminant occurrence and exposure is to:
  • Estimate the number of public water systems in which contaminants occur at levels between the existing MCL and a new potential MCL, or other threshold value, based on health effects or analytical methods information, and
  • Evaluate the number of people potentially exposed to these levels of interest.

This occurrence and exposure information indicates how changing an MCL or required "treatment technique" may affect health risks of water consumers and impact compliance costs for public water systems. For example, if the Agency were to lower an MCL for a particular contaminant, the occurrence analysis can estimate the additional number of water systems that will have to incur costs to treat or otherwise reduce the level of the contaminant in drinking water. Determining the number of people currently exposed to these levels of contaminant occurrence can help determine if a meaningful opportunity exists for health risk reduction by modifying a drinking water regulation.

To obtain contaminant occurrence data, EPA may issue an “Information Collection Request (ICR)" as a one-time request for state drinking water regulatory agencies to submit historical drinking water system monitoring data for a specified period of time. This data represents the national occurrence of regulated contaminants in public drinking water systems.

EPA may also analyze available source water data obtained from the United States Geological Survey (USGS) National Water Quality Assessment (NAWQA) database and EPA’s STORET database.

Read more about Six-Year Review occurrence data.