Water Quality Monitoring


By Section 305(b) of the Clean Water Act (CWA), states, territories, and tribes must assess waters every two years to determine whether they are meeting water quality standards (WQS). States issue a biennial 305(b) report that includes results from these assessments and explanation of assessment methodologies and data collection processes.

Waters that are identified through the assessment as not achieving water quality standards (in other words, too polluted to support the water’s uses) are called “Impaired” and enter into a list of Impaired Waters under CWA Section 303(d). Waters that achieve water quality standards but are predicted to violate standards by the time the next 303(d) list is completed are considered “Threatened.” Based on the 303(d) list, state clean water agencies are required to develop priority restoration rankings for waterbodies and develop Total Maximum Daily Loads (TMDL(s)) for these waters. TMDLs form the basis for efforts to restore impaired water bodies.

How can I use water quality monitoring and assessment process to protect drinking water sources?

States and tribes have discretion in prioritizing water bodies for monitoring of water quality and for TMDL assessment and development. A state may prioritize a water body that is designated as public drinking water supply for monitoring, assessment, and, if necessary, listing as in impaired water body under section 303(d) and subsequent protections through the TMDL program. Even if a water body is not designated for use as a public drinking water supply, stakeholders can encourage states to prioritize monitoring and assessment of their drinking water sources and to use assessment methodologies that specifically monitor for contaminants of concern to drinking water sources. Stakeholders are encouraged to collect and submit substantive water quality data to state authorities to help address gaps in water quality data collected by CWA program, and identify and characterize threats to and impairments of drinking water sources.

  • Encourage states to prioritize monitoring in waters that are public drinking water supplies
  • Encourage states to use assessment methodologies that include monitoring of contaminants of concern to drinking water
  • Collect and share substantive water quality data for better assessment of source water quality

Opportunities for involvement

There is ample opportunity for public involvement during state water quality assessment processes. Stakeholders should submit comments and/or data after release of Draft Assessment Methodologies and during state Triennial Review of Water Quality Standards processes. Watch for other public notices from your state environmental or water quality agency. Find more information at your state environmental agency website, including resources for collecting and sharing data, found here.

For further discussion on protecting drinking water through the Clean Water Act, see Opportunities to Protect Drinking Water and Advance Watershed Goals through the Clean Water Act: A Toolkit for State, Interstate, Tribal, and Federal Water Program Managers.