Water Quality Standards adopted by the states include both designated uses and criteria to protect those uses.
Drinking water is one of the uses that can be protected.
States and Tribes describe water quality in terms of a waterbody either fully supporting, threatened, partially supporting, or not supporting the drinking water designated use. The percent of assessed waterbodies in a watershed meeting their drinking water designated use is a partial proxy for the condition of the source waters used by drinking water systems. It is an indicator of how well drinking water uses are protected by surface and ground water quality standards promulgated through the Clean Water Act. The data set containing State assessments for drinking water use is incomplete either because some States did not designate or assess their waters for drinking water use, or they assumed that other uses that were assessed served as surrogates for drinking water use attainment. Drinking water assessments are inconsistent across states.