EPA Ambient Water Quality Criteria (Aquatic Life)

The criteria are scientific recommendations rather than direct regulations; states and tribes adopt them, often with local modification, into their enforceable water-quality standards. The parameters and their criteria are given in the limit table on this page.

State and tribal environmental agencies use the criteria as the scientific basis for their surface-water standards, which in turn govern discharge permits, impaired-water listings, and cleanup plans. Industrial and municipal dischargers must keep their effluent within the resulting limits, and consultants, researchers, and watershed groups measure rivers, lakes, and estuaries against the criteria to judge ecological health. Anyone assessing whether a water body can support fish and other aquatic organisms (regulators, permit holders, and scientists alike) works from these values.

The core aquatic-life parameters (dissolved oxygen, pH, temperature, and conductivity) are exactly those a multiparameter water-quality sonde measures continuously in situ. Dissolved oxygen is often the decisive one, because it swings over a day-and-night cycle and can crash below the criterion during warm, low-flow conditions that a single daytime grab sample would miss. A sonde on a Thiamis gateway captures these diurnal patterns and short-lived excursions against the values in the limit table on this page, while toxic pollutants such as metals, ammonia, and pesticides are quantified by laboratory analysis of collected samples. Continuous monitoring reveals when and why conditions breach a criterion; laboratory work supplies the definitive figures for the substances a sonde cannot measure.