Ambient Water Quality Criteria

Ambient water quality criteria are numeric or narrative limits for pollutants in receiving waters set to protect aquatic life and human uses, distinct from end-of-pipe effluent limits. In the United States, the EPA publishes recommended criteria under Clean Water Act section 304(a), which states adopt into enforceable water quality standards alongside designated uses and antidegradation policy. Criteria for aquatic-life protection typically pair a criterion maximum concentration (acute) with a criterion continuous concentration (chronic), each with averaging periods and allowable exceedance frequencies. Criteria exceedance places a water body on the section 303(d) impaired list, triggering a total maximum daily load. Bacteria, nutrients, dissolved oxygen, and metals are common criteria parameters.