What distinguishes ambient water quality criteria from enforceable water quality standards, and how do they relate to permits?

• A. Standards are guideline values; ambient criteria are legally enforceable thresholds; permits are designed to enforce those standards.
• B. Criteria are guideline values; standards are legally enforceable thresholds; permits set limits to meet standards and protect designated uses.
• C. Ambient criteria and enforceable standards are identical in effect and are both used to set emission allowances directly.
• D. Criteria specify water uses only, while standards specify numeric limits for all pollutants.

Answer: (B)

Ambient water quality criteria are scientifically derived guideline values that indicate what water quality is needed to protect designated uses of a water body. They are not legally binding on their own.

Water quality standards are the legally enforceable thresholds that states (with EPA approval where required) establish to protect those designated uses. They set the actual limits and conditions that must be met.

Permits connect these concepts by translating standards into enforceable requirements for dischargers. A discharge permit specifies effluent limits and monitoring so that a facility’s releases do not cause or contribute to violations of water quality standards and thereby protect the designated uses of the water body. Criteria help shape and inform the development of standards, and the standards, in turn, drive the limits that permits enforce.