Water Quality Standards

Potomac River Basin Drinking Water Source Protection Partnership

The Clean Water Act (CWA) is the primary federal law in the U.S. that protects the health of our nation’s waters, including rivers and reservoirs. The CWA requires that ambient water quality criteria be established to protect functions that each water body is intended to support. In the CWA, these are referred to as “designated uses.” Water Quality Standards (Standards) form the basis for controls on the amount of contaminants entering waters from sources such as industrial facilities, wastewater treatment plants and storm sewers. A standard can consist of either numeric or narrative limits for a specific physical or chemical parameter. Water quality standards are developed to help protect and maintain water quality necessary to meet and maintain designated uses, including public water supply. When water quality standards are adopted to protect public water supplies, these standards can be enforced under state or federal laws to control pollution.

Water quality standards consist of four basic elements:
1. Designated uses of the water body (e.g., public water supply, aquatic life, recreation).
2. Water quality criteria to protect designated uses by limiting chemical constituents that may be present in the water body. The criteria consist of numeric concentrations and/or narrative requirements.
3. An anti‐degradation policy to maintain and protect existing uses and high quality waters.
4. General policies addressing implementation issues (e.g., low flows, variances, mixing zones).

Standards are used in various types of permits (e.g. industrial and municipal discharge permits) and set permit limits based on applicable water quality standards. Water quality standards are also used when assessing conditions of a waterbody. When assessments identify a water body that is not meeting adopted water quality standards, the assessment may lead to a determination of impairment, initiating further action such as a Total Maximum Daily Load (TMDL) or other regulatory procedure aimed at addressing the impairment.

Potomac Basin Designated Uses

Maryland – The Potomac River is designated as a “public water supply” by the Maryland Department of the Environment.

Water quality standards in the Potomac watershed

Learn more about state water quality standards.