Although the Clean Water Act does not specify an approach for identifying high quality waters, EPA provides guidance on two approaches that are generally in use.

Parameter-by-Parameter Approach: Pollutant Specific

This approach to identification of high quality waters is typically conducted as part of the permit review process. The basis for the determination can be chemical data as well as biological data or indices. The approach involves the following steps:

This increment — termed the "assimilative capacity" of the water body — is considered the resource to be protected.

Waterbody-by-Waterbody Approach: Combination of Characteristics

This approach to identification of high quality waters is usually based on monitoring data collected either prior to or as part of the permit review process. This so-called "designational approach" involves the following:

• Determine whether a combination of chemical, biological, or other parameters distinguish the water body as high quality.

The State or Tribe's determination about whether to authorize a degradation of high quality waters should involve the following questions:

Is it necessary?

That is, are cost-effective alternatives available that would eliminate or reduce the reduction in water quality?

Does the activity associated with the discharge support important economic or social development?

That is, what benefits are forgone if the reduction in water quality is not allowed?

That is, what benefits are forgone if the reduction in water quality is not allowed?

Has the public been involved?

Public participation—as well as intergovernmental review—generally takes place through comment on the draft permit.

Public participation—as well as intergovernmental review—generally takes place through comment on the draft permit.

Will existing uses be protected?

Degradation can only be allowed if existing uses are protected.

For example, assume a State or Tribe has classified a water body for "fishable/swimmable" uses and has identified a criterion of 5.0 mg/L for dissolved oxygen (DO) as protective of that use. If the DO level is subsequently found to be better than identified as protective for the use, the State/Tribe could consider allowing a use that would degrade water quality back to the criterion level. However, it could not allow the level of DO in the water body to degrade any further than that established by the criterion. Also, the State or Tribe must follow its established procedures when allowing the use.

Significance of Discharge

Some States/Tribes do not require an antidegradation review if the lowering of water quality will be "de minimis"—that is, if the reduction would be so small as to be considered insignificant. Such a policy can enable States/Tribes to focus resources on questions involving the potential for more significant degradation.

Key Point. For point sources, the State/Tribe must ensure that the most stringent technology-based controls required by statute and regulation will be implemented.

Key Point. The danger of such a policy can be the potential cumulative effect on a water body’s assimilative capacity of allowing a number of de minimis degradations of the same pollutant. Hence, EPA recommends that where de minimis exclusions are allowed that a cumulative cap on these degradations be applied to a waterbody's assimilative capacity.

Review of Point and Non-point Sources

Generally, antidegradation reviews for high quality waters take place when a point source discharger requests a new National Pollutant Discharge Elimination System (NPDES) permit or an increase in allowable loadings under an existing permit. The discharger generally submits the analyses needed for the State/Tribe to weigh considerations and make a determination.

Key Point. For point sources, the State/Tribe must ensure that the most stringent technology-based controls required by statute and regulation will be implemented.

Because the Clean Water Act (CWA) does not authorize regulatory non-point source control programs, EPA cannot require States/Tribes to implement such programs. Nonetheless, where States/Tribes have established an approach for evaluating non-point sources, the process must be followed before allowing degradation of high quality waters.

Key Point. For non-point sources, the States/Tribes with a program should ensure that reasonable and cost-effective best management practices for control will be implemented.

Resource. Enforceable State Mechanisms for the Control of Nonpoint Source Water Pollution (Environmental Law Institute, 1997) (Note: 1998 and 2000 editions also extant.)

If the antidegradation review involves the dredging or filling of a wetland, Section 404 of the CWA requires that any permit issued be consistent with the State or Tribe's water quality standards and antidegradation policy.

The U.S. Army Corps of Engineers reviews Section 404 discharge permit applications to avoid "significant degradation" of wetlands, as defined by the regulations (40 CFR 230.10(b))

Resource. Wetlands Fact Sheets

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