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WQS: Key Definitions

To understand the regulations that apply to designating uses under WQS, several key terms must be defined. As noted previously, a designated use is a use specified in water quality standards for each water body whether or not conditions currently support that use. (It might be helpful to think of these as “desired” uses). For example, a water body may be designated by state regulations for “aquatic life support” even though it might not contain a healthy aquatic ecosystem now.

The term existing use has a somewhat different meaning, in the context of the CWA, than one might expect. Rather than actual or current uses, it refers not only to those uses the water body is capable of supporting at present but also any use to which the water body has actually attained since November 28, 1975. Even if the water body is currently not supporting a use attained since November 28, 1975, for purposes of the CWA, it is still an “existing use.” Even if there has been no documentation that a use has occurred since November 28, 1975, evidence that water quality has been sufficient to support a given use at some time since November 28, 1975 can be the basis for defining an “existing use” for a water body.

The process of changing a use designation is called use reclassification. The terms downgrading and upgrading are sometimes used in this context. Removing a designated use and replacing it with a “lower” use is often referred to as “downgrading." “Upgrading” is just the reverse. It is important to note, however, that in the parlance of the CWA, the difference between a “higher” and “lower” use reflects the quality of water needed to support each use. Those uses needing cleaner water are considerably “higher.” The terms “high” and “low” are not intended to suggest that one use of a water body (fishing, for example) is inherently more important than another (industrial water supply, for example). Hence, removing from the designated uses of a water body one that required an average daily concentration of pollutant “x” of 20 mg/L or less, so that the next highest use was one needing concentrations of 30 mg/L or less would be a “downgrading.”