ABSTRACT

Although the U.S. Environmental Protection Agency (U.S. EPA) has not used the term

species sensitivity distribution

(SSD) in its work on water quality criteria for aquatic life, this concept has been important since the agency decided that such criteria should be derived using written guidelines. Prior to the development of written guidelines, aquatic life criteria for the U.S. EPA, such as those in the “Red Book” (U.S. EPA, 1976), were derived using the “ad hoc approach.” The ad hoc approach consisted of reviewing all data available concerning the toxicity of a pollutant to aquatic life and then using the data as deemed best by those selected to derive the criterion for that pollutant. The ad hoc approach allowed substantial inconsistencies among aquatic life criteria regarding how toxicity data were used and regarding the level of protection provided. This approach might also be called the “lowest number approach” or the “most sensitive species approach” because most of the criteria were derived to protect the most sensitive species that had been tested. This approach is usually criticized as resulting in criteria that are too low, but the resulting criteria can be too high if, for example, the most sensitive tested species is not as sensitive as one or more untested important species (Stephan, 1985).

Late in 1977, David J. Hansen at the EPA laboratory in Gulf Breeze, Florida suggested to Donald I. Mount at the EPA laboratory in Duluth, Minnesota that the ad hoc approach for deriving aquatic life criteria for the U.S. EPA should be replaced by an approach based on written guidelines. In the new approach, guidelines describing the methodology to be used to derive aquatic life criteria would be written before criteria were derived so that, to the extent possible, all aquatic life criteria would be derived using the same methodology. The guidelines were intended to provide a systematic means of interpreting a variety of data in an objective, consistent, and scientifically valid manner and were to be modified only if sound scientific information for an individual pollutant indicated the need to do so (U.S. EPA, 1978a). Mount convinced the U.S. EPA to accept the idea of written guidelines and then formed an EPA aquatic life guideline committee consisting of Hansen; Gary A. Chapman at the EPA laboratory in Corvallis, Oregon; John (Jack) H. Gentile at the EPA laboratory in Narragansett, Rhode Island; and Mount, William A. Brungs, and Charles E. Stephan at Duluth.