At the request of U.S. Environmental Protection Agency's (EPA) Office of Water and Regions, the EPA Office of Research and Development has developed an aquatic life benchmark for conductivity for the Appalachian Region. The benchmark is applicable to mixtures of ions dominated by salts of Ca(Sup 2+), Mg(sup 2+), SO(sub 4)(sup 2-) and HCO(sub 3)(sup -) at a circum-neutral to alkaline pH. The impetus for the benchmark is the observation that high conductivities in streams below surface coal mining operations, especially mountaintop mining and valley fills, are associated with impairment of aquatic life. However, application of the benchmark is not limited to that source. The benchmark was derived by a method modeled on the EPA's 1985 methodology for deriving ambient water-quality criteria for the protection of aquatic life. The methodology was adapted for use of field data, by substituting the extirpation of stream macroinvertebrates for laboratory toxicity data. The methodology and derivation of the benchmark were reviewed by internal reviewers, external reviewers, and a review panel of the EPA's Science Advisory Board (SAB). The SAB panel's review was in turn reviewed by the Chartered SAB. The SAB concluded that the benchmark is applicable to the regions in which it was derived and the benchmark and the methodology may be applicable to other states and regions with appropriate validation. In addition, hundreds of public commenters provided their views. Comments from all of these sources were considered and used to improve the clarity and scientific rigor of the document.