The indicator species procedure enables modification of a national water quality criterion to account for differences in the biological availability/toxicity of a chemical introduced into site waters. To evaluate this procedure, a case study was conducted with zinc using the Trinity River near Denton, Texas as an example site.

Acute and chronic toxicity tests were performed with Pimephales promelas and Daphnia pulex in both natural river and reference waters. Toxicity results were expressed as a function of both total and soluble metal.

An acute water effects ratio (WER) was used to calculate the site-specific criterion maximum concentration. The site-specific value showed excellent agreement with the national water quality criterion adjusted for the site water hardness when based on total zinc. These findings were altered when toxicity results from a different reference water were used in calculations, but were unaffected if test organisms were not acclimated in site water prior to testing. Three different methods were used to calculate the site-specific criterion continuous concentration, which was found to be up to four times lower than the national criterion. Toxicological differences observed between lab and site waters were reduced when expressed in terms of mean soluble metal.

Additional experiments demonstrated that particulate zinc was not biologically available under test conditions and that soluble zinc may vary in its toxicity. Data also suggest that kinetic factors influence zinc toxicity. The implications of this research for the development of site-specific metal criteria are discussed.