USING A WATER-EFFECT RATIO APPROACH TO ESTABLISH EFFECTS OF AN EFFLUENT-INFLUENCED STREAM ON COPPER TOXICITY TO THE FATHEAD MINNOW

Water-effect ratio (WER) testing, examining the relative toxicity of c opper in various dilutions of effluent and stream water as compared wi th laboratory dilution water, was conducted during January, April, and June, 1995, to assess the potential of seasonal and flow effects on s ite water quality and toxicity of copper to the fathead minnow. Total organic carbon (TOC) and dissolved solids were significantly related t o effluent concentration in the site water (R-2 = 0.92, p < 0.01), res ulting in higher copper median lethal concentrations (LC50s) and highe r WERs as effluent contribution in site water increased. Total recover able copper LC50s were similar to dissolved copper LC50s in laboratory and upstream water tests (p > 0.10), while effluent and stream water tests had significantly higher total recoverable than dissolved copper LC50s (p < 0.01), suggesting more solids or complexing agents when ef fluent was present. The LC50 and WER for upstream water were significa ntly higher in April than in January (p < 0.05), consistent with the h igher TOC, alkalinity, and lower stream flow observed during April. Th e WER tests, using different proportions of upstream water and effluen t (tests of additivity) in both January and April, indicated that tota l recoverable copper WER was predictable under a variety of stream flo w conditions. Dissolved copper WERs were less predictable, in part bec ause effluent and upstream water WERs were similar, particularly in Ap ril. Low-flow stream conditions (when effluent concentration was great est) appeared to be the limiting condition in terms of greatest copper toxicity at this site. Furthermore, winter low-flow conditions appear ed to be more limiting (less water effect on copper toxicity) than sim ilar or even lower flows in spring (April) or summer (June). This was probably due to the higher TOC and dissolved solids present in upstrea m water in the warmer seasons. All analyses indicated that copper was at least five times less toxic in the effluent-influenced stream water than in typical laboratory test dilution water. Our data indicate tha t seasonal effects on water quality, as well as stream flow, can be im portant in determining limiting conditions on copper toxicity in efflu ent-dominated stream systems.