Application of the water-effect ratio for site-specific copper criteria in the Clark Fork River and tributaries

Copper Water-Effect Ratios (WER -- a numerical representation of the relative toxicity of a metal when tested side-by-side in a natural surface water and in a laboratory reconstituted water) were determined for rainbow trout (RBT) and Ceriodaphnia dubia at multiple sites on the Clark Fork River, Montana and several of its headwater tributaries (i.e., Silver Bow Creek, Warm Springs Creek, Willow Creek, and Mill Creek). Test procedures were in accordance with recent (1994) guidance on the development and use of WERs for metals and were intended to provide the necessary data for derivation of site specific copper criteria in the waters sampled. Water samples were collected at 13 field sites during four events (characterized by different seasonal/flow conditions). WERs ranged from 1.1 to as high as 8.9, depending on test species, sampling location, and sampling time. Elevated concentrations of apparently non-toxic (to rainbow trout) background copper in some samples resulted in higher WERs for these sites. Also, WERs tended to increase with decreasing water hardness. Study results indicate that national AWQC overestimate copper toxicity at the sites surveyed; although WERs will be variable across sites characterized by differing water quality parameters. Results of this testing program were consistent with thosemore » of previous studies investigating WERs at sites further upstream on the CFR. Mean copper WERs for the section of the CFR bracketed by those studies were 3.04, based on dissolved copper, and 4.60, based on total recoverable copper.« less