Changes in copper water-effect ratios in toxicity tests conducted at varying water hardness levels

Side-by-side acute toxicity tests (Oncorhynchus mykiss and Ceriodaphnia dubia) were conducted in SITE waters collected from a western Montana river and in laboratory waters reconstituted to match the hardness and alkalinity of the SITE water samples. Tests were conducted according to USEPA guidance on the determination and use of Water-Effect Ratios (WER). Multiple WERS, calculated as the ratio of LC50 values from SITE and LAB water tests, were regressed against water hardness and provided significant correlations (r{sup 2}=0.7 to 0.8 (logarithmic)). WERs increased with decreasing water hardness ranging from 1.1 at 282 mg/l hardness to 8.9 at 60 mg/l hardness. Additional tests showed that WER values for single water samples tested at multiple hardness levels (samples augmented with calcium and magnesium salts) again significantly correlated with water hardness. These results are independent of the known ameliorating effect of water hardness on metals toxicity, since WERs compared LC50s for tests conducted at identical water hardness levels. One explanation for this hardness-WER relationship may be that both hardness and certain other toxicity-reducing water quality parameters (e.g., suspended solids, organic carbon) compete in providing protection from the toxic effects of metals in surface waters. As hardness decreases, the importance of these other watermore » quality parameters may increase, thus enhancing the difference between toxicity in soft laboratory reconstituted waters (lacking organic enrichment or suspended solids) and soft site-waters. These results have potentially important implications for the application of national Ambient Water Quality Criteria (Gold Book values) to soft surface waters moderately enriched in organic carbon, suspended solids, or other toxicity-mitigating factors.« less