An Ecotoxicological Assessment of a Treated Coal-mining Effluent in the Leading Creek Watershed, Meigs County, Ohio
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Further study using strength of evidence, regression analysis and manipulation of laboratory formulated media, indicated that the mine effluent was the primary causal agent of the observed biotic impairment, and its toxicity could be attributed to sodium/sulfate-dominated TDS, which is significantly ameliorated by water hardness. Finally, although testing with lentic cladocerans, such as Ceriodaphnia, is consistent, cost-effective and sensitive to TDS related toxicity, the ecological relevance and protective capability of such testing is questionable when assessing contaminant effects on sensitive macroinvertebrates indigenous to lotic systems. A more ecologically relevant laboratory bioassay using the mayfly, Isonychia bicolor, in simulated lotic microcosms provided more sensitive endpoints than Ceriodaphnia and Pimephales promelas. Although the heartiness of Isonychia in laboratory conditions is poorly understood relative to standardized test organisms, these results, along with potential toxic impacts from numerous sodium/sulfate-dominated wastewaters discharging into freshwater systems, may have important implications to future national pollution discharge and elimination system (NPDES) permit testing. Currently, however, strong recommendations can only be made using Ceriodaphnia endpoints. Potential acute toxicity to aquatic organisms in high hardness solution (~790 mg/L as CaCO3) is possible where sodium/sulfate-dominated TDS levels exceed ~7000 uS/cm (5167 mg/L), with potential chronic toxicity occurring at ~3200 uS/cm (2342 mg/L). These endpoints were significantly reduced in solutions of lower hardness (88 mg/L as CaCO3), with acute and chronic toxicity occurring at 5100 uS/cm (3754 mg/L) and ~2100 uS/cm (1523 mg/L), respectively. Point source discharges causing instream TDS concentrations to exceed these levels risk impairment to aquatic life.
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