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Not all questions fit in beakers - direct and indirect toxic effects of metal mixtures and the application of ecotoxicological experiments to derive better water quality standards and predict recovery after abandoned mine reclamation




Cadmus, Peter, author
Clements, William H., advisor
Kondratieff, Boris C., committee member
Winkelman, Dana L., committee member
Ranville, James F., committee member

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Aqueous discharges from abandoned metal mines include complex mixtures of physical and chemical stressors. Consequently, identifying mechanisms and causal relationships between acid mine drainage (AMD) and community responses in the field is challenging. In addition to the direct toxicological effects associated with elevated concentrations of metals and reduced pH, mining activities influence aquatic organisms indirectly through physical alterations of habitat, including increased sedimentation, turbidity and substrate embeddedness. Although direct toxicity can sometimes be effectively studied in the laboratory, the indirect toxicity of toxicants rarely manifests into a measurable endpoint in the short duration and limited ecological realism of traditional laboratory toxicity experiments. The installation of a mine effluent treatment plant near Blackhawk Colorado (USA), had potential to remove the majority of aqueous metals from a mountain stream heavily degraded by Acid Mine Drainage (AMD). To investigate direct and indirect effects of Acid Mine Drainage (AMD) a series of field biomonitoring, field experiments, and mesocosm experiments were conducted. These studies quantified the relative importance of chemical (direct) and physical (indirect) stressors associated with AMD discharges and predicted recovery potential for dominant macroinvertebrate taxa. Ferric Fe is often a dominant toxicant present in AMD but is largely believed to be non-toxic to aquatic life. Results of toxicity tests reported here suggest that the current USEPA chronic Fe criterion is underprotective and that the current criterion should be reduced to 25% of its current level (251 µg/L). These studies demonstrated additional risk to aquatic insects and periphyton in metal mixtures that included ferric Fe. Responses were primarily a result of indirect physical effects associated with Fe oxide deposition rather than direct toxicity. All aquatic insects hatch as nearly microscopic organisms and small size classes were consistently the most sensitive in numerous experiments. Sampling small age classes in nature and conducting toxicity trials with small age classes is difficult and therefore these studies are lacking from the scientific literature. Failure to characterize sensitivity of early size classes may lead to gross overestimation of tolerance. Mesocosm experiments conducted using natural benthic communities provide a unique opportunity to quantify the relative importance of these indirect physical effects.


Per author permission, embargo lifted January 3, 2020. ac.

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benthic macroinvertebrates
acid mine drainage
mesocosm experiments


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