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Mesocosm Community Response Sensitivities to Specific Conductivity Comprised of Different Major Ions
Citation:
Nietch, C. AND Jim Lazorchak. Mesocosm Community Response Sensitivities to Specific Conductivity Comprised of Different Major Ions. Society of Environmental Toxicology and Chemistry North America 36th Annual Meeting, Salt Lake City, UT, November 01 - 05, 2015.
Impact/Purpose:
Traditional toxicity test assays have been used to evaluate the relative sensitivity to different major ion mixtures as a proxy for understanding what the response of aquatic species growing in their natural environment would be during exposure to specific conductivity or total dissolved solid (TDS) stress emanating from resource extraction activity. To increase the field relevance of these laboratory exposures we have conducted several stream mesocosm studies that pair single-species and community-level responses in-situ to the mesocosms that allow for direct comparison among different ionic mixtures of specific conductance.
Description:
Traditional toxicity test assays have been used to evaluate the relative sensitivity to different major ion mixtures as a proxy for understanding what the response of aquatic species growing in their natural environment would be during exposure to specific conductivity stress emanating from resource extraction activity. To increase the field relevance of these laboratory exposures we have conducted several stream mesocosm studies that pair single-species and community-level responses in-situ and allow for direct comparison among different ionic mixtures of specific conductivity. Dose-response models have been fit to numerous single-species and group-level (e.g., ephemeroptera emergence, total insect drift, periphytic diatom density) effects obtained from 42 day dosing periods imposed on the naturally colonized mesocosms. Significant 25% inhibition concentration (IC25) effects were estimated, and then were collectively ranked to produce a mesocosm response sensitivity distribution for specific conductance for each of five major ion mixtures tested. After selecting an acceptable effect threshold for the response distribution one can compare relative sensitivities among the major ion mixtures. If we select the 20th percentile of the response sensitivity distribution, for example, this is the point that 80% of all the estimated mesocosm IC25 responses would lie above, and, therefore would be less sensitive to. The analysis is akin to the 20 percentile hazard concentration (HC20) derived from a species sensitivity distribution. A preliminary comparison of HC20 specific conductance values among 5 different ion mixtures tested showed that mesocosm responses were most sensitive to excess specific conductivity dominated by magnesium and sulfate ions (HC20 = 588 uS/cm) followed by conductivity from sodium and bicarbonate (HC20 = 695 uS/cm), then sodium and calcium chloride mixtures (HC20 = 1087 uS/cm). These results suggest that stream community responses would be significantly more sensitive to excess specific conductance from coal mining activities that leave leachable spoils rich in carbonate and sulfate salts compared to sources emanating from deep well extraction works, which tend to be dominated by ions from sodium and calcium chloride salts.
