You are here:
Individual and community responses in stream mesocosms with different ionic compositions of conductivity and compared to a field-based benchmark
Citation:
Nietch, C., Jim Lazorchak, Dave Mount, AND W. Clements. Individual and community responses in stream mesocosms with different ionic compositions of conductivity and compared to a field-based benchmark. To be Presented at 7th SETAC World Congress/SETAC North America 37th Annual Meeting, Orlando, FL, November 06 - 10, 2016.
Impact/Purpose:
Evaluate stream mesocosm study results within the context of laboratory and field results to help inform the utility of aquatic life benchmarks for conductivity
Description:
Several anthropogenic activities cause excess total dissolved solids (TDS) content and its correlate, specific conductivity, in surface waters due to increases in the major geochemical ions (e.g., Na, Ca, Cl, SO4). However, the relative concentrations of major ions varies with the source, and single-species tests have revealed that ion compositional differences can affect their toxicity. Within the central Appalachian region of the U.S., oil and gas production and surface coal extraction activities are two sources of excess TDS for surface waters; releases from the former are typically dominated by Na, Ca, and Cl, and the latter by Ca, Mg, SO4, and HCO3. A stream mesocosm study evaluated the effects of excess TDS from both discharge types against a natural background chemistry by dosing two recipes mimicking each type of excess TDS over a 56d period. Hazard concentrations (HCs) for specific conductivity from EC50 response sensitivity distributions derived from the mesocosm data differed, with HC5 estimates of 792 µS/cm (685 to 933, 95% CI) for the simulated oil and gas produced water excess TDS, and 434 µS/cm (338 to 584, 95% CI) for the recipe simulating streams receiving surface coal mining discharges. While the mesocosm study was originally intended to evaluate TDS limits in NPDES permits, it also serves a dual purpose to evaluate the field based conductivity benchmark of Cormier et al. The benchmark, circa 300 µS/cm, was derived from analyses of benthic samples taken from central Appalachian streams, and was meant to protect communities exposed to ion mixtures comparable to the ionic composition of the surface coal mining recipe dosed to the mesocosms. As the lower 95% confidence interval of the HC5 estimate for the coal mining recipe was 338 µS/cm, these results support the field benchmark as protective for the specific conditions for which it was developed. In keeping with the applicability restrictions imposed by Cormier et al however, the higher HC5 value from the mesocosms exposed to the oil and gas produced water recipe highlights how important it is to keep the context for the benchmark in mind when evaluating risks of discharges with different ionic compositions or for different receiving waters.
