Assessing causation of the extirpation of stream macroinvertebrates by a mixture of ions

Abstract

Abstract-Increased ionic concentrations are associated with the impairment of benthic invertebrate assemblages. However, the causal nature of that relationship must be demonstrated so that it can be used to derive a benchmark for conductivity. The available evidence is organized in terms of six characteristics of causation: co-occurrence, preceding causation, interaction, alteration, sufficiency, and time order. The inferential approach is to weight the lines of evidence using a consistent scoring system, weigh the evidence for each causal characteristic, and then assess the body of evidence. Through this assessment, the authors found that a mixture containing the ions Ca þ , Mg þ , HCO À 3 , and SO À 4 , as measured by conductivity, is a common cause of extirpation of aquatic macroinvertebrates in Appalachia where surface coal mining is prevalent. The mixture of ions is implicated as the cause rather than any individual constituent of the mixture. The authors also expect that ionic concentrations sufficient to cause extirpations would occur with a similar salt mixture containing predominately HCO À 3 , SO 2À 4 , Ca 2þ , and Mg 2þ in other regions with naturally low conductivity. This case demonstrates the utility of the method for determining whether relationships identified in the field are causal. Environ. Toxicol.

A method for deriving water‐quality benchmarks using field data

“…This field-based method helps to address some of the difficulties encountered using SSDs derived from toxicity testing when the agent is not amenable to toxicity testing or effects are not measured by standard toxicity tests [8]. Use of field data allowed us to evaluate realistic exposures to mixtures for many species throughout all life stages and complex interactions involving mating, competition, and predation.…”

“…Some considerations are size of the data set, ability to characterize effects on many sensitive taxa, and a range of exposures sufficient to cause a full range of effects. For more discussion, see the section Data Set Selection and Adequacy in the present report and an example in Cormier and Suter [8], this issue.…”

“…Specific conductance, hereafter referred to as ''conductivity,'' was selected as the measurement method because the biological effect is due to the mixture, not a single ion, and because measuring conductivity is fast, inexpensive, and reliable. A literature review [19] and a regional causal assessment [8] were performed to document what was known about the toxicity of the ionic mixture, to ensure that the benchmark would address a true cause, and to communicate the extensive but dispersed scientific knowledge about the causal relationship. The causal assessment described sources of bicarbonate and sulfate ions, mechanisms of action, and specific effects.…”

“…The causal assessment described sources of bicarbonate and sulfate ions, mechanisms of action, and specific effects. It showed that high conductivity causes extirpation of sensitive species [8]. Conceptual models of the source to effect pathways were provided along with a literature review [19].…”

“…Approaches using field data have properties that are advantageous [6,8]. The option of using field data is permitted by the European Water Framework Directive [3] and has been recommended by the U.S. Environmental Protection Agency (U.S. EPA) Science Advisory Board [9].…”