There are a number of state and federal agencies calling for development of aquatic life criteriaor thresholds for conductivity, including a field data-derived benchmark of 300 μS/cm that wasrecently proposed for central Appalachian streams chemically dominated by sulfate andbicarbonate
salts. Such directives are based on observed correlations between conductivity andbenthic macroinvertebrate community composition. However, development of a true aquatic lifecriterion for a composite variable like conductivity is made difficult by a number of factors. Forexample, consistent
stressor-response data are typically required to derive regulatory thresholdsusing species sensitivity distributions – i.e., organisms should have a consistent response to atoxicant, only differing in sensitivity. In addition, other states have evaluated this issue anddetermined that
a composite variable, like conductivity or TDS, is not appropriate for criteriadevelopment, as toxicity varies strongly as a function of specific ion composition and ispotentially mitigated by elevated hardness. Finally, our independent statistical analyses of theWest Virginia Watershed Assessment
Branch Database (WABbase) used to create thisbenchmark indicate there is a real possibility that observed patterns of invertebrate communitycomposition versus conductivity may, in fact, be related to a combination of abiotic (ioniccomposition, habitat, temperature, flow, channel form, etc)
and biotic (competition, predation,life history patterns) factors – potentially adding an insurmountable level of confounding. Weconclude that the relationships between conductivity and changes in benthic macroinvertebratecommunity composition are neither strong, consistent, nor reliable
enough to warrant derivationof a criterion at this time.
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