Predicting the Aquatic Toxicity of Mixtures of the Major Ions: Application to Additional Test Species

Elevated concentrations of the seven major ions (Ca2+, Mg2+, Na+, K+, Cl-, SO42-, and HCO3-), usually not considered toxic, have increasingly become a concern in freshwater ecosystems. The development of water quality criteria for major ions has proven to be a challenging endeavor because toxicity of any specific ion has been found to be influenced by both the concentrations of other dissolved ions as well as by the overall mixture composition. The use of an integrative measure such as TDS or conductivity to evaluate multi-ion toxicity (MIT), on the other hand, provides an indication of the total concentration of the ions but still fails to address the mixture composition. Because of the aforementioned limi-tations of relating aquatic toxicity to specific ion concentrations or conductivity, we have pursued the problem from a different perspective – by relating toxicity primarily to trans-epithelial potential (TEP). This physiological parameter is computed based on internal and external major ion concentrations and calibrated model parameter values. This approach has been successfully applied to a few relatively large acute toxicity data-sets (C. dubia, D. magna, P. promelas and N. triangulifer) with widely varying ion compositions. Such datasets are limited in number and, as a result, it is necessary to also utilize smaller, less comprehensive datasets for model development. This presentation will describe an approach that has been used to incorporate these smaller data sets in the TEP model. The approach will be illustrated using results obtained with data for spe-cies not previously evaluated to date. We will also describe how the MIT approach might be extended to chronic toxicity datasets.