Predicting the Aquatic Toxicity of Mixtures of the Major Ions: Technical Update – Chronic Exposures and Additional Test Species

Elevated concentrations of mixtures of seven major ions (Ca2+, Mg2+, Na+, K+, Cl-, SO42-, and HCO3-) have become a concern in some freshwater systems. The development of water quality criteria for these constituents has proven to be a challenging endeavor because toxicity of any specific ion has been found to be influenced by both the total concentration of ions in the mixture and, importantly, mixture composition. Use of an integra-tive measure such as TDS or conductivity to evaluate multi-ion toxicity (MIT), provides an indication of the total ion concentration, but fails to address mixture composition. Because of the aforementioned limita-tions of relating aquatic toxicity to specific ion concentrations or TDS/conductivity, we have pursued the problem from a different perspective – by relating toxicity to trans-epithelial potential (TEP), a physiological parameter that is computed based on blood or hemolymph and external major ion concentrations and calibrated model parameter values. This approach has been successfully applied to a limited number of relatively large acute toxicity datasets (C. dubia, D. magna, P. promelas and N. triangulifer) with widely varying ion compositions and, more recently, to much smaller, less robust datasets for species that would ultimately be included in the species sensitivity distribution (SSD). This presentation reviews how the approach has been extended to additional small acute and chronic effects datasets with multiple species for the purpose of testing the existing approach as well as expanding it to additional organisms and exposure durations.