A Novel Approach to Developing Thresholds for Total Dissolved Solids Using Standardized and Experimental Toxicity Test Methods.

The increasing salinization of freshwater streams from anthropogenic land uses and activities is a growing global environmental problem. Increases in individual ions (such as sodium or chloride) and combined measures such as total dissolved solids (TDS) threaten drinking water supplies, agricultural and economic interests, and the ecological health of freshwater streams. Because the toxicity of high ionic strength waters depends on the specific ion composition, few water quality standards exist to protect freshwater streams from salinization. In the present study, we used a novel approach to develop site-specific and ecologically relevant TDS thresholds for the protection of aquatic life. The first step of the approach was to characterize the ion composition of the waterbody or region of interest and prepare artificial samples to match that composition. Using a combination of standardized toxicity test species and more ecologically relevant field-collected species, toxicity tests were then conducted on these artificial samples prepared at a range of TDS concentrations. The advantage of this approach is that water quality criteria can be developed for easy-to-measure generalized parameters such as TDS while ensuring that the criteria are protective of instream aquatic life and account for the complex interactions of the various ions contributing to salinization. We tested this approach in Sand Branch, Loudoun County, Virginia, USA, where salinization from hard rock mining and urban runoff has impaired aquatic life. Acute and chronic TDS thresholds of 938 and 463 mg/L, respectively, were developed in this stream and used for total maximum daily load development in the watershed. The approach provides a potential model for establishing protective thresholds for other waterbodies impacted by salinization. Environ Toxicol Chem 2022;41:2782-2796. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.