Watershed scale chloride storage across a gradient of urbanization

Abstract:

Sodium chloride is the main de-icing agent used during the winter in Canada and the northern United States. However, little is known about the long term fate, residence time, and ecological effects of chloride. This talk integrates work taking place across three sites in Southern Ontario, Canada: Hamilton Harbour, the Toronto lakeshore, and Lake Simcoe. We quantify chloride inputs, outputs, and changes in storage for a number of watersheds across a gradient of urbanization. For the three winter months (January, February, March), we show that stream water chloride concentrations approach those of brackish waters for urban watersheds. Chloride is also highly persistent, with stream water chloride concentrations decreasing from the winter months and approaching baseline levels only in July. These baseline levels are greater than 100 mg Cl/l in the urban watersheds, suggesting high levels of chloride storage in soil and groundwater. Using road salt application rates and groundwater levels and chloride concentrations, we estimate the magnitude and residence time of the chloride pools in a number of watersheds across a gradient of urbanization. Our results suggest that the magnitude and residence time of chloride storage varies with urbanization and other factors. We show that summer baseflow concentrations do approach the EPA’s chronic exposure guideline of 230 mg Cl/l, implying more work is needed to understand the in stream and downstream ecological effects of chloride.