Estimates of Chloride Storage Across a Gradient of Urbanization in Watersheds in Southern Ontario, Canada

English Abstract:

In seasonally frozen environments such as Canada, de-icers (chloride salts) are widely used to maintain safe driving conditions. While the beneficial role of winter salt usage for public safety is unequivocal, recent studies suggest that chloride accumulates in watersheds causing summer baseflow concentrations to approach chronic exposure levels. The potential risks that elevated chloride concentrations pose to aquatic ecosystems drive our need for a better understanding of the movement of chloride through watersheds. In this study, we carry out watershed-scale chloride mass balance estimates for multiple watersheds in southern Ontario that fall along a gradient of urbanization. Chloride inputs to the study watersheds are estimated using municipal, regional and provincial road salt usage data coupled with geospatial information on road length and classification. Watershed chloride outputs are calculated using chloride concentrations measured by regional and provincial authorities and stream discharge measured by conservation authorities and the Water Survey of Canada. While our mass balance calculations are designed to estimate the mass of chloride storage within each watershed they also incorporate significant uncertainty related to inputs of chloride salts for winter maintenance on private properties. To address this uncertainty, we use geospatial analysis of remote sensing imagery to estimate privately-owned impervious areas and apply a range of salt application rates to these areas to determine potential private inputs. The results of our mass balance estimates are used to examine the timing and magnitude of chloride movement through a watershed with respect to the extent, topology and connectivity of impervious areas.