Spatial distribution and extent of urban land cover control watershed-scale chloride retention

In some cold regions up to 97% of the chloride (Cl^-) entering rivers and lakes is derived from road salts that are applied to impervious surfaces to maintain safe winter travel conditions. While a portion of the Cl^- applied as road salt is quickly flushed into streams during melt events via overland flow and flow through storm sewer pipes, the remainder enters the subsurface. Previous studies of individual watersheds have shown that between 28 and 77% of the applied Cl^- is retained on an annual basis, however a systematic evaluation of the spatial variability in Cl^- retention and potential driving factors has not been carried out. Here we used a mass balance approach to estimate annual Cl^- retention in 11 watersheds located in southern Ontario, Canada, which span a gradient of urbanization. We evaluated the influence of multiple landscape variables on the magnitude of Cl^- retention as well as the long-term rate of change in stream Cl^-concentration for the same systems. We found that mean annual Cl^- retention ranged from 40 to 90% and was higher for less urbanized watersheds and for watersheds with urban areas located farther from the stream outlet. This result suggests that less urbanized watersheds and ones with longer flow pathways have more Cl^- partitioned into storage and hence the potential for legacy Cl^- effects on aquatic organisms. While we did measure statistically significant increasing trends in stream Cl^- concentration in some watersheds, there was no consistent relationship between the long-term rate of change in stream Cl^- concentrations and patterns of urbanization and the magnitude of Cl^- retention. Based on our results we present a detailed conceptual model of watershed Cl^- dynamics that can be used to guide future research into the mechanisms of Cl^- retention and release within a watershed.