Mallory Carpenter

Modelling the Transport and Retention of Chloride using INCA-CL in an Urbanizing Watershed

In northern environments such as Canada, road salt (e.g. sodium chloride, NaCl) has been used as a de-icing agent to improve winter driving conditions since the 1950’s. While research has shown that the application of salt to roadways can reduce accident rates, the use of road salt has been linked to increasing concentrations of chloride (Cl) in ground and surface waters. High Cl concentrations have been found to be potentially harmful to freshwater organisms, and long-term exposure can have detrimental effects on human health.  The Integrated Catchment Model for Chloride (INCA-Cl), is a sub-watershed scale model that requires land cover, meteorological, and Cl surface application data as inputs to model the transport of Cl through a watershed.   Here, we apply INCA-Cl to the rapidly urbanizing East Holland River sub-watershed located within the Lake Simcoe watershed in Southern Ontario, Canada. Results showed good agreement between measured and modelled stream flow, however the model fails to capture the in-stream Cl concentration dynamics.  We reworked the spatial aspect of the model in an attempt to more accurately target Cl inputs to appropriate areas of land use, and still failed to obtain a satisfactory simulation. A comparison of modelled and measured Cl inputs, show that the model is able to balance mass on an annual basis, however, looking at the modelled results on a seasonal basis shows a strong negative relationship in summer months, suggesting that that the model is failing to capture the timing of the low-concentration summer flows. This is likely due to inadequacies in the model structure, such as an inability to model impervious surfaces, or to constrain salt application to specific land covers.  Further work will be necessary to make INCA-Cl a viable option for Cl modelling in urban environments.