Chloride concentrations in natural surface and groundwaters of the Western Lake Ontario Basin are rising, surpassing guidelines for the protection of aquatic organisms. Thus far the observed increases of chloride have been attributed primarily to the application of winter de-icing salts on an increasing impervious area. To develop effective best management practices for chloride application, a complete understanding of the sources of anthropogenic chloride is required. Continuous monitoring and discrete sampling were performed at four research sites within the Credit River Watershed for one hydrological year. To determine sources of chloride, geochemical and isotopic source characterization techniques were applied, including Cl/Na ratios, Cl/Br ratios, deuterium, iodine, artificial sweeteners, tritium, and helium analysis. The highest chloride concentrations in surface and groundwater samples occurred during the winter at the two urban sites. Cl/Na plots suggest that three of the four sites were impacted by road salt to varying extents. Cl/Br ratios revealed further sources including wastewater, and landfill leachate at the most urbanized site. These sources were further confirmed using analysis of deuterium, iodine, tritium-helium and artificial sweeteners. The findings of this study indicate that various anthropogenic sources of chloride must be considered and can be discerned using geochemical and isotopic techniques.