Chloride trends in Ontario’s surface and groundwaters

The Ontario Ministry of the Environment, Conservation and Parks leads 11 long-term monitoring programs at over 2,500 surface and groundwater sampling locations across the province that report chloride (Cl−) concentration, some dating back to the 1960s. This study integrates these disparate datasets to provide comprehensive evidence relevant to spatial and temporal Cl− trends in the Laurentian Great Lakes, and Ontario's inland lakes, streams and groundwaters. While the vast majority of historical Cl− data indicate concentrations are well below the chronic exposure Canadian Water Quality Guideline (120 mg L−1), many lake, stream and groundwater sampling locations proximal to roadways or in urbanized areas meet or exceed this guideline, and concentrations in these regional areas are persistently increasing. The current evidence implicates road salting activities for winter safety as a primary contributor to elevated Cl− concentrations, and the trends may be exacerbated with urbanization and population expansion, particularly in southern Ontario.