EVALUATING GROUNDWATER CHLORIDE IN THE GREAT LAKES BASIN: GEOSPATIAL ANALYSIS AND SUBSEQUENT FIELD INVESTIGATIONS IN A HOT SPOT AREA

In cold regions, the increase in urbanization, along with the need to ensure safe winter roads, frequently results in excessive use of road salt and the ensuing release of chloride into the adjacent environment. Southern Ontario, Canada, due to its northern location and climate, is especially susceptible to this issue. Rivers and streams within the province exhibit concentrations that regularly surpass recommended chloride guidelines. Correspondingly, there has also been a marked increase in groundwater chloride concentrations in urban areas of Ontario within recent decades. Although the impact of surface runoff on elevated chloride levels in surface water is relatively well-researched, the influence of groundwater in this regard remains largely unexplored.

To address knowledge gaps, publicly available data was acquired to evaluate the concentrations and spatio-temporal patterns of chloride in both groundwater and surface water across different time periods. Connections between groundwater and surface water chloride concentrations, land use patterns, and hydrogeological features were examined using geospatial analysis. The outcomes of this assessment were used to identify areas with high vulnerability or hot spots that require further site-specific and targeted road salt management, one of them being the Western Lake Ontario Basin. Currently, detailed field investigations are being conducted in the Credit River watershed, located within this basin, to enhance the understanding of how groundwater receives chloride and delivers it to surface waters in the watershed. Sampling (major ions and stable isotopes) and continuous monitoring (levels and electrical conductivity) of shallow and deep groundwater and surface water has transpired at four field sites since May 2022. Both groundwater and surface water routinely exceed guidelines for chloride in urban and rural settings. Urban surface water concentrations remain elevated throughout the non-salting season, likely due to groundwater contributions of chloride during this time. Additional groundwater-surface water interaction investigations are underway to elucidate the dynamics of this transport. The project results will provide support for recommendations regarding future road salt best practices and policies in Great Lakes Basin.