Hydrogeochemical implications of groundwater contamination by road de-icing chemicals

Abstract This hydrogeochemical study concerns the distribution, origins and behaviour of groundwaters in Quaternary sediments of southern Ontario, Canada, containing elevated concentrations of chloride. Major-ion, minor-ion and trace-metal analyses of 37 potential chloride sources and over 400 well waters, 70 urban springs and 30 pore waters are reported. Source chloride concentrations ranged up to 230,000 mg L−1 for brines from southwestern Ontario; for groundwaters in Quaternary sediments, the chlorides range up to 700 mg L−1 for domestic wells, 2,840 mg L−1 for urban springs and 13,700 mg L−1 for shallow pore waters. Regional studies performed in support of the study suggest that very little chloride enrichment is associated with natural chemical evolutionary processes. Background concentrations are in the range 15–20 mg L−1 and these levels are exceeded in over half the wells in the area; potential sources include road salts, landfill leachates, agricultural fertilizers and saline bedrock waters. Most of these sources are found to be chemically pure with 2 or 3 major ions occurring to the virtual exclusion of all other constituents. Source trace-metal concentrations are low and none are diagnostic of origin. Only iodide and fluoride prove useful indicators of source. Iodide is especially useful for differentiating between road salts and saline bedrock waters, sources which are normally indistinguishable using major-ion criteria alone. From an inorganic water quality perspective, trace-metal concentrations in the chloride sources do not appear to be an environmental concern and only the primary chemical components constitute a serious threat to groundwater potability.