HYDROGEOCHEMICAL IMPLICATIONS OF GROUNDWATER CONTAMINATION BY ROAD DEICING CHEMICALS

This hydrogeochemical study concerns the distribution, origins and beh aviour of groundwaters in Quaternary sediments of southern Ontario, Ca nada, containing elevated concentrations of chloride. Major-ion, minor -ion and trace-metal analyses of 37 potential chloride sources and ove r 400 well waters, 70 urban springs and 30 pore waters are reported. S ource chloride concentrations ranged up to 230,000 mg L-1 for brines f rom southwestern Ontario; for groundwaters in Quaternary sediments, th e chlorides range up to 700 mg L-1 for domestic wells, 2,840 mg L-1 fo r urban springs and 13,700 mg L-1 for shallow pore waters. Regional st udies performed in support of the study suggest that very little chlor ide enrichment is associated with natural chemical evolutionary proces ses. 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 sou rces include road salts, landfill leachates, agricultural fertilizers and saline bedrock waters. Most of these sources are found to be chemi cally pure with 2 or 3 major ions occurring to the virtual exclusion o f all other constituents. Source trace-metal concentrations are low an d 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 norma lly indistinguishable using major-ion criteria alone. From an inorgani c water quality perspective, trace-metal concentrations in the chlorid e sources do not appear to be an environmental concern and only the pr imary chemical components constitute a serious threat to groundwater p otability.