The application of road salt or de-icing chemicals to southern Ontario roadways has been the standard practice for the past 70 years. Approximately 100,000 tonnes of road salt is used on southern Ontario roadways, and 5 million tonnes is applied to roads across Canada annually. Groundwater protection and management are vital in southern Ontario and are a topic with increased interest as the impacts of road salt application are becoming more wildly known and understood. The Oak Ridges Moraine (ORM) is recognized as a regional groundwater recharge area and has an extensive history of groundwater use, with records of approximately 150,000 private and public wells drilled in the region’s watershed. Samples of salts and bedrock brines, in addition to suspected anthropogenic contributors, are compared with groundwater and pore water samples from some regional and municipal wells across the ORM.
Stable chlorine and possibly bromine isotope analyses can aid in the identification of anthropogenic salt contamination in groundwater and help along with other geochemical methods such as Cl/Br mass ratio to identify additional sources of salt contamination. This study attempts to determine what contributes to the presence of chloride in groundwaters of the ORM. The 37Cl isotopic signature of road salt is ±0.2 per mil SMOC and the 81Br isotopic signature ranges from +0.8 to -0.2 per mil SMOB. The standard mass ratio of chloride to bromide (Cl/Br ratio) in the ocean, is about 300 and the value of Cl/Br ratios of deep formation water from under the ORM is lower than 300, whereas the value of Cl/Br ratio of shallow groundwater affected by road salt is greater than 300. In this study, the isotopic value of bromine (Br) ranged from -1.9 per mil to +2.8 per mil, which may be the result of several processes such as road salt dissolution and recharge into groundwater, saline sedimentary formation fluid upward advection from the bedrock and/or organic and microbial release of organically bound bromide from subsurface sedimentary reservoirs. The highest chloride concentrations (800-1400 mg/L) are found at shallow depths and carry the localized isotopic signature for road salt. High concentrations (>200 mg/L) are also found in both older and recently recharged waters in deeper aquifers within the ORM. Some of these waters based on other parameters such as tritium, 18O, and Cl/Br ratios appear to be recent recharge of anthropogenic fluids at depth. Additional high concentrations of salt-contaminated groundwaters appear to be sourced from the upper migration of high salinity bedrock formation fluids based on novel isotopic tools and a lack of tritium.