Contamination of Subsurface Waters by Road De-icing Chemicals

Abstract

Abstract The application of road de-icing chemicals to urban and rural roads in southern Ontario represents a serious potential source of shallow subsurface contamination. These chemicals are highly soluble and are readily mobilized by surface water run-off. This run-off can subsequently enter the soil zone and migrate through the shallow subsurface resulting in degradation of groundwater quality in the underlying aquifer. Recent work has investigated the extent to which the inorganic constituents of road de-icing chemicals constitute a threat to groundwater in the shallow subsurface environment. This study has focused on a) determining the extent to which road de-icing chemicals are accumulating in the shallow subsurface; b) identifying diagnostic chemical parameters which will enable conclusive contaminant source identification; and c) assessing rates of road de-icing contaminant migration. Fifteen field sites were selected to sample sediments adjacent to urban and rural roads. A total of forty seven interstitial water samples were submitted for major ion (Na, K, Mg, Ca, Cl, SO4, HCO3, NO3), minor ion (F, I, Br), pH, electrical conductivity and trace constituent analyses. Preliminary results indicate that the shallow subsurface adjacent to the urban road sites contain considerably elevated Na, Cl, Ca, and Mg. Interstitial waters from urban road sites may contain from 103 to 104 mg/L Cl and 103 mg/L Na as compared to less than 102 mg/L Cl and less than 102 mg/L Na for interstitial waters sampled from rural road sites and park locations. The ratio I/Cl exhibits potential as a diagnostic indicator of source origin. Interstitial waters contaminated by road salt regularly show I/Cl ratios two orders of magnitude lower than I/Cl ratios for saline bedrock waters.