Numerical Simulations of Road Salt Impact at a Municipal Water Supply

Chloride concentrations at major water supply wells within the Regional Municipality of Waterloo in Ontario have been increasingly rapidly over the past 20 years. If present trends continue, drinking water limits may be reached within the next decade. Road salt has been identified as the prime source of the contamination, and various remediation strategies are being investigated. As part of a overall comprehensive chloride reduction study, advanced numerical simulation tools are used to investigate the mechanisms of chloride transport from ground surface to water supply wells, and to assess the effectiveness of various remediation scenarios. Several options were simulated such as reducing chloride application by 25% over the entire study area, eliminating applications on secondary roads, and eliminating applications within the 5 and 10-year capture zones. The numerical simulations include a 2D variably-saturated model and a fully 3D model of chloride transport from a growing road network. The results from the variably-saturated model show that chloride application to the water table can be assumed to be a steady state process where seasonal transients can be neglected. This assumption was used in the 3D model, which showed that the option of reducing salt application by 25% was sufficient to protect the water supply. Eliminating application on some secondary roads could further decrease concentrations at the wells with very little effort. Eliminating application from within the capture zones results in a dramatic decrease in chloride at the wells, however the chloride trends in the wells continue upward after time as chloride arrives from outside the capture zone.