Rinsing of Saline Water after Frequent Road Salt Applications from an Unsaturated Sandy Soil by Rainwater Infiltration: Significance of Rainfall Duration

Numerical one-dimensional (1D) model simulations were performed on the infiltration of freshwater as saturated plug flow into an unsaturated sandy soil at field capacity composed of coarse sand or sands (with saturated hydraulic conductivity, $K_s$, ranging from 0.004 to $0.1\mathrm{cm}/\mathrm{s}$ according to the USDA). Simulation used the Richards equation for the flow of water and an advection-dispersion equation for the transport of salt in aqueous solution. The 1D model had been validated previously against experimental laboratory data. Existing simulation models of transport in porous media, e.g., HYDRUS, are more powerful than what was needed in this study, therefore we chose to use our own model. In the regions where road salt is used, the saline pore water initially near the ground surface is from road salt (NaCl) applications during winter; in the following warm season, the soil thaws, the saline water percolates, and some amount of saline water is left in the pores of unsaturated soil. Infiltration of freshwater from rainfall can rinse accumulated saline water stored in the pores of the soil, the depth of which is up to 5 m. In time, the saline water will reach shallow groundwater and pollute aquifers because ${\mathrm{Cl}}^{-}$ can be toxic. The simulation results show that only a few hours of continuous infiltration are needed for highly permeable soil ($K_s$ from 0.01 to $0.1\mathrm{cm}/\mathrm{s}$) in order to reduce the salinity in the soil to less than 1% of its initial mass averaged across the entire modeling domain. For $K_s=0.004\mathrm{cm}/\mathrm{s}$, a few days (depending on the soil depth) of continuous freshwater infiltration is needed. The long required times indicate that the saline pore water is difficult to remove in one rainfall event from soils consisting of finer sands. The results are consistent with findings of significant salt concentrations in surface and shallow groundwater, long after road salt has been applied.