Transport and Storage of Chloride in the Unsaturated Zone Following Deicing Applications

The application of road salts as deicing agents within the snowbelt areas of the United States and other higher latitude countries contributes to the anthropogenic loading of sodium chloride on the environment, specifically surface water and ground water. Within central Illinois, elevated chloride (Cl^-) in streams during the winter and early spring and low Cl^- concentration in summer suggest that deicing agents are impacting stream quality. Additionally, the occasional spike of Cl^- during the summer and spring allude to the potential storage of Cl^- in the unsaturated and saturated zones with subsequent release to streams. Using a 2D solute model, the potential of the unsaturated zone to serve as a reservoir and the mechanisms controlling the movement of Cl^- were examined. Physical properties and initial pore^-water Cl^- concentrations were derived from an array of soil borings. The initial pore^-water concentrations show the presence of a Cl^- “slug” approximately 0.75 m below the surface. Modeled simulations show that vertical transport is dominant within the unsaturated zone. After one year, the high vertical transport coupled with limited horizontal transport created a Cl^- “slug” similar to the background observation. While residual Cl^- is retained in the unsaturated zone, a net loss of Cl^- from the unsaturated zone was simulated for the first ten years. In year eleven, an equilibrium between the Cl^- input and output is achieved. The presence of Cl^- in the unsaturated zone would become a continuous source of Cl^- to the ground water, which would eventually discharge into the surface water. Historical surface water chemistry data signify imply the continual discharge of chloride to the surface water in the area, further supporting the hypothesis that the unsaturated zone serves as a Cl^- reservoir.