DO SOILS SLOW THE TRANSPORT OF WINTER DEICING SALTS TO SHALLOW GROUNDWATER?

Road deicing salts are widely used to improve winter travel safety. However, their application causes the release of Na, Cl, and other contaminants into the environment. While Na may temporarily be retained in soil, it can gradually be released over longer timescales via cation exchange. Most studies that evaluate road salt retention and transport in soils have relied on lab experiments. To our knowledge, these effects have not been tested in soil porewaters under field conditions. Thus, we collected in situ soil water samples from a roadside soil immediately after road salt applications using suction lysimeters; samples were obtained weekly for 3 months and collection is ongoing. Lysimeters were arranged by depth (10, 20, and 50 cm) into four nests that increased in distance from the road (1.0, 1.5, 4.0, and 18.0 m), allowing us to assess salt transport in three dimensions. Water samples were also collected weekly from a nearby karst spring. Waters were analyzed for stable isotopes (δ¹⁸O and δD) and Na and other major ions (Ca, Mg, K, Si, and Cl). Stable isotope results show that soil water retention time increases with depth and distance from the road. Soil water Na levels decreased significantly (p < 0.01) with distance from the road at all studied depths, though they were low on average (0.6 mg/L) compared to the spring (3.0 mg/L). This indicates differences in Na retention times between the soil and karst aquifer. Na levels increased significantly with depth at the three nests that were farthest from the road (p < 0.01), but there was no significant (p = 0.22) change in Na levels with depth at the nest closest to the road, evidencing higher Na inputs at the soil surface from road salting. Following road salt applications, the three nests farthest from the road featured decreasing Na concentrations over time (p < 0.02). At the nest nearest to the road, Na increased at 10 cm (p = 0.03) and did not change at 20 cm (p = 0.17) during the monitoring period, suggesting delayed release of the higher Na levels at the roadside via cation exchange. Our temporal field results support previous lab findings for this site that showed Na retention for up to 5 months in the soil after road salt application. Understanding the three dimensional transport of road salt through multiple reservoirs will allow us to better assess the harmful impacts it has on the environment.