ROAD SALT RETENTION AND TRANSPORT IN SOILS AND SUBSEQUENT RELEASE OF BASE CATIONS AND TOXIC TRACE ELEMENTS TO POREWATERS

Increased salinity in surface waters due to road salt application has been observed in cold regions across the globe. Salt accumulation in watersheds has largely been attributed to retention in groundwaters because of their long residence times. Lab studies have shown that soils may also store salts (either in porewater or adsorbed on particles) that can gradually be released. These deicing salts can trigger ion exchange processes that cause leaching of other elements to porewaters. To our knowledge, no study has yet explored how road salts and their related contaminants are transported across multiple reservoirs (from soil to groundwater). Thus, suction lysimeters were used to collect porewaters from a roadside soil weekly for 1.5 years. Samplers were arranged by depth (10, 20, 50 cm) into nests that increased in distance from a road (1.0, 1.5, 4.0, 18.0 m) allowing us to assess salt transport in three dimensions. Samples from a nearby karst spring were also obtained weekly as a proxy for groundwater chemistry. Waters were analyzed for Na, Cl, other major ions (Ca, Mg, K, Si), toxic trace elements (As, Cu, Zn), and stable isotopes (δ¹⁸O, δ²H). Soil water Na and Cl levels were highest after salting and decreased with distance from the road at all studied depths (p < 0.01). Average soil water Na and Cl concentrations were low (0.35 and 0.48 mM) compared to the spring (1.76 and 2.60 mM). After salting events, base cations peaked with Na and Cl and toxic trace elements were higher near the road (p < 0.02). Our isotope results showed that soil water retention time increased with depth and distance from the road, while previous findings for the spring revealed that there were both slow (diffuse) and rapid (conduit) flowpaths in the karst aquifer. In soil, Na and Cl were retained by 1) slow porewater movement, causing a 5-8 day lag near the road, and 2) ion exchange processes (evidenced by increased concentrations of other ions), enhancing porewater Na levels 2-5 months after the salting season. In groundwater, higher baseline Na and Cl levels indicated accumulation in the aquifer due to diffuse transport. Yet, spring Na and Cl peaked during salting events, suggesting that faster flowpaths were also important. Understanding road salt-related contaminant transport through multiple reservoirs will allow us to better assess resulting harmful impacts.