Road salt retention and transport through vadose zone soils to shallow groundwater

Increasing background salinity in watersheds has largely been attributed to road salt retention in groundwaters due to their long residence times. However, laboratory studies demonstrate that soils temporarily store salts, either in porewater or adsorbed onto particles. Field studies of road salt retention in soils are nevertheless rare, and mechanisms of salt transport across multiple hydrological reservoirs (e.g., from soil to groundwater) are unknown. Thus, we collected roadside soil porewater and karst spring water weekly for similar to 1.5 yr to determine salt transport through the vadose zone into the phreatic zone. We observed dual retention mechanisms of sodium (Na+) and chloride (Cl-) in soils due to slow porewater movement, causing ion movement through the soil as slow as 1.3 cm/day, and cation exchange processes, leading to initial Na+ retention followed by later release months after application. Cation exchange processes also caused base cation loss fromexchange sites intomobile porewater. RapidNa(+) and Cl- delivery to groundwater occurred through karst conduits during thewinter. However, elevated background levels of salt ions in groundwater during the non-saltingmonths indicated accumulation in the catchment due to slower porewater flow in the soil and rock matrix and delayed Na+ release fromsoil exchange sites. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

Laboratory studies show temporary salt storage in soils, but field studies are rare, and the mechanisms of salt transport across hydrological reservoirs remain unknown. The research observed dual retention mechanisms of Na+ and Cl- in soils, along with ion exchange processes causing slow movement and retention of ions in porewater.