WATERSHED LITHOLOGY INFLUENCES RETENTION TIMES FOR POLLUTANTS ASSOCIATED WITH ROAD DEICING EVENTS

Salt is commonly used as a roadway deicing agent to improve safety in areas that receive snow and ice in the winter. Excessive use of winter deicers adds salt-associated ions (e.g., Na, Cl) to streams following precipitation or melt events, impacting ecosystem health. The movement of road salt pollutants through urban watersheds may be impacted by the geologic setting, which modulates the flow of water and ions through the system. Areas dominated by carbonate rocks can have dissolution features connecting surface and subsurface water resources, allowing for rapid transport of contaminants on the landscape to groundwater reservoirs. Silicate-hosted watersheds usually feature slower transmission of water and ions due to diffuse flow through the rock matrix combined with retention of certain species along the flowpath due to sorption processes (e.g., Na interactions with clay minerals in the rock). Despite urban areas occurring across many lithologies, few studies compare how rock types influence contaminant retention times. Our study addresses how geology impacts road salt pollution transport in urban watersheds by comparing two catchments near St. Louis, Missouri, which have similar development levels but differing lithologies (~100% Mississippian carbonates versus ~25% Mississippian carbonates and ~75% Pennsylvanian silicates). Variability across each catchment was captured with “spatial” samples that were collected at baseflow at least bimonthly for ~1 year from >10 sites per catchment in 2018-2020. Weekly “temporal” samples were collected at the basin outlets to determine short-term responses due to changing flow regimes. All samples were analyzed for geochemical signatures indicative of water-rock interactions (e.g., Ca, Mg) and road salt pollution (e.g., Na, Cl). Both the sample suites showed lower Na:Cl ratios in the silicate-dominated catchment (temporal = 0.89 ± 0.18 mM/mM; spatial = 0.88 ± 0.22 mM/mM) compared to the carbonate-dominated basin (temporal = 0.95 ± 0.16 mM/mM; spatial = 0.99 ± 0.25 mM/mM). These results suggest that silicate-hosted watersheds retain Na from deicers longer than those in carbonate landscapes. Our research will help identify how watershed geology influences contaminant transport response, which can inform watershed stakeholders on best management practices.