Modeling Fate and Transport of Chloride from Deicers in Urban Floodplains: Implications for Urban Planning

Abstract:

Road salting in urban areas of the northeastern United States increases chloride concentrations in urban streams. Groundwater storage of saline road runoff results in increased surface water chloride concentrations through time, even in non-winter months. Stream-groundwater (SW-GW) interactions promote buffering of large seasonal swings in stream chloride concentrations, resulting in lower surface water chloride in winter and higher concentrations in summer, relative to streams hydrologically disconnected from riparian floodplains. However, the hydrogeologic processes controlling salt storage and transport in urban floodplain aquifers have not been fully investigated. We developed a 3D numerical groundwater flow and solute transport model of an urban floodplain in Syracuse, New York, using MODFLOW and MT3DMS. We ran the model for 1 year, calibrating to three conditions: water table elevations along a riparian transect, measurements of net groundwater flux to the stream along the 500-m reach, and chloride concentrations in groundwater through time in riparian wells. Chloride enters the riparian aquifer via three pathways: hillslope groundwater discharge, hyporheic exchange, and groundwater recharge during overbank flooding events. Winter overbank flooding events are the primary source of chloride to floodplain sediments. While hillslope groundwater discharge results in relatively uniform chloride through time in high conductivity units, surficial floodplain sediments with lower conductivity have high chloride concentrations from winter overbank flood events. When compared to road salt application rates (up to 20 tons of salt per lane kilometer per year), the 0.013 km² floodplain holds only a tiny fraction of chloride applied in a watershed (>100 km of road in the watershed). To promote riparian aquifer storage of road salt and buffering of stream chloride concentrations, urban planners should design urban floodplains for frequent winter flooding events, and allow ample floodplain surface area to accommodate typical road salt application rates.