STREAM-GROUNDWATER INTERACTION MECHANISMS THAT PROMOTE STORAGE OF DEICING CHEMICALS IN URBAN FLOODPLAINS

The impact of road de-icers on streams in the northeastern United States has been well documented, with research showing that groundwater storage of road de-icer runoff results in increased surface water chloride concentrations over time, even in summer months. However, the mechanisms controlling the seasonal exchange of chloride between streams and floodplains, particularly in urban systems, have not been adequately explored. Previous research on an urban stream in Syracuse, New York, indicates that, when present, riparian aquifer discharge buffers surface water chloride concentrations throughout the year, resulting in smaller annual ranges of chloride relative to reaches that are disconnected from riparian groundwater discharge due to bank armoring. Stream reaches with an intact riparian floodplain, which discharges groundwater year-round, had annual chloride concentrations that ranged from 252.0 to 657.0 mg/L, while armored reaches had chloride ranging from 161.2 to 2173 mg/L. We developed a heuristic, 3D numerical groundwater flow model of an urban riparian floodplain with Visual MODFLOW to study how chloride from road de-icers is exchanged between streams and riparian floodplains. We initially hypothesized that winter overbank events recharged the floodplain with high chloride stream water heavily impacted by snowmelt and road runoff and that the stored chloride was then slowly released as relatively saline baseflow discharge throughout the year. Model results indicate chloride is more likely transported from the stream to the floodplain through hyporheic interaction and down-valley groundwater flow parallel to the stream channel. Modeled chloride concentrations were compared to surface and groundwater chemistry observations from the floodplain taken between May 2013 and June 2014 to qualitatively calibrate the heuristic model and determine controls on chloride storage and transport in this urban, floodplain system.