Spatial and Temporal Patterns of Chloride Contamination in a Shallow, Urban Marsh

High chloride concentrations occur in inland waters from road salt applications (NaCl) as deicers. We studied seasonal and spatial distributions of chloride contamination in a shallow, urban marsh in Madison, Wisconsin, during the warm-wet winter of 2012-13, the cold-dry winter of 2014-15, and in the ice-free season of 2015. Chloride concentrations at the open water sites ranged from 10 to 1,261 mg L-1. Marsh inflows include a storm sewer, direct precipitation, and distributed surface flow and inferred groundwater flow around the marsh's perimeter. The upstream storm sewer had more influence on chloride concentrations in the upper marsh, whereas runoff from a large snow storage pile had more influence in the lower marsh. In the cold-dry winter, the storm sewer and ion exclusion from ice formation greatly influenced chloride concentrations. In the warm-wet winter, snow melt, and rain on snow events diluted chloride concentrations in the open marsh, while more snow and deicer use increased the role of a snow storage pile as a source of chloride. In the open-water sites in 2015, chloride concentrations decreased following rain, but continued to increase in drier periods, which suggests distributed overland and groundwater inflows provide a source of legacy chlorides to the marsh.