Temporal and spatial patterns of Cl- and Na+ concentrations and Cl/Na ratios in salted urban watersheds

The study of sodium and chloride in the environment has a long history with a particular focus on road salting in urban areas. In many studies, spatial scales are limited (e.g., city) and temporal measurements are coarse (e.g., monthly), with the result that our understanding of the hydrogeochemical dynamics is constrained. Through a unique set of spatial and temporal measurements from the State of Michigan we a) examine the spatial distribution of chloride across a broad geographic area, b) explore the temporal behavior of chloride and sodium over hydrologic events capturing snowmelt and rain through salting seasons, c) evaluate the use of chloride/sodium ratios as a tool for linking sources to concentrations, and d) develop a conceptual framework for processes responsible for their environmental concentrations. Results show 1) the short-term and local impact of urban areas on chloride concentrations is clearly delineated, 2) concentration and ratio variations over the hydrographs differ during salting and post-salting periods, 3) chloride/sodium ratios do not clearly indicate a halite source and can be very high (>5) and this is interpreted to be due to the different environmental behaviors of the two ions, and 4) during salting periods, chloride and sodium are quickly removed from the landscape during first flush and diluted as event water begins to dominate, but in post salting periods, only chloride is diluted. We also find evidence for upwelling of brine in some locations. These two solutes are easily measured indicators of human influences on water quality and it is recommended that they routine be included in water quality assessments. However, we suggest more research is necessary to better understand their cycling on shorter time scales and then how this knowledge can be used to inform our understanding of other chemical cycles in the environment.