Freshwater Salinization Syndrome creates organic and inorganic chemical cocktails across five major metropolitan regions in the U.S.
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Along urban streams and rivers, various processes, including road salt application, sewage leaks, and weathering of the built environment, contribute to novel chemical cocktails made up of metals, salts, nutrients, and organic matter. In order to improve our understanding of how contaminants can be transported and retained together within urban watersheds, we investigated how both organic and inorganic chemical cocktails varied along nine rural-to-urban flowpaths across five major U.S. cities. In seven of nine streams, salty chemical cocktails made up of salt-derived ions (Ca2+, Mg2+, Na+, and K+) and commonly correlated elements (e.g. Sr, N, Cu) increased along these urbanization gradients. Principal component analysis identified two principal components that explain 48.9% of the variance across all sites. Total dissolved nitrogen, BIX, and FI along with the salty chemical cocktails were positively correlated with one another, while HIX and SUVA254 were negatively correlated. These results suggested that labile organic matter and nitrogen increase with elevated salinity. To further investigate this link between salinity, nitrogen, labile organic matter, we conducted high frequency sampling during road salt events at several Washington, D.C. sites. With increasing specific conductance, the ratio of protein-like to humic-like organic matter significantly increased at all sites and total dissolved nitrogen significantly increased at two sites (p < 0.05). Urban salt pulses mobilized nitrogen and labile dissolved organic matter pulses in streams. Our results show that salinity interactions play a role in modification of nitrogen and organic matter transport and cycling along the urban watershed continuum, which has implications for eutrophication and hypoxia of sensitive receiving waters.