Freshwater Salinization Syndrome Alters Retention and Release of 'Chemical Cocktails' along Flowpaths: from Stormwater Management to Urban Streams.

We investigate impacts of Freshwater Salinization Syndrome (FSS) on mobilization of salts, nutrients, and metals in urban streams and stormwater BMPs by analyzing original data on concentrations and fluxes of salts, nutrients, and metals from 7 urban watersheds in the Mid-Atlantic U.S. and synthesizing literature data. We also explore future critical research needs through a survey of practitioners and scientists. Our original data show: (1) sharp pulses in concentrations of salt ions and metals in urban streams directly following both road salt events and stream restoration construction ( e . g ., similar to the way concentrations increase during other soil disturbance activities); (2) sharp declines in pH (acidification) in response to road salt applications due to mobilization of H ⁺ from soil exchange sites by Na ⁺ ; (3) sharp increases in organic matter from microbial and algal sources (based on fluorescence spectroscopy) in response to road salt applications likely due to lysing cells and/or changes in solubility; (4) significant retention (~30-40%) of Na ⁺ in stormwater BMP sediments and floodplains in response to salinization; (5) increased ion exchange and mobilization of diverse salt ions (Na ⁺ , Ca ²⁺ , K ⁺ , Mg ²⁺ ), nutrients (N, P), and trace metals (Cu, Sr) from stormwater BMPs and restored streams in response to FSS; (6) downstream increasing loads of Cl ^- , SO ₄ ^2- , Br ^- , F ^- , and I ^- along flowpaths through urban streams, and P release from urban stormwater BMPs in response to salinization, and (7) a significant annual reduction (> 50%) in Na ⁺ concentrations in an urban stream when road salt applications were dramatically reduced, which suggests potential for ecosystem recovery. We compared our original results to published metrics of contaminant retention and release across a broad range of stormwater management BMPs from North America and Europe. Overall, urban streams and stormwater management BMPs consistently retain Na ⁺ and Cl ^- but mobilize multiple contaminants based on salt types and salinity levels. Finally, we present our top 10 research questions regarding FSS impacts on urban streams and stormwater management BMPs. Reducing diverse 'chemical cocktails' of contaminants mobilized by freshwater salinization is now a priority for effectively and holistically restoring urban waters.