Episodic salinization and freshwater salinization syndrome mobilize base cations, carbon, and nutrients to streams across urban regions

Urbanized watersheds in colder climates experience episodic salinization due to anthropogenic salt inputs and runoff from impervious surfaces. Episodic salinization can be manifested as a pulse' in concentrations and fluxes of salt ions lasting from hours to days after snowstorms in response to road salting. Episodic salinization contributes to freshwater salinization syndrome, characterized by cascading mobilization of chemicals and shifting acid-base status. We conducted laboratory experiments and analyzed high-frequency sensor data to investigate the water quality impacts of freshwater salinization syndrome and episodic salinization across 12 watersheds draining two major metropolitan regions along the U.S. East Coast. Sediments from 12 watersheds spanning land use gradients across two metropolitan regions, Baltimore, Maryland and Washington DC, were incubated across a range of replicated salinity treatments (0-10g/L sodium chloride). There were statistically significant linear increasing trends in calcium and potassium concentrations with experimental salinization across all 12 sites and in magnesium concentrations at 11 of 12 sites (p<0.05), with mean rates of increase of 1.92 +/- 0.31mg-Ca per g-NaCl, 2.80 +/- 0.67mg-K per g-NaCl, and 1.11 +/- 0.19mg-Mg per g-NaCl, respectively. Similarly, there were statistically significant increasing linear trends in total dissolved nitrogen (TDN) concentrations with experimental salinization at 9 of the 12 sites, with a mean rate of increase of 0.07 +/- 0.01mg-N per g-NaCl. There were statistically significant increasing linear trends in soluble reactive phosphorus (SRP) concentrations with experimental salinization at 7 of the 12 sites (p<0.05), with a mean rate of increase of 2.34 +/- 0.66 mu g-P per g-NaCl. The response of dissolved inorganic carbon (DIC) and organic carbon (DOC) concentrations to experimental salinization varied between sites, and dissolved silica did not show any significant response. High-frequency sensors near the experimental sites showed statistically significant positive linear relationships between nitrate concentrations, specific conductance, and chloride concentrations similar to relationships observed in laboratory incubations. Our results suggested that episodic salinization and freshwater salinization syndrome can mobilize base cations and nutrients to streams through accelerated ion exchange and stimulate different biogeochemical processes by shifting pH ranges and ionic strength. The growing impacts of freshwater salinization syndrome and episodic salinization on nutrient mobilization, shifting acid-base status, and augmenting eutrophication warrant serious consideration in water quality management.