Effects of Episodic Salinization on Carbon, Nutrients, Base Cations, and Trace Metals Fluxes Across Urban Metropolitan Regions

Urbanized watersheds in colder climates experience episodic salinization due to anthropogenic salt inputs and runoff from impervious surfaces. Episodic salinization manifests as a 'pulse' in concentrations and fluxes of salt ions lasting from hours to days after snowstorms in response to road salting. We conducted lab experiments and analyzed high-frequency sensor data to investigate the water quality impacts of episodic salinization across 12 watersheds that span a land use gradient and drain two major metropolitan regions of the Chesapeake Bay watershed (Washington DC and Baltimore MD). Sediments and streamwater were incubated across a range of replicated salinity treatments (0-10 g/L NaCl). We observed significant (p < 0.05) linear increasing trends in calcium and potassium concentrations with salinization across all 12 sites and in magnesium concentrations at 11 of 12 sites, with mean rates of increase of 1.92 ± 0.31 mg-Ca per g-NaCl, 2.80 ± 0.67 mg-K per g-NaCl, and 1.11 ± 0.19 mg-Mg per g-NaCl, respectively. We observed significant increasing linear trends in total dissolved nitrogen (TDN) concentrations with salinization at 9 of the 12 sites, with a mean rate of increase of 0.07 ± 0.01 mg-N per g-NaCl. We observed significant increasing linear trends in soluble reactive phosphorus (SRP) concentrations with salinization at 7 of the 12 sites, with a mean rate of increase of 2.34 ± 0.66 μg-P per g-NaCl. We observed increases in dissolved trace metal concentrations with salinization at some of the sites. The response of dissolved inorganic carbon (DIC) and organic carbon (DOC) concentrations to salinization varied between sites, and dissolved silica did not show any significant response. High-frequency sensors near the experimental sites showed significant positive linear relationships between nitrate concentrations and salinity, similar to relationships observed in laboratory incubations. Our results suggest that episodic salinization 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 on contaminant mobility, shifting acid-base status, and augmenting eutrophication warrant serious consideration in water quality management.