Influence of Varying Porewater NaCl Concentrations on Mobilization of Metals, Arsenic, and Phosphorus Stored in Streambed Sediments

Abstract

Winter deicing salt use has increased freshwater salinization, with groundwater-surface water interactions exposing streambed sediments to large salinity fluctuations. This study examines how salinity fluctuations mobilize metals (Al, Cr, Cu, Fe, Ni, Pb, Zn), arsenic (As), and phosphorus (P) from streambed sediments. We conducted column experiments, exposing sediments to increasing (salting) and decreasing (freshening) NaCl concentrations. Across four scenarios with varying salinity changes and sediment sources, most elements were mobilized, with the greatest releases occurring during freshening. Rapid decreases in porewater salt concentration triggered significant increases in soluble reactive phosphorus (SRP) and dissolved Al, As, Cr, Cu, Fe, Ni, Pb, and Zn, with many exceeding Canadian aquatic life guidelines. Total metal concentrations exceeded dissolved concentrations by more than 3-fold, which together with turbidity and exchangeable sodium percentage (ESP) data suggest colloid dispersion was the dominant mobilization mechanism during freshening. In contrast, total P and As were similar to SRP and dissolved As concentrations, with data indicating that their mobilization may be due to pH-driven desorption. Mobilization decreased with smaller salinity changes but increased with higher sediment contamination. These findings demonstrate that porewater salinity fluctuations can generate hot moments of contaminant release, particularly during freshening, compounding the water-quality impacts of freshwater salinization.