76-1 Phosphorus and metal(oid) mobility induced by changing salinity near groundwater-surface water interfaces

Session: Contaminants Near Groundwater-Surface Water Interfaces

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Abstract:

Sediments in many groundwater-surface water interaction environments can be exposed to temporally variable porewater salinity, typically driven by fluctuations in sodium chloride (NaCl) concentrations. In cold climate regions, this is driven by seasonal salt inputs from winter de-icing practices that may impact surface waters and groundwater differently.  In coastal settings, salinity fluctuations occur near the sediment-water interface of surface and subterranean estuaries as saltwater zones shift in response to changing coastal and hydrologic forcings.  While studies have shown that salinization can mobilize metals and phosphorus (P), the effects of freshening are poorly understood.  This paper will present laboratory sediment column experiments, informed by field data from urban stream sites in Ontario, Canada, to investigate the impacts of salinization and freshening on the mobilization of metal(oid)s and P.  Results show that metal (Al, Pb, As) and P mobilization were highest during freshening - when the NaCl concentrations decreased from high (4125-8250 mg/L) to low (50-165 mg/L). Mobilization was reduced when the decrease in NaCl concentration was less. Colloid dispersal, as suggested by increased turbidity, is proposed as the main mechanism responsible for this release during freshening. Notably, during this period Al, Pb, and As concentrations exceeded the Canadian water quality guidelines for protection of aquatic life and P concentrations reached up to 40 mg/L - 400 times greater than the hypereutrophic threshold. More broadly, this paper highlights the complex interactions among chemical species that influence contaminant fate near groundwater-surface water interfaces, and underscore the need to consider transient dynamics in understanding biogeochemical processes.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6567

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