Studies of a subarctic coastal marsh, II. Salinity

The salinity of James Bay coastal marshes is more strongly affected by fossil salt from the underlying sediments than from tidal sources. A numerical model simulates the transport of this salt towards the marsh surface by molecular diffusion. However, the more permeable sediments near the surface transmit salt by advection in the groundwater flow, notably at the beach ridges, where the vertical hydraulic gradients are the strongest. The diffusion and advection transport processes supply chloride (Cl−) to the surface at rates of 0.4 and 6.0 g m−2 d−1 respectively. Although the diffusion transport rate is low, this process contributed nearly half as much chloride to the marsh surface as advection did, because diffusion operates over a much larger area. These estimated chloride fluxes were within 15% of the measured average Cl− export in the stream channel which drains the marsh. A model simulating the daily rate of Cl− export indicates that its day to day variability is due to the near-surface hydrological processes, but over the long term is limited by the vertical advection and molecular diffusion processes that bring the salt to the surface.