Tidally driven groundwater flow and solute exchange in a marsh: Numerical simulations

Tidal fluctuations drive groundwater flow in salt marsh sediments. This flow could cause significant chemical exchange across the sediment‐water interface and could affect marsh ecology. Numerical models of a generalized tidal creek and marsh were constructed to calculate flow patterns and solute exchange between the marsh and creek. The governing equation for saturated/unsaturated flow was modified to account for tide‐related changes in total stress. Groundwater flow occurred primarily in the creek bank, even when the marsh platform was inundated at high tide. For marsh sediments with a hydraulic conductivity of 10 −4 m s −1 , groundwater ages in simulations lasting 60 days were on the order of days near the creek bank and increased to 50–60 days with distance into the marsh. The volume of water that discharged between high and low tide was 0.22–0.31 m 3 per meter length of channel, which, for a creek drainage density of 0.012 m −1 , corresponds to 10–14 L m −2 d −1 . Sediment permeability and capillarity were important controls on flow and groundwater age in the marsh. Sediment compressibility affected groundwater age for compressible sediments representative of mud but not for sediments with lower compressibilities representative of sand. Simulations were relatively insensitive to dispersivity. A comparison of simulation results with other estimates of groundwater exchange from the North Inlet, South Carolina, suggest that tides could drive observed exchange there only if the Pleistocene sands underlying muddy marsh sediments outcrop within the tidal range.