Tidal wetlands are a critical interface between land and ocean providing habitat for a large range of flora and fauna. The health and evolution of these wetlands depend on the hydrodynamically-controlled physical, chemical, and biological processes. That is, these processes are affected by tides, water density variations and evapoconcentration of salt. Infiltrated rainwater forms a terrestrial groundwater lens which overlies intruding seawater and flows to the ocean. Tides drive mixing of these waters and have a strong influence on pathways and residence times. The distribution of salt, its concentration and the pathway of nutrients all influence marsh vegetation.
Field monitoring campaigns were carried out to map the temporal and spatial salt distribution across a subtropical wetland system located in Southeastern Queensland, Australia. 2-D numerical simulations were conducted to examine the pore water flow and salt distribution patterns in a cross-shore section of the marsh soils under the influences of the spring-neap tide and evaporation. Simulation results, consistent with the field data, showed that soil surface (especially root zone) salinity in general increases with the marsh surface elevation due to evapoconcentration. The surface topography governs the area and duration of seawater inundation over one tidal cycle. A hypersaline zone may form near the spring high tide boundary, as long as a hydraulic connection exists between the evaporating soil surface and the water table. This connection is the key to maintaining evaporation. The evaporation induced salt accumulation on the marsh surface can result in unstable flow driven by the upward density gradients, depending on the slope of the marsh surface. The findings from this study may help understand the ecohydrological functioning of these estuarine systems.