A field study on groundwater dynamics in a salt marsh – Chongming Dongtan wetland

Subsurface hydrology plays an important role in governing the behaviour of salt marshes, in particular, nutrient exchange, plant zonation and carbon cycling. The objective of this paper is to examine the hydrological characteristics of a salt marsh based on a field investigation conducted at a cross-creek section in the Chongming Dongtan wetland (Shanghai, China). Field measurements showed complex groundwater table fluctuations, controlled by multiple factors. The groundwater table near the creek responded quickly to the creek water level fluctuation, in contrast with the weak water table response in the marsh interior. The average elevation of the groundwater table near the creek was lower than that in the interior, suggesting that the optimal soil aeration condition occurred near the creek. The variations of pore water salinity along the cross-creek transect also appeared to be complex even though a low salinity zone remained present near the creek levee over the investigation period. Relatively strong pore water circulation near the creek prevented local solute accumulation. These results are essentially consistent with the hypothesis that the area near the creek is better suited for plant growth. In the marsh interior, weakened drainage and flushing effects may leave the soil largely saturated for a long period and suppress carbon exchange between the marsh soil and ambient environments (coastal water and atmosphere).