Numerical Modeling Of The Groundwater Flow In A Brackish Tidal Marsh In Chongming Island, Estuary Of The Yangtze River

Groundwater discharge from the inland in a brackish tidal marsh is an important pathway for fluxes of nutrients and contaminants to estuaries. The research on the mechanism of groundwater-surface water interactions with the distribution of plant types is of practical significance. To better understand the geologic and hydrologic controls on these fluxes, a 1km-long cross-shore transect was chosen in the saltwater-freshwater transition zone of the Yangtze River estuary to conduct field work of measurements and survey including vegetation characteristics.The whole transect has two slope break points (terrain change points) and is submerged during high tides of spring tides. Five monitoring wells were set along the transect and hourly observed data of water pressure, temperature, electrical conductivity were obtained. Two wells which were closer from the inland and river were kept monitoring one year, and the others were kept for about 31 days during the summer of 2013. The soil saturation conditions and vegetation species have good relativity. The groundwater salinity of tidal marsh is insensitive for periodic submerge of tidal river. In addition, data from the two long-term observation wells indicated that groundwater temperature and electricity conductivity are of high positive correlation.The two dimensional finite element numerical model MARUN was used to simulate the field observations. On the basis of field experiments and recent literature, a two-layered hydraulic structure model was established. The vertical hydraulic conductivity was set to be 10-7m/s for the ～3 m thick surface layer and 10-5m/s for the-22 m thick bottom layer. The horizontal and vertical anisotropy ratio equals 1.0 for the surface layer and 3.0 for the bottom layer. The simulated water tables matched the observations reasonably well.Numerical simulation results show the following. Over the transect, tidal forcing was found to generate an averaged outflow of 0.33 m3m-1d-1 during a spring-neap period. The inland freshwater recharge flux was estimated to be 0.21m3m-1d-1. The groundwater discharge area located in the area which is 400-1000m away from the land and river’s infiltration area located in the 180-400m’s area. The groundwater drainage area and river infiltration area are just Spartina alterniflora and reed growth area, respectively. The terrain plays a very important role in the groundwater circulation. The average flow velocity normal to the surface in the discharge area is 4 mm/d, and the average rate in the infiltration area is 3 mm/d. The maximum velocity occurs in the slope break points. Moreover, the river-groundwater interaction along the transect is finally graphically conceptualized.