Quantitative Submarine Groundwater Ischarges In Caofeidian

The Caofeidian project started in2002. With the large-scale construction inCaofeidian, the geological environment protection and the virtuous circulation of thegroundwater system become very important. Studying the characteristics ofgroundwater-seawater circulation in coastal aquifers, including groundwater flowdynamics, solute transport and quantitative estimates of submarine groundwaterdischarge (SGD), has important theoretical and practical significance for waterresources, environmental geological survey and protection, as well as construction ofthe building project.Generally, the length of the intertidal zone in Caofeidian is several kilometers. Weinvestigated a remaining lower intertidal zone abutting a land reclamation area, whichis located in Caofeidian Ecological City. Short-term and long-term monitoring wasconducted in a cross-shore transect which was consisted of three pits. Pressure sensorswere used to automatically record the pressure, temperature and salinity ofgroundwater and tidal seawater. The two dimensional finite element model MARUNwas used to simulate the observed pressure and salinity of groundwater. Themedium-scale numerical model was eatablished. By analyzing the observation data, anew method which was based on Darcy’ law, was used to estimate the SGD.The seepage faces were observed during several low tides, which indicated theexistence of SGD in the intertidal zine. The observation data indicate that there mostprobably exist the upper saline plume (USP), the freshwater discharge “tube”(FDT)and the classical salt wedge in the aquifer. The slight salinity fluctuating in phase withthe tides strongly indicated the vertical advective flow. The salinity variation in theshallow aquifer is closely related to the variation of the terrestrial freshwater recharge.Seasonal variations in precipitation led to corresponding seasonal changes in thegroundwater salinity of the aquifer.The simulation results of our model without tidal effects indicate that, the totalSGD is influenced by terrestrial freshwater hydraulic gradient, permeability of theunconfined aquifer and silt covering layer. The change of permeability of silt covering layer doesn’t affect the terrestrial freshwater SGD. The freshwater flushing process ofsaline water is related to the freshwater head on left boundary and permeability of theunconfined aquifer. The simulation results of the model considering the tidal effectsshows that, SGD changes with time and space in the intertidal zone and has thefollowing characteristics. There is large amount of groundwater discharges near thenew coastline. SGD increases during falling tide, and decreases during rising tide.SGD is closely related to tidal range and low tides. In an entire spring-neap tide cycle,the volume of groundwater discharging into the sea is1.43m3/m/d (in the sense that inunit length in the along-shore direction per day) and the net freshwater recharge frominland (terrestrial freshwater SGD) is18.2%of total SGD. But within a24-hour tidecycle, the SGD is1.44m3/m/d. Numerical simulations show the presences of USP,FDT and the classical salt wedge in the aquifer. Since the simulation results ofgroundwater head and salinity have considerable differences from the observations,some hydrogeological parameters need further adjustment in the future work.SGD in a mudflat in Caofeidian estimated by the vertical hydraulic gradientmethod is23.2m3/m/d。...