Submarine Groundwater Discharge And Associated Of Nutrients In A Tidal Sandy Beach Of Daya Bay,China

Studying groundwater hydrodynamics and submarine groundwater discharge(SGD)in coastal aquifer systems,which includes the groundwater flow and solute transport,the mechanism and process of land-sea interface flux exchange and their impacts on the coastal ecosystem and biodiversity can prevent ecological environment problems in coastal areas.In addition,Researches on groundwater hydrodynamics and SGD can provide a reliable basis for relevant departments.In this paper,we investigated the groundwater flow and solute and nutrient transport in a typical transect of a tidal beach at the Xunliao bay area located in the east coast of Daya bay.The data of the pressure,salinity and nutrients of groundwater and seawater were obtained from either,five observation wells at the Transect,or the water samples near the Transect.The two-dimensional numerical model MARUN was used to simulate the groundwater flow and the transport and decay of nutrients along the Transect.Submarine groundwater discharge(SGD)and and fresh submarine groundwater discharge(FSGD)and associated nutrient fluxes are estimated based on the numerical simulations.The results of field observation and numerical simulation show that nutrient concentration has a strong negative correlation with and salinity in the study area,i.e.,the nutrients are low where the salinity is high.Specifically,the salinity is low in the upper and lower intertidal zones,and high in the middle of the intertidal zone,while the nutrient concentration is high in the upper and lower intertidal zones,and low in the middle of the intertidal zone.It is concluded that there are an up salinity plume(USP),a fresh salt water mixing zone and a fresh water discharge tube(FDT)in the modeling domain.The nutrient of the groundwater in the tidal beach is higher.There is no large river flowing into the sea in this area.The nutrient input to the sea is mainly from groundwater.Numerical simulation results show that the total submarine groundwater discharge(SGD)from the beach is 8.68m2/d,and fresh submarine groundwater discharge(FSGD)is 2.03m2/d,accounting for 23.4% of the total SGD during an entire spring-neap tide cycle.Inorganic nitrogen flux from inland is 18.67 g/(d·m),and inorganic nitrogen flux associated with SGD is 14.39 g/(d·m).That is to say that 23% of DIN mass flux has been degraded.Previous studies indicated that SGD in the subtidal zone is very small.Numerical simulation results of this study,however,show that SGD in the subtidal zone is 1.84m2/d,accounting for 23.4% of total SGD during an entire spring-neap tide cycle.