Quantitative Study On Seawater And Groundwater Exchange Rate In Muddy Tidal Flat In South Coast Of Laizhou Bay,China

With the development of new methods and new technologies,the exchange of seawater and groundwater has become a hot topic in the field of oceanography and hydrogeology.Not only Submarine Groundwater Discharge(SGD)is an important part of participating in the global water cycle,but also it is an important way to transport marine nutrients,pollutants and other chemicals to the inland.SGD has an important impact on coastal ecological environment.As an important fishing and salt base,Laizhou Bay is affected by human activities,which leads to serious pollution.The amount of pollutant output is more than the amount that input into Laizhou Bay.The south coast of Laizhou Bay is a muddy tidal flat with gentle slope and relatively low permeability coefficient,and there are few studies for this area now.In this paper,the tidal flat with large exudation surface in the southern coast of Laizhou Bay is taken as the research object.The typical section selected was about3800 m.Five monitoring points(W1,W2,...,W5 from inland to coast)were selected along the profile,and applying CTD-Diver to observe the groundwater head and salinity.Monitoring time was from 15:00 on August 7,2015 to 15:00 on August 12,2015 and data was recorded every hour.Vertical penetration coefficient in the top layer of aquifers(silt layer)was measured by"vertical tube dewatering head experiment".Based on the analysis of the variation of water head and salinity with time,groundwater head and salinity of W3 were special.It was suspected that there was a more permeable“skyhole”in the weakly permeable layer at W3.Brine extraction and seepage in the inland area,resulted in groundwater salinity increasing.In the meanwhile,the more permeable aquifer allowed W3 to receive more supplies from the inland.Based on the available data,field investigation and generalization of hydrogeological conditions,a conceptual model of hydrogeology was established.The constructed hydrogeological conceptual model was simulated by SEAWAT-2000module in Visual MODFLOW,and the simulation values and observations of water head and salinity of W2~W5 are fitted.Based on the fitting results,groundwater flow and solute transport in aquifer were simulated.Based on the above method,during one tidal cycle,submarine groundwater discharge value was 17.46 m~3/m/d,and the discharge caused by seawater recirculation was 10.74 m~3/m/d,and seawater infiltration capacity was 2.71 m~3/m/d.In the rising tide stage,submarine groundwater discharge decreased,and the direction of the flow in permeable muddy layer was roughly from seawater into aquifers,and seawater infiltration capacity increased,while the ebb phase was reversed,which was due to the rising or ebb tide phase,the tidal head was gradually higher or lower than the groundwater head,and the hydraulic gradient increased between tidal head and groundwater head,and the seawater-groundwater exchange took place.As the slope of muddy tidal flat was small(less than 0.04%),permeability was poor,solute transport within a short time was not obvious.This study further clarified the mechanism of seawater-groundwater exchange and provided some basis for the management of the groundwater in the coastal zone.