Influence Of Infiltration On The Salinization Time Scale Of An Estuary Reservoir And Heat Tracing Of The Surface-subsurface Exchange

Pollutants accumulated in sediments often affect the quality of overlying water through the molecular diffusion mechanism.The surface water-groundwater exchange,especially slow exchange,will promote the formation of a unique vertical profile of pollutants at the sediment-water interface,which has an important impact on the transport and fate of pollutants.It is generally believed that with time elapsing,slow infiltration can restrain the influence of upward diffusion of solutes from sediments on the quality of overlying water.Quantifying the influence of slow infiltration on the time scale of overlying water affected by contaminated sediments can provide important information for decision-making of water environment remediation.However,there is a lack of accurate criteria to identify the critical time node of polluted sediments affecting their overlying water and a mathematical model to effectively simulate sediment-reservoir water coupling.In this dissertation,a method for quantifying the influence time of saline sediments on reservoir water in the presence of slow infiltration was established,and the influence of infiltration velocity on the time scale of salinization was discussed by taking the proposed Muguandao Estuary Reservoir in Qingdao as an example.After establishing the method to determine the time scale,quantifying the exchange rate between surface water and groundwater became the key to discuss the time scale for a specific Lake or reservoir.Therefore,a potential seepage surface water environment was selected to invert the vertical steady and unsteady exchange velocity between surface water and groundwater by the heat tracing numerical method,which is realized in an open source software R.And then the influencing factors of inversion flow velocity and the spatio-temporal pattern of surface-subsurface exchange were explored.The research results are as follows:（1）By modifying the traditional Péclet number and redefining Pe=|uC/D（（?）C/（?）x）|x=0,the transient relative roles of downward advection and upward diffusion on the transport of inactive solutes can be characterized.Pe=1 is an accurate criterion for determining the critical time node when saline sediments no longer affect the overlying water.A single-domain model was developed to simulate salt transport in the case of seepage-diffusion coexistence.This model thoroughly couples the interaction between sediment（one-dimensional）and reservoir water（zero-dimensional）interactions,and its computational efficiency is significantly higher than that of the current alternating sediment and water column model（In the simulation scenario,the running time is 8.79 seconds and 84.43 seconds,respectively）.In combination with the newly defined Pe criterion,the established single-domain model can accurately determine the influence time.On the above basis,an empirical formula,t=5.25D/u²,for estimating the time scale is derived.（2）Infiltration can effectively decrease the salt concentration and the influence time of saline sediments on reservoir water.The calculation results for the Muguandao Reservoir indicate that the salt concentration in reservoir water decreases with infiltration rates at the same moment;the influence time of saline sediment on reservoir water decreases with infiltration rates in a power function.The infiltration of centimeter per year plays a decisive role in the change of influence time,which is reduced from 970 to 19.3 years in the range of（1-7）×10^-2 m/year.Therefore,slow infiltration cannot be neglected in evaluating the long-term effects of internal pollution sources.（3）One-dimensional heat transport model is built based on the open source software R to automatically invert surface-sub surface exchange rates.The boundary conditions of the model are to directly input the upper and lower boundary temperature time series data in the time step without data preprocessing.The inversion results show that exchange velocities of No.2,No.4 and No.5 monitoring points are 0.0018 m/h,0.024 m/h and 0.012 m/h respectively,all of which are groundwater recharged by surface water;but No.3 is groundwater recharging surface water at a rate of 0.006 m/h（No.5,3 and 2 are at the upstream of the river rubber dam and away from the rubber dam in turn;No.4 is the same distance as No.3 to the rubber dam,but far from the shore）.（4）High sensitivity is the premise of successful inversion of unknown parameters,and some neglected model assumptions are related to the accuracy of inversion.The results of simultaneous inversion of the two parameters（velocity and thermal dispersivity）and sensitivity analysis of the parameters indicate that the thermal dispersivity can be ignored,and the exchange velocity is of high identifiability.Accuracy of vertical one-dimensional velocity inversion may be influenced by some neglected factors such as non-vertical component of flow and free heat convection at the sediment-water interface.（5）The unsteady exchange velocity can be inverted by splitting the monitoring temperature data into shorter time series（6 h or 12 h）.The results from simulated temperature show that the shorter the time series of the split boundary,the more fitting the simulation temperature to the measured temperature.The errors caused by initial condition will also increase with the number of segments.Therefore,the inversion of dynamic velocity by splitting short time series is more suitable for the situation where the exchange rate changes greatly with time.In addition,the inversion results indicate that the vertical surface-subsurface exchanges exhibit significant differences in time and space,which exist not only in exchange intensity but also in direction.The spatio-temporal difference may be related to sediment heterogeneity,riverbed topography,water head difference,and free convection driven by temperature difference.