Studies Of The Numerical Simulation Of The Water Quality And Quantity Transport Under Sea Surface Water And Groundwater Interaction

The studies of the interaction between the surface water and groundwater are the key and hot problems, which is also the important physical process in the interaction of the land and water. International Association of Hydrological Sciences (IAHS) in 1986 and International Association of Hydro geologists (IAH) in 1994 submited this topic into the congress, and then it came to the hot problems of the water resource in hydrology and hydrogeology. However, due to the limitation of the computer and other matters, the modeling of the interaction between the surface water and groundwater, always trended to either surface water modeling or groundwater modeling, and the surface water movement is described by the 1-D flow or 2-D flow. In fact, the sea water movement is 3-D. With the development of the economy and the computer, the accuracy of numerical simulation should be higher and higher. Therefore,3-D groundwater and 3-D surface water were coupled to simulate the spatial distribution of water quality and quantity with time in this paper.The first part of this thesis is on numerical simulation of the interaction between the groundwater and surface water as boundary condition of the aquifer, that is the movement of the surface water is ignored in the stress period. Firstly, the traditional finite difference scheme and semi finite difference scheme are established under the condition that the difference of density between the fresh water and salt water is ignored. The influences of the initial condition, time step and spatial step to the results of finite difference method are discussed, and the relations and difference among semi finite difference method, traditional finite difference method and analytical solution were studied. Secondly, the finite difference scheme was established if the density of salt water was considered. By comparing the numerical results from semi finite difference method, traditional finite difference method and analytical solution, the limitations and demerits of these models were obtained by numerical simulations. The results shows(1) The semi-analytical numerical solution of the groundwater level, which is numerical for the spatial variables and analytical for the time variable. The results from numerical experiments show that the dimension of spatial step does affect the numerical results very much, by comparing the traditional finite difference method, semi finite difference method and analytical method. In contrary, the time step dimension is an important factor to the accuracy of the numerical simulations.(2) If the tidal amplitude is small in relevant to the aquifer's thickness in the unconfined aquifer, the error caused by the linearization for the nonlinear model can be ignored, or the error is large.(3) The groundwater flow in coastal aquifer is simulated by the traditional finite difference method, the initial condition is necessary for the unsteady flow. The semi finite difference method and analytical method do not need it, due to the tidal period. By comparing the simulations among traditional finite difference method, semi finite difference method and analytical method, the results show the average sea level is better as the initial head for the unsteady flow.(4) The storage coefficient is a very sensitive factor for the groundwater level movement caused by the sea intrusion. If it is small, the error, caused by neglecting the difference between the fresh water and salt water, can not be ignored. However, as for the unconfined aquifer where the storage is small, the error exists in the area not far from sea water, and is small for the area far form sea water.(5) The sea water moves to the inland with time. Due to the fluctuant of sea water, it moves fast at the beginning, but slowly later.The second part corresponding to the chapter three is about the interaction between the groundwater and surface water whose movement has been considered by the 3-D Navier-Stokes equations. Firstly, the upstream weighted semi implicit finite difference scheme was established, based on the Katerina's numerical method. Secondly, the character of this finite difference scheme was analyzed, and then the improved finite difference method was presented——weighted factor predicting method, and the fomula of the weighted factor was derived. Lastly, the rationality of the improved method was verified by two numerical simulations, at the same time, the influence of the aquifer property to the water quality and quantity distribution spatially was simulated.The results show(1) The computation of Katerina's numerical method is small to simulate the surface water and groundwater interaction, but the convergent condition is difficult to be satisfied. If it can not be met, mass in region can not be balanced and the water level is fluctuant, more than the maximum or smaller than the minimum. Therefore, the improved numerical method was presented, and the numerical simulations show it is more reasonable.(2) If the permeability of the aquifer is low, and storage is high, the interaction between the surface water and groundwater is strong, in the contrary, it is not strong and the surface water level fluctuant decrease drastically.The third part corresponding to the chapter four is about the contaminant transport numerical simulation under the flow field of the groundwater-surface water. Firstly, the finite difference scheme of the solute transport governing equation was established, and then its law in the surface water and groundwater coupling flow field is simulated by the numerical experiments. Results show:the velocity of the surface water flow is very fast, comparing with groundwater flow velocity, so the contaminant moves faster in the surface water than in the groundwater. If some region was polluted, the contaminant should be limited in small zone by some measures, such as impermeable wall, and some biochemistry solvents could be injected into the aquifer, simultaneously, in order to purify the groundwater.The fourth part is about the practical application. Using the numerical method, the special distributions of the water quality and water quantity were simulated under the interaction of surface water and groundwater in Lei Zhou Peninsula. Firstly, the hydraulic background in this area should be made clear through arranging and analyzing the data from the field investigation. Secondly, hydrageological conceptual model was generalized, and the corresponding mathematic model and numerical model were built. Lastly, the special distribution of the water quality and water quantity was simulated by computer programs written by myself. The results show hydraulic head in aquifer in 100m from sea changes with the fluctuant of surface water, and it is almost not affected by the sea water tidal movement in the aquifer more than 100m from sea. As for the sea water intrusion, salt water moves into the aquifer in the beginning, but the interface of the salt water and fresh water does not move later. In Leizhou Peninsula, if the exploitation is not strong, the sea water does not move into the aquifer.