擬似三維地下水變密度質量傳輸模式之發展

Abstract

本研究之目的係發展一套擬似三維地下水變密度質量傳輸模式，藉以探討海水入侵之過程與釐清其他物理特性對其入侵深度之影響。模式中結合流體質量守恆方程式與溶質質量守恆方程式，並利用濃度與密度之本構關係以及 建立耦合計算關係。考量地下水層空間尺度在水平方向遠大於垂直方向，故在垂直方向以水平切層，並假設每層中之物理量為均值，以分層積分方式將三維計算轉換為水平二維與垂向一維之計算架構。 分別以垂直流濃度場、水平流濃度場、 三種不同案例驗證模式之正確性後，再探討水力傳導係數、淡水補注量、表層補注、表層抽水對趾部海水入侵之影響。模擬結果顯示水力傳導係數與入侵距離呈正相關，而淡水補注量與入侵距離呈負相關。表層補注與抽水將會影響海水自然對流狀態，表層補注量增加則入侵距離隨之減少，表層抽水量過大將導致所抽取地下水鹽化，並加重海水入侵之趨勢。

In this study, a quasi 3-D groundwater density-dependent mass transport model was developed to investigate the process and the impact of physical properties on depth of seawater intrusion. The mass balance of fluid with the interaction of Darcy’s law in the model and solute mass transport equations are numerically coupled. In addition, the variation of density is assumed as a linear relation of concentration. Furthermore, the horizontal length scale of the computational domain, referring to the natural geometry, is assumed much larger than the vertical length scale. Thus, this study employs the layer concept to divide the three-dimensional solute transport equations into depth averaged (two-dimensional) and vertical variation (one-dimensional) equations.

The three different cases i.e. vertical solute transport, horizontal solute transport , were used to verify the model, and then hydraulic conductivity, amount of fresh water recharge and surface recharge or discharge are respectively investigated by a series of hypothetical scenarios. The simulation results show positive correlations between hydraulic conductivity and intrusion of the distance, and negative correlations between amount of fresh water recharge and intrusion of distance. Surface recharge and discharge will affect free convection. Amount of surface recharge increases with decreasing of distance of intrusion. Surface discharge will lead to salinization of groundwater and aggravate situation of seawater intrusion.