雙層含水層中含滯水層之污染物傳輸研究

Abstract

本論文主要目的，在研究雙層含水層系統中污染物傳輸問題，此雙層水層系統包含兩個水層中間夾著有限厚度的滯水層，形成含水層-滯水層-含水層的系統。就此，我們發展一個描述在此系統污染傳輸的一維數學模式，在含水層中傳輸的機制主要是傳流與延散，滯水層中傳輸的機制主要是傳流與擴散，在上端含水層移行的污染物，可穿透滯水層而進入下端含水層。所建立的傳輸模式，利用拉普拉斯轉換方法，可導得拉普拉斯域解析解，透過數值逆轉，可求得污染物在各水層中濃度對時間的分布。同時，本文也發展顯式有限差分模式，以模擬污染物在該水層系統之二維傳輸。數值模擬的結果，顯示位於上、下水層之深度平均濃度與本論文推導的解析解預測結果，僅存在些微差異。而研究結果顯示，上端水層的污染物遷移，受到滯水層的影響，速度明顯的減緩，若忽略滯水層內的傳流，會低估下端含水層的污染物濃度，尤其污染物釋放時間很長時。反之，若增加滯水層的流速，則下端含水層的污染物濃度會顯著增加。

This study deals with the issue of one-dimensional contaminant transport in an aquifer-aquitard-aquifer system, where the aquitard lies between two aquifers. Different from all the previous studies on analysis of the contaminant transport in a leaky aquifer system only consider the diffusion mechanism in the aquitard with a thin or thick thickness. We developed a mathematical transport model in the aquifer system with considering transport of solutes in aquifers controlled by advection and dispersion processes and in aquitard by advection and diffusion. The solutes in the upper aquifer may penetrate the aquitard and enter the lower aquifer. The Laplace-domain solution of the model for concentration distributions is obtained by the Laplace transform technique and its corresponding time-domain results are computed numerically by using Laplace numerical inversion. Meanwhile, an explicit finite difference model is also developed to simulate two-dimensional contaminant transport in the system. The simulated depth-averaged concentrations in the lower and upper aquifers slightly differ from those predicted by the present solution. The results show that the movement of contaminant in the upper aquifer is slowed down considerably due to the advective transport in aquitard. When neglecting the aquitard advection (a zero Peclet number), the concentration level in the lower aquifer will be underestimated, especially at late times. In addition, the contaminant concentration in the lower aquifer increases significantly with aquitard’s Peclet number.