異質性含水層中水文地質參數之有效推估

Abstract

抽水試驗是現地實驗中推估水文地質參數相當常見的方法。已有許多研究探討及推估均質等向性含水層中之水文參數，但僅有少部分研究著重在非等向異質性含水層中之水文特性。 實際上，地下含水層參數均為異質性，但卻利用均質且等向的方式『有效參數』來描述異質含水層參數。本研究主要探討在抽水試驗中，由觀測水位資訊，來推估非等向性含水層之有效參數。因此本研究利用VSAFT2模式產生異質性含水層之場址，進行抽水試驗模擬，利用下述方法推估非等向性有效參數並加以比較：(1)Papadopulos解析解－曲線法(2)權重平均(3)達西定律及(4)洩降空間矩分析。由結果得知，利用Papadopulos解析解推估參數，長時間的抽水試驗分析所得的流通係數是代表在抽水井有效半徑內所有流通係數的某種平均值；此平均值與抽水井和觀測井附近的地下水參數數值有很大的關係，而且可能受到有效半徑內地質異質性的影響。利用權重幾何平均的方式對異質性地層做參數推估比較，權重算術平均的方式會有較好的代表性。利用達西定律得到的非等向性情況下之水文地質參數與利用Papadopulos解析解方法求出之非等向性情況下水文地質參數有很大的差異。應用修正空間洩降矩分析推估在任意時間內隨機場中之有效參數，如有效流通係數張量或有效儲水係數。

Pumping test is the most common way to estimate hydro-geological parameters in the field experiment. There are a lot of investigations to estimate hydraulic parameters of homogenous aquifer, but merely fewer researches focus on the hydro-geological characteristics of the anisotropic and heterogeneous aquifer. Actually, the hydro-geological parameters of aquifer are heterogeneous in the field, but many people use the effective parameters which are anisotropic and homogeneous in heterogeneous aquifers to describe the parameters of heterogeneous aquifer. In this paper, we will use the groundwater level observed from pumping test to estimate the effective parameters of the anisotropic aquifer. At first, we use the numerical model, VSAFT2, to generate a heterogeneous study aquifer. And then, we proceed the pumping test on this study area. Finally, we use the four methods, Papadopulos analytical solution, weighting average, Darcy’s law and spatial moment analysis, to indentify the anisotropic effective parameters and analyze the results. Results show that the effective transmissivities identified by the Papadopulos analytical solution represent an averaged value of all transmissivities within the effective cone of depression. This value is strongly related to the hydro-geological parameters near the pumping well and observation well, and the hydro-geological heterogeneity within the effective cone of depression. The optimums of heterogeneous aquifer by the geometric weighted mean method are better than ones by the arithmetic weighted mean method. The hydro-geological parameters of anisotropic aquifer estimated by Darcy’s law are very different to ones by Papadopulos analytical solutions. It is successful to apply spatial moment analysis to estimate the effective parameters, such as the effective transmissivity coefficient tensor or effective storage coefficient in any duration for any data from the curve of drawdown versus time.