應用一維地電阻於水力傳導係數推估 -以濁水溪沖積扇為例

Abstract

水力傳導係數為一重要之水文地質參數，傳統上多以抽水試驗求得，因此需花費相當之成本與時間，取得資料之數量也因而受到限制。近年來已有許多實驗室或現地尺度研究，整合地表地電阻資料及抽水試驗資料推估水文地質參數，而其推估方法大都利用地電阻資料所推估之地層因子(Formation factor)與抽水試驗所得之水力傳導係數建立簡單之線性關係，惟前人研究通常忽略泥層之影響，且其結果僅適用於含泥量較少之區域。惟對於一完整之沖積扇而言，扇央至扇尾通常為泥層出現頻率較高之區域，因此泥質之影響實際上應不可忽略。 本研究將以前人研究為基礎，以沉積物分布，進行研究區域分區，再將試驗資料依區域分區進行分群，建立各分群之水力傳導係數推估式。接著再應用此水力傳導係數推估式，建立濁水溪沖積扇第一含水層於主扇範圍，包含舊濁水溪以南至新虎尾溪以北區域之水力傳導係數分佈。分群分析結果顯示，濁水溪沖積扇之主扇，在第一含水層可分為扇頂與扇頂以外區域兩個分群，各分群之水力傳導係數與地層因子之回歸關係成線性相關。水力傳導係數推估部分，比較本研究推估之水力傳導係數值、現地試驗資料及應用以往文獻推估式之推估值等，結果顯示本研究與現地試驗值之誤差介於11~58 m/day之間，遠較以往推估式所得結果合理，顯示本研究所建立推估式之正確性與適用性。 本研究目前在濁水沖積扇所推估之水力傳導係數場，可作為濁水溪沖積扇後續地下水相關研究，如地下水數值模式建置之依據，所發展之水力傳導係數推估流程，亦可推廣至其他相似之沖積扇，而有助於更廣泛的地下水相關研究。

Hydraulic conductivity (K) is an important parameter of an aquifer and is usually obtained using conventionally pumping test method. However, the pumping test method is time consuming and expensive so that only limited data can be collected. In recent years, some studies estimated hydraulic conductivity using surface electrical resistivity survey along with pumping test data. These studies demonstrate its efficiency with low requirement of pumping test data and low cost. However, majority of these studies apply simple linear regression to modeling the relationship between pumping test data (K) and formation factor derived from electric resistivity data without consideration of clay layers. Thus, the equations derived by these studies are only suitable for the study areas without clay layers. In fact, clay layers are commonly distributed in middle-fan and distal-fan. Therefore, this study divides study area, Zhuoshui River Alluvial Fan, into several zones based on the sediment distribution. Each zone has a linear regression equation derived from the pumping test data and formation factors. Moreover, this study applied these equations to develop the hydraulic conductivity distribution of the study area, which locates on the shallow aquifer of the major fan, which is bounded by the Old Zhuoshui River at the north side and by New Hu-Wei River at the south side. The result shows that the shallow aquifer of the major fan of Zhuoshui River can be divided into two zones, which are top-fan and non-top-fan areas. The regression results show good correlation between hydraulic conductivity and the formation factor in each zone. These regression equations are then used to estimate hydraulic conductivity in the study area. The results are compared between the field measurement and the results obtained from Khalil’s equations. The results indicate that the estimation error, between 11m/day and 58m/day, is much smaller than the estimation error obtained using Khalil’s equation. The results of this study can be further applied to other analyses such as groundwater modeling or water fluctuation method.