應用示蹤劑試驗調查裂隙岩層中優勢地下水流路徑

Abstract

裂隙岩層中地下水流及汙染傳輸主要受控於少數的透水性較佳的裂隙，然而裂隙水流分布複雜，調查極為不易。岩芯分析與傳統井測不易判釋透水裂隙，而水力試驗難以估算裂隙的水文地質參數，常用的示蹤劑試驗在裂隙岩層中經常失靈，因此本研究嘗試整合運用複井水力試驗、熱脈衝流速儀量測及示蹤劑試驗，在研究井場進行現地試驗，探討偵測裂隙岩層中優勢水流路徑的方法，期能對岩層地下水資源調查與地下水污染傳輸過程等議題，建立有效的調查方式。 本研究之試驗井場位於南投縣信義鄉台大實驗林和社營林區內，鑽取的岩芯主要是輕度變質的頁岩或粉砂岩，岩層多處呈現破裂，地表裂隙調查指出井場附近的砂岩層至少存在三組裂隙。本研究首先進行熱脈衝流速試驗，偵測各個井孔中透水性較佳的岩層裂隙所在位置，試驗結果指出岩層中裂隙的透水性與裂隙密度大小並無絕對關係。然後進行複井抽水試驗，確認井孔之間的水力連通程度，再依據水力試驗所得之初步調查結果，選取水力連通性較佳的兩個井孔進行示蹤劑試驗。繼而改良過去的示蹤劑試驗方法及材料，分別使用奈米鐵及氯化鈉水溶液，運用灌注方式製造強制流場，調查井孔之間岩層裂隙水力連通性。研究結果發現，氯化鈉示蹤劑可以證實兩個井孔的水力連通性，然而氯化鈉的高擴散性難以定位透水裂隙所在；具磁性的奈米鐵示蹤劑，則可成功偵測出監測井與投注井水力連通裂隙的位置，因此奈米鐵示蹤劑試驗材料與方法有潛力發展成為未來調查岩層優勢地下水流路徑工具之一。

Groundwater flow in the fractured rock is mainly controlled by a few permeable fracture. Core analysis and acoustic televiewer can be used to examine the fracture density and orientation, but difficult to characterize whether rock fractures are permeable. In this study, we integrate a variety of field tests, including tracer test, hydraulic test, and heat-pulse flowmeter test, to locate the permeable fractures and to detect the hydraulic connections between boreholes. The field study was conducted at Heshe hydrogeological experimental well station in central Taiwan. There are eight test wells and two observation wells at the site where the in-situ rock below the overburden is primarily shale and siltstone. Surface geological survey shows three sets of joint planes. In order to detect the preferential pathway of groundwater flow, heat-pulse flowmeter measurement was adopted to identify the depth of permeable fractures in the boreholes. It was followed by the multi-well pumping test for investigating the hydraulic connectivity between these wells. The tracer tests were then used to detect the hydraulic connectivity of permeable fractures between two wells. By injecting saltwater or nano zero-valent iron into one well, it is possible to detect the variation of tracer in the nearby wells. We found nano zero-valent iron adsorbed by a magnet array in the detection well in two tracer tests. The adsorbed nano iron can specifically locate the position of permeable fractures connecting to the injection well. Our study results show that the nano iron tracer test is a potential useful tool to investigate the preferential groundwater flow in the fractured rock.