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年度	105	專案性質	實驗性質
專案類別	研究專案	研究主題	評估
申請機構	逢甲大學	申請系所	環境工程與科學學系
專案主持人	陳建隆	職等/職稱	副教授
專案中文名稱	以地電阻影像探測技術推估地下水流速流向：實場驗證
中文關鍵字	地電阻,地下水流速,地下水流向
專案英文名稱	Determining groundwater velocity and direction with electrical resistivity tomography technique: field validation
英文關鍵字	resistivity, groundwater velocity, groundwater flow direction
執行金額	1,240,000元
執行期間	2015/11/17 至 2016/11/16
計畫中文摘要	本計畫為去年度計畫的延續。主要目的為對以地電阻探測技術量測地下水流速流向方法進行實場驗證。此方法主要目的為在只有一口地下水監測井的限制下，發展一套可靠且有效率之地下水流速流向測定技術。去年度計畫砂箱實驗結果顯示量測地電阻變化可有效量測地下水流速流向。因此本年度以實場展示此技術之可行性。並藉由實驗建立現場量測基本流程與方法。實驗共可分為五個主要部分：(1)場址及實驗設置；(2)示蹤劑試驗；(3)場址示蹤劑傳輸模式模擬；(4)地電阻試驗；(5)地電阻推估地下水流速流向。 本計畫選擇雲林科技大學校園內做為進行實場示蹤劑及地電阻試驗場址，該場址內既有監測井為17口，含水層約20 m，監測井完全貫穿含水層，開篩長度從地表下1.5 m至20 m。地電阻試驗分為兩階段，第一階段為方法驗證。第二階段為地下水流速流向推估。本計畫搭配雲科大溫志超教授研究團隊進行的抽水試驗，抽水井連續抽水三天，在抽水試驗的第三天流場穩定後，即進行示蹤劑地電阻試驗，以固定體積的高導電度鹽水(NaCl；濃度約2.45 g/L，導電度5000 μS/cm)的鹽水從上游的注入井注入，同時下游抽水井以固定流量持續抽水。 本計畫地電阻監測共執行六次，分別編號為試驗No. 1至No. 6，試驗No. 1至No. 3為五測線、試驗No. 4為三測線、試驗No. 5至No. 6為單測線。每1 m設置一電極，共設置16至32支。示蹤劑以脈衝式點源 (instantaneous point source)注入，成分為濃度約為2.45 g/L NaCl水溶液(導電度5000 μS/cm)，注入體積為100至250公升，注入時間約1至2小時。當鹽水注入後，注入井將停止注水，但抽水井則會繼續抽水。同時開始進行監測井中導電度與地電阻量測，地電阻間測時間分別為5、5、8、8、23及21小時。 本計畫目前獲得之研究數據指出，地電阻確實能感測到實場地下水受高導電度示蹤劑影響，產生較背景值為低的電阻，且電阻峰值隨時間有降低的趨勢。透過此電阻變化量隨時空變化的趨勢，應能解析地下水流場流速流向等訊息。但目前數據分析結果顯示，地電阻資料未能明確掌握示蹤劑的移動變化，主要原因可能是流場流速太慢，以至於在有限時間內測量的視電阻峰值的移動距離較地電阻電極間距短(1 m)，故無法解析出示蹤劑的移動方向。 地下水流場以MODFLOW模擬，而示蹤劑傳輸以MT3D模擬，模擬結果顯示抽水產生的人為流場，其地下水流速約為0.5至1.2 m/d，說明地下水流場流速確實較慢。實際監測時間內示蹤劑團移動的距離小於地電阻電極間距1 m。表示監測時間8至24小時的地電阻試驗，不足以偵測到示蹤劑團有顯著的移動，然而，注入井位置基本上仍會有高導電度的示蹤劑團，此現象與實場試驗結果相符。 根據本研究實場試驗結果，發現砂箱試驗與實場規模的試驗結果的不同，注入點及附近除了有電阻較背景值降低的現象，亦會發有電阻較背景值增加情況發生。表示高導電度示蹤劑進入地下水，不止是改變電場的分佈，更可能改變電流的路徑分佈，產生電壓差上升，結果以電阻增加的結果呈現，與一般的定電流的假設有所差異。
計畫英文摘要	This research extends the works of last year’s project. The main objective is to directly measure groundwater velocity and direction based on electrical resistivity tomography (ERT). The motivation for this technique is to be able to accurately and effectively identify the direction and velocity of groundwater flow with only one monitoring well. Last year’s project has validated the feasibility of this technique by conducting sandbox experiments. This research will validate the effectiveness of this technique in a real groundwater flow field. This research includes five main tasks: (1) setup experimental site; (2) conduct tracer tests; (3) construct and validate a numerical model for groundwater and solute transport of the tracer tests; (4) conduct electrical resistivity tomography tests; (5) measure the direction and velocity of groundwater flow of the test site. The site for this study is located in National Yuanlin University of Science and Technology. The aquifer within the site is about 20 m in depth. There are 17 monitoring wells, each screened between depths of 1.5 and 20 m, within the site. The tracer study is divided into two stages: verification of technique and measurement of groundwater velocity and direction. Concurrent pumping tests were conducted during the ERT experiments. Therefore, the tracer experiments were conducted after the groundwater flow reached equilibrium, which was approximately 2 days after the pumping started. The tracer tests are conducted using an injection approach. Solutions containing 2.45 g/L (5000 S/cm) of sodium chloride was injected into the aquifer. The ERT technique in the test site was consisted of 5 testing lines, each with 16 electrodes of 1 m apart. The 5 testing lines were parallel with each other with a spacing of 1 m. The injection volume is 250 L and the injection process lasted 5 min. One tracer was conducted so far. The results of the tracer study indicate the change in groundwater electrical conductivity can be detected by the ERT technique. However, the ERT results show insignificant movement of the concentration plume. The main reason is that the groundwater velocity induced by the pumping process is only 0.1 m/h, which means the concentration front moved less than 1 m during the 5 h of ERT monitoring period. Consequently, the setup of the ERT electrode needs to be modified to address this problem. In future tracer studies, the equilibrium groundwater flow will be analyzed before the tracer studies and the spacing between each ERT line will be decreased to 0.25 m, whereas the distance between electrodes will be still 1 m. A model for the groundwater flow within the test site will be constructed using MODFLOW. The model will be calibrated and validated with the data from the pumping tests. Afterwards, the model will be used to generate groundwater flow field for a solute transport model constructed using MT3D. The expected outcome of this project is to validate the technique for effectively and accurately measuring groundwater velocity and direction.