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年度	104	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：方法建置及驗證
中文關鍵字	電阻率,地下水流速
專案英文名稱	Determining groundwater velocity and direction with electrical resistivity tomography technique: method development and validation
英文關鍵字	Resistivity, groundwater velocity
執行金額	1,000,000元
執行期間	2014/12/1 至 2015/11/30
計畫中文摘要	本研究提出以地電阻探測技術量測地下水流速流向方法並評估其敏感度及準確度。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。由於污染物在含水層傳輸主要藉由地下水流之對流 (advection) 及延散而使污染團向下游移動擴大。因此地下水流速流向的掌握對描繪及建構污染場址污染物在含水層傳輸的概念模式有關鍵性影響。而此概念模式正確與否，對於污染團之圍堵攔阻或污染復育對策擬定及執行具有決定成敗之重要性。本研究另一個目的為建立地電阻影像探測所得之表面導電度 (apparent conductivity) 與孔隙水導電度之關係。 技術研發將藉由在實驗槽中進行示蹤劑試驗。實驗主要步驟如下：(1)在實驗槽中填充細砂，安裝地下水監測井並維持一穩定之地下水流場；(2)以高導電度之氯化鈉溶液為示蹤劑導入至流場中；(3)以地電阻探測儀量測含水層導電度變化並推估示蹤劑濃度分布；(4)分析實驗數據與理論濃度分布值比較推估地下水流速流向；(5)利用Archie’s law 分析地電阻測得導電度與地下水導電度之關係。 實驗槽為一長1.5公尺，寬0.5公尺，高1.0公尺之壓克力槽。槽體長端兩端各有一寬5公分之定水頭槽體。細砂填充以飽和填充法以避免造成層化現象。填充完後，將安裝地下水監測井以量測水頭及地下水導電度。本研究之示蹤劑試驗將進行三維之脈衝點源及連續式點源。地電阻電極之排列為溫奈及雙偶極，電極棒之間隔為5，10及20公分，運用不同之電極排列及間距可以分析評估此方法之準確度及敏感度。另外所測得之地下水導電度與表現導電度，可由Archie’s law 算出含水層之formation factor。本研究將探討不同之電極排列及間距和輸入電流對formation factor 之影響。 本研究預期將可研發出一經濟且準確之地下水流速流向測定技術，在未來污染場址調查或場址復育，污染源調查乃至於地下水污染潛勢調查及使用管理都具有重要貢獻。 研究結果顯示，ERT重複性良好、敏感性佳，可測得鹽水導電度600、800、1000、1200 μS/cm之差異。由Archie’s law可驗證鹽水電阻與ERT量測含鹽水之石英砂電阻之關係，顯示施測結果具有準確性。Dipole-dipole排列法下，輸出電流大小對於靜態實驗槽之ERT視電阻率量測結果無顯著差異。相同電極數量，偶極間距越短，施測時間越長，可探測地層範圍之資料點數越多，解析度越高。改變電極插入地表的深度，可明顯發現越接近地下水，探測得電阻率越低。 脈衝式與連續式示蹤劑試驗結果，顯示ERT可明顯地捕捉到示蹤劑於地下水中流動造成電阻值改變之訊息，此成果可應用於地下水流速流向之推算。而增加測線數量，可提高流場流向判斷的解析能力。由單測線試驗計算地下水流速與控制流速相差±10%以內，並發現深度增加，流速有降低的趨勢。多測線試驗計算流速高於控制流速，可能受流場非均質性及短流影響，實際流動面積小於槽體截面積，或槽體底部石英砂密度與空隙率受重力壓實影響，導致地下水主要流動發生在上層，亦可能為ERT示蹤劑試驗計算流速較高的可能原因。以監測井量測的地下水電阻變化與ERT監測結果，透過Archie’s law關係修正後比較，顯示觀測值與推估值兩者相近，表示驗證結果良好。
計畫英文摘要	The main purpose of this research is to develop a technique for directly measuring 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 of groundwater flow and direction with only one monitoring well. Accurate knowledge of groundwater velocity and direction is essential for constructing a conceptual model for groundwater water and soil transport in a contaminated site as advection is the main mechanism for contaminant transport. Without an accurate model for the transport of contaminants, the containment and remediation efforts for groundwater and soil contamination have very little chance of success. The other objective of this research is to evaluate the relationship between the apparent electrical conductivity and groundwater electrical conductivity. Tracer studies will be conducted to achieve the objectives. The main procedures for the tracer studies are as the following: (1) fill the sandbox with sand, install groundwater monitoring wells, and maintain a steady groundwater flow in the sandbox; (2) inject high electrical conductivity sodium chloride solution into the flow; (3) use ERT to calculate the change in electrical conductivity and use it as the change in sodium chloride concentration; (4) obtain groundwater velocity and direction by comparing the observed concentration with the theoretical values; (5) evaluate the relationship between apparent electrical conductivity and groundwater electrical conductivity using Archie’s las. he sandbox is made of acrylic plates and has dimensions of 1.5 m in length, 0.5 m in width, and 1.0 m in height. One 5-cm-wide compartment is attached to each end of the sandbox for use as a constant head boundary. The packing of sand will be conducted using sequential saturation method to prevent layer formation. Groundwater monitoring wells for water head and pore water electrical conductivity measurements will be installed after packing. The tracer experiments will be conducted under instantaneous point source and continuous point source modes. The ERT will be obtained using two different electrode arrays (Wenner and dipole-dipole array) and three electrode spacing (5, 10, and 20 cm). Results from these different settings of electrodes will be used to evaluate the accuracy and sensitivity of the technique. Archie’s law will be used to evaluate the groundwater electrical conductivity and apparent conductivity and obtain the formation factor for the simulated aquifer. The effect of electrode arrays and spacing and input current on formation factor will be evaluated. The research will develop a new technique for accurately and cost-effectively measuring groundwater velocity and direction with only one monitoring well. Therefore, it could be very useful in the application of site characterization and site remediation, locating contaminant sources, and even in groundwater management and containment of groundwater pollution plumes. The results from reliability and sensitivity of the ERT experiments are acceptable. The Archie’s law can be validated with the apparent resistivity from the ERT experiments and the solution resistivity, which means the ERT technique can accurately detect the change in solution resistivity. Using a dipole-dipole array setup, the effect of the applied current strength on the apparent resistivity is insignificant. The apparent resistivity decreases with the depth of the electrodes. The velocity and direction of groundwater was measured using the ERT technique under both impulse and continuous tracer injection schemes. The results from both schemes show the technique can detect the groundwater velocity and direction. The measured groundwater velocity from both schemes is within 10 percent of the actual velocity. It is observed that the flow regime in the sandbox was non-homogeneous although great effort was put into packing the sand during experiment setup; this now-homogeneous flow regime is detected by the ERT technique. The results indicate that it is feasible the measure groundwater velocity and direction using the ERT technique presented in this report. Additional study in a real test site is needed to further verify the practical and potential use of this technique.