利用地震後地下水位變化推求侷限含水層之水文地質參數

Abstract

本研究之目的在於提出新的方法論與研究分析步驟，以推估侷限含水層之導水係數及儲水係數。研究蒐集地下水位歷線資料，時間點的選取以集集地震發生後48小時，觀察地震造成地下水位變化與抽水試驗水位洩降之曲線相似，將此現象解釋為地震造成全域地下水位抬升，以自然界執行全域均勻抽水使水位洩降，藉由此現象建立概念模式，從模式推導過程中設定抽水量與抽水距離，同時以虛擬案例證明概念模式之可行性。本研究分為兩種概念模式，其抽水量設定皆以複合井流場概念，將全域抽水量線性疊加作為抽水量之給定，但抽水量資訊取得有限，所以參考現地水井清查各鄉鎮每層之抽水量作為本研究抽水量之設定；影響半徑之判識為當地下水位變化小於一公分時，視為抽水井影響觀測井之最遠距離，在此從地下水位資料判識，影響半徑為1.4公里；第一種概念模式之抽水距離，以等間距均勻抽水之抽水距離為影響半徑0.36788倍，而第二種概念模式之抽水距離，以空間均勻抽水之抽水距離為影響半徑0.60650倍。兩概念模式之抽水量與抽水距離設定，以不同的概念想法呈現，合理推導過程設定的抽水量與抽水距離具有物理意義，推求的水文地質參數具有參考之價值，而非粗估地質所判識給定之參數值。從本研究針對濁水溪沖積扇區域推估之參數其成果顯示數值合理，並補充未試驗觀測井之水文地質初步評估，對於各含水層之局部水文地質特性、水流快慢與儲蓄涵養水源能力皆有參考依據，並發現增加觀測井數，含水層特性有顯著的變化，同時也可解釋該研究區域特性不應該僅由數口現地試驗得到之參數，就簡單說明含水層特性，因此對於濁水溪沖積扇含水層整體特性應該深入探討，使往後進行水資源管理時，才會能做出更明確的決策。

This study proposes a new methodology to estimate transmissivity and storage coefficient of confined aquifer. The hourly groundwater level observations after Chi-Chi earthquake in 48 hours are collected. It is found out the groundwater variations after earthquake acts as that of pumping test. Such phenomenon explains the earthquake is like a regional uniform pumping test, and the drawdown can be used to identify the hydrogeological parameters of aquifer. Two conceptual models are established to simulate the uniform pumping test within the radius of influence (R) of monitoring well, one is radial uniform pumping; the other is spatial uniform pumping. Moreover, the pumping wells clusters are simplified by linear superposition as an equivalent single well with the same drawdown observed at monitoring well. The equivalent quantity of pumping is both equal to summation of all pumping rate; while the equivalent distance from monitoring well to pumping well is 0.36788R for radial uniform pumping and 0.60650R for spatial uniform pumping, respectively. The simplification is verified by a virtual case and shows success in identifying transmissivity and storage coefficient. Lastly, the proposed parameter estimation method is applied to Jhuoshuei River alluvial fan for identifying transmissivity and storage coefficient of confined aquifer. The identified value of parameters falls in a reasonable range of typical sand-deposited aquifer, which shows the proposed method is suitable for a preliminary assessment of hydrogeological characteristic. Additionally, the identified parameters varies significantly from different locations, which supposes further investigate of the groundwater system to clarify the hydrogeological distribution is suggested. Furthermore, compare the results of viewing earthquake as a pumping test with that of in-situ pumping test, the identified parameters are generally smaller. This may infer the scale effect of hydrogeological characteristics, that is, the analysis of earthquake provides a larger-scale character of aquifer and is more close to the scale of groundwater flow simulation of a groundwater district.