定水頭部分貫穿汲水推估非受壓含水層水文參數之方法; Parameter Estimation of Constant Head Test in the Unconfined Aquifer with Partially Penetrating Effect

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题名:	定水頭部分貫穿汲水推估非受壓含水層水文參數之方法;Parameter Estimation of Constant Head Test in the Unconfined Aquifer with Partially Penetrating Effect
作者:	郭綉娟;Hsiu-Chuan Kao
贡献者:	地球物理研究所
关键词:	定水頭抽水試驗;部分貫穿效應;異質性;非受壓含水層;水文地質調查;參數推估;unconfined aquifer;heterogenous;partially penetrating effect
日期:	2004-05-25
上传时间:	2009-09-22 09:54:53 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	許多污染場址位於低滲透性未受壓含水層中，必須使用定水頭試驗進行參數推估以瞭解水文地質狀況；然而，參數推估需要使用合適的水井水力學理論才能避免錯誤的解釋。我們用三種方法分析美國Iowa冰磧土定水頭洩降資料，證實資料分析疏於試驗場址真實狀況會造成含水層異質性的假象。冰磧土含水層的厚度約2.5公尺厚，抽水試驗用井皆為全程貫穿井且井篩開口至整個含水層，由於抽水井維持固定1.5公尺的固定水位，造成1.5公尺的井篩段為不進水情況，形成部分貫穿井的特徵。觀測井與抽水井的距離分別為0.87、1.80、2.71與3.62公尺，其最大洩降變化分別為含水層厚度的15.43%、11.12%、10.96%與5.62%，也有部分貫穿井特徵存在。方法一將各井忽略部分貫穿效應，而假設為全程貫穿井來進行推估，參數推估結果有流通係數(Transmissivity, T)與視儲水係數(Apparent Storage, Sa)隨觀測井距離越遠其值越大之趨勢；方法二則考慮了抽水井因為人為汲水造成井中洩降已占含水層60%的厚度，全程貫穿井必須視作部分貫穿井來考慮，參數推估結果隨距離之趨勢變化減少許多；方法三將觀測井加入部分貫穿效應，由各別觀測井推估出接近一組的水文地質參數組：垂直水力導數(horizontal hydraulic conductivity, Kz)、水平水力導數(vertical hydraulic conductivity, Kr)、儲蓄係數(specific storage, Ss)與比出水量(specific yield, Sy)。以距離抽水井不到4公尺的觀測井中推估出接近的參數組是合理且可以接受的。為了證明此參數組是合理的，我們使用複合式洩降方法(Composite Drawdown Method)來佐證此組參數，我們得到正面的結果。此外，在同一含水層中，執行不同的抽水試驗並不會改變當地的水文地質參數，我們用Jones et al.在同一場址所執行定流率試驗洩降資料來驗證此參數組的正確性，然而，定流率抽水井有水位落於井篩段位置之現象，我們嘗試用數值方法來解決此問題，得到不錯的參數驗證結果。 Transmissivity (T) values estimated using pumping test data normally vary with distance, suggesting the aquifer be heterogenous. However, we found that this result may be not so much due to the spatial variability of T as due to the data analysis method failing to account for field test conditions. Here, we use three different methods to analyze drawdown data produced from a constant-head pumping test in an unconfined till aquifer in Iowa. The saturated till thickness is about 2.5 meter. The water depth (the constant head) maintained in the pumping well was only 1.5 meter. There were four observation wells, and their maximum drawdown changes from 5.62% to 15.43% of the saturated thickness. Method I ignores the possible partial penetrating effect in the wells by assuming a fully penetrating condition for all the wells. The estimates of T vary relatively randomly, lacking a sound explanation. Method II assumes that the pumping well is partially penetrating while the observation wells are fully penetrating, and the spatial variability of the T estimates decreases. Method III takes into account the partial penetrating effect of the wells, and a constant T value is obtained using the drawdown data of all the observation wells. This constant T is also used to analyze drawdown data from a constant-rate test conducted in the same aquifer and the same wells. Considering the five observation wells are located within a short extent of 5 meters surrounding the pumping well, the T value being constant is plausible. For this particular case, it is thus concluded that the spatial variability of the estimates of T is an artifact due to the negligence of the partial penetrating effects of the pumping well and the observation wells. For the constant head data analysis, we develop a mathematical model which is suitable for the unconfined aquifer and involves four pertinent parameters; namely, the horizontal and the vertical hydraulic conductivity, the storage coefficient, and the specific yield.
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