以數值及試驗方法探討非飽和水力特性對非受壓含水層抽水洩降之影響;Using Numerical and Experimental Methods to Assess the Effect of Unsaturated Hydraulic Characteristics on Drawdown Behavior in Unconfined Aquifer Tests

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Title:	以數值及試驗方法探討非飽和水力特性對非受壓含水層抽水洩降之影響;Using Numerical and Experimental Methods to Assess the Effect of Unsaturated Hydraulic Characteristics on Drawdown Behavior in Unconfined Aquifer Tests
Authors:	陳昶宏;Chen, Chang-Hung
Contributors:	應用地質研究所
Keywords:	非受壓含水層;非飽和帶;抽水試驗;FEMWATER;van Genuchten 土壤水分特徵曲線
Date:	2017-07-27
Issue Date:	2017-10-27 16:18:14 (UTC+8)
Publisher:	國立中央大學
Abstract:	非受壓含水層抽水試驗的影響範圍包含初始非飽和層以及地下水位面下降形成的非飽和層，為探討非飽和層以及材料土壤水分特徵參數在抽水試驗過程中，對非受壓含水層洩降特徵的影響。本研究使用數值模式配合現地與室內試驗方法，嘗試從非受壓含水層洩降資料中，評估非飽和水力特性水力參數對抽水洩降之影響。研究過程中先以FEMWATER地下水模式建立符合抽水試驗場址的三維變飽合地下水模式，參考試驗場址土壤樣本取得的非飽和土壤van Genuchten特徵曲線，進行數值試驗，量化非飽和層水力特性對抽水洩降之影響。之後再以數值模式套配現地非受壓含水層抽水試驗洩降資料。研究結果顯示，本研究所選用的場址，雖然地下水位面接近地表，但是非飽和層中的大時間段洩降特徵極不明顯，現地抽水試驗洩降資料利用Neuman解析法得到的流通係數(transmissivity)T值為0.004 m2/min，儲蓄係數(storage coefficient)S值為0.00058，平均K值為0.001 m/min，從場址地質鑽探資料以及推估出之異向比Kz/Kh中我們推論本研究含水層上部非飽和層對抽水試驗影響甚小，且由於試驗沒有出現大時間洩降特徵因此推估出的Sy皆不在合理範圍。FEMWATER雖成功模擬出非受壓含水層的抽水洩降，從數值試驗中，抽水井開篩段為0~6公尺以及0~8公尺的結果顯示離抽水井越遠抽水井為非全程開篩的影響越小。且非飽和層對非受壓含水層抽水試驗的影響不僅於地下水位面下降所產生之非飽和層，初始非飽和層對其的影響也不可忽略，其影響程度與抽水井距離成正相關。van Genuchten參數敏感度分析的結果顯示，van Genuchten參數中的值越大，中時間段洩降特徵越明顯；越小，中時間段洩降越不明顯。而van Genuchten參數中的值越大，中時間段洩降維持時間越長，越晚進入大時間段特徵；相反地，值越小，中時間段洩降時間越短，越快進入大時間段特徵。但在與現地資料套疊過程中，擬合效果不佳，尤其是在中時間段重力排水的部分，推估其原因可能是本研究使用的壓力鍋試驗尺寸太小，未明確掌握土壤中礫石材料混合土壤的排水特性。;The influence of pumping test on unconfined aquifer includes the initial unsaturated zone and the unsaturated zone in the cone of depression. To investigate the effects of unsaturated zone on the drawdown behavior in the unconfined aquifer tests, this study uses numerical model, field and laboratory tests to quantify the influence of unsaturated soil parameters on unconfined aquifer tests. The study first employs FEMWATER model to develop the site-specific variable saturated groundwater model. The sampled soil characteristic parameters are obtained based on the van Genuchten soil characteristic model. The parameters are the basis to conduct the sensitivity test for quantifying the influence of the unsaturated zone on the site-specific unconfined pumping test. In the aquifer test, the estimated transmissivity and storage coefficient are 0.004 m2/min and 0.00058 respectively, and the average hydraulic conductivity is 0.001 m/min. According to geological drilling data and anisotropic ratio obtained from pumping test, we concluded that the initial unsaturated zone above the aquifer has little effect on the pumping test. Moreover, the results indicate that the unclear behavior of large time drawdown at the site produces an unreasonable specific yield value. Numerical simulation results show that the drawdown is inversely proportional to the length of well screen, they also show that initial unsaturated zone can not be ignored when pumping in the unconfined aquifer, its influences are positively correlate to the distance from the pumping well. The numerical sensitivity analyses show that the value in van Genuchten model controls the drawdown behavior in the middle time. However, the value in van Genuchten model influences the time elapse of drawdown behavior in the middle time. The FEMWATER model reproduces well the multi-step drawdown behavior in this study. However, the numerical model cannot fit well the field pumping test data, especially the drawdown behavior in the middle time. Such results might be induced by the insufficient information of material with large grain size in the soil sample at the test site.
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