地層下陷乃台灣水資源和土地資源管理不善所造成的結果。近年來，濁水溪沖積扇之深部地層有持續下陷地情況並在部分地區達到總沉陷量的50％。而農業抽水被認為是彰化縣和雲林縣主要的地下水使用類別。然而，農業用水從淺層含水層抽水的取水型態所引起深部地層下陷的問題仍然存在爭議。我們首先描述濁水溪沖積扇的水文地質特性，以確定含水層系統的連通性。接著使用數值軟體COMSOL對地下水流和土壤壓密進行數值模擬。結果顯示，從淺層含水層取水之農業抽水行為確實對不同深度含水層的地下水位造成影響。淺層農業抽水與含水層一、二、三和四之地下水位的呈負相關，平均相關係數分別為-0.404，-0.380，-0.303和-0.271。結果亦顯示，淺層農業抽水與淺層及深層含水層下陷量的平均相關係數分別為0.458和0.322。結果表明淺層抽水不僅會影響含水層系統的淺層含水層，亦會對深部含水層造成影響。數值模擬結果顯示，當淺層含水層與深層含水層的抽水量為1：1、 2：1、 3：1和3.65：1（經建會，2011）時，淺層農業抽水造成深層含水層下陷量占整體下陷量比例分別為25.9％，41.1％，51.2％和56.1％。雖然從淺層含水層中抽水不能直接抽取深層含水層所含之地下水，但結果顯示，地下水會通過滲漏的含水層棲移至淺層含水層並導致深層含水層孔隙水壓下降，進而產生下陷。

Land subsidence is the result caused by the inappropriate management of water and land resources in Taiwan. Continuous compaction of deep stratum in Chou-shui River alluvial fan has reached up to 50% of the total subsidence. Agricultural pumping has been recognized as the major groundwater use in Changhua and Yunlin counties. However, the question on whether the agricultural pumping from the shallow aquifer to cause the compaction of deep stratum is still under debate. We first characterize the hydrogeology to identify the connectivity of aquifers. Then numerical modeling is performed for the groundwater flow and soil compaction using COMSOL. The result shows that agricultural pumping in shallow aquifer did affect the groundwater level of aquifer at different depth. Agricultural groundwater pumping and groundwater level are negatively correlated with average coefficient in F1, F2, F3 and F4 are -0.404, -0.380, -0.303 and -0.271, respectively. The results also show that the average of correlation coefficient of agricultural groundwater pumping and land subsidence at (a) shallow, (b) deep aquifer are 0.458 and 0.322, respectively. It indicates that the shallow pumping will influence the aquifer system not only at shallow aquifer but also at that of deeper aquifer. The result of numerical modelling shows that when the ratio of discharge of shallow aquifer to deep aquifer is 1:1, 2:1, 3:1 and 3.65:1 (CEPD, 2011), the proportions of deformation in the deep aquifer due to agricultural pumping at shallow aquifer are 25.9%, 41.1%, 51.2% and 56.1%, respectively. Although pumping in shallow aquifer does not directly pump deep aquifer, water moves through leaky aquitard to reach upper aquifer and causes the compaction of the deep aquifer.

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