近幾十年來濁水溪沖積扇由於抽取地下水而造成使得地層下陷問題日益嚴峻。雖然近年受到有關單位控管措施使得沉降速率已經明顯降低，但雲林縣的沉降速率仍然很高。農業和工業用水為雲林縣地下水利用的主要標的，不同事業標的分別由淺層和深層含水層中抽用地下水。本研究中擇定土庫中學站做為調查各層壓密對總地面沉降的貢獻，以及不同含水層抽水對地層壓密的影響。透過應用FLAC3D來分析地下水抽取引起的地面沉降行為，該程式是基於有限差分法。推估地層壓實度與觀測數據進行比較以進行模式校正，以檢定數值模型的性能。從觀測數據來看，從第一含水層到第四含水層的壓密率分別為 1.1%、 20.1%、 11.9%和 50.1%。第二、第四含水層占比最大，佔地面沉降總量的70%以上。這可能是由於該含水層厚度較大、旱季和雨季地下水水位差異大，以及存在許多粘土夾層。研究中數值模擬係以假設無限橫向地下水補注為前提，模擬結果顯示最大抽水率應小於 218 m3/day，以最大限度地減少粘土夾層的非彈性壓密。在第二含水層抽水會導致最大的地層下陷，而在第一含水層或淺層抽水對其他含水層造成更大的壓實，但在深層含水層抽水主要對自身影響較大。此模擬結果對於雲林縣制定有效的減緩地層下陷管理方案提供了重要的參考方向。;Choushui River alluvial fan is a severe land subsidence area due to groundwater extraction in the past decades. Although the current subsidence rate has reduced significantly, Yunlin county still has a high subsidence rate. Agricultural and industrial pumping is the main activity of groundwater use in Yunlin county, which extracts in shallow and deep aquifers. In this study, Tuku Junior High School station is a specific site to investigate the contribution of each layer compaction to the total land subsidence, and the effect of pumping in different aquifers on compaction. The finite-difference algorithm in FLAC3D software was applied to analyze the land subsidence behavior due to groundwater pumping. The predicted layer compaction will be compared to observed data in the calibration stage to verify the numerical model performance. From observation data, the compaction percentage of four aquifers from aquifer 1 to aquifer 4 is 1,1%; 20,1%; 11,9%; and 50,1%, respectively. The second and fourth aquifers contribute the most, with more than 70% of the total land subsidence. That may be due to a large thickness, a high difference in groundwater level between dry and wet seasons, and the presence of many clay-interbedded layers. The assumption of the numerical modeling would be an unlimited supply of lateral groundwater. The calibrated model suggested that the pumping rate should be less than 218 m3/day to minimize inelastic compaction at clay-interbedded layers. Pumping at aquifer 2 caused the most total land subsidence. Pumping at aquifer 1 or shallow aquifer caused more compaction on other aquifers, but pumping at deep aquifers mainly affected themselves a lot. The simulation results provide critical references to reaching an efficient management plan to mitigate subsidence in Yunlin county.
