| 摘要: | 湄公河三角洲(Vietnamese Mekong Delta: VMD)為南越主要經濟中心之一，約有1800萬居住人口，對於該地區相關民生、農業以及工業活動而言，地下水資源為關鍵水源來源。由於數十年來對於地下水資源超限利用緣故，造成地下水位快速下降，進而產生地下水鹽化等問題，不僅限制了地下水的可用性，對於居住人民生活亦產生負面影響。因此有必要了解該區地下水文循環機制，並預估地下水位及鹽度變化，以提供管理權責單位制訂相關措施。過去已有許多研究針對此議題進行相關研究，然而前人研究多將VMD劃分成小區塊進行研究，而非就整體VMD進行探討。地下水為集水區尺度系統，局部區域系統之活動對於周邊系統會產生連動之影響，因此對於整體地下水系統模式之建立，對於預測預測地下水資源及相關評估共作是必要的。本研究利用USGS-SEAWAT模組進行模式建立，該模組結合MODFLOW以及MT3D，以模擬不同密度條件下地下水流及溶質傳輸行為。該模式進行相關檢定及驗證程序，包含邊界條件之修正以及地層材料參數率定，使之模擬能重現真實地下水反應。模式參數修正後能準確模擬地下水位，其RMSE < 1 m 且NSE > 0.95。此外模式能模擬2000年至2021年地下水鹽度變化，其模擬結果RMSE < 3 g/l且 NSE > 0.85在接受範圍。驗證模式後，本研究就此模型進行不同情境下之地下水位及鹽度之預測，其情境包含於本世紀終海平面上升約0.77 m，於2030年最大地下水補注體積達56 萬 m3/day ，並以每年0.5%速率降低至2100年。結果顯示7個含水層之地下水位持續降低，且深層含水層有較顯著之下降，截至2100年，推估Holocene含水層最低地下水位下降至-17m，Pleistocene含水層為-30m，Pliocene含水層為-38m，Miocene含水層為-47m。而地下水鹽度與地下水位相反，推估地下水鹽度持續升高，對於淺層含水層而言更是顯著，在深層含水層鹽度為中等程度增加。再進一步分析，Holocene、Pleistocene以及 Pliocene含水層鹽化增加面積分別為939 km2、5632 km2以及992 km2。於本模型推估中值得注意的是，Miocene含水層相較於鹽化，該含水層去鹽化程度較高，其結果顯示至2100年有43 km2鹽水可能轉換為淡水，推估地下水位及鹽度結果進一步呼應該區域對於有效利用地下水資源之迫切性。本研究成功模擬VMD地區7個含水層地下水位及鹽度變化，及結果有助於相關管理權責單位制訂有效管理計畫及策略方針。;The Vietnamese Mekong Delta (VMD) is one of the largest economic centers in southern Vietnam, home to approximately 18 million inhabitants. Moreover, groundwater is a crucial water resource for domestic, agricultural, and industrial uses in the VMD. For decades, due to over-extraction and urbanization, groundwater levels have been depleting rapidly and being salted, restricting groundwater usability and negatively impacting human lives. Therefore, it is necessary to understand the hydrological mechanisms and forecast groundwater levels as well as groundwater salinity in the VMD, supporting groundwater resource management. Several studies have been conducted to understand the mechanism as well as forecast groundwater reserves and quality in the VMD. However, most of them consider particular areas rather than the entire VMD. Since groundwater is a basin-wide system, where activities in a local area affect the surrounding area, it is necessary to develop a complete basin groundwater model in order to forecast groundwater resources and relevant issues for the entire VMD basin. This study adopted the USGS-SEAWAT, which is the coupled version of MODFLOW and MT3D, to simulate groundwater flow and solute transport in variable-density conditions. The model was developed and calibrated by modifying boundary condition values and physical properties of materials such that it could reproduce groundwater responses. The model can accurately simulate groundwater levels after calibration (RMSE < 1 m, NSE > 0.95). Additionally, the model shows that it can simulate groundwater salinity from 2000 to 2021 with acceptable statistical parameters (RMSE < 3 g/l, NSE > 0.85). After demonstrating its ability to reproduce the past, the model was applied to forecast groundwater levels and salinity by using reasonable scenarios. The scenarios applied in this study include the following: the sea level would rise to around 0.77 m by the end of this century, and the groundwater recharge volume could reach a maximum of 0.56 million m3/day in 2030 before decreasing by 0.5% annually until 2100. The result shows that the groundwater levels for 7 aquifers will continue to decrease, with deeper aquifers having a more significant decline. By 2100, the lowest groundwater levels could drop to -17 m in the Holocene aquifer, roughly -30 m in Pleistocene aquifers, -38 m in Pliocene aquifers, and -47 m in the Miocene aquifer. In contrast to groundwater levels, groundwater salinity is predicted to increase significantly in shallow aquifers. Meanwhile, deep aquifers would only see a modest groundwater salinity increase. In detail, in the Holocene, Pleistocene, and Pliocene aquifers, it is anticipated that the saline area would grow by 939 km2, 5632 km2, and 992 km2, respectively. Noticeably, the Miocene aquifer is predicted to experience more desalination than salinization, resulting in 43 km2 of saline water potentially changing to freshwater by 2100. The forecasted groundwater levels and salinity results further emphasize the necessity of efficiently managing groundwater resources. Finally, this study successfully forecasts the groundwater levels and salinity of 7 aquifers for the Vietnamese Mekong Delta, which may support policymakers in developing effective plans and strategies for managing groundwater resources in the VMD. |
