濁水溪沖積扇地下水鹽化機制與未來趨勢探討

Abstract

濁水溪沖積扇是臺灣地下水資源最豐沛的地區，然而，隨著產業發展，地下水超抽情況逐年惡化，也導致後續出現地層下陷、海水倒灌、地下水鹽化等自然災害。其中地下水鹽化因範圍廣大、整治不易，更容易導致長期的用水問題。目前對於此區的鹽化程度及範圍已有諸多研究成果，但關於未來鹽化趨勢卻少有討論。此外，過去多以單次或一年一次之採樣結果進行分析，容易忽略季節對地下水鹽化的影響。因此本研究統整歷史地下水數據，採樣分析豐枯水期之水質，試圖以水化學的角度，了解濁水溪沖積扇的地下水鹽化分布、來源、機制、季節性變化與未來趨勢。 研究成果顯示地下水鹽化集中在沿海第一與第二含水層，其鹽化特徵包括高導電度、高氯鹽鹼度當量濃度比值以及Na-Cl型之水化學相。主要離子與氯鹽之關係圖中，多數資料點落在淡海水理論混合線附近，代表鹽化來源自海水，且由海水的比例決定其鹽化程度。鹽化機制方面，鹽化程度與井位離海距離關係不大，加上沿海表層缺乏泥層覆蓋，推測地表鹽分入滲是造成第一含水層鹽化最主要的機制，而第二含水層的鹽化則可能是鹽分沿深井井管入滲所造成。 現地採樣分析結果顯示，雖然鹽化程度並無季節上的差異，但BEX指示部分地區豐枯水期間有鹽化/淡化傾向反轉的現象。各井BEX的長期趨勢顯示出三種型態：持續為正、持續為負、由正轉負，其中BEX持續為負的井位鹽化加劇的可能性最高，BEX持續為正則最低。且鹽化加劇可能性較高的井位多位於含水層具鹽化傾向之區域，代表可先找出具鹽化傾向的區域，再依據各井BEX趨勢判斷各井未來鹽化趨勢，以擬定更完善且彈性的管理政策。

Choushui River Alluvial Fan is the area with the most abundant groundwater resources in Taiwan. However, due to industrial development, excessive groundwater extraction has led to the deterioration of groundwater levels and subsequent natural disasters such as land subsidence, seawater intrusion, and groundwater salinization. Among them, groundwater salinization poses long-term water supply issues owing to its extensive scope and challenging remediation. Currently, the extent and distribution of salinization in this area have been well known, but there is limited discussion on future trends. Additionally, previous studies have often focused on single or yearly sampling results, neglecting the seasonal impact on groundwater salinization. Therefore, this study integrates historical groundwater data and samples the water quality during wet and dry periods. The objective is to understand the distribution, sources, mechanisms, seasonal variations, and future trends of groundwater salinization in the Choushui River Alluvial Fan from a hydrochemical perspective. The results indicate that groundwater salinization in the Choushui River Alluvial Fan is concentrated in the first and second aquifer (F1&F2) near the coast. Salinization characteristics include high electrical conductivity, a high chloride/alkalinity equivalent concentration ratio, and a Na-Cl type hydrochemical face. In the major ions and chloride bivariate diagrams, most data points fall near the theoretical mixing line of freshwater and seawater, indicating that salinization originates from seawater, with the degree of salinization determined by the proportion of seawater. Regarding the salinization mechanisms, the degree of salinization does not show a significant correlation with the distance of wells from the sea. Additionally, the lack of surface clay coverage in the coastal area suggests that salt leaching from the surface is the primary mechanism causing salinization in F1, while salinization in F2 may result from salt infiltration through deep wells. Results from the sampling analyses show that although there is no seasonal difference in the degree of salinization, there is a phenomenon of salinization/freshening reversal in certain areas between the wet and dry seasons, indicating by the Base Exchange Index (BEX) values. Long-term trends in BEX for each well indicate three patterns: consistently positive, consistently negative, and a transition from positive to negative. Wells with consistently negative BEX values have the highest likelihood of intensifying salinization, while those with consistently positive BEX values have the lowest likelihood. Wells with a higher likelihood of salinization intensification are often located in areas where the aquifer tends to be saline. This suggests that it is possible to identify regions with salinization tendencies and then formulate more comprehensive and flexible management policies based on the BEX trends of individual wells.