以天然放射性同位素探討台灣東部海底地下水滲流之研究

Abstract

本研究聚焦於探討台灣東部海底地下水滲流(Submarine Groundwater Discharge, SGD)的動態及其機制，分析其與區域地質和氣候條件之間的關聯，並估算淡水海底地下水滲流(Submarine Fresh Groundwater Discharge, SFGD)的通量。研究區域的地質背景以花崗岩、片麻岩及大理岩為主，具有高透水性及發達裂隙系統，有助於地下水快速流動，通過文獻回顧與數據分析，結合多種測量和計算方法，系統探討變質岩區域地下水滲流的特徵。為了更深入了解這些變化，將2024年第二季的數據作為地震後的樣本進行單獨分析，並將其他季節之數據比較乾季和濕季之間的滲流通量和流速變化。 研究數據顯示，台灣東岸地質導致滲流通量高於其他地區，破裂帶的存在進一步促進了地下水滲流，尤其是B-02井的鐳同位素(如ex224Ra)數據顯示該地區淡水滲流活動旺盛。乾季期間，鐳同位素活性顯著升高，反映出乾季地下水補注的增加。其SFGD之估算滲流通量範圍為3.93×10-1 - 8.23×10-1 m·d-1，平均值為 0.90 m·d-1，並藉由達西定律後可計算出水力傳導係數約為5.55×10-5 - 1.16×10-4 m·s-1符合現地調查及前人文獻。滲流活動在乾濕季節及地震前後均呈現顯著差異。地震前乾季的滲流通量與流速普遍高於濕季2022(Q4)與2023(Q1)的乾季滲流通量分別達454和489 dpm·m⁻²·day⁻¹，顯著高於2022(Q3)濕季之53.8 dpm·m⁻²·day⁻¹；兩個地下水井(Site-#1、Site-#2)亦呈現類似趨勢，乾季滲流通量明顯高於濕季，這反映了乾季水力條件對地下水滲流的增強作用；流速方面，SFGD之乾季流速(2022(Q4))的0.789 m·day⁻¹亦高於濕季(2022(Q3))的0.393 m·day⁻¹，說明降水減少及水力梯度變化可能加強了地下水的滲流速度。然而，地震後2024(Q2)之滲流活動達到歷史高峰，SFGD和Site-#1的滲流通量顯著增加，分別達804和923 dpm·m⁻²·day⁻¹，超越地震前乾濕季之數據；流速亦提升至0.823 m·day⁻¹；相較之下，Site-#2的滲流通量卻略有下降，維持濕季之變化趨勢，研究指出地質條件是影響滲流的重要因素。 這些地質與水文特徵強調了地震可能加劇地下結構破裂及水力梯度調整，進一步影響滲流模式。結合分析，本研究展示了季節性變化和地震對地下水滲流動態的顯著影響，為進一步理解台灣東部變質岩區域淡水海底地下水動態的特徵，也顯示出天然示蹤劑法和多方法綜合應用在滲流研究中的潛力，對未來的環境監測和資源管理具有重要意義。

This study focuses on exploring the dynamics and mechanisms of submarine groundwater discharge (SGD) in eastern Taiwan, analyzing its relationship with regional geological and climatic conditions, and estimating the flux of submarine fresh groundwater discharge (SFGD). The geological background of the study area is dominated by granite, gneiss, and marble, characterized by high permeability and well-developed fracture systems that facilitate rapid groundwater flow. Through literature review and data analysis, combined with various measurement and calculation methods, the characteristics of groundwater discharge in the metamorphic rock regions were systematically investigated. To gain deeper insights into these variations, data from 2024 (Q2) were analyzed separately as post-earthquake samples, and seasonal data were compared to examine the differences in discharge flux and flow rates between the dry and wet seasons. Research data reveal that the geology of Taiwan's east coast results in higher discharge flux compared to other regions, with fault zones further enhancing groundwater discharge. Radium isotope measurements, particularly from well B-02 (e.g., ex224Ra), indicate active freshwater discharge in the area. The estimated SFGD flow rates ranges from 3.93×10⁻¹ to 8.23×10⁻¹ m·d⁻¹, averaging 0.90 m·d⁻¹, and the hydraulic conductivity calculated via Darcy's Law is approximately 5.55×10⁻⁵ to 1.16×10⁻⁴ m·s⁻¹, consistent with field observations and previous studies. Prior to the earthquake, dry season discharge flux and velocity were generally higher than in the wet season, with dry season discharge flux reaching 454 and 489 dpm·m⁻²·day⁻¹ in 2022(Q4) and 2023(Q1), significantly exceeding the 53.8 dpm·m⁻²·day⁻¹ recorded in the wet season. A similar trend was observed in the two groundwater wells (Site-#1 and Site-#2), where dry season discharge flux was markedly higher than in the wet season. Post-earthquake discharge activity in 2024(Q2) reached a peak, with SFGD and Site-#1 discharge flux increasing to 804 and 923 dpm·m⁻²·day⁻¹, surpassing all pre-earthquake seasonal data. These geological and hydrological characteristics highlight the potential for earthquakes to exacerbate subsurface structural fractures and adjust hydraulic gradients, further impacting discharge patterns. By integrating these analyses, this study demonstrates the significant influence of seasonal variations and earthquakes on groundwater discharge. It provides a deeper understanding of the characteristics of SFGD in the metamorphic rock regions of eastern Taiwan and showcases the potential of natural tracer methods and multi-method approaches in discharge studies. The findings are of critical importance for future environmental monitoring and resource management.