Spatiotemporal Variation of Submarine Groundwater Discharge in Taoyuan Coastal Area, NW Taiwan: Numerical and Geochemical Approaches

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Title:	Spatiotemporal Variation of Submarine Groundwater Discharge in Taoyuan Coastal Area, NW Taiwan: Numerical and Geochemical Approaches
Authors:	鄧明君;Quan, Dang Minh
Contributors:	應用地質研究所
Keywords:	海岸帶地下水出流;海岸帶自流井;地下水源;多相地下水模型;鐳同位素;桃園-中壢台地;Submarine groundwater discharge;Coastal flowing artesian well;Source of groundwater;Multi-phase groundwater model;Radium isotopes;Taoyuan-Chungli Tableland
Date:	2025-01-15
Issue Date:	2025-04-09 18:27:36 (UTC+8)
Publisher:	國立中央大學
Abstract:	海岸帶地下水出流（submarine groundwater discharge, SGD）為淡水、營養物質和溶質從陸地向海洋輸送的重要媒介，在乾季時亦可作為關鍵的淡水資源。台灣桃園沿海有相當強的海岸帶地下水出流跡象，該地區有多個沿海自流井（flowing artesian wells, FAWs），在2021年極為乾旱條件下仍有顯著出流。海岸帶是一個動態且複雜的環境，包含潮汐變化、密度驅動的海水與淡水交換以及季節性海水與淡水交互作用，形成了海水與淡水之間錯綜複雜的水文交互作用。本研究結合數值模擬和地球化學分析，探討了自流井和海岸帶地下水出流季節性變化的特性與發生機制，特別是本研究觀測資料為極端乾旱條件下（2021年大旱）取得，更凸顯本研究的重要性。本研究分析了海岸帶自流井和陸地水井的水力水頭數據，並結合潮汐觀測，探討與提出沿海自流井的發生機制。多相地下水流模型顯示，沿海自流井導因於海淡水界面引起的地下水上升流驅動，這一現象與傳統的自流井概念不同。而透過該模型估算日均海岸帶地下水出流量，可為當地人口提供約23.18天的用水需求。此外，鐳同位素示蹤劑（223Ra、224Raex 和228Ra）之季節性變化分析結果顯示，海岸帶地下水出流量在濕季（841.47 ± 156.61 cm d⁻¹）約是乾季（391.34 ± 79.59 cm d⁻¹）的2.7倍，而較深層含水層可能為乾旱期間海岸帶地下水出流的主要來源。穩定同位素（δ18O、δD）和鐳活度比之分析結果亦支持該結論。本研究之結果加深了我們對不同水文條件下海岸帶地下水出流的了解，強化了該出流水在緩解乾旱中的可能貢獻，並凸顯了以海岸帶自流井作為研究海岸帶動態地下水出流研究的重要性。 ;Submarine groundwater discharge (SGD) plays a vital role as a source of water, nutrients, and solutes from land to sea, and might serving as a crucial freshwater resource, particularly during drought conditions. The northwestern coast of Taiwan, known for its strong SGD, features several coastal flowing artesian wells (FAWs) that become prominent under such conditions. This coastal area presents a dynamic and complex environment where tidal variation, density-driven seawater circulation, and seasonal fresh groundwater discharge creates intricate hydrological interactions between seawater and freshwater. This study integrates physical modeling and geochemical analysis to investigate the mechanisms driving FAWs and seasonal variations in SGD, with a particular focus on extreme drought conditions. Hydraulic head data from coastal FAWs and onshore wells were analyzed alongside tidal observations to elucidate the mechanisms driving coastal FAWs. Multi-phase groundwater modeling revealed that coastal FAWs are driven by upward flow caused by a sharp freshwater-seawater interface, which acts as a barrier and deviates from conventional geological concepts. The daily SGD volume was estimated from this model to sustain the local population’s water needs for approximately 23.18 days. Complementing this, radium isotope tracers (²²³Ra, ²²⁴Raₑₓ, and ²²⁸Ra) provided valuable insights into seasonal variations in SGD. The result of SGD during the wet season (841.47 ± 156.61 cm d⁻¹) was 2.7 times higher than those in the dry season (391.34 ± 79.59 cm d⁻¹), with deep aquifers likely contributing to SGD during droughts, as evidenced by stable isotope (δ¹⁸O, δD) and radium activity ratio analyses. These findings enhance our understanding of SGD under contrasting hydrological conditions, underscore its role in mitigating drought impacts, and highlight the importance of coastal FAWs as key tools for investigating SGD dynamics.
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