利用鐳同位素探討大鵬灣地區海底地下水滲流輸出

Abstract

陸域物質藉由河川和海底地下水等傳輸途徑輸送陸域物質進入海洋，而傳統上河川被認為是輸出營養鹽、微量元素和陸域物質至海洋的主要的途徑。河川傳輸的議題被廣為研究，研究河川傳輸營養鹽到海洋的資訊唾手可得。相對於河川而言，海底地下水滲流（submarine groundwater discharge, SGD）存在空間時間的非均性，導致通量的測量極為困難。然而近年研究顯示SGD對於營養和微量元素的輸出不亞於河川輸出的程度，因此備受注目。 大鵬灣，位居地下水資源豐沛之屏東平原，是個由沙嘴和海岸圍成的半封閉淺水潟湖，是陸域與海域交互作用的緩衝地帶。灣內水體由雨水、家庭廢水、養殖廢水、農業廢水及海水混合而成，並無河川注入。灣內僅有一個潮口與海水接觸，故灣內水體與外洋水交換速率不佳，導致水質欠佳。水文地質條件顯示，大鵬灣灣內可能存在海底地下水滲流輸入的潛勢。本研究藉由水體中的放射性鐳同位素（224Ra, 226Ra, 228Ra）評估大鵬灣灣內SGD通量，並進一步探討SGD的季節性變化及其輸入灣內營養鹽通量。 結果顯示，相較於海水、河水等水體，灣內水體四季皆有明顯高鐳同位素活度現象，表示灣內有額外高鐳同位素活度水體補注，額外水體為SGD自然滲流入灣以及由人為方式排放入灣的陸域水體。各季節鐳同位素活度的空間分布存在變化，但大抵呈現灣內東南方水體具有較高鐳同位素活度。此外不同潮時水體鐳同位素變化，與潮汐活動相呼應，呈現高潮位時活度低，此現象表明大鵬灣內水體受Tidal pump影響。根據質量守恆模式（Mass balance model）計算出大鵬灣的每日SGD通量約9.18×104-9.16×105 m3/day，每日流量大約為灣水體積的1%至9.5%左右。

River discharge is a major pathway for transporting terrestrial materials to the ocean, including nutrients and trace elements, and their fluxes have been widely studied. Although not as obvious as river, the underground flow across the seabed to the coastal ocean, referred to as submarine groundwater discharge (SGD), now has been recognized as a significant contribution to the coastal ocean. However, little is known and studied about SGD in Taiwan. The Dapeng Bay, located at the Pingtung Plain where is characterized by abundant groundwater resource, is a shallow lagoon with only one outlet for exchanging water with the ocean. Although the water in the Dapeng Bay mainly consists of seawater with domestic, fishing, and agricultural waste waters, the reported hydrogeological conditions in the Pingtung Plain indicate a potential of SGD in the Dapeng Bay. Therefore, this study purposed to investigate SGD in the Dapeng Bay by using naturally occurring radionuclides, Radium (224Ra, 228Ra, and 226Ra), as tracers and further to understand its seasonal variation. Our results show that high Ra water was observed at the inner part of Dapeng Bay. The high activity at high tide and low one at low tide indicates that the study area is highly influenced by tidal activity. As well, Ra activities in the Dapeng Bay water is much higher than offshore seawater, indicating the possibility of SGD input. Based on mass balance model of Ra, we estimated the SGD flux is 9.18×104-9.16×105 m3/day, accounting for 1~9.5 % of the total volume of the Dapeng Bay.