利用水氡及鐳同位素建立高屏沿岸海底湧泉輸出及通量

Abstract

本研究旨在以放射性同位素示蹤劑估算高屏沿岸海底地下水輸出通量（submarine groundwater discharge, SGD）。在陸域調查方面，2014年於屏東平原的河川及湧泉進行水體採樣工作，而海域調查方面則以海研三號研究船在高屏沿海分別於五月及九月進行兩個航次（OR3-1768以及OR3-1799）採樣工作。 本研究利用同步延時計數計（Delayed Coincidence Counter, RaDeCC）以及RAD7進行短半衰期鐳同位素（224Ra、223Ra）及水氡活度測量。陸域分析結果顯示，水氡能反應短時間區域性的降雨事件，濕季時淺層地下水水氡活度高達26000 dpm/100 L以上，但鐳同位素活度低於4 dpm/100 L。鐳同位素活度受到溶解時間以及地層岩性差異的影響，河水及湧泉的長半衰期的核種活度普遍高於短半衰期核種，惟高屏溪下游在山區降雨增加時224Ra活度明顯上升，其來源可能為地下水因地下水位升高而進入到河水中。海域調查結果顯示，平原區降雨增加會使表層海水的水氡及224Ra活度升高1.5倍，山區降雨則會反應在沿海底層的226Ra活度。 在海底地下水輸出通量估算方面，五月高屏沿岸的輸出主要發生在底部的地層，其輸出通量為26890 m3/day是表層輸出量的五倍；九月則集中在表層的輸出，其通量為981295 m3/day是底層八倍。以放射性同位素估算之SGD流速範圍約為2×10-10－4×10-8 m/sec，滲透係數約為1.1×10-7－2.2×10-5 m/sec，與地調所調查資料中顯示之屏東平原全區透水層的滲透係數範圍10-3－10-6 m/sec相符。

This study aims to estimate the submarine groundwater discharge (SGD) flux in Pingtung coastal zone with radioisotope tracers. For SGD fluxes estimation, radon and radium isotopes in river and estuary waters were measured in dry and wet seasons, and coastal investigations were conducted by using Ocean Researcher 3 in May (OR3-1768) and September (OR3-1799) 2014. Delayed Coincidence Counter (RaDeCC) and RAD7 instruments were used for measuring the short-lived radium isotopes (224Ra, 223Ra) and 222Rn. The analysis results of river and spring samples in Pingtung Plain showed the variation of radon-in-water activities may reflect short-term regional rainfall events. In wet season, radon-in-water activities in shallow groundwater can reached up to 26000 dpm/100 L or more. In contrast, the 224Ra activities were less than 4 dpm/100 L. Owing to radium isotopes activities are controlled by the dissolution time and lithology of aquifer, activities of long-lived radionuclides in rivers and springs were generally higher than the short-lived radionuclides. In the downstream of Gaoping River, 224Ra activities are significantly increased with rainfall which implied the deep groundwater input by the rise of water table. Marine investigation results showed the rainfall in plain area will increased activities of radon-in-water and 224Ra in surface waters by 1.5 times, but on the other hand, the variation of 226Ra activities in coastal bottom waters will be reflected on precipitation in mountain range. In May 2014, the main SGD output occurred in the bottom layer, the estimated flux of SGD is 26890 m3/day, which is five times higher than the surface flux. However, the SGD output in September is mainly on the surface layer, and the flux (981295 m3/day) is eight times higher than the bottom.