利用水氡與鐳同位素評估宜蘭地區海底湧泉輸出與通量

Abstract

本研究旨在以放射性同位素方法估算宜蘭地區海底湧泉的輸出通量及其乾濕季變化。 研究結果發現海水222Rn活度在2014年八月介於0至2.26 Bq/m3，2015年一月活度為4.86至11.02 Bq/m3，海水224Ra活度在2014年八月活度介於0.43至1.14 Bq/m3， 2015年一月由0.48至1.98 Bq/m3。222Rn與224Ra活度冬季高於夏季，顯示地下水輸出訊號會隨著季節改變。 2014年四月至2015年一月，每三個月至蘭陽平原湧泉進行淺層地下水採樣，分析結果發現湧泉224Ra活度均不高，介於0.02至0.29 Bq/m3間，四季活度無明顯變化。222Rn活度介於386至32220 Bq/m3，活度差異甚大，其高值出現在雪山山脈旁之湧泉，越往平原與沿海地區活度越低，雪山山脈旁之湧泉出現氡高鐳低之現象。除湧泉採樣外，2015年一月及三月至礁溪溫泉與冬山600公尺深井做深部來源地下水採樣，發現其224Ra活度比地表湧泉高出許多，礁溪溫泉為2.26 Bq/m3、冬山深井為20.97 Bq/m3。 將本研究海水與湧泉之224Ra活度做比較，發現海水224Ra活度值高於湧泉，有別於地下水224Ra活度高於海水之認知，比對2015年三月之冬山深井，其224Ra活度20.97 Bq/m3遠高於淺層地下水0.02至0.29 Bq/m3，推測海域中可能有深層地下水體輸入。 宜蘭灣海底湧泉輸出通量計算結果顯示，2014年八月輸出量約4.3×106 m3/day，約為每日河水輸出通量的8%；2015年一月輸出量約為8.4×106 m3/day，約為河水輸出通量的16%。營養鹽中硝酸鹽輸出量在2014年八月約為7.7×106 mole/day，高出河水硝酸鹽輸出通量約六倍，一月約為1.5×107 mole/day，高出河水硝酸鹽輸出通量約12倍，硝酸鹽類藉由海底湧泉輸出至宜蘭灣，其通量大於河川輸出，顯示海底湧泉在沿岸生態環境扮演重要角色。

In this study, radon and radium isotopes are used as tracers for SGD survey. The samples were collected from spring-water, coastal seawater, the Lanyang River and its tributaries in dry and wet seasons. Our results show the activities of 224Ra in coastal seawater ranged from 0.43 to 1.14 Bq/m3 in August 2014, 0.48 to 1.98 Bq/m3 in January 2015, and the differences between summer and winter were observed. In spring-water samples, the activities of 222Rn and 224Ra are ranging from 386 to 32220 Bq/m3 and 0.02 to 0.29 Bq/m3, and no significant difference between wet and dry seasons. Unexpectedly, the spring-water samples are characterized with high 222Rn and low 224Ra activities. Compare with the coastal seawater and spring-water samples, the 224Ra activities in coastal seawater are 3-4 times higher than spring-water. A deep groundwater sample which collected from a deep well at 600 meter depth shows high 224Ra activity (20.97 Bq/m3). It reveals that the SGD in the coastal area of the Yilan Plain may derive from deep aquifers. The evaluation results of SGD flux in Yilan Bay indicate the minimum flux is 4.3×105 m3/day in August 2014, accounting for 8% of the Lanyang River output; 8.4×105 m3/day in January 2015, accounting for 16% of the Lanyang River output. Nitrate flux is 7.7×105 mole/day in August 2014, about 6 times higher than the Lanyang River output；1.5×106 mole/day in January 2015, about 12 times higher than the Lanyang River output.