台灣小河川溶解性物質之季節性變化與極端事件影響

Abstract

台灣地區雨量豐沛且平均每年有四個颱風侵襲，降雨之時間與空間分布不均，河川流域母岩性質脆弱易斷裂，且經高度的風化，造成台灣河川每年向海洋傳輸大量物質，單位面積的傳輸量甚至遠超過世界主要大河，故台灣地區小河川極具特殊性，是研究化學風化之絕佳地點。本研究藉由研究台灣地區11條主要河川之溶解態主要離子，探討台灣地區河川化學風化特性、與流域環境變化之關係，及對海洋所帶來的影響程度。 本研究發現台灣河川溶解態主要離子濃度皆呈現明顯的區域與季節性變化。本研究大部分測站之溶解態主要離子因降雨造成流量上升，產生離子濃度的稀釋效應，造成濕季濃度較低，乾季濃度較高，相差可達數倍之多，此乾濕季分明之季節性變化南部地區更為明顯。河川溶解態主要離子濃度之控制因素主要為蒸發作用、岩石風化與降雨，本研究發現台灣地區河川溶解性物質濃度之主要控制因子為流域母岩的岩石風化，而台灣地區岩層主要由矽酸岩組成，故矽酸岩貢獻程度高達80%以上。 颱風事件對於台灣河川溶解性物質是非常重要的特殊自然極端事件。颱風期間河川溶解態物質的濃度變化主要有下列三種現象：稀釋作用(Dilution effect )、潤濕作用(Enhanced hydrologic access)和海水飛沫的影響(Sea spray effect)。本研究估算台灣地區河川溶解性物質於颱風期間之輸出量與輸出效率皆高於平日，與世界河川相比，化學風化產率更高出數十倍之多，顯示颱風事件是台灣地區小河川溶解態物質進入海洋的一個關鍵。 本研究估算之台灣小河川總溶解物質年平均輸出產率為623 tons/km2．yr，與世界其他河川相比，明顯高出數倍至數十倍不等，顯示台灣小河川總溶解物質單位面積向海洋之輸出量遠高於世界河川，化學風化非常強盛，顯現出台灣地區小河川化學風化對海洋的重要性。

Small rivers in Taiwan export large amounts of substances to the ocean each year due to an uneven distribution of rainfall in time and space, and a high degree of weathering in rock of the river basin. As a result, chemical weathering rate of rivers in Taiwan are higher than other rivers in the world, and make Taiwan the ideal place to study properties of chemical weathering. The objectives of this study are to understand the characteristics of chemical weathering in Taiwan rivers and the extent of their impacts on the ocean by studying the major ions dissolved in 11 major Taiwan rivers. Significant regional and seasonal changes were found in major ions dissolved in the Taiwan rivers. Most of the dissolved ions showed a dilution effect due to the rise of discharge by rainfall. Therefore, the concentrations of the dissolved major ions were high in the dry season and low in the wet season, especially in the southern region. The difference in the ion concentrations is up to 7 fold. The three main mechanisms that affect the concentration of ions in rivers are precipitation, evaporation and rock compositions. The major mechanism influencing the concentration of dissolved major ions in Taiwan rivers is rock composition. The rock composition in Taiwan is mainly silicate rock and contributes to more than 80% of the weathering. Typhoons are very important for the major dissolved ions in Taiwan rivers. The three important mechanisms that influence the ions in the rivers during typhoon events are the dilution effect, enhanced hydrologic access and sea spray effect. TDS load and chemical weathering rate during typhoon events are both higher than normal. This chemical weathering rate is up to 70 times higher when compared with other rivers in the world. This indicates that typhoon events are a key factor in determining the dissolved ion carried by the rivers into the ocean. Chemical weathering rate in Taiwan small rivers is estimated to be 711 tons/km2．yr. This chemical weathering rate is up to 30 times higher when compared to other rivers in the world. This result shows that effect of chemical weathering is very powerful in Taiwan rivers and very important to the ocean.