翡翠水庫及其集水區之氫氧碳同位素 水文學研究

Hydrogen, Oxygen, and Carbon Isotope Compositions of Meteoric Waters in Feitsui Reservoir and Its Watershed, in Northern Taiwan

莊凱勳(Kai-Hsun Chuang)

指導教授 ： 汪中和 ； 劉康克

國立中央大學/地球科學學院/水文與海洋科學研究所/碩士(2006年)

摘要

翡翠水庫集水區是大台北地區主要用水來源。水體中氫、氧同位素組成的變化記錄了水的來源與混合，是研究水文學的重要工具之一。因此，本研究探討翡翠水庫之庫水與集水區之雨水、河水間之關係，以提供水庫水資源管理所需的基本同位素水文資訊。本研究自2003年4月至2004年4月以連續方式收集雨水，並按月採集水庫以及周圍集水區三大主要支流的水樣，進行氫、氧、碳同位素值之分析，並配合氣象及水文資料，進一步了解集水區中的水文循環變化。 研究結果發現翡翠水庫雨水中氫氧同位素值與氣溫呈負相關，與一般大陸地區之情況不同。推論台灣北部翡翠水庫集水區內雨水氫氧同位素組成之變化主要受不同來源氣團的影響，氣溫並非主要因素。由於氣團的來源及路徑依季節變化，故其間的水循環之同位素分化過程也相異，本研究主要區分為兩個不同時段探討天水線關係式。5月到9月為西南季風盛行期，δ18O值分佈自-0.7‰至-11.7‰，而δD則由1.1‰至-87.4‰；迴歸的天水線為???。10月到次年3月為東北季風盛行期，δ)7.04.7()1.08.7(18±+±=ODδδ18O值分佈從0.6‰至-6.2‰，而δD則由20.5‰至-38.4‰；迴歸的天水線為：。研究期間內全年的雨水天水線的迴歸式為：。 )4.19.13()5.02.7(18±+±=ODδδ)79.005.13()17.02.8(18±+±=ODδδ 翡翠水庫集水區之河水同位素組成變化趨勢與雨水之變化差異很大。將雨水進行5個月移動平均並依雨量加權後，發現雨水與河水在季節性的變化趨勢上較為吻合，可能受雨水入滲的遲滯效應的影響。河水同位素值的變化幅度小於雨水，且同位素值偏輕，推測可能的影響因素有：(1)夏季受雨量效應影響，同位素值相對較低，入滲量亦可能較大；(2)海拔較高之雨水同位素值相對較輕；(3)過去數年的雨水同位素值可能偏低。 庫水的來源主要由上游支流匯入，而支流的基流係來自雨水入滲後形成的地下水，因此所觀察到的同位素組成的季節性變化幅度很小，δ18O值分佈範圍自-4.7‰至-8.3‰，全年平均為-5.8‰。在本研究中發現水庫進水來源除了低流量之河川基流外，在颱風大雨期間發生的漫地流亦對水庫同位素組成產生重大影響。例如水庫中所量測的最低氧同位素值(-8.3‰)，即是2003年9月時的颱風過後，漫地流進入水庫的高濁度中層水。 研究期間水庫表水無機碳同位素值分佈在-11.7‰到-16.0 ‰之間，以6月最重而3月較輕。主要原因係6月藻類大量繁殖，行光合作用優先利用較輕的碳同位素，因此剩下的水體碳同位素組成就相對趨重。7月以後葉綠素含量開始減少，由呼吸作用產生的二氧化碳持續增加，加之有機碳同位素值偏輕，因此水庫中碳同位素值也就相對較輕。 本研究結果顯示，氫氧碳同位素組成是水文研究的良好示蹤劑，可有效解釋翡翠水庫的庫水、河水和雨水之來源與混合，並可深入了解集水區之水文循環及與生地化作用的關係，同時亦對翡翠水庫的水量及水質管理提供了有價值的資訊。

關鍵字

翡翠水庫 ； 天水 ； 氧同位素 ； 氫同位素 ； 碳同位素 ； 水文學 ； 水循環 ； 地球化學

並列摘要

Feitsui Reservoir provides water supply in Taipei City as well as parts of Taipei County in northern Taiwan. The variations of hydrogen, oxygen, and carbon isotope compositions of waters record the sources and mixtures of waters; they are ideal and important tools to study hydrology. This study discussed the relationships among the waters in Feitsui Reservoir, rain waters and river in its watershed in order to provide basic isotope information in hydrology for the appropriate water resource management. From April 2003 to April 2004, rain waters of respective rainy days were collected at Feitsui Reservoir; waters of the reservoir and major branches of its watershed were also sampled monthly. These water samples were analyzed for hydrogen, oxygen, and carbon isotope compositions. In addition, information of meteorology and hydrology was also compiled and integrated with isotope data to study the hydrology cycle of Feitsui Reservoir.
Analytical results showed that the hydrogen and oxygen isotope compositions of rain waters in Feitsui Reservoir exhibit a negative correlation with air temperature; an opposite feature with those observed in the continent. Apparently, the variations of hydrogen and oxygen isotope compositions of the rain waters in Feitsui Reservoir’s watershed have been primarily affected by the air masses of different seasons rather than the air temperature; because the sources and the paths of air masses change with seasons and cause distinct isotope fractionations accordingly. Two intervals (southwest and northeast monsoons) were differentiated to discuss the meteoric water lines in this study. Southwest monsoon prevails from May to September, and the ranges of δ18O and δD were -0.7‰ ~ -11.7‰ and1.1‰ ~ -87.4‰, respectively; the least-squares regression line was: ? ?Northeast monsoon prevails from October to March, and the ranges of δ)7.04.7()1.08.7(18±+±=ODδδ18O and δD were 0.6‰ ~ -6.2‰ and 20.5‰ ~ -38.4‰, respectively; the least-squares regression line was: .Over this study period, the least-squares regression line of all precipitation data was: . )4.19.13()5.02.7(18±+±=ODδδ)8.01.13()2.02.8(18±+±=ODδδ
The hydrogen and oxygen isotope compositions of the river waters in Feitsui Reservoir’s watershed showed significant and systematic discrepancy with those of rain waters. The isotope data of rain samples were processed as five-month moving average and weighted according to the precipitation amount. The resulting trends of processed rain waters showed similar seasonal pattern with those of river waters probably due to the hysteresis effect of rain infiltration. The isotopic ranges of the river waters were relatively smaller and depleted than those of the precipitation. The possible causes are: (1) Rain waters had a larger precipitation amount effect in summer with relatively depleted isotope signatures and higher amount of infiltration;(2) The isotope compositions of the rain waters have an altitude effect that provides relatively depleted isotope values at higher locations; (3) The isotope compositions of the rain waters for previous years might carry relatively depleted isotope values.
The main source of the Reservoir derives from upstream branch waters, and base flows of those branches are groundwaters after rain infiltration. The observed seasonal isotope fluctuations of base flows were small, δ18O ranging from -4.7‰ to -8.3‰ with a mean value of -5.8‰. On the other hand, the overland flow displayed a large impact on the isotope compositions of the Reservoir during the typhoon periods. For example, the lowest δ18O (-8.3‰) value was detected in the middle water layer of the Feitsui Reservoir after the typhoon of September 2003.
The dissolved δ13C values of water samples ranged from -11.7‰ to -16.0 ‰, with relative enriched compositions in June and depleted values in March. The main cause of this season discrepancy is due to the higher growth of algae in June, which absorbed the lighter carbon, leaving the water carbon isotope relatively enriched. The chlorophyll reduced significantly after July with carbon dioxide continually introduced from respiration, resulting lighterδ13C values in the reservoir.
In short, results of this work demonstrated that hydrogen, oxygen, and carbon isotope compositions are good tracers in studying hydrology. These isotope tracers not only can effectively explain the sources and mixtures from various sources in Feitsui Reservoir but also enhance our understanding of the hydrology cycle in its watershed and the close relationship between the biology and geochemistry. The isotope data collected in this study are also valuable in the management of the Feitsui Reservoir waters in terms of quality and quantity.

並列關鍵字

carbon isotopic ； hydrogen isotopic ； oxygen isotopic ； meteoric water ； Feitsui Reservoir ； hydrology cycle ； geochemistry ； hydrology

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