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標題: 梨山農作區水體之水化學及硝酸鹽的氮氧同位素研究
Hydrochemical and Isotopic (δ15N and δ18O in Nitrate) Studies of Waters in Li-Shan Agricultural Area
作者: 范家華
Fan, Chia-Hua
關鍵字: 硝酸鹽;水化學特徵;氮同位素;氧同位素;硝化作用
出版社: 土壤環境科學系
梨山地區為德基水庫的重要集水區之一,近年來當地農業活動施用的肥料與農藥對水源區水質的影響逐漸受到重視,其中NO3- 是重要的污染指標離子之一。本研究分析雨水、地表水及地下水的主要離子組成以了解當地水體之水化學特徵及其環境意義;同時也測定該區地表水和地下水中NO3- 之氮及氧同位素組成來評估水體中NO3- 的可能來源與轉化作用。
由水化學的結果顯示,梨山地區水體中的主要離子大部分來自鹽類溶解及/或岩石的風化作用,源自大氣沉降的貢獻很小。碳酸鹽溶解是本研究區水體中的主要水化學作用,且在地下水中尚有硫酸鹽溶解及矽酸鹽風化作用等反應;隨著地下水愈往下游之路徑過程,漸有較為明顯的硫酸鹽溶解及矽酸鹽風化作用現象。水化學結果亦顯示出約有52 % 的地下水中,NO3- 濃度高於飲用水水質標準(45 ~50 mg/L),此硝酸鹽污染情形可能是受到農業活動施用肥料的影響。
比較梨山地區水體中NO3- 與環境中可能的硝酸鹽來源之氮、氧同位素組成特徵,結果顯示地表水中的NO3- 可能主要源自未施肥土壤的有機氮。在地下水中,NO3- 出現有明顯的禽畜糞堆肥之同位素訊號,在斜坡上端處之地下水較斜坡下端處有較高的NO3- 濃度及較輕之δ15N值,造成此現象之原因可能是不同區域的施肥方式相異所導致,或是斜坡下端處的地下水有較強烈的同化或脫氮作用。比較不同季節的地下水硝酸鹽氮同位素組成與NO3- 濃度、pH及DO濃度顯示,在雨季(3月及6月)地下水受到的同化作用或脫氮作用較旱季(12月)明顯。然而,從目前的數據並不能辨別同化作用或脫氮作用對水體影響的程度。

Li-Shan area is one of the important catchments of Te-Chi Reservoir. Agricultural activities with application of fertilizers and insecticides affect water quality of study area are increasingly recognized in recent years and the NO3- is one of the important pollution indexes. In this study, we analyze the major ions compositions of rain, surface water and groundwater for understanding the hydrochemical characteristics of such water and its meanings to the environment of Li-Shan agriculture area; in the meantime, we also determined the nitrogen and oxygen isotope compositions in NO3- of surface waters and groundwaters in order to evaluate the possible sources and transformations of NO3- in waters.
The chemical result showed that major ions of waters are mainly contributed from salt-dissolution and/or rock-weathering, and atmosphere source is few. Carbonate dissolution is the basic hydrochemical reaction in waters of this study and additional sulfate dissolution and silicate weathering process are in groundwaters. The more downstream groundwaters, the more significant in sulfate dissolution and silicate weathering. The chemical result also showed about 52 % of NO3- concentrations of groundwater are higher than the water quality standard (45 ~50 mg/L)of drinking water. The NO3- contamination may be induced from fertilizers while applied by the agricultural activities.
Comparing the nitrogen and oxygen isotopic characteristics of NO3- in waters of Li-Shan agriculture area with the potential sources of nitrate indicated that NO3- in surface water might be derived from organic nitrogen of unfertilized soil, and NO3- of groundwater had obvious isotopic signals of poultry manure. In groundwaters, groundwater at the upper slope part (upstream) has higher NO3- concentration and lighter δ15N value than that at lower slope part (downstream). The reason that it may be caused by the application of fertilizers in various ways in different parts, or the groundwater at the lower slope part is stronger in assimilation and/or denitrification. Further to compare the relationships between δ15N value and NO3- concentrations, pH, DO of groundwaters in different season are showed that the groundwater exhibited assimilation and/or denitrification significantly in the rainy season (March and June) than that in dry one (December). However, the present data couldn't differentiate the degree that assimilation and denitrification affect to the waters.
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