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  • 學位論文

同位素闡釋濁水溪沖積扇含砷地下水氮化合物之來源及轉化

Isotopic evidence and simulation of nitrogen sources, transformation, and transport in arsenic-contaminated groundwater

指導教授 : 劉振宇
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摘要


本研究區域濁水溪沖積扇南端之淺層地下水含有高濃度砷,歷年研究報告指出此區砷主要釋出機制為含砷之無晶型氧化鐵的還原溶解作用,另過去研究報告曾探討濁水溪沖積扇沿海區域之鹽化地下水中硫的氧化還原循環對砷移動性之影響,結果顯示高砷鹽化類地下水砷主要受硫歧化作用與含砷氫氧化鐵還原溶解影響,高砷非鹽化類則顯示有人為抽水行為改變地下水氧化還原條件,促使含砷硫化物再氧化,吸附於硫化物表面上的砷因而釋放至地下水中。除硫酸鹽類外,根據相關化學反應式,氮的氧化還原循環亦可能影響地下水中砷的傳輸與宿命,惟其在砷的生地化循環過程中所扮演的角色目前尚無人研究。 根據本研究結果,濁水溪沖積扇之扇頂區域之地下水含有高濃度硝酸鹽,扇尾區域則普遍存在高濃度砷及氨氮,透過δ15NNO3及δ18ONO3繪圖,可得扇頂地下水中硝酸鹽來源主要由含氮肥料、動物糞肥、及人類排泄物等所貢獻,扇央及扇尾地下水中硝酸鹽則主要來自含氮肥料及海洋中硝酸鹽,並發現自扇頂至扇尾有脫硝現象發生。氮循環系統方面,透過氮氧同位素化學反應方程式及其與硝酸鹽濃度之繪圖,可得扇頂地下水有顯著硝化作用,扇央地下水中硝酸鹽之植物同化為硝酸鹽削減之主要控制因子,惟脫硝現象並不顯著,而高濃度之砷、氨氮、鐵及δ15NNO3之削減隱含著feammox作用的發生;扇尾地下水有顯著脫硝作用發生,並且可能伴隨著植物同化、含氮物質礦化、硝酸鹽異化還原氨氮等作用,促成一個硝酸鹽削減及氨氮增加的地下環境。δ15NNO3、δ18ONO3與砷濃度繪圖以及相關化學反應方程式,指出扇央地下水之feammox作用及扇尾地下水的脫硝作用為導致含砷之鐵氫氧化物還原溶解,並且釋出吸附砷至地下水的主要反應促使過程。 爾後,利用PHREEQC模擬軟體進行前述研究成果之模擬,除藉由現地環境氧化還原狀態及實驗數據驗證模擬結果外,亦推估了未來反應終止之可能最終狀態,以及可能傳輸情形,進一步瞭解濁水溪沖積扇地下水含氮化合物影響砷之生地化循環過程。由PHREEQC模擬結果顯示,硝化或feammox主要發生在扇頂及扇央,扇尾則無明顯此反應,而根據硝酸鹽從扇頂至扇尾的濃度空間分布,指出脫氮及硝酸鹽異化還原氨氮從上游至下游漸序發生。依現地溶氧及氧化還原電位數據分析,扇央及扇尾屬較還原狀態,可促使脫氮及硝酸鹽異化還原氨氮反應的發生。針對砷的價態轉換模擬,扇頂三價砷減少而五價砷增加,扇央及扇尾,由溶氧及氧化還原電位觀察結果,模擬砷由三價轉化為五價之狀況非常顯著。δ15N的差異模擬結果,在扇央及扇尾均顯示增加,符合現地採樣結果及理論依據,即脫氮反應發生時,δ15N將會增加。 一維傳輸結果,硝酸鹽同化作用發生在扇央,而氨氮硝化作用則發生在扇頂,但不同年份模擬結果均有不同程度之時間位移。五價砷濃度於扇央開始增加,乃因含砷之鐵氫氧化物還原溶解並使砷脫附所造成;三價砷濃度則在扇尾開始時增加,主要由五價砷的還原轉換及持續性的鐵氫氧化物還原溶解所導致。二價及三價鐵在扇央初期,因鐵氫氧化物還原溶解而使濃度增加,而三價鐵接著轉換為二價鐵。二價鐵為主的環境,除了與地下水還原態環境有關外,也可能與feammox反應而直接產生二價鐵有關。

關鍵字

地下水 氮同位素 生地化循環

並列摘要


In this study, on the basis of physicochemical characteristics of groundwater and the nitrogen and oxygen isotope composition of NO3−, it was inferred that the main sources of NO3− in the proximal fan of the Choushui River alluvial fan are likely to be ammonium fertilizers, manure, and septic waste; that in the mid-fan and the distal fan, the possible sources are nitrate fertilizers and marine nitrate. In the proximal fan, the oxidative state obviously promotes microbial nitrification. High DO concentrations and relatively low values of δ18ONO3 in the deeper aquifer of the proximal fan may be attributed to unconfined granular nature and groundwater pumping by agricultural activities. In the mid-fan, NO3− assimilation is the dominant response to NO3− attenuation, and denitrification is insignificant; however, high concentrations of As, NH4+ and Fe and depletion of δ15NNO3 imply the occurrence of feammox process. By contrast, denitrification evidently occurs in the distal fan, through assimilation, mineralization, and dissimilatory NO3− reduction to NH4+, resulting in depletion of NO3− and increase in NH4+ in groundwater. Feammox in the mid-fan and denitrification in the distal fan may be the main processes leading to the release of As from As-bearing Fe oxyhydroxides into groundwater. The simulation result of nitrification shows that the nitrification and/or feammox mostly occur in the proximal fan and mid-fan, whereas they slightly occur in the distal fan. The concentrations of NO3− and NH4+ in the proximal fan evidently support the occurrence of NH4+ nitrification. The spatial concentration distribution of NO3− from the proximal fan to the distal fan indicates the gradual occurrence of NO3− denitrification and/or DNRA from upstream to downstream of the Choushui River alluvial fan. The mid-fan and the distal fan were assessed on the basis of the local DO and ORP values to be in relatively more reductive conditions, driving the occurrence of denitrification and/or DNRA. In the proximal fan, As3+ decreased and As5+ increased, and this valence transformation of As species and As concentration difference seem comprehensible. In the mid-fan and the distal fan, the reductive state was observed base on the DO and ORP data of the groundwater, and the circumstance of reduction from As5+ to As3+ was obvious. The discrepancy of δ15N in NO3− in groundwater was simulated on the basis of the influence of the reaction of NO3− denitrification. The values of δ15NNO3 increased in the groundwater of the mid-fan and the distal fan; in theory, the denitrification increases δ15N values of the residual NO3−. The 1-D transport simulation result suggested that NO3− assimilation occur from the mid-fan of the Choushui River Alluvial Fan to the distal fan, whereas NH4+ nitrification is observed at the beginning of the proximal fan. The concentration of As5+ increased at the beginning of the mid-fan, which may be caused by the reductive dissolution of As-bearing Fe oxyhydroxides and the desorption of adsorbed As. The concentration of As3+ increased obviously at the beginning of the distal fan, which may be related to the transformation of As5+ to As3+ in the reductive environment, and the continuous desorption of As from Fe oxyhydroxides simultaneously. Both the concentrations of Fe3+ and Fe2+ increased at the end of proximal, causing by the reductive dissolution of Fe oxyhydroxides. The transformation of Fe3+ to Fe2+ occurred soon when the groundwater reached the mid-fan. The increase in Fe2+ is not only related to the reductive environment, but also attributed to the reaction of feammox, which Fe oxyhydroxides react with NH4+ and produce Fe2+ in the groundwater.

參考文獻


Agricultural Engineering Research Center. Analysis and evaluation of the groundwater quality survey in Taiwan, 2010; 2012. Taiwan Water Resource Bureau. Taipei.
Akai, J., Izumi, K., Fukuhara, H., Masuda, H., Nakano, S., Yoshimura, T., Ohfuji, H., Anawar, H.M., Akai, K., 2004. Mineralogical and geomicrobiological investigations on groundwater arsenic enrichment in Bangladesh. Applied Geochemistry. 19(2), 215-230.
Amberger, A., Schmidt, H.L., 1987. Natürliche isotopengehalte von nitrat als indikatoren für dessen Herkunft. Geochimica et Cosmochimica Acta. 51(10), 2699-2705.
Anawar, H.M., Tareq, S.M., Ahmed, G., 2013. Is organic matter a source or redox driver or both for arsenic release in groundwater? Physics and Chemistry of the Earth. 58-60, 49-56.
Andersson, K.K., Hooper, A.B., 1983. O2 and H2O are each the source of one O in NO2- produced from NH3 by Nitrosomonus: 15N-NMR evidence. FEBS Letters. 164(2), 236-240.

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