應用氮同位素模擬濁水溪沖積扇地下水含氮化合物與砷之生地化循環關係

濁水溪沖積扇為臺灣重要地下含水層，地下水含氮化合物汙染主要源於農業活動發展，扇尾區具高濃度砷汙染，而該區域砷釋出機制仍未明確，且砷汙染因子與氨氮群集。本研究目的為建立濁水溪沖積扇地區地下水含氮化合物與砷之生地化循環關係模型，模擬並評估地下水之水文化學特徵及傳輸路徑之影響。應用地質化學模式PHREEQC進行模擬，研究主要分為三部分。第一部分為水質參數模擬，透過呼吸作用及脫氮能力模擬，評估扇尾區域地下水環境為脫氮作用主導，扇頂區域需要較高濃度之有機物濃度進行呼吸作用方能使環境缺氧進入脫氮反應，而扇頂區有機物並未具所需之濃度，因此無法進行脫氮反應；透過硝化作用模擬，相同歷時，儘管扇頂區氨氮濃度消耗比例大，然並未有高濃度硝酸鹽氮生成，評估扇頂區之高硝酸鹽氮濃度並非因硝化作用產生，並從同位素驗證及文獻回顧得知，其原因為施用肥料所產生；透過脫氮作用模擬，溶氧耗盡後硝酸鹽氮消耗速率增快並伴隨亞硝酸鹽氮及氮氣生成，且於長時間(50年)作用方有氨氮產生，扇尾區硝酸鹽消耗比例較高，顯示該區域明顯脫氮作用發生；將砷物種加入脫氮作用模擬，砷物種之加入使硝酸鹽氮消耗速率加快，表示脫氮作用因此加快進行，並且當脫氮作用發生時，伴隨著三價砷氧化成五價砷，評估缺氧狀態下三價砷之氧化促使硝酸鹽氮還原進行脫氮作用。第二部分進行同位素驗證，經水樣分析評估扇頂區域之低及低概因過量肥料施用造成高濃度硝酸鹽氮汙染，且此區域比例為硝化作用產生之硝酸鹽氧同位素範圍－10‰～＋10‰間，驗證此區域發生硝化作用，將氨氮反應為硝酸鹽氮，而扇央及扇尾區域之同位素為分化作用之結果；並經同位素脫氮作用模擬，比值隨著脫氮作用發生而增高，其分化作用映證扇尾區及扇央區脫氮作用之發生，且扇尾區較為明顯；透過同位素富集因子計算，由同位素分化作用評估濁水溪沖積扇地下水脫氮作用速率於反應初期較為劇烈。第三部分為模擬地下水砷移流傳輸及表面錯合反應，扇央區流入扇尾區之硝酸鹽氮濃度隨反應時間減少，映證脫氮作用之發生，且流入扇尾區之五價砷易吸附於鐵(氫)氧化合物形成錯合物，而脫氮作用產生之氨氮易與鐵(氫)氧化合物反應，並使表面之五價砷脫附為三價砷進入地下水體，使扇尾區域三價砷濃度增高。

關鍵字

砷；脫氮反應；濁水溪沖積扇；地下水；同位素； PHREEQC

並列摘要

Choushui River alluvial fan is the important aquifer in Taiwan. The nitrogen pollution in the proximal fan is mainly associated with agriculture activities. Moreover, the high arsenic concentration has been identified in the distal fan, but the arsenic release mechanism remained unclear. The factor analysis showed that arsenic pollution was correlated well with ammonium. The objective of this study is to investigate the biogeochemical processes of nitrogen and arsenic in Choushui River alluvial fan. The hydrogeochemical model was used to simulate the groundwater geochemical characteristics and transport process. The study adopted the geochemical model PHREEQC for simulation. The simulation result of respiration and denitrification capacities reveals that denitrification is the dominant reaction in the distal fan. In the proximal fan, the groundwater requires sufficient microorganisms to promote the aerobic respiration for denitrification. The insufficient microorganisms were not able to exhaust dissolved oxygen. In the same time, despite the high ratio of ammonium reacted in the proximal fan, the product of nitrate concentration was low. The result showed that the high nitrate pollution in the proximal fan originate from nitrification. In the simulating denitrification process, after the dissolved oxygen was consumed, the rate of nitrate consumption became faster with generating nitrite and nitrogen products, and the ammonium may produce after a long period of time. The high ratio of nitrate reactant in the distal fan indicated that denitrification occurred in the area. The rate of denitrification became faster if the arsenic species existed in the environment, the oxidation of arsenite (As3+) was stoichiometrically coupled to the reduction of nitrate. In other words, the nitrite could be oxidized to nitrates because of arsenite. The isotope simulation is used to verify the proposed reactions. According to the sampling for stable isotope analysis, high nitrate concentrations are associated with low and low values and these low values represent the isotopic composition of nitrate resulting from organic N fertilizers in the proximal fan. After all, the range of is -10 ‰ to +10 ‰, suggested that the nitrification did occurr in the area. And the analysis showed the isotope fractionation reaction existed in the mid fan and distal fan. The denitrification could cause the isotopic fractionation, therefore, the value would be higher. The result of isotopic modeling demonstrated that the denitrification occurred in the area and the reaction in the distal fan was more likely. The isotopic enrichment factor suggested relatively rapid denitrification in the initial reaction and declined with time. The simulation of reactive groundwater transport with surface complexation of arsenic to iron (hydr)oxides from the mid fan into distal fan showed that the denitrification occurred, arsenic might be sorbed to iron (hydr)oxides, and the denitrification product of ammonium could reduce iron (hydr)oxides to release the arsenite to groundwater.