| Summary: | 碩士 === 國立臺灣大學 === 生物環境系統工程學研究所 === 107 === 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.
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