蘭陽平原地下水及地層中砷之分布與特徵:意涵砷之釋出過程

Geochemical Characteristics of Arsenic in Groundwater of the Lanyang Plain, Taiwan: Implication to Arsenic Release Processes

吳明哲(Ming-Che Wu)

指導教授 ： 劉振宇

國立臺灣大學/生物資源暨農學院/生物環境系統工程學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.02787

摘要

砷(Arsenic)為一種類金屬(metalloid)普遍存在於地球之地殼中，台灣的蘭陽平原有高濃度的砷存在於地下水中(平均0.091 mg/L)。本研究對蘭陽平原的29口地下水監測井之水質檢測資料，應用統計的因子分析法判斷主要造成地下水中砷濃度高的因子，且使用PHREEQC計算地下水平衡狀態下物種生成及飽和指數，再採取9口地質鑽探井共403個岩心樣本，分析岩心中的總砷與總鐵濃度，由地下水中含高砷濃度監測井中選取19和9個岩心土壤樣本，分別進行連續萃取、礦物主成分及表面分析，礦物主成分及表面分析是利用X光螢光分析儀(XRF)、高解析電子能譜儀(XPS)和電子顯微鏡暨能量分散光譜儀(SEM-EDS)。因子分析結果顯示，在淺層地下水中砷濃度主要受到土地利用汙染入滲引發還原作用所致，而深層地下水砷濃度主要為還原環境由沉積物中釋出。在岩心結果總砷與總鐵於海相沉積環境有相關性，而在非海相沉積環境皆無關，高解析電子能譜儀進一步分析鐵的成分指出海相沉積岩心以FeOOH、FeO與Fe3O4為主，且FeS出現在較下游處與PHREEQC計算結果一致。當地下水系統趨於還原時，造成原本吸附於顆粒表面之鐵(氫)氧化物便開始溶於地下水中，並釋放Fe(Ⅱ)，與原本吸附於鐵(氫)氧化物表面之As(Ⅲ)與As(V)離子，推測為蘭陽平原地下水中砷濃度的主要來源。

關鍵字

砷 ； 因子分析 ； 蘭陽平原 ； 地球化學模式 ； 連續萃取 ； X光螢光分析儀 ； 高解析電子能譜儀 ； 掃描式電子顯微鏡暨能量分散光譜儀

並列摘要

Arsenic (As) is an ubiquitous metalloid in crust of the earth. High arsenic concentrations (average 0.091 mg/L) of groundwater were found in Lanyang plain of Taiwan. Groundwater from 29 wells was sampled and 14 hydro-geochemical parameters were analyzed. Factor analysis was applied to determine major influence factors of the arsenic, and PHREEQC was used to calculate the distribution of aqueous species and saturation index of which affected groundwater quality. 403 geological core samples from 9 drilling wells were collected and analyzed the contents of total arsenic and iron. Meanwhile, 19 and 9 core samples around high arsenic concentration groundwater were selected, respectively, for additionally sequential extraction and mineralogical phases analyzes, using x-ray fluorescence (XRF), high resolution x-ray photoelectron (XPS) and scanning electron microscope and energy dispersive spectrometer (SEM-EDS). Results of the arsenic-pollution factor determined by factor analysis indicated that infiltration of the pollutants from human activities to shallow groundwater, and the reductive dissolution from iron oxyhydroxides in the deep aquifer were the main processes of arsenic release to groundwater from the sediment. Total arsenic and iron contents of the core samples were well correlated in marine sequences. Surface analyses of core sample performed by XPS shown that arsenic was adsorbed copricipitated with noncrystalline iron oxyhdroxides. After a long term burial of sediment, microbial metabolism of organic matter creates a more reducing environment, arsenic may then be gradually released from iron oxyhydroxides by reductive dissolution or desorption to aqueous phase. Reduction dissolution or desorption of As from iron oxyhdroxides is considered as the most likely release process to groundwater in the Lanyang plain.

並列關鍵字

Arsenic ； Factor analysis ； Lanyang plain ； Geochemical modelling ； sequential extraction ； X-ray fluorescence(XRF) ； High resolution x-ray photoelectron spectrometer(XPS) ； Scanning electron microscope and energy dispersive spectrometer(SEM-EDS)

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