關渡濕地砷在固相、液相及植物相之分布及累積

Abstract

本研究針對關渡濕地之兩岩心沉積物及孔隙水樣本(S2及S5)，分析乾季岩心沉積物及孔隙水之化學組成及同位素組成，濕季岩心沉積物之礦物組成，並採集植物樣本，分析其總砷結果並計算其遷移因子(translocation factor, TF)及生物濃縮因子(bioconcentration factor, BCF)，說明關渡濕地砷在固相、液相及植物相之分布及累積特性。隨深度分析乾季岩心中孔隙水及沉積物之化學組成，應用統計方法因子分析(factor analysis)及群集分析(cluster analysis)，依據孔隙水中的水質特徵，將岩心隨深度劃分為氧化帶(淺層)、緩衝帶(中層)及還原帶(深層)，瞭解各分層帶砷可能移動的機制；由濕季岩心沉積物樣本，以X光繞射鑑定(X-ray diffraction, XRD)分析礦物種類，但未存在含砷礦物；進一步應用同步輻射技術X光吸收光譜(X-ray absorption spectra, XAS)中X光吸收近邊緣結構(X-ray absorption near-edge structure, XANES)分析沉積物之砷原子氧化價態，顯示深層沉積物有些微As(V)轉變為As(III)的現象，此結果佐證關渡濕地隨深度增加越趨於還原狀態；此外應用同位素群集分析之結果，將乾季岩心樣本隨深度分成淺層及深層，深層產生硫同位素分化係數之峰值，說明此區明顯受到異質性硫酸鹽還原作用，再利用孔隙水中硫酸鹽之硫氧同位素值，推測關渡濕地不同深度之水體來源，淺層及深層之水體來源分別為海水及北投地下水。關渡濕地主要的植物為水筆仔(Kandelia obovata)，分析其總砷濃度結果顯示水筆仔(23.69 mg/kg) >土壤(17.24 mg/kg) >水體(0.018 mg/L)，並分析水筆仔各部位之總砷濃度，結果為：根部(19.74 mg/kg) >莖部(1.76 mg/kg) >葉部(1.71 mg/kg) >胎生苗(0.48 mg/kg)，顯示砷主要累積在水筆仔根部；從TFstems/roots (0.088)、TFleaves/roots (0.088)、TFseedlings/roots (0.024)，顯示砷從根部傳輸至莖部及葉部的效應較胎生苗高，從TF皆小於1，說明砷由根部往地上部傳輸之效應差，其與水筆仔耐鹽機制一致；由於關渡溼地受潮汐影響劇烈，表水中的砷易被海水稀釋造成BCFplant/water>>BCFplant/soil，因此BCFplant/soil較可用來說明水筆仔的生物濃縮行為，由BCFplant/soil>1說明水筆仔吸收及累積周圍土壤中的砷之能力佳，其吸收機制為水筆仔藉其特殊通氣系統將氧氣輸送至根部，造成根系周圍形成鐵氧化物，砷易吸附在鐵氧化物的表面上，因此水筆仔利用本身對土壤中重金屬的吸收能力佳，使砷累積於體內。

The purpose of this study was to determine the distribution and accumulation of arsenic (As) among aqueous, solid, and plant phases in the Guandu Wetland of Taiwan. Chemical compounds and isotopic compositions in porewater samples of dry season and mineral compositions in the sediment samples of wet season were analyzed to characterize the As distribution with depth. Arsenic concentrations in plant samples were analyzed to assess the translocation factor (TF) and bioconcentration factor (BCF), and to understand the distribution and accumulation of As in the plant, solid, and liquid phases. Chemical compounds concentration were applied the factor and cluster analyses in porewater samples to delineate the vertical profile of oxidation zone (shallow layer), buffer zone (mid layer), and reduction zone (deep layer); and to elucidate the mechanism of As possible mobile in each zone. As-bearing minerals were not found in all sediment samples by using X-ray diffraction (XRD). However, the result of As species transformation indicated mild As(V) (arsenate) shifting to As(III) (arsenite) in the deep layer by X-ray absorption near-edge structure (XANES) of X-ray absorption spectra (XAS). The results implicate that reduction gradient was tended to increase with depth in this wetland systems. The cluster analysis was applied to determine the boundary of shallow zone and deep zone using isotopic compositions data of the sediment samples. The high values of sulfur isotope fractionation factor (ε) occurred in deep zone which was caused by the bacterial sulfate reduction. The results of sulfur isotope indicate that the source of water in shallow zone and deep zone are mainly from mixing of the seawater/surface water and Batou groundwater, respectively. Kandelia obovata is one of the most dominant plant species in mangrove ecosystems of the study area. In this study, As concentrations in plants (23.69 mg/kg) were higher than in the surrounding water (0.0018 mg/L) and soils (17.24 mg/kg). The order of arsenic concentration in various plant tissues at maturity was as follows: roots (19.74 mg/kg) > stems (1.76 mg/kg) > leaves (1.71 mg/kg) > seedlings (0.48 mg/kg), suggesting a high accumulation in the roots. The low translocation factors (TFstems/roots= 0.088, TFleaves/roots= 0.088 and TFseedlings/roots= 0.024) indicate that the translocation of As in various plant tissues is extremely low depending on salt tolerance mechanism of K. obovata. The significant tidal effects result in high BCFplant/water that the As concentrations in wetland water is diluted by seawater. A high BCFplant/soil (BCFplant/soil> 1) between plants and soil indicates that the uptake and bioaccumulation of As in K. obovata are significant; therefore, K. obovata is an As accumulator. The uptake mechanism of plants might depend on the formation of As-contained iron oxides in the root zone, causing the uptake of As in plants to be from soil.