Biogeochemical characteristic of arsenic distribution and mobilization in the Guandu wetland of northern, Taiwan

High arsenic (As) contaminated geothermal spring water has been observed in world widely. In Taiwan, high As contents spring water may be mobilized downstream of wetland ecosystem, causing hazard of ecological environment. However, both the distribution and mobilization of As in the wetland and the impact of As on ecosystems are still poorly defined at present in Guandu Wetland, Taiwan. Chemical (As, Fe, Mn, TOC, SO42-, FeS2…), isotopic (δ34S and δ18O) and mineralogical (HR-XPS, SEM-EDS and XANES) compositions of the pore water and sediment samples were analyzed, to characterize arsenic distribution and understand the key mechanisms of biogeochemical processes controlling the fate and transport of As in wetland ecosystems. The analytical results of pore water and solid samples revealed the oxidation condition and consist of low As(III) and Fe(II) concentrations in shallow layer. Highest As and/or Fe concentrations in pore water were found at the depths with decreased Eh in mid layer and deep layer. Factor analysis and cluster analyses were applied (18 chemical parameters) in pore water samples to delineate the vertical profile of redox potentials. The results show that vertical variability of redox zonation divided into three zones, namely oxidation zone (shallow layer), buffer zone (mid layer), and reduction zone (deep layer); to evaluate the possible As mobility mechanism in each zone. In oxidation zone, liberation of aqueous As and Fe species were restrained by oxidation condition. Besides, highest As and Fe concentrations in sediments were found at 0-15cm depth, suggesting the oxidation of As-contained Fe oxides was the major process. In buffer zone, decrease in solid phase of Fe and As contents and increase in aqueous phase of Fe and As contents may indicate that reductive dissolution of As-contained Fe oxides in the Fe-reducing condition was likely to occur with increasing organic matter. The high δ34S and δ18O values in reducing zone caused by significant bacterial sulfate reduction, resulted in increased As and FeS2 concentrations in sediments. Moreover, the occurrence of elevated aqueous As occurred in the buffer zone situated at the boundary between Fe-reducing and sulfate-reducing conditions. According to enrichment fractionation factor (ɛ) of sulfur isotope, the bacterial disproportionation is other important process for As mobility, accompanying with the reduction of As-contained Fe oxydroxides. Therefore, the results of As K-edge XANES spectra suggested that the significant mixture of As(III) and As(V) in the sediments were found in buffer and reducing zones. Transformation of As(V) to As(III) indicates that the process occurred in the reducing condition. Integrated results showed that the distribution and mobilization of As in this wetland systems is primarily associated with the oxidation/reduction of As-contained Fe oxides, bacterial sulfate reduction and bacterial disproportionation, which were governed by the redox condition in each zone.