INFLUENCE OF BIOGEOCHEMICAL CHARACTERISTIC ON ARSENIC DISTRIBUTION AND MOBILIZATION IN THE GUANDU WETLAND OF NORTHERN, TAIWAN

High arsenic (As) concentration (~4.32 mg/L) spring water from upstream geothermal spring area discharges to the downstream wetland ecosystem via surface water and groundwater routes in northern, Taiwan. Chemical (As, Fe, Mn, TOC, SO₄^2-, FeS₂), isotopic (δ³⁴S and δ¹⁸O) and mineralogical (HR-XPS, SEM-EDS and XANES) compositions of the pore water and sediment sample were analyzed, to characterize the key mechanisms of biogeochemical processes controlling the distribution and mobilization of As in wetland. Factor analysis and cluster analyses were applied (18 chemical parameters) in pore water samples to delineate the vertical profile of redox potentials. The 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 the oxidation of As-contained Fe oxides as major process. In buffer zone, the reductive dissolution of As-contained Fe oxides resulted in decrease in solid phase of Fe and As contents and increase in aqueous phase of Fe and As contents. 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 (ε) 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 buffer and reducing zones caused by Fe and S 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.