INFLUENCE OF HYDROGEOCHEMICAL FACTORS ON THE ARSENIC DISTRIBUTION IN WETLAND OF NORTHERN TAIWAIN

In northern Taiwan, high 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. The wetland located in an estuary is closely affected by tidal fluctuation. Mixing of seawater and fresh water results wildly varied in salinity, pH, and redox condition with depths, which may control geochemical reactions of As release/retention. The purpose of this study is to evaluate the associated geochemical factors affecting As distribution and mobility in wetland. Arsenic species and other concerned chemical compounds of depth-varied pore water and solid samples were analyzed. In pore water, nitrite concentration increases with depth whereas nitrate showed an inverse relationship, indicating the downward redox condition tends to more reductive. In upper sediment (0-20cm), liberation of aqueous As and Fe species was restrained by oxidation condition, causing a low concentration of arsenite and ferrous iron. Highest As and Fe concentrations in pore water were found at 20-30cm depth suggesting the reductive dissolution of As-contained Fe oxides was the major source. A decrease of solid phase Fe and Mn contents with depth in both core samples indicated that reductive dissolution of Fe and/or Mn oxides in the lower layer was likely to occur with increasing organic matter. In the lower layer (>30cm), aqueous As concentrations were constrained by reducing condition causing precipitation of sulfide minerals and retention of As on the particle surface of lower layer sediment. Elevated As occurred in the shallow (20-30cm) redox transition zone situated at the boundary between Fe-oxidizing and sulfate-reducing zones. Moreover, the ratio of pore water sulfate and chloride markedly declined at depth below 60cm. Semi-diurnal tidal carried sufficient amount of sulfate as electron acceptor creating an alternative-varied redox condition at 30-60cm sediment depth. The alternated redox condition controlled As mobility within this interval. Accordingly, distribution and mobilization of As in this wetland systems is primarily associated with Fe/Mn (hydr)oxides and sulfides, which governed by the cyclic redox condition in the ambient environment.