Application of diffusive gel-type probes for assessing redox zonation and mercury methylation in the Mekong Delta sediment

The vertical profiles of PO43-, Mn, Fe, S2-, Hg, and CH3Hg+ in sediment pore water were investigated using DGT and DET probes in the Tien River, the northern branch of Vietnam's Mekong Delta. Although some of the DGT measurements could be lower than the actual pore water concentrations due to the depletion of the species, the measurements provided information for understanding redox zonation and Hg methylation. The gradual increases in the measured species concentrations with the sediment depth were observed and the diffusive fluxes of the species to overlying water were expected. The vertical profiles suggested that (1) SO42- seemed to be reduced before Fe3+, or the two electron acceptors were reduced simultaneously; (2) the release of PO43 was more closely related to S-2 than Fe release; and (3) Hg methylation was active in the micro-niche between the aerobic and anaerobic transition zones. The maximum pore water CH3Hg+ concentrations were observed at depths just above where the maximum S2- concentrations were detected. Hence, the maximum CH3Hg+ concentration was observed near surficial sediments (less than 1 cm from the surface) in brackish water, and at a depth of 3 cm in fresh water. The different vertical profiles led to a CH3Hg+ diffusive flux eight-times greater in brackish than in fresh water. The present study showed that the in situ application of DGT and DET probes was helpful to understand coupled biogeochemical reactions and mercury methylation by measuring pore water redox species.