Effect of cyclic redox oscillations on water quality in freshwater acid sulfate soil wetlands

Restoration of acid sulfate soil (ASS) wetlands by freshwater re-flooding can lead to the reformation of various Fe(II) and reduced inorganic sulfur (RIS) species in surface soil layers. However, in many locations, wetland water levels undergo large seasonal fluctuations that drive extreme redox oscillations. Newly formed RIS species [e.g. greigite, mackinawite, nano-pyrite and S(0)] and Fe(II) are vulnerable to rapid oxidation during dry periods and may generate substantial acidity. Rainfall following a dry period may then mobilise acidity and metal cations in surface waters prior to eventual recovery in pH by re-establishment of reducing conditions. We explore this dry-wet transition by subjecting soil samples from two freshwater re-flooded ASS wetlands to oxidative incubation for up to 130days followed by re-flooding simulation for 84days. During very early stages of re-flooding (up to 7days) there was an initial pulse-release of acidity, and trace metals/metalloids (Al, Mn, Zn and As). This was followed by a rapid reversion to anoxia, and Fe(III) and SO4 reducing conditions which generated alkalinity, ameliorated acidity and sequestered Fe, S, Zn, Mn and As. Field-observations of surface water quality in an ASS wetland at a sub-catchment scale also confirms re-establishment of SO4 reducing conditions and recovery of pH within ~4-8weeks of re-flooding after dry periods. These observations suggest that retaining surface water in ASS wetlands for ~8weeks after a dry-wet transition will allow sufficient time for alkalinity producing reductive processes to ameliorate most surface water acidity. Although management of freshwater re-flooded ASS wetlands in a highly dynamic climate will remain challenging over the long term and the post-remediation effectiveness of the method depends on initial soil characteristics, knowledge of the timing of redox oscillations and the associated changes in water geochemistry can be helpful for mitigating the risks to downstream estuarine water quality.