A complex interplay between hydrology, redox conditions and Fe mineralogy leads to diverse reactions that determine contaminant fate and general water quality in acid sulfate soil (ASS) wetlands. Many of the Fe minerals that commonly occur in ASS environments are both metastable and capable of being potent scavengers for toxic trace metals and metalloids under oxidising acidic conditions. Some trace metals and metalloids (such as arsenic and antimony) may have a large impact on environmental quality. Therefore, a sound understanding of metastable host-mineral phases and the corresponding behaviour of associated trace metal/metalloid contaminants under fluctuating redox conditions is essential for ASS wetlands management. My research focuses on how mineral formation and evolution over time affects the geochemical behavior of toxic metalloids arsenic and antimony in the geo-environment using a wide range of advanced analytical techniques including X-ray Absorption Spectroscopy (XAS). A major focus is on improving our understanding of the potential effects of seasonal redox oscillations on Fe and S speciation and exploring the consequences for water quality. There is a need for improved interpretation of the important processes which are likely to occur in ASS and acid mine drainage (AMD) under fluctuating redox conditions in order to better predict prospective environmental outcomes and choose optimal management strategies.