Mobilisation and precipitation of ferrous iron on the SO_4~(2-)/S~(2-) redox boundary in a fractured bedrock aquifer

摘要

The transport of ferrous iron from recharge to discharge zones in a fractured bedrock aquifer of Ordovician age near Yass, NSW, Australia is discussed. The quartz-rich metasediments contain disseminated pyrite in fractures and veins. The mobilisation of Fe(Ⅱ) through the fracture network is evident in the recharge zone, where fresh oxidised waters react with FeS_2 producing Fe(Ⅱ) and SO_4~(2-). Ferrous iron in the presence of oxygen is oxidised to Fe(Ⅲ) and is removed from solution either as orange lepidocrocite or as amorphous iron oxide ferrihydrite. Some Fe(Ⅱ) remains in solution depleted in dissolved oxygen (DO) and is transported into discharge zone by flowing groundwater. In the zone of depleted DO, and in the presence of organic matter and/or other reducing agents, Fe(Ⅲ)-precipitate is reduced to Fe(Ⅱ) and a significant amount of SO_4~(2-) is reduced to S~(2-). As conditions are strongly reduced reaching -300 mV, black FeS is precipitated inside the casing where groundwater is flowing under artesian pressures in discharge zones.