Geochemical Dynamics of Sedimentary Iron in Waterways from Acid-Sulfate Soil Landscapes.
Edward Burton, Richard T. Bush, and Leigh A. Sullivan. Southern Cross University, 1 Military road, Lismore, Australia
The geochemical cycling of iron (Fe) is of great significance as a regulator of pH and contaminant mobility in soil, sediment and water. Pyrite (FeS2(s) oxidation is a particularly important aspect of Fe behaviour associated with ore/coal mining and coastal lowlands. The geochemical dynamics of Fe in the context of Acid Mine Drainage (AMD) environments has been studied intensively. In contrast, Fe geochemistry in waterways associated with coastal lowland Acid-Sulfate Soils (ASS) has received comparatively little research. Therefore, the objective of this study was to determine the solubility, mineralogy and geochemical dynamics of sedimentary Fe in acid-sulfate waterways associated with ASS. This was achieved by employing selective extractions, geochemical modeling, X-ray diffractometry and analytical scanning electron microscopy to describe sedimentary Fe behavior in ASS-associated waterways in eastern Australia. Schwertmannite was abundant in near-surface sediment and appeared to control surface-water FeIII solubility and possibly pH (which spanned pH 3.3 – 3.5 in surface water). Subsurface sediments contained abundant pore-water HCO3 and were reducing (Eh < -100 mV) with pH 6.0 – 6.5. Pore-water FeII concentrations were high (> 2 mM) and were constrained by precipitation-dissolution of siderite. The development of anoxic conditions at depth caused reductive dissolution of schwertmannite. This process, along with conversion of schwertmannite to goethite, provided an in-situ reservoir for pore-water SO4. This promoted SO4-reduction, resulting in precipitation of abundant amounts of disordered mackinawite. The presence of high pore-water FeII concentrations in the sediments described here caused the rate of pyrite formation to be sufficiently slow (due to very low S-II concentrations) to allow accumulation of anomalously large concentrations of disordered mackinawite. Overall, this study provides new insights into redox-driven Fe geochemistry in acid-sulfate waterways.