Redox changes in a small wetland with potential acid sulfate, saline and sodic soils

The study site is a typical, perched wetland in a saline, sulfidic discharge area in the eastern Mt Lofty Ranges near Adelaide, South Australia. The wetland is part of the 10% of strongly waterlogged landscape identified by GIS analysis of a larger 80 km area. Approximately 40% of this area was identified as being poorly drained, but only a small proportion, less than 10%, supports wetland vegetation. The climate is Mediterranean with hot, dry summers and cool, moist winters. Better management of these wetlands can be attempted if more is known of the redox processes associated with soils in them, their likely response to erosion, drainage, vegetation changes and grazing, and their effect on water quality. Consequently, the objective of this work is to report on redox changes that occur seasonally, diurnally and after rainfall in soils along a 25 m transect with soil conditions that range from sodic (margin area) to saline (seepage area) to potential acid sulfate (marsh area). The transect was instrumented at five points from the margin area (re-vegetated with tall wheatgrass), saline seepage area (little or no vegetation) and marsh area (vegetated with native wetland plants). A data logger recorded Eh from eight platinum electrodes, temperature at two points, and rainfall. The sensors for Eh were placed at 5 and 20 cm depths and for temperature at 5 cm. Data records were captured at six hourly intervals and were accumulated continuously from February 1999 to February 2001. Daily redox cycles and some seasonal effects measured in the permanently wet marsh soils are thought to be associated with the activity of C plants and oxygen diffusion. Subsoils (at 20 cm) in the marsh remain reduced and sulfidic throughout the year. Eh sensors, either at 5 or 20 cm depths, indicated increasingly reduced conditions as a direct response to rainfall events and, eventually, a greater, lagged decrease to about -200 mV over several days after significant rain events in May that remained reduced until spring (September). Lowering of the groundwater table in summer resulted in oxidation, formation of ferrihydrite, precipitation of sulfate salts and formation of magnesium-substituted calcite in the upper few centimetres of the marsh soil. Should the marsh erode or drain, there are likely to be significant consequences in the release of salts, iron minerals and protons mobilising toxic metals into drainage lines. The main sources of sulfur are significant sulfide mineralisations that occur in the region. Relict sulfidic wetlands can be recognised in stream lines within the same MERRY ET AL. 17 WCSS, 14-21 August 2002, Thailand 2066-2 catchment. They produce significant acidification on exposure through erosion and are local sources of salinity, generate sodic soils and release iron and aluminium minerals.