Science, Technology and Engineering

Environmental Geoscience

Honours Projects

Inland acid sulphate soils: a study of three wetlands along the Murray River, Australia

Fiona Glover

Supervisor: Dr John Webb

The Murray-Darling Basin is suffering from a severe water shortage given the current drought. This has led to the drying out of some wetlands along the Murray River, exposing sulfidic sediment, which in one case has led to significant acidification (Bottle Bend Lagoon). To investigate the problem of acid sulfate soils (ASS) in inland wetlands and determine the factors controlling acidification, three wetlands along the Murray River were chosen for this study: Bottle Bend Lagoon, Psyche Bend Lagoon and Tareena Billabong.

Sulfidic sediments are formed by the reduction of sulfate (SO₄^2-), which in the wetlands along the Murray River was provided by saline water associated with dryland salinity. It is an ideal environment for the production of sulfide minerals when the wetlands become waterlogged, so the build up of sulfidic material has been further increased by the removal of natural wetting and drying cycles of the wetlands due to the changed hydrology of the Murray River, which means the wetlands are continuously flooded.

Unlike coastal ASS, which is characterised by abundant pyrite, the most common sulfide mineral found at the three wetlands studied was the iron monosulfide greigite (Fe₃S₄). Mackinawite (Fe₉S₈) was also present at Tareena Billabong and Bottle Bend Lagoon, but no pyrite was found. The other minerals were also similar across the three sites, with the most common minerals being quartz and illite. Carbonates (aragonite) were only found at Psyche Bend Lagoon, providing buffering capacity to the sediment of ~1127 moles H+/tonne. The lack of carbonates at Bottle Bend is believed to be the main reason for acidification.

Tareena Billabong was sampled over a three month period and showed high variability; the sulfidic material appeared as mottles within the sediment, rather than the distinct reduced layers seen at Bottle Bend and Psyche Bend. Despite this, the potential acidity at Tareena Billabong (up to 592 mol H⁺/tonne) was very similar to the levels found at Bottle Bend (523 mol H⁺/tonne). Psyche Bend had a lower level (235 mol H⁺/tonne), perhaps due to incomplete removal of carbonates during analysis.

Owing to the similar potential acidities at Tareena Billabong and Bottle Bend, Tareena Billabong was also expected to acidify when it dried out, but this did not occur. Oxidation experiments on sediments from all three sites showed that the intermediate oxidation products elemental sulfur and thiosulfate formed in each case, but the elemental sulfur concentration at Tareena Billabong (0.5mM) was much less that those found at Bottle Bend and Psyche Bend (~4 mM), indicating a slower rate of oxidation. The inhibition of catalysing bacteria by toxic metals may be the cause of the slower rate of oxidation at Tareena Billabong and therefore the lack of acidification, but more investigation into the microbiology of the sites is needed.

The project is co-supervised by Dr. Ewen Silvester of La Trobe University in Albury-Wodonga and Dr. Darren Baldwin of the Murray Darling Freshwater Research Center (MDFRC).