Outcomes and Key Findings from East Trinity
A National ASS Demonstration Project
Re-establishment of tidal inundation at East Trinity has led to:
- Major improvements in surface water quality of tidal estuarine creeks
- Large decreases in acidity and exchangeable Al of former sulfuric soils
- Reformation of pyrite and other reduced inorganic S species in former sulfuric soils
- Large-scale reductive dissolution of secondary Fe minerals (e.g. jarosite)
- Enhanced surface accumulation of reactive Fe(III) minerals (e.g. schwertmannite)
- Reductive mobilisation of As in soil porewaters
Strategic research has been conducted which explains how, where, and most importantly - why - these environmentally relevant changes have occurred across the landscape.
Prolonged tidal seawater inundation of acid sulfate soils initiates radical changes in shallow groundwater hydrology and sediment geochemistry. It stimulates Fe and SO4 reducing conditions, generating internal alkalinity and greatly decreasing groundwater acidity.
However, these changes also have profound consequences for the mobilisation, redistribution and transformation of Fe minerals and co-associated trace elements such as arsenic.
Distinct hydro-geochemical zones have been created in the landscape which are controlled by tidal zonation. A conceptual hydro-geochemical model has been developed which describes the fundamental characteristics of each zone.
Zone A and B: Reductive dissolution of As(V)-bearing Fe(III) minerals, including jarosite (KFe3(SO4)2(OH)6), has resulted in highly elevated concentrations of Fe2+ (2000 mg L-1) and As (~400 ug L-1) in the groundwater of former sulfuric horizons in the upper-intertidal zone.
Zone C and D: Former sulfuric horizons of the shallow fringing aquifer have effectively been transformed into a subterranean estuary, with extreme redox gradients and tidally oscillating hydrology. This creates potential for dynamic exchange of aqueous species with overlying surface waters.
Fe(III) (hydr)oxides at the sediment-water interface are poorly crystalline and display a diverse mineralisation sequence related to tidal zonation. These Fe(III) (hydr)oxides act as a natural reactive-Fe barrier and help retard As flux from groundwater to overlying surface waters. However, they also represent a highly transient phase that is prone to reductive dissolution during future redox boundary migration. This has uncertain consequences regarding the potential future release of co-associated trace elements.
Oscillating vertical and horizontal hydraulic gradients caused by tidal pumping promoted upward advection of As and Fe2+-enriched groundwater within the intertidal zone. This causes some flux of Asaq and Fe2+aq to overtopping tidal surface waters and the accumulation of As(V)-enriched Fe(III) (hydr)oxides at the oxic sediment-water interface.
Zone E: The sub-tidal zone is characterised by extensive sulfidisation and reformation of FeS and FeS2 minerals including pyrite. Reformed pyrite is providing a sink for the sequestration of trace elements, including arsenic.
The coupling of a redox transition and gradient with physical forcing processes is an important feature of seawater inundation of acid sulfate soils. The extreme enrichment of poorly crystalline Fe(III) (hydr)oxides (~40% Fe w/w) near the surface is a function of the interplay between tidally influenced hydrology, topography and geochemistry. It has important consequences for As partitioning as well as short- and long-term sulfur cycling processes.
Johnston S.G., Burton E.D., Bush R.T., Keene A.F., Sullivan L.A., Isaacson L. (2009) Pore water sampling in acid sulfate soils: a new peeper method. Journal of Environmental Quality 38, 2474-2477
Johnston S.G., Burton E.D., Bush R.T., Keene A.F., Sullivan L.A., Smith D., McElnea A.E., Ahern C.R., Powell B. (2010) Abundance and fractionation of Al, Fe and trace metals following tidal inundation of a tropical acid sulfate soil. Applied Geochemistry 25, 323-335.
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Johnston S.G., Keene A.F., Burton E.D., Bush R.T., Sullivan L.A. (2011) Iron and arsenic cycling in intertidal surface sediments during wetland remediation. Environmental Science and Technology 45 (6), 2179-2185
Johnston S.G., Keene A.F., Bush R.T., Burton E.D., Sullivan L.A., Isaacson L.S., McElnea A.E., Ahern C.R., Smith C.D., Powell B. (2011) Iron geochemical zonation in a tidally inundated acid sulfate soil wetland. Chemical Geology 280, 257-270.
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Johnston S.J., Keene A.F., Bush R.T., Burton E.D., Sullivan L.A., Smith D., McElnea A.E., Martens M.A., Wilbraham S. (2009b) Contemporary pedogenesis of severely degraded tropical acid sulfate soils after introduction of regular tidal inundation. Geoderma 149, 335-346.
Keene A.F., Johnston S.G., Bush R.T., Burton E.D., Sullivan L.A. (2010) Reactive trace element enrichment in a highly modified, tidally inundated acid sulfate soil wetland: East Trinity, Australia. Marine Pollution Bulletin 60 (4), 620-626.
Keene A.F., Johnston S.G., Bush R.T., Sullivan L.A., Burton E.D., McElnea A.E., Ahern C.R., Powell B. (2011) Effects of hyper-enriched reactive Fe on sulfidisation in a tidally inundated acid sulfate soil wetland. Biogeochemistry 103, 263-280.
Keene A.F., Johnston S.G., Bush R.T., Burton E.D., Sullivan L.A. Dundon M., McElnea A.E., Smith C.D., Ahern C.R., Powell B. (2014) Data from: Enrichment and heterogeneity of trace elements in redox-interfacial coastal sediments.
Ward N.J., Shepherd T. (2014). Data from: Geochemical properties of acid sulfate soils collected from the East Trinity coastal wetland, Cairns 2013.
Ward N.J., Shepherd T., Wang Z. (2014). Data from: Changes in the surface water chemistry at low tide in drainage channels at East Trinity coastal wetland, Cairns (August 2013).