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Freshwater remediation of acid sulfate soils and biogeochemical redox cycling of Fe-S-C in the rhizosphere

Australian Research Council Project: LP120100238

Project Partners

• SCU
• Port Macquarie Hastings Council
• Great Lakes Council

Background

A variety of remediation techniques have been developed to reduce the impacts of acid sulfate soil (ASS) wetland drainage on downstream water quality.

However, most remediation techniques involve relatively small-scale changes to floodplain management, such as manipulating floodgates to enhance tidal exchange and dilution, or partially blocking drains to decrease acid export.

Reflooding ASS wetlands with freshwater is an alternative, and far more substantial, remediation strategy. Freshwater reflooding may achieve a variety of environmentally beneficial aims simultaneously, including:

• preventing further pyrite oxidation
• containing acidity in the landscape / decreasing acid export
• ecological restoration and
• neutralising in situ acidity within the wetland by reversing key geochemical processes.

The in situ generation of alkalinity by iron and sulfate reduction leads to the formation of iron sulfides (FeS₂ and AVS) and elemental sulfur S(0), otherwise known as RIS. RIS become important in remediation of ASS, as they are a potential store of acidity.

This study explores the geochemical consequences of freshwater re-flooding of two ASS wetlands, Darawakh in the Great Lakes catchment and Partridge Creek in the Hastings. Both are floodplain wetlands that contain acid sulfate soils. The wetlands have been drained, and are impacting on downstream water quality. Both sites have been reflooded with freshwater as a strategy to remediate ASS.

In this study we quantify the abundance and vertical distribution of RIS species, carbon and reactive iron species after freshwater re-flooding for 8-9 years. We examine the beneficial environmental outcomes for water quality, explore the factors that appear to be influencing RIS abundance, speciation and spatial distribution and explore possible longer-term management implications.

Results

Increase in soil and water pH.

• Sulfuric horizon: soil pH increased by 2-3 pH units
• Drain discharge water: increase in pH by 2-3 units

Soil-accumulation of organic carbon

• Organic carbon accumulation near surface (upper ~20cm) - up to 20-30% dry weight, making peat.
• Accumulating fuel to drive generation of alkalinity by iron and sulfate reduction.

RIS formation

• RIS [FeS₂, S(0), AVS] reforming - greater at lower elevation
• 45% of values greater than 0.1%S - NSW ASS guideline action criteria
• More accumulating near the surface, upper 0.2m
• Dominated by pyrite, with substantial S(0)

RIS speciation

• Broadly similar at both wetlands, some differences
• Dominated by pyrite (S_CR)
• Abundant elemental sulphur [S(0)]
• Variable AVS-S (FeS)

Pyrite morphology

• Old pyrite has large crystals and clusters, tens of microns in size
• Newly formed pyrite is dispersed, and has very small crystals (200-300nm). It has high surface area, and is prone to faster oxidation.

Consequences of new RIS characteristics

• RIS oxidation would release sulfuric acid, and it's unknown how these 'new sediments would behave in a future drought.
• It's currently unknown how much acidity they would produce or how quickly.
• Drought simulation experiments are currently in progress to help answer these questions.

Seasonal context

• The region where the two study sites are located experiences large annual/decadal scale oscillations in rainfall.
• The region has been in a wet period since around 2007.

Key findings

Reflooding with freshwater has led to:

• There have been large increases in the pH of soil and drainage water following re-flooding. This is due to reflooding stimulating anaerobic geochemical processes that generate alkalinity (ie. Fe and SO₄^2- reduction).
• Substantial organic carbon is accumulating in surface sediments as vegetation recolonizes the wetlands.
• Diverse RIS species (mackinawite, elemental sulfur, greigite, pyrite) have formed near the sediment surface.
• Reactive Fe is abundant while SO₄^2- is limiting the formation of RIS.
• Oxidation of near-surface RIS during the next ENSO induced drought episode may generate H⁺ and cause future oscillations in water and sediment acidity.

Relevant publications

Johnston S.G., Burton E.D., Aaso T., Tuckerman G. (2014) Sulfur, iron and carbon cycling following hydrological restoration of acidic freshwater wetlands. Chemical Geology, 371 9-26.

Johnston S.G., Burton E.D., Hagan R., Aaso T., Tuckerman G. (2015) A revised method for determining existing acidity in re-flooded acid sulfate soils. Applied Geochemistry, 52, 16-22.