Remediation of coastal acid sulfate soils by tidal inundation: effectiveness and geochemical implications

The effects of restoring marine tidal inundation to a severely degraded acid sulfate soil landscape were investigated. Five years of regular tidal inundation led to substantial improvements in a range of key parameters used to assess soil and water quality. The pH of estuarine creeks improved dramatically following reintroduction of tidal inundation. Time series water quality and climatic data indicate a substantial decrease in the magnitude of creek acidification per given quantity of antecedent rainfall. The soil pH also increased by 2–3 units and titratable actual acidity (TAA) decreased by ~40–50 μmol H+ g-1 within former sulfuric horizons. Tidal inundation stimulated Fe and SO4 2- reduction within the landscape, leading to internal alkalinity generation and the reformation of considerable quantities of pyrite within former sulfuric horizons. In addition, there were large decreases in water-soluble and exchangeable Al fractions within former sulfuric horizons, which is an important finding from an eco-toxicology perspective. However, the radical change in hydrology and geochemistry initiated by tidal inundation has had profound consequences for the fate, mobilisation, redistribution and transformation of Fe minerals and co-associated trace elements. There was substantial diagenetic enrichment of poorly crystalline Fe-oxides near the soil surface following tidal inundation. This was also associated with enrichment of some trace metals. High concentrations of arsenic were observed in porewaters (~300 μg L-1) and were associated with reductive dissolution of secondary iron minerals, including jarosite, which had formed during the previous oxic / acidic phase. This study demonstrates that marine tidal inundation can be an effective method for remediating acid sulfate soils at a landscape-scale. However, there are a range of potential geochemical complexities which need to be considered prior to implementing this technique.