Acid Sulfate Soils and Their Impact on Surface Water Quality: A Case Study in Southeast Brazil

Re-flooding of previously drained and oxidised acid sulfate soils (ASSs) can stimulate reduction processes while decreasing soil acidity, thereby mitigating the harmful effects of acidity. The aim of this study is to explore the pedogenesis of ASSs in a tropical climate and to assess the impact of ASS on surface water quality at the end of the seasonal re-flooding period. An integrated study of soil, water and sediment was carried out in an area in the Doce River Delta, Espírito Santo state, Brazil. The results indicate that seasonal re-flooding restores reduction processes of inorganic species in soils, but with low proton consumption. Re-flooding results in the preservation of chemical characteristics resulting from sulfurization, as verified mainly in pH values lower than 4.0, characterising ASSs with a thionic horizon. The maintenance of acidic conditions is strongly influenced by the complexation of exchangeable acidity, as well as the acidity retained in Fe hydroxysulfates, both generated in the sulfurization process. As a result, the activity of sulfate-reducing microorganisms is limited, restricting proton consumption through reductive dissolution of minerals containing Fe³⁺, supplying iron and other potentially toxic elements to the acidic soil solution. Drainage and impact of these soils are shown by the deterioration of the surrounding surface water quality at the end of the wet period, with high concentrations of acidity, sulfate and potentially toxic elements. The acidic and reduced conditions accentuate iron exports that precipitate onto bottom sediments, which is an important scavenger of potentially toxic elements. This in situ study concludes that the remediation efficiency of ASSs exposed to oxidising conditions through seasonal re-flooding is dependent on the degree of sulfurization of soil, and that aggravating effects of re-flooding can be observed in the surrounding surface water.