Effect of season and landscape position on the aluminium geochemistry of tropical acid sulfate soil leachate

Abstract

Acid sulfate soils (ASS) occupy an estimated 5.8 × 106 ha of coastal Australia. In tropical Australia, the processes operating in these soils, and their environmental hazards, are poorly understood. Drainage of a tropical estuarine wetland containing extensive ASS deposits left the area in a highly degraded condition. Surface and soil water pH values from the site were consistently <5 and commonly <3.5. Aluminium activity was several orders of magnitude greater than the level set for the protection of aquatic ecosystems, with a seasonal variation of 3 orders of magnitude. Aluminium behaved conservatively at the discharge point to receiving waters. In drainage lines and soil solution, aluminium activity was limited by elevated sulfate activity. Aluminium was commonly supersaturated with respect to alunite and behaved as though an aluminium species with the stoichiometry Al:OH:SO4 regulated its activity, which was 2–5 orders of magnitude lower than if gibbsite or amorphous aluminium hydroxide solubility was the control. While jurbanite (AlOHSO4.5H2O) is no longer considered a potential mineral in ASS, these data again raise the question of a satisfactory explanation of aluminium activity. Sulfate activity was influenced by seasonal factors. Wet season conditions were reducing and favoured the dissolution of acid iron oxidation products. The dry season oxidising and drying conditions favoured their precipitation, resulting in seasonal cycling. Based on our findings we developed a landscape geochemical process model for the site.