Acid drainage from canelands: Quantities exported and mechanism of production

The drainage water from two acid sulfate soils paddocks planted to sugarcane has been measured during two storms. One of these paddocks had been drained for more than 30 years and the main source of acidity was jarosite (site J). The other paddock was drained within the past 5 years and the main source of acidity was pyrite (site P). At site J, the drainage water contained high concentrations of iron and lower concentrations of aluminium. At site P, the drainage water contained low concentrations of iron and high concentrations of aluminium. It is postulated that oxidation of pyrite forms iron and acid, the iron forms jarosite, while the acid dissolves clay minerals liberating aluminium. When jarosite concentrations are high, jarosite is hydrolysed to form iron and acid, the iron is exported, and the acid liberates some remaining aluminium. The drainage water at sites P and J had aluminium and iron concentrations, which are many times greater than recommended levels for freshwater ecosystems. Other heavy metal (zinc and copper) concentrations in the drainage water had levels 10 times greater than recommended for fresh water ecosystems.