Behaviour of aluminium and iron in acid runoff from acid sulphate soils in the lower Richmond River catchment

Aluminium and iron concentrations and partitioning between particulate, colloidal and dissolved forms were examined in acid run• off from known acid sulphate soil environments in the lower Richmond River catchment during dry season conditions. Chronically acid drains in the Tuckean Swamp and Rocky Mouth Creek exhibited extremely high concentrations of dissolved metals (8- 10 mg/L AI , 5mg/L Fe). Dissolved aluminium and iron were quickly transformed to solid hydroxide species, which were rapidly removed from the water column by the aggregation and precipitation of diaspore and hematite in distinct flocculation zones, as water was subjected to steep pH and salinity gradients. This suggests that high metal concentrations may be found in benthic sediments and biota. Dissolved metals in acid runoff represent a major source of environmental pollution and, combined with the effects of acidity and low dissolved oxygen levels, pose a significant threat to estuarine ecosystems.