Leachate quality from gypsum neutralized red mud applied to sandy soils

Mixtures of fine bauxite refining residue (red mud), waste gypsum and local sandy soil that are proposed to be used in a catchment nutrient management program, were watered in columns simulating rainfall over a period of 2 yr and the quality of the leachate determined. The major salts released were sodium sulphate, a product of red mud alkalinity neutralization by gypsum, and excess gypsum released at its solubility concentration. At an application rate of 850 t ha^?1 of red mud the salts leached to groundwater (40 kg t^?1 red mud) would be equivalent to salts leached by rainfall over a 20 yr period from soil without red mud application, and would pose no significant impact when applied to the sandy agricultural soils in the catchment area of an estuary. The leaching of Al, Fe, and Cd from the red mud and gypsum was negligible, while the retention of superphosphate was over 99%. Fluoride from the waste gypsum was leached rapidly and reduced to background concentration (less than 1 mg L^?1) within one winter rainfall.