PHOSPHORUS RETENTION AND LEACHATES FROM SANDY SOIL AMENDED WITH BAUXITE RESIDUE (RED MUD)

Bauxite residue (red mud) is a finely crushed, alkaline by-product of the alumina industry. The application of red mud to soil has the poten tial to reduce eutrophication of rivers and waterways by retaining nut rients on infertile sandy soils. The areas which may benefit from amen dment with red mud are often groundwater recharge areas for drinking w ater and those near environmentally sensitive waterways, because of th is, the off-site effects of red mud must be assessed before its widesp read use. This research aimed to assess the length of time that phosph orus continued to be taken up by red mud and the best application rate of red mud to retain applied phosphorus. The effect of gypsum-amended red mud on phosphorus retention was examined. The composition of leac hates from the red mud was compared with drinking water standards for humans and an untreated control. Monthly rainfall was simulated and le achate was collected from lysimeters filled with bleached grey sand am ended with 5-80 t/ha of red mud, with and without gypsum. Leachates fr om over 12 months of simulated rainfall were tested for potential poll utants (Cd, Al, Fe, As, F, SO42-), electrical conductivity, pH, and P. The rainfall simulation was continued for the equivalent of 5 years a nd P levels were monitored during this time. The ionic concentrations of the leachates from columns treated with red mud were similar to the concentrations in the controls or fell to similar levels after the eq uivalent of 3 months of rainfall. The concentrations of these leachate s were below the maximum recommended limits for drinking water, except in the case of fluoride which only occurred when gypsum was applied. The concentration of fluoride that leached from the gypsum-amended red mud dropped to drinking water standards within the equivalent of 7 mo nths of rainfall. The best application rates of red mud which will red uce phosphorus leaching are 10-20 t/ha, without gypsum. The improved n utrient retention from red mud continues for the equivalent of at leas t 5 years of fertiliser application.