OPTIMISING NUTRIENT MANAGEMENT TO ACHIEVE SUSTAINABLE REHABILITATION OF BAUXITE RESIDUE DISPOSAL AREAS

Environmental management of bauxite residue areas (RSA) during both the operating and closures stages is a long term commitment of alumina producers. A series of studies have been carried out over the past few years using bauxite residue sand from Alcoa’s three Western Australia refineries (Kwinana, Pinjarra and Wagerup). The aims of these studies were to (1) investigate nutrient availability and dynamics in residue sand of varying pH, (2) identify nutrient indicators for evaluating rehabilitation performance, and (3) assess the impact of organic and inorganic amendments on improving the characteristics of residue sand. Up to 85% of ammonium-nitrogen (NH₄-N) applied as di-ammonium phosphate was lost as NH₃ within 7 days of application. This loss was due to the initially alkaline nature of residue sand, and the percentage loss decreased as pH decreased. Field sampling and pot trials revealed that nitrate-N (NO₃--N) was the dominant form of N in residue sand, and this anion plays a key role in plant nutrition and growth. Combined residue sand and plant nutrition analyses showed that 2M potassium chloride (KCl) extractable NO₃--N, and anion exchange membrane extractable phosphate were the best indicators of N and phosphorus availability in residue sand from established (>5 years old) rehabilitation. Both zeolite and greenwaste compost were found to be the most suitable amendments for residue rehabilitation due to their ability to supply plant nutrients. This research has highlighted the benefits of fertiliser combinations (organic and inorganic) and diagnostic analysis of residue and plant materials as a means of optimising nutrient management in residue rehabilitation.