Germination and Survival of Grasses and Legumes Are Enhanced on Blended Byproduct Caps Compared to Bauxite Residues

Abstract Establishment of vegetation cover is one of the key steps in the final closure strategy for tailings storage facilities. Bauxite residues (Bayer process tailings) are highly alkaline, saline-sodic materials that make direct revegetation challenging unless the harsh chemical properties of residues are improved towards plant tolerable values. Currently, either ‘cap and store’ or ‘in situ remediation’ approaches are used to close and revegetate bauxite residue storage facilities. A new approach called ‘blended byproduct capping’ (blending tailings with available byproducts such as fly ash, eucalypt mulch and sewage water on-site) has been developed to avoid common drawbacks of existing closure methods. In this study, a germination assessment of two grass species (barley ( Hordeum vulgare ) and ryegrass ( Lolium multiflorum )) and three annual legume species (lupin ( Lupinus angustifolius ) , mungbean ( Vigna radiata ) and serradella ( Ornithopus sativus )) was used to evaluate substrate suitability of the developed blended byproduct caps of different textures (bauxite residue fines, bauxite residue sand and bauxite residue fines plus 10% bauxite residue sand). Potting mix and unblended and unleached bauxite residues were used as positive and negative controls, respectively. Barley, ryegrass, mungbean and serradella had germination rates of > 80% in blended byproduct caps and potting mix, whereas lupin germination rates were ≤ 50%. Barley germinated (< 10%) on unblended residues, while lupin germinated only with bauxite residue fines (0.8%). However, plant survival in the blended byproduct caps was also poor, suggesting that capping materials require further optimisation to support vegetation efforts. The pH and Na concentrations in the blended byproduct caps remained above target values for vegetation establishment, and were the likely causes of poor plant survival. Addition of acidifying agents in future blended byproduct cap trials is recommended to improve conditions for plant growth.