Ameliorating caustic properties of aluminum extraction residue to establish a vegetative cover

Abstract

High pH, electrical conductivity (E.C.), and extractable Na levels in the residue remaining after aluminum was extracted from bauxite were greater than can support vigorous plant growth. These caustic properties must be ameliorated so that vegetation could be established on the surface of large disposal ponds. Fresh residue was added to 14 field lysimeters of dimensions 3 m long × 3 m wide × 1 m deep. Internal drainage was enhanced in some lysimeters and restricted in others. Gypsum applied at the rate of 7.5% of the weight of the top 15 cm of residue in selected lysimeters. These management issues were studied to determine how their influence would lower chemical properties to support vegetation. Residue samples were removed annually for 5 years after the lysimeters were established to determine changes in pH, E.C., and extractable Na and Al over time. Plant species C. dactylon (bermudagrass), Atriplex nummalari (oldman saltbush), and Atriplex canescens (fourwing saltbush) were planted either as seeds or transplants beginning the third year after lysimeter establishment. Residue pH, E.C. Na, and Al decreased annually, but more gradually in restricted drainage. Species survival in the enhanced drainage treatments was more vigorous with gypsum amendments beginning in the third year after lysimeter establishment. This coincided with a decrease in the mean pH below 9.5 and decrease in mean E.C. below 10 dS/m. Extractable Na and Al did not seem to influence survivability. Survival of seeded and transplanted C. dactylon was greater than the two Atriplex spp. each year. This study proved that the caustic properties of this residue could be lowered enough to support vegetation, but not without appropriate drainage. The addition of gypsum amendments and appropriate amounts of supplemental water would accelerate the timeframe for vegetation establishment probably at a level commensurate with the application rate of both inputs in a well-drained system.