EROSIONAL STABILITY OF REHABILITATED URANIUM-MINE STRUCTURES INCORPORATING NATURAL LANDFORM CHARACTERISTICS, NORTHERN TROPICAL AUSTRALIA

Australian Government guidelines specify that tailings containment str uctures at rehabilitated uranium mines in the Alligator Rivers Region of tropical northern Australia should have an engineered structural li fe of 1000 years. As part of the containment structure design process, erosion plots incorporating both regional geomorphological characteri stics (concave hillslope profiles and a weathering-resistant rock cove r of schist) and more conventional engineering design parameters (stra ight slopes and mine waste rock) were constructed at the Ranger Uraniu m Mine. The plots were monitored for storm runoff, and concentrations of solutes, suspended solids and selected ions over successive wet sea sons. The concave slopes (the hillslope analogues) had lower peak disc harges and lower concentrations of suspended solids than the straight slopes. However, solute concentrations in runoff from the schist cover ed (hillslope) slopes were higher than from the waste rock covered plo ts. Solute (mainly magnesium sulfate) concentrations for both rock typ es decreased by about an order of magnitude over the wet season. High sulfate concentrations are also likely to decrease substantially after several wet seasons, due to settlement of the waste rock and a reduct ion in rates of weathering. Development of a vegetation cover on the r ehabilitated landforms will reduce the high suspended sediment concent rations. These initial results suggest that rehabilitated uranium mine structures which utilise selected features of stable natural landform s in their design may have greater erosional stability than more conve ntionally engineered structures.