The natural denudation rate of the lowlands near the Ranger mine, Australia: A target for mine site rehabilitation

Abstract

This paper provides a robust estimate of the total natural denudation rate for the area surrounding the Ranger uranium mine in the Northern Territory, Australia. This rate will be used to assess whether the final rehabilitated landform of the Ranger mine will denude at the natural rate after the landform has stabilized. The approach adopted was to compile and analyze all existing estimates of the total denudation rate.(including both physical erosion rates and solution losses) from areas near the mine site. Where available, solution losses are based on local measurements. These were used to adjust estimates for other local sites, where only physical erosion rates are available. Adjustments were based on a global relationship between physical erosion and solution loss rates. The adjusted total denudation rate estimates were shown to have no statistically significant relationship with the time over which they have been estimated (the averaging time, maximum period about 59,000 years), which suggests that they conform to mass flux equilibrium. This form of equilibrium implies an equivalence between inputs and outputs of sediment and solutes for which there is some evidence. Because the denudation rates are independent of averaging times, the long-term average total natural denudation rate can be calculated from all of the data, giving a value of 0.075 ± 0.013 mm year−1 (75 mm 1000 years−1, 75 m Ma−1). This value is almost twice the previously quoted rate of 0.04 ± 0.03 mm year−1 (40 ± 30 mm 1000 years−1), which was calculated from a mixture of physical erosion rates and the total denudation rates including some from large river catchments. The derivation of a revised denudation rate provides confidence that the revised value, based on all available data, is representative of the region; and is therefore an appropriate value with which to assess the success of the rehabilitation of a post-mining landform.