USING THE RUSLE TO IDENTIFY FACTORS CONTROLLING EROSION RATES OF MINESOILS

Observable differences in particle size, smoothness and compaction bet ween cap site (slope 2 . 8 per cent) and batter site (slope 20 . 7 per cent) surfaces on the waste rock dump at Ranger Uranium Mine were qua ntified in terms of revised universal soil loss equation (RUSLE) param eter values. Cap site surface material had a K-m (erodibility correcte d for sediment density) of 0 . 030 and batter site surface material ha d a K, of 0 . 0056. Using these K-m values (derived from particle size distributions), slope length and steepness (LS) factors of 0 . 36 for the cap site and 3 . 66 for the batter site, and a cover (C) factor o f 0 . 45 for the cap site and 0 . 16 for the batter site, the RUSLE pr edicts an erosion rate from the cap site that is 1 . 9 times greater t han erosion from the much steeper batter site. The RUSLE indicates tha t the finer particle size and blocky soil structure of the cap site (D -50=0 . 91 mm) compared with the looser granular structure of the batt er site (D-50 = 1 . 74 mm) strongly influence erosion. The predictions are similar to observed soil losses from erosion plots on these sites under rainfall simulation events, for which the measured erosion rate from the cap site was approximately twice that from the batter site. For the RUSLE to predict the observed erosion rates, the support pract ice (P) factor for the cap site would have to be approximately 30 per cent greater than the P factor for the batter site. The higher cap sit e P factor probably results from smoothing and compaction caused by ve hicle movement across the surface. Compaction is considered to have gr eatly reduced infiltration capacity, thus increasing the erodibility o f the cap site. Vehicles probably also crushed the surface material at the cap site, creating the observed finer particle size distribution and further increasing the erodibility. Compaction, through its effect s on erodibility (K-m) and surface roughness (P), is concluded to be t he major cause of higher erosion from the cap site, even though the sl ope steepness is 10 times less. Parameterisation of the RUSLE quantifi es the differences between sites and explains the unexpected erosion r ates observed. The results highlight the need for careful management o f rehabilitated sites to avoid increases in erosion which may arise fr om compaction by machinery.