The water balance and position of the phreatic surface in a tailings dam are significantly controlled by infiltration and evaporation in the near-surface layers. Deposition onto tailings beaches provides an excess of water, which is also subject to evaporation losses, both during and after deposition. However, the fate of the deposited water is difficult to assess, particularly at high rates of rise, due to the partition of fluxes, comprising surface runoff to the pool, infiltration during deposition, percolation from the deposited layer after deposition, entrainment of interstitial water and evaporation from wetted and drying surfaces. Understanding these concurrent processes is difficult and practitioners have often resorted to empirical approaches, where evaporation from wet and dry beaches is considered a
fraction of the potential evaporation (PET). This can result in significant error to the overall water budget, poor estimates of recharge to the phreatic surface and complex conditions for water management.
Accordingly, a series of physical observations were conducted on a TSF that stores platinum tailings. The purpose of these observations was to quantify evaporation losses and rates of infiltration so that critical elements of the water balance could be refined.
In comparison to normally wetted agricultural soil, expectedly, tailings material behaves differently. Most notable findings were the behavior of actual evaporation (ET) at the various stages of deposition. ET losses were found to be lower than PET demand during the early stages of deposition, most likely due to the turbidity of the slurry. Once settling of the particles commences, ET equals PET, which can continue for 4-10 days after cessation of deposition. Further drying of the tailings surface generally occurs for two weeks, after which evaporation losses are practically nil, unless further wetting of the surface occurs during rainfall. ET may drop well below PET during the drying cycle when demand is high, but may subsequently continue to evaporate at PET, should PET drop.
The observations and understanding gained should be used to provide guidelines for future estimation of evaporation, infiltration and percolation fluxes during tailings dam operation and closure. Typical estimates of 80% of PET over wetted beaches and 20% over dry beach areas need to be revised based on these observations. Continued observations should be made in different tailings materials and different climatic periods. Local meteorological observations or proper A-Pan measurements should be made to allow continued comparisons of responses to the atmospheric demand to be made.
