In the semi-arid areas of southern Africa, Annual Potential Evapotranspiration (PET), the atmospheric demand for water uptake, exceeds annual rainfall by more than threefold. Nevertheless, the intensity and volumes of daily rainfall result in water ingress into tailings porous media and can result in periodic recharge to a phreatic surface. For this semi-arid environment, evaporation losses from bare surfaces are primarily dependent on the hydraulics of the porous medium, which controls the fluxes of water and vapour to the surface, rather than the atmospheric demand. The water balance fluxes in a tailings dam, are significantly controlled by the fluxes of 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 post-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 post-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 are considered as some fraction of the PET or Lake Evaporation. This can result in a significant error to the overall water budget.
The quantification of these near-surface fluxes and the ability to predict these accurately over varying climate and material property types will contribute directly to:
- The Water Balance of the overall inputs and effluents from the tailing’s impoundment, either as a high-level manual calculation of water balance or as a simulation of the tailing’s material;
- Estimation of Seepage losses through the base of the impoundment, as simulated in finite element models;
- Design of Drains through simulation of fluxes to the drains; and
- Definition of the phreatic surface and interstitial water contents to allow for Slope Stability analysis.
Accordingly, a series of physical observations were conducted over a platinum TSF. 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. Further to this, 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.
