The water balance of vermiculite covers on mining wastes in a semi-arid environment.

Abstract

Deposition of large quantities of mining waste is an activity that can have a negative impact on surface and ground water resources. These impacts necessitate reclamation and amelioration of the wastes through various measures to reduce these impacts. Specific reclamation measures are therefore needed for mine closure and legal requirements. These requirements include covering the waste with a sustainable cover material which is then vegetated. Covers are mostly designed to restrict the ingress of water into the mine waste, from where water might move pollutants into the environment. Cover designs could also allow for natural attenuation/degradation of pollutants within the waste and often contribute to design stability. To act effectively, the covers need to be constructed of materials matched to the design and task. The primary task of the cover soil is to hold water for vegetation and and subsequently release it into atmosphere and so contribute to the integrity of the cover. Ideal materials are not always readily available and so some understanding of cover material characteristics are required before effectiveness of the cover’s contribution to the site water balance can be evaluated. The interaction of the waste cover with the waste material should also be evaluated for its contribution to sustainability and to the cover’s water balance. Knowledge of these interactions will always be beneficial, as the availability of water for the vegetation in the cover stays important for the stability of the cover. This benefit, though, is dependant on the pollution potential of the waste in relation to the cover material. Through a literature review the studied vermiculite cover design was identified as a type of store and release cover. The study subsequently evaluated the water balance mechanisms of vermiculite covers on mining wastes in its semi-arid environment. The cover material and the waste material’s hydraulic properties were evaluated to determine likely flux responses. Soil water volumetric water content status was used to confirm fluxes and water status in cover and waste materials. Surface runoff and lysimeter drainage measurements added information for the assessment of fluxes onto and below the cover material. The HYDRUS unsaturated/saturated soil water physics model was used to model and derive general estimates of the overall waterbalance, having been calibrated with physical results obtained from field observations and material characteristics that were determined in the field. The study yielded promising results for the use of vermiculite waste as a cover material in that it showed good store and release properties under varying rainfall intensities and should therefore not contribute excessively to ground water pollution under similar circumstances.