Modeling variably saturated water flow within planned covers of a hypothetical potash tailings pile

Abstract

Potash mining to obtain potassium generates large quantities of solid waste composed mainly of sodium chloride, accompanied in lesser quantities by magnesium sulfate and magnesium chloride, as well as insoluble clay minerals. The solid waste is disposed off on the ground surface to form tailings piles. In central Germany, there are dozens of potash tailings piles, some of which are more than two hundred meters high and form a unique landscape due to their appearance and lack of vegetation. Potash tailings are therefore exposed to climatic conditions such as rainfall, which may potentially dissolve salt, thus affecting the surrounding ecosystem including surface water bodies and groundwater. The environmental impact can likely be mitigated by applying soil covers over the potash tailings to limit and, in the best case, prevent the contact of the percolating water with the saline core of a pile. The efficiency of the soil covers is increased by applying a perennial vegetation layer to reduce the amount of water infiltration by interception, evaporation and plant transpiration. Therefore, the design of a cover system is based on the hydrological processes of the site, which include precipitation, surface runoff, evapotranspiration, in- and exfiltration and the water storage capacity of the soil. In addition, the type of soil cover and thus its hydraulic characteristics, the number of layers, the thickness of each layer, as well as the angle of slope of the layers can significantly influence the long-term efficiency of the cover. In conclusion, understanding the synergy between the soil cover and the potash tailing is essential to determine the effects caused on the surrounding environment, in particular on groundwater. The objective of this research work focuses on the numerical simulation of variably saturated water fluxes, in particular the creation of a water balance between surface runoff, in- and exfiltration and evaporation loss to the atmosphere in the soil cover. For this purpose, different configurations of vegetation covers are investigated and the capillary barrier effects are analyzed for each of them. The water flow through the potash tailing produced by the remaining infiltration through the vegetation cover is modeled for a hypothetical geometrical setting of a representative tailings pile.