Modeling and measurement of evaporation in moisture-retaining soil covers

Abstract

Soil covers are widely used in mine waste and landfill applications to protect the environment. For an effective soil cover, the infiltration and oxygen barrier must have a low hydraulic conductivity and also maintain a high degree of saturation. Soil water evaporation significantly affects water content, and as a result the degree of saturation of the soil. Therefore, knowledge of the rate of evaporation at the soil–atmosphere interface is required to estimate the water content of candidate cover soils. Clayey soil is commonly used, either separately as a single cover or in various combinations with other soils as a layered cover system. In this study, water flow through a single clayey till cover and a layered soil cover were modeled using a coupled liquid flow, vapor diffusion and heat transfer finite-element model (SoilCover). The layered soil cover studied was a typical mine-waste soil cover intended to control oxygen diffusion and infiltration in a temperate climate and consisted of three layers: coarse sand and fine sand as upper and lower capillary barriers, respectively, and clayey till as an infiltration and oxygen barrier. The water level was located at the bottom of the cover. Evaporation and drainage predicted by the model reasonably agreed with experimental results and showed that the clayey till would be an effective oxygen barrier in sulfide-bearing mine waste covers, for which a very low oxygen flux is desired in order to achieve environmental protection. The results emphasize the need to have evaporation and drainage barriers above and below clay barriers designed to retain moisture. Further analysis using SoilCover showed that coarse sand would perform better than either fine sand or silt as a protective top layer over a clayey till barrier.