IMPROVING WATER STORAGE OF RECLAMATION SOIL COVERS BY FRACTIONATION OF COARSE-TEXTURED SOIL

Mining operations lead to considerable land disturbance and accumulation of large amounts of waste rock that may contain elevated concentrations of hazardous substances. Without proper capping, they may have considerable negative environmental impact on different spheres of the Earth. Capping of waste rock with a soil cover re-creates the water and nutrient regimes required for the growth of native plants and returns biological productivity and biodiversity of the land to a condition similar to that existing before site disturbance. In many cases the area of disturbance is composed of coarse-textured materials with low water retention properties, which are not desirable in semi-arid zones. This study was conducted to determine (1) whether a considerable increase of water storage is possible after separation of coarse-textured soil into size fractions and layering them in such a way that the finer fraction overlies the coarser fraction; and (2) whether such soil covers are susceptible to preferential flow under various initial and boundary conditions and what influence this type of flow has on residence time. Four types of soil covers were constructed in chambers: homogeneous covers composed of natural sand, two-layered covers with abrupt and gradual interlayer transitions, and four layered soil covers with abrupt transitions. Soil water storage was measured at field capacity (FC). Soil covers were tested under two types of lower boundary conditions: gravel layer and -25-cm matric potential. Flow stability was assessed during intermittent and constant ponded infiltrations. Water storage capacities (WSCs) for soil covers with -25-cm matric potential at the bottom of a cover were additionally simulated in HYDRUS-1D. Water storage capacities increased with the number of layers under both lower boundary conditions. Two-layered covers with a transition layer had slightly lower water storage than the same cover without the transition, due to a decreased hydraulic contrast at the layer interface. Simulated WSCs under -25-cm matric potential at the bottom were in satisfactory agreement with measured WSCs. The wetting front was stable in the homogeneous cover under both initially dry and FC conditions and in the two-layered cover with a gradual transition under initially dry water content during intermittent ponded infiltration. Unstable flow was observed only in the two-layered soil cover under both initial water contents. Other covers were partially unstable under initially air-dry and FC conditions. Generally, the wetting front was more diffuse at FC. Flow in all covers was stable under constant ponded infiltration. The residence time of water increased with the increase in the number of layers under both types of infiltration. Results of the study show that WSC and residence time do increase with increasing number of layers in soil covers, where layers are composed of different fractions of coarse-textured soil. In addition, tested soil covers have shown limited susceptibility to preferential flow even when layered into finer-over-coarser soil systems