Water balance during evaporation and drainage in cover soils under different water table conditions

Abstract

Experimental measurement and analytical methods were performed to study water flow through four different cover soils: clayey till, silt, coarse sand and fine sand. The upward and downward water fluxes (i.e. evaporation and drainage) were measured in the laboratory under different water table conditions. Both experimental and analytical results indicated that downward water flow (drainage) significantly affected upward water flow (evaporation) during changes in the water table elevation. Deepening water table elevation decreased evaporation in the sands and to a lesser extent in the silt, by promoting gravity-driven drainage and hence limiting water supply to the soil surface. Evaporation and drainage in the fine-textured clayey till did not change significantly as the water table dropped from the soil surface to 1 m below the soil column. The results suggest that the clayey till would be an effective oxygen barrier in sulfide-bearing mine waste covers, while the sands would be effective evaporation and drainage barriers for the till. The experimental results showed that the air entry value and hydraulic conductivity of the soil are critical parameters controlling drainage and evaporation in soils under different water table conditions. The study emphasizes the need to have evaporation and drainage barriers above and below a soil layer intended to maintain high water saturation under adverse environmental conditions, such as high evaporation and a fluctuating water table.