The use of "enhanced" moisture store-and-release cover systems over reactive mine waste in cold and warm semi-arid climates

Abstract

Moisture store-and-release cover systems utilize the high evaporative demand in semi-arid and arid climates to reduce the infiltration of meteoric waters to underlying reactive mine waste. This paper describes the benefits of adding a thin, reduced hydraulic conductivity layer below the overlying non-compacted layer to "delay" the downward percolation of infiltrating moisture and enhance the overall performance of the cover system. The additional cover layer is not required to be a high quality "barrier" layer and therefore does not necessarily require a clay-rich material. For example, a compacted waste rock layer as a result of haul truck traffic on top of a dump lift could serve as the low hydraulic conductivity layer. A one-dimensional numerical modeling program was used to compare the performance of a conventional moisture store-and-release cover system and an "enhanced" moisture store-and-release cover system. The modeling program was based on actual climate conditions and measured soil properties from a cold semi-arid site in northern Canada and a warm semi-arid site in Australia. Two cover system alternatives examined at each site found a decrease in the average annual net percolation for the enhanced store-and-release cover system of approximately 4%-7% of annual precipitation compared to the conventional store-and-release cover system. Analysis of the simulations showed net percolation occurred during the spring after snow melt and following the autumn rainfall events at the northern Canadian site and after intense rainfall events during the rainy season at the Australian site. The thin, reduced hydraulic conductivity layer improved the performance of the store-and-release cover system by holding water within the cover system for an increased period of time, allowing increased actual evapotranspiration in the subsequent dry periods.