Simulation of vertical transport in mining pit lake

Subaqueous disposal is a technique that can, under suitable circumstances, delay or mitigate the release of material containing high levels of dissolved compounds, for example, acid rock drainage, into the surrounding environment. The technique places the material in question under a relatively inert cap of Lighter fluid in a deep basin, such as that left after mining. In many situations, because of low diffusion rates, the material may be considered as being isolated from the environment. However, there are a number of naturally occurring physical mechanisms that can quite efficiently bring this material to the surface, and hence, to the surrounding environment. We describe a modeling application to a deep and steep-sided chemically stratified lake using an extended version of the lake and reservoir water quality model, DYRESM, incorporating algorithms for detailed ice cover, heat fluxes, and also internal wave-driven boundary mixing. Sheltering and shading of the meteorological forcing is taken into account in the model. Both the field data and the model confirm the capping effects of the freshwater cap (S < 0.7 g/L) overlying the relatively salty water (S > 0.85 g/L) in the pit. Examination of the mechanistically determined vertical eddy diffusivities suggest that at depths below the surface mixed layer, double diffusion dominates over vertical mixing due to bottom-generated turbulence stemming from basin-scale internal waves. The ability of the model to simulate for periods longer than about 6 months is not addressed in this study.