We study the behaviour of the residual contamination in the former uranium leaching site in Straž pod Ralskern in the north of the Czech Republic. We constructed a numerical model of density-dependent coupled now and solute transport. We present results or several model scenarios how the contaminants spread in the complicated inhomogeneous geological structure. We show how the effect of gravity substantially changes the transfer of contamination between two main aquifers, Cenomanian and Turonian, in particular the relation between the hydraulic water movement upwards and density-driven gravity force downwards.
The numerical modelling of groundwater contamination is widely used for efficient management or artificial operations and risk studies. The basic processes in the models arc water now and solute transport (Bear and Verruijt, 1990). In the usual approximation, the water flow is determined by both the natural pressure conditions and injecting/drawing wells. The solute transport is then calculated with use of the velocity field resulting from the water now model. In some cases, the inhomogeneity or the solution density (caused by inhomogeneous solute concentration) can have substantial effect on the water now. Such case is denoted as density-driven or variable-density flow (Holzbccher, 1998).
The two-directional coupling of now and transport leads to qualitatively different behaviour of the real systems but brings many difficulties in numerical calculations. For practical application, it is important to decide if the variable-density approach is necessary or the constant density approximation is appropriate. The density effect appears for systems, where both high Concentration gradients exist and the now caused by external forces is relatively small. As well, the effect is strongly influenced by a particular geometrical configuration.
There exist several model problems for density-driven now, mostly constructed as benchmarks for the numerical codes. The studies aimed to analyse the importance of density effects were performed for problems of freshwater and saltwater interaction in the coastal areas. In this paper we study a problem with density effects caused by high concentration or contaminant together with a specific hydrogeological configuration of two interacting aquifers. The model problem represent the ecological problem of large extent, the residual contamination after the remediation of the Straž pod Ralskcm site, which was one of the world's largest uranium leaching operation in 70s (Novak, 200 I). The numerical experiments compare the antagonistic forces on the contamination movement: the piezometric gradient upwards and the gravity (density effect) downwards.
The Straž deposit is located in the Straž block of North-bohemian Cretaceous basin. There are two sandstone aquifers in the block, lower Cenomanian and upper Turonian aquifers, isolated by 60 m thick serniaquitard created of muddy limestones, marlstones and sandy siltstones (Figs 1,2). The ore mineralization is embedded on the base of the Cenomanian aquifer, while the upper Turonian aquifer represents a source of drinkable water for wider region.
(Figure in full paper)
The most contamination is situated in the Cenomanian aquifer, composed of the two layers: friable sandstones and fucoid sandstones.