Density-dependent solute transport in discretely-fractured geologic media: is prediction possible?

The development of a dense solute plume in a fractured geologic medium can be highly irregular due to both the complexity of the fracture network as well as the presence of convection cells that may arise as a result of the density contrast between the invading solute and the ambient groundwater. A two-dimensional numerical model has been developed here to investigate density-dependent groundwater flow and solute transport in geologic materials that contain discrete fractures in order to examine some of the complex forms into which plumes can evolve, particularly with regard to fracture–matrix interactions. Results from simulations which involve parallel vertical fractures show that the evolution of the solute plume is affected by the development of convection cells in the porous matrix blocks between the vertical fractures. In a geologic medium containing a network of regularly spaced horizontal and vertical fractures, complex migration pathways can develop that are unexpected even though the geometry and interconnectivity of the fractures are known a priori. Downward solute migration can occur in some vertical fractures, while upward migration of less dense fluid can occur in others with transient circulation patterns developing in the intervening porous matrix. Because of the inherent uncertainty associated with fracture delineation, and because of the irregular nature of unstable dense plumes, deterministic prediction of dense-plume migration pathways and travel times in fractured geologic media will be subject to considerable uncertainty.