Simplified Modelling of Pollutant Transport in Stratified Groundwater Aquifers

Chemical species such as tracers or dissolved pollutants are dispersed by water flowing within a permeable matrix. The species move not only “down-stream”, but also spread in all directions. The rate of dispersion depends on the permeability structure and the fluid speed. Generally, groundwater systems have layered structures determined by different events in the geological processes that formed them. The layers in a system have different physical properties, and their thicknesses are not uniform. The system of advection-dispersion equations that model the fluid and species transport then have coefficients that depend mainly on depth, but with a layer composition that may change with horizontal distance. A single-layered homogeneous, confined aquifer is considered first, where the matrix and flow parameters are assumed constant. The aquifer thickness is assumed small compared to its lateral extent, and slopes of its boundaries are also small. The steady-state fluid flow and the associated pressure distribution may be readily computed. The vertically-averaged pollutant concentration within the singlelayered aquifer varies in the plane of the flow; lateral changes in concentration within the aquifer may occur by advection and dispersion. The idea is extended to multi-layered non-homogeneous aquifers where each layer may have properties different from the others. In some cases, full or partial analytic solutions for the species concentration can be found, thereby saving computational effort.