AQUITARD CONTROL ON SOLUTE TRANSPORT IN AN AQUIFER: A TWO-DIMENSIONAL APPROACH

Aquitards consist of fine grain materials coming from a variety of geological environments such as glacial till sheets, ancient lacustrine deposits, and floodplains. They are important layers for protecting underlying aquifers. We have investigated solute transport in an aquifer bounded by an upper aquitard and a lower aquitard or bedrock. The present conceptual models of solute transport in aquitard-aquifer systems often ignore the solute diffusion into the aquitards. A few studies have considered aquitard diffusion, but they have adopted a methodology used in fracture-matrix systems by treating the solute flux across the aquitard-aquifer interface as a volumetric sink/source term in the governing equation of transport in the aquifer. This simplification does not satisfy the mass conservation requirement rigorously and its accuracy is unclear. To illustrate the importance of aquitard diffusion, we have investigated two-dimensional transport of a fully penetrating and laterally infinitely long source in an aquitard-aquifer system. We have maintained continuity of concentration and vertical solute flux at the aquitard-aquifer interface. The concentrations in the aquitards and aquifers are obtained by applying Laplace transform to time and finite Fourier Transform to the vertical coordinates of the transport equations. The new solutions can be used to calculate the spatial and temporal distributions of solute in the aquifer and aquitard, and to calculate the ratio of mass diffused into the aquitard over the mass remained in the aquifer. After cutting off the source, we can quantitatively discuss the “tailing” effect of the breakthrough curve because the solute trapped inside the aquitard will now come back to the aquifer via back-diffusion. If the aquitard diffusion is ignored, the problem becomes the well-known one-dimensional transport problem for which analytical solutions are available. These analytical solutions are used as benchmarks to test the special cases of the new solutions.