Cleanup of fractured rock aquifers: Implications of matrix diffusion

Abstract

As contamination moves through a fractured rock aquifer, it tends to diffuse from the flowing fracture water into the rock's essentially stagnant pore water. This process tends both to retard a contamination plume's advance through a fractured rock aquifer and to substantially increase the difficulty of purging contamination from the aquifer. A mathematical model has been developed to evaluate the potential impact of this phenomenon upon water quality restoration in fractured rock aquifers. The numerical modeling reveals that cleanup of fractured rock aquifers will, in many cases, require many decades, even centuries, to achieve, particularly where substantial improvements in water quality are sought. The parameters which most strongly govern the degree to which matrix diffusion prolongs the aquifer restoration process are the rock's matrix porosity, fracture spacing, and matrix diffusivity, the chemical identity of the contaminant(s), and the length of time the aquifer has been contaminated.Since sedimentary rocks tend to have both relatively high matrix porosities and matrix diffusivities, it can be particularly difficult to purge contamination from sedimentary rock aquifers. Crystalline rocks, in contrast, typically have lower matrix porosities and matrix diffusivities, and therefore undergo more rapid cleanup. However, even in crystalline rocks, attainment of very high degrees of water quality improvement may be problematic. Numerical modeling also indicates that conventional groundwater 'pump and treat' programs are not likely to be very effective in speeding up aquifer restoration if the rate limiting step in the process is diffusion of contaminants from the rock matrix.