diffusion and dispersion in groundwater flow

diffusion and dispersion in groundwater flow Dissolved constituents, or solutes, that are introduced to groundwater in a small volume of an aquifer are usually observed to spread throughout large volumes of the aquifer as they migrate in the direction of groundwater flow. The general process of spreading is known as dispersion. While dispersion reduces contaminant concentrations in the aquifer, a potentially positive effect, it also results in larger total volumes of contaminated water to remove and treat when an aquifer is cleaned up. Molecular diffusion, driven by concentration gradients, is one contributor to the observed dispersion. Variations in the microscopic velocity within a given pore, and between pores of different sizes, lead to further ‘mechanical’ dispersion, a process that is generally more significant than diffusion except where groundwater velocities are very small. Observed dispersion at the scale of laboratory columns can often be explained by expected rates of molecular diffusion and mechanical dispersion. In aquifers, however, dispersion tends to occur at much higher rates than would be predicted from laboratory experiments. Enhanced dispersion at the field scale is primarily the result of larger-scale variations in groundwater velocity resulting from variations in permeability in the aquifer. Development of improved models to predict field-scale dispersion is a continuing area of research.

J. Bahr