Behavior of Aquifers Concerning Contaminants: Differential Permeability and Importance of the Different Purification Processes

Abstract

Contaminants in transit through groundwater systems cross different types of rocks, each one with different water-bearing characteristics. It is imperative to understand the behavior of different aquifers (porous or karstic) concerning contaminants and the relative functions and mechanisms of major processes in a given system before planning remedial measures. From the surface systems to groundwater four main types of filter can be considered. They intervene most often simultaneously and the interactions are very strong. The activities of the different filters lead us to the concept of permeability, which varies according to the efficiency of the filter, ranging from the most impermeable porous aquifers to karstic aquifers. Among major processes we have chosen to consider three groups: retention, selfpurification and dilution and to discuss their importance and their rate of change. We focus more specifically on the problem in karstic systems. An example concerning nitrate profiles in a French karstic system is given, showing an increase of nitrate concentration in the epikarstic zone due to the impact of agriculture. Very few studies on karstic system contamination are mentioned because the processes controlling contaminant cycling inside the karst are as yet not very well understood.