Prediction and assessment of the fate of xenobiotics in soil requires the participation of many different disciplines: microbiology, hydrology, soil science, process engeneering, geochemistry and mathematical modeling. Using a system dynamics approach, the different mechanisms associated with the hydrodynamic transport, the physicochemical interactions and the biodegradation of pentachlorophenol (PCP) were successfully identified with the aid of soil columns. These soil columns were either filled with sand or a brown leached soiland were operated with well defined boundery and initial conditions. The major mechanisms of interaction between PCP and the solid matrix were characterized in both batch and column studies demonstrating their dependence on organic matter concentration, pH and the "immobile" water content. We isolated 4 bacterial strains able to utilize the PCP as the sole energy and carbone source. After one year of cryoconservation, these strains lost their degradation capabilities, leading us to employ a weil known microorganism : Rhodococcus chlorophenolicus for the study of microbiol behaviour in column and batch (microcosms). Experimental results concerning hydrodynamic behaviour, as studied by use of a chloride tracer and the fate of PCP in columns have been compared with results from mathematical modelling in order to verify hypothesis. The processes identified as being major contributors to the laboratory observations are : The convective transport, which varies with the nature and the water content of the porous solid matrix and the water flow. The physico-chemical interactions, pH and organic carbon content dependent. The biodegradation, regulated by many biotic factors such as microscopic microbial distribution, adaptation phenomenom, introduced population dynamics (predation and competition), and abiotic factors such as chemical toxicity, water and oxygen content ... The particle transport, which is influenced by the soil solution composition and ionic strength variation and can be very important in leached soils in case of intensive rains. This last mechanism has not been modelised.
