Comparison of ex situ and in situ techniques for bioremediation of hydrocarbon-polluted soils

In situ and ex situ biodegradation have been proven to be very effective methods for remediating organic contaminations from soil and aquifers. A four-year field study of a hydrocarbon-polluted site showed discrepancies between results of groundwater and subsoil samples clearly demonstrating, that monitoring of water samples (circulation water) cannot replace the detailed investigation of subsoil samples. Ex situ techniques should consider the possible removal of volatile compounds by physical processes, which must be taken into account in the biodegradation balance. The use of pre-cultivated microorganisms to enhance degradation processes in a heap consisting of hydrocarbon-polluted subsoil had no benefit. By optimization of the environmental conditions, the indigenous microorganisms were shown to degrade a significant portion of the hydrocarbon mixture. In situ bioremediation of a site containing high amounts of chlorinated compounds raised the problem of hazard assessment. In this case, a discrepancy between the results of laboratory and field studies was observed, in addition to the problem of mass balance, which is a general problem in all bioremediation processes. Each site has its special characteristics which makes it unique, and this requires special preinvestigations, including hydrology, chemistry and microbiology, in order to find the best methods for optimization of degradation capacities of the indigenous microflora. Special remediation strategies have to be developed for each case, taking into account a critical assessment of the chosen methods.