Microbial processes govern the fate of pollutants in contaminated aquifers to a major extent. Therefore, the assessment, quantification and monitoring of in situ biodegradation is crucial for the implementation of NA concepts in ground water management strategies which is very often a labour intensive and complicated task. The purpose of this contribution is to give an overview on two isotope geochemical concepts offering perspective for ground water monitoring operations in the context of monitored natural attenuation.

The first concept employs stable isotope labelled tracer substances incubated directly in the aquifer within a small in situ cosmos test system. The in situ degradation is proved by the transformation of labelled carbon into the biomass upon anabolic processes. This indicates that the contaminant is used as a carbon source for microbial growth. The isotope composition of the membrane lipids (fatty acids) derived from microorganisms colonising the in situ microcosm is used as an indicator for in situ biodegradation. Molecular biological techniques can be employed to analyse the composition of the microbial community colonising the in situ test system. The microcosms can be further used as inoculum for conventional laboratory studies to investigate the physiology of contaminant degrading bacteria.

The second concept takes advantage of isotope fractionation processes to quantify in situ degradation on a flow path between the source of contaminants and a monitoring well. Microbial degradation alters the isotope composition of contaminants and stable isotope fractionation processes have been used to assess the in situ biodegradation of contaminants in aquifers. Many case studies demonstrate that the isotope fractionation concept can be applied to predict the in situ degradation of aromatic and chlorinated hydrocarbons as well as fuel oxygenates. To quantify the in situ degradation using the Rayleigh equation the isotope composition and concentration of contaminants in the field and the compound specific fractionation factor ( ) is needed. The extent of isotope fractionation is dependent on the degradation pathway and other biological factors. Therefore a careful selection of an appropriate fractionation factor representative for the geochemical conditions at the field site can improve the quantitative assessment of biodegradation.

Recent advantages in employing the isotope fractionation concepts will be summarised and discussed with respect to quantification of in situ biodegradation in heterogeneous aquifers. The isotope fractionation concept will be discussed with respect to uncertainty of the method and boundary conditions.