Compound-specific stable isotope analysis (CSIA) is an advanced monitoring tool for evaluating the fate of organic pollutants at contaminated field sites and is increasingly applied in many countries. CSIA is clearly the most conclusive method to provide a sound source characterization by the isotopic fingerprint as well as for evaluation of contaminant degradation, as changes in isotope ratios are directly linked to this process. In addition, the composition of the microbial populations as well as the abundance of functional genes, encoding specific enzymes for pollutant degradation quantified by qPCR analysis, highlights characteristic areas of in situ pollutant degradation potentials. Using the example of a former industrial site that is contaminated with organic pollutants (mainly chlorinated ethenes, BTEX, & PAHs), a combination of innovative CSIA investigation methods were used to determine parameters to evaluate pollutant degradation processes. Two distinctive sources of contamination were identified, and sustainable natural pollutant degradation was demonstrated both at the source areas and in the contaminant plume. The determined degradation rate constants were applied to a numerical site model in order to create forecasts for the future development of the contaminant plume at the site and the stimulated aerobic mineralization of the pollutants was demonstrated and quantified in a laboratory approach by highly sensitive 13C-microcom studies under in situ-like conditions. Based on model calculations, site-specific remediation options were derived, and the future development of the contaminant plume was estimated.