[en] The investigations at the Frankenthal site are performed as a part of the German funding priority KORA (TV 3.6) of the BMBF (Federal Ministry of Education and Research). The aim of KORA is to develop technical and legal instruments, which will facilitate the evaluation and customised use of Natural Attenuation (NA) processes in the risk assessment and remediation of contaminated sites. The investigations show that by using compound specific isotope analysis (GC-IRMS, Gas Chromatograph Isotope Ratio Mass Spectrometry) evidence of NA processes on chlorinated hydrocarbons (CHC) can be provided. The amount of microbial degradation was quantified by using a Rayleigh-type-model approach with the weighted mean ¹³C-CHC-signature, the derived starting ¹³C-signature and the sum of site specific ¹³C-enrichment factors ε that were determined in laboratory experiments. The results indicate an ongoing degradation process, which is shown in diagrams under minimum criteria, which is the basis of the used quantification method. The diagram shows that the state of degradation is increasing with time (sampling campaign 2000-2004-2005-2006) and distance from the source. The methods will be implemented into guidelines for the use of Monitored Natural Attenuation processes in the risk assessment and remediation of contaminated water resources. (author)