Compound-specific stable isotope analysis (CSIA) has proven a useful tool for the quantification of the extent of degradation (QED), and for source identification and source apportionment (SA) in contaminated environmental systems. However, the simultaneous occurrence of degradation processes and mixing of emission sources complicates the use of CSIA in combined SA and QED. In a companion study, we developed a mathematical model that allows for combined SA and QED of organic pollutants (and inorganic compounds such as nitrate) in a scenario of two emission sources and degradation via one reaction pathway. This work presents a validation of the model against virtual data from a two-dimensional reactive transport model. The model calculations for SA and QED were in good agreement with the simulation results, which suggests the correctness of the model assumptions. However, the application of the model to field data of benzene contamination was challenged by large uncertainties in CSIA data and the unknown interplay between competing degradation pathways. Nonetheless, the use of the model allowed for the identification of a prevailing contribution of one emission source and revealed a low overall extent of degradation at the field site. This indicates that the model can, for example, facilitate the characterization of air pollution or aquifer contamination with organic pollutants. © 2014 American Chemical Society.