Abstract A portable stable carbon isotope ratio analyzer for carbon dioxide, based on wavelength scanned cavity ringdown spectroscopy, has been used to detect, locate, and characterize an intentional leakage of CO2 from an underground pipeline at the ZERT experimental facility in Bozeman, Montana. Rapid (1 h) walking surveys of the 100 m × 100 m site surrounding the pipeline were collected using this mobile, real-time instrument. The resulting concentration and 13C isotopic abundance maps were analyzed using Keeling plots, permitting not only the identification of specific leakage locations, but providing the ability to distinguish petrogenic sources of CO2 from biogenic sources. The ability to rapidly and reliably detect 12CO2 and 13CO2 concentration anomalies and identify the isotopic composition of the source flux provides a powerful and practical tool for detecting leakage from CO2 sequestration projects.