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 x 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.