Continuous Monitoring of the Vadose Zone Gas Phase by Mass Spectrometry

Abstract

Core Ideas Continuous monitoring of vadose gases was performed with a process mass spectrometer. Large spatial and temporal variability was observed for reactive and inert gases. Gas dynamics were controlled by water fluxes, O2 depletion, and diffusion. Long‐term continuous measurements of gas concentrations in the vadose zone are required to identify gas transport processes. A magnetic sector mass spectrometer, originally developed for industrial process monitoring, was deployed in the field, and its performance was assessed for continuous monitoring of vadose zone gases. This instrument allows unattended concentration measurements of a large number of gas species without any purification. With a detection limit on the order of 10 nL L−1, it allows determination of trace gases such as Kr and Xe isotopes at their natural atmospheric abundances. Good precision and accuracy were obtained for major, minor, and trace gases with a daily calibration. Continuous monitoring of vadose zone gases for 6 mo in two boreholes at various depths at the Roselend Natural Laboratory (French Alps) showed strong spatial and temporal variability of gas concentrations, not only O2 and CO2 but also inert gases. Such time series allow determination of transport processes and interactions between water and gas fluxes in the vadose zone.