AbstractThis paper describes a new technique for vadose zone monitoring above CCUS sites, its use as a response tool to landowner concerns about CCUS, and its wider implications for vadose zone monitoring at CCUS sites. The process- based method uses ratios of coexisting gases (CO2, O2, N2 and CH4) to distinguish promptly a leakage signal from natural vadose zone CO2 without the use of background monitoring. This method was applied at the Kerr farm, a site of alleged leakage from the IEAGHG Weyburn-Midale CO2 Monitoring and Storage Project (WMP) reservoir and showed that no leakage had occurred. These results agreed with two other studies conducted at the Kerr site that used a variety of methods including comparisons to the large historical WMP data set. The successful use of the process- based method at the Kerr site shows that background monitoring may not be necessary for leakage detection by soil gas methods. Instead, we introduce the concept of vadose zone characterization which requires a one-time assessment of spatial chemical variability pre-injection, rather than repeated background measurements. In addition, we suggest that a monitoring approach not requiring prolonged background measurements is most efficient as a response tool targeted to specific events and areas of concern thereby simplifying vadose zone monitoring without sacrificing accuracy.