Soil gas monitoring at the Illinois Basin – Decatur Project carbon sequestration site

•IBDP is a large-scale CCS demonstration project.•Six years of soil gas monitoring yielded a valuable data set from the IBDP site.•There were Large temporal and spatial variations of CO2 concentrations and δ13C.•Lines of evidence indicate no CO2 leakage occurred at the site.•δ13C, but not 14C, can serve as an effective natural tracer at this site.