Illinois Basin – Decatur Project: Soil Carbon Dioxide Flux Monitoring

Circular 599: Illinois Basin – Decatur Project: Soil Carbon Dioxide Flux Monitoring

Carl H. Carman, Curt S. Blakley, Christopher P. Korose, and Randall A. Locke II

This report summarizes the soil carbon dioxide flux measurements collected throughout the Illinois Basin – Decatur Project (IBDP), a geologic carbon storage project in Decatur, Illinois, that began injecting CO2 into the Mt. Simon Sandstone on Nov. 17, 2011, and concluded on Nov. 26, 2014. The monitoring, verification, and accounting program estimated soil CO2 fluxes at a network of locations within the IBDP study area weekly from June 2009 to June 2015 as part of an extensive monitoring program to ensure the safety of human health and the environment. The 109 discrete monitoring installations in the network consisted of three installation types—bare-shallow, natural-shallow, and bare-deep—to examine the effects of vegetation removal and ring insertion depth (3.1 vs. 18.1 in.) on the magnitude and variability of fluxes. In total, 12,904 flux measurements were collected during the project. Nonparametric statistics were used to test fluxes at each location to evaluate whether CO2 injection had affected fluxes at the IBDP site. Flux data indicated that soil CO2 fluxes at the IBDP site were not affected by CO2 injection.

Soil CO2 fluxes varied with seasonal temperature cycles and were greatly affected by extremes in soil moisture. For example, a drought in 2012 caused fluxes from April to July to be 37% lower than the site average for that period across all monitoring years. The IBDP monitoring network was not expected to provide a protocol for how to deploy soil flux monitoring, but rather to provide a detailed understanding of flux behaviors at one site so that those experiences could be used to guide the development of monitoring programs at other carbon capture and storage sites. Fluxes at the bare-shallow installations were smaller and less variable than those at the natural-shallow installations and would be more effective in identifying a surface leak signature if one were to occur. Therefore, the bare-shallow installation was considered the preferred type for monitoring soil CO2 fluxes at an industrial carbon capture and storage site. Given the anticipated nature of leaks (e.g., diffuse, with small surface expression, possibly sporadic, with potentially low flux rates compared with the range of natural variability, and having potential surface expression as methane), soil flux was not used as the primary indicator of leakage at the IBDP. Instead, it was used as a point of reference to define flux variability over the life of the project and detect anomalous signals, and it is expected to be able to provide estimates of leak quantification.

A digital version of this report is available for free download in the University of Illinois IDEALS repository. To order a printed publication, visit our onlien shop at or call the Illinois State Geological Survey information office: 1-217-244-2414, 8 a.m. to 1 p.m. Central time Monday through Thursday.