Plumbing the Depths: testing natural tracers of subsurface CO2 origin and leakage, Utah, USA

The effective geological storage of fluid CO2 in porous subsurface rock will require the ability to track, and identify the origin of, any CO2 seepage measured at the near surface. In a study relevant to postemplacement seepage, we examine natural CO2 springs around Green River on the Colorado Plateau, United States, in an attempt to determine the CO2 origins. We use a combination of natural geochemical and isotopic tracers to fingerprint the origin of CO2 and water erupting at the surface. Stable oxygen and hydrogen isotope data enable the estimation that 80–90% of the spring water is derived from shallow groundwater, with the remainder originating from a deep saline aquifer. Carbon dioxide and water volumes imply the ascent of CO2 as a separate fluid or gas, not just in solution in the pore water. Stable carbon isotopes of CO2 are unable to discriminate between possible sources. However, the CO2 contains trace amounts of noble gas isotopes, which can be used to demonstrate that approximately 0–20% of CO2 erupted at the surface originates from the mantle, with the remainder originating from the crust. Within the study area, diverse noble gas signatures at different springs are observed, implying that no homogeneous free-phase accumulation at shallow depths exists as has been previously proposed. Natural tracers are, in this case, capable of resolving CO2 origins and migration pathways. From this study, we conclude that the errors and uncertainty in tracing anthropogenically stored CO2 will be reduced dramatically if a baseline survey is conducted to unambiguously characterize aquifer water chemistry, CO2, and noble gas content, before the addition of anthropogenic CO2 to the subsurface.