Tracking natural CO2 migration through a sandstone aquifer using Sr, U and C isotopes: Chimayó, New Mexico, USA

• Published in: International journal of greenhouse gas control Vol. 104; no. C; p. 103209
• Main Authors: Gardiner, J.B., Capo, R.C., Newell, D.L., Stewart, B.W., Phan, T.T., Keating, E.H., Guthrie, G.D., Hakala, J.A.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier Ltd 01.01.2021; Elsevier
• Subjects: Carbon isotopes; CO2 sequestration; Geochemical tracer; Natural analogs; Strontium isotopes; Uranium isotopes; CO2 sequestration; Natural analogs; Geochemical tracer; Carbon isotopes; Strontium isotopes; Uranium isotopes
• ISSN: 1750-5836; 1878-0148
• DOI: 10.1016/j.ijggc.2020.103209

•Groundwater isotope analyses can distinguish between CO2-transport mechanisms.•Strontium isotopes are sensitive to saline water intrusion into shallow groundwaters.•Uranium isotopes suggest two distinct uranium sources in Chimayó groundwaters.•Multi-isotope model identifies groundwaters affected by diffuse CO2 gas migration.•Diffuse CO2 gas migration into groundwaters is associated with elevated uranium. The geochemical and isotopic characteristics of groundwaters in Chimayó, New Mexico, reflect processes that affect water quality in the Tesuque Aquifer, which overlies a leaking natural CO2 source in a structurally complex region. In this study, select isotopes (δ13C, 87Sr/86Sr, 234U/238U) are applied to groundwaters to better understand CO2 transport mechanisms and related water-rock interactions that impact Chimayó groundwater. Carbon stable isotope ratios of dissolved inorganic carbon (DIC; δ13CDIC = -15.10‰ to 4.50‰) identify a distinct source of upward-migrating CO2 that interacts with the groundwater. Groundwater 87Sr/86Sr compositions (0.7098 to 0.7154) reflect intrusion of varying amounts of saline water associated with the high CO2 source, while 234U/238U ratios corroborate presence of deep groundwater and suggest impacts from distinctive natural uranium sources are affecting groundwater. Previous work proposed two CO2 transport mechanisms at the site: (1) dissolved in deep brine that underlies the aquifer and (2) in the gas phase; this study uses isotope mixing models to identify wells that are affected by these CO2 transport mechanisms and demonstrates that both transport mechanisms are associated with impaired groundwaters. Overall, this study demonstrates that applying multiple isotope systems (δ13CDIC, 87Sr/86Sr, 234U/238U) is a dynamic tool for identifying and measuring the impact of CO2 leakage from a sequestration site.