CO2-RICH MINERAL SPRINGS – NATURAL ANALOGS FOR A LEAKING CARBON SEQUESTRATION SCENARIO?

The growing scientific consensus is that the sharp increase in atmospheric CO₂ levels and the observed warming worldwide are primarily anthropogenic impacts. A reduction in CO₂ emissions from the developed and developing world to levels needed to mitigate additional warming seems unlikely in the near future, and thus, the pressure for engineered solutions, such as CO₂ sequestration is mounting. The long-term storage of CO₂ in the subsurface warrants natural analog studies where geological sources of CO₂ are known to evaluate topics such as water quality impacts and quantification of CO₂ loss due to leakage.

The geochemistry of CO₂ –rich mineral springs in the western U.S. and Tibet are used to evaluate the impacts of CO₂ migration on groundwater geochemistry and estimate the flux of CO₂ to the atmosphere. Springs generally issue along normal faults related to extensional tectonics, and are often associated with travertine accumulations. Spring waters range in pH from 5.5 – 8.5, from 6 – 58°C, are dilute to saline (100 to > 20,000 ppm total dissolved solids), and have alkalinities of ~100 to > 3000 mg/l (as HCO₃^-). Arsenic concentration ranges from not detected to nearly 5 ppm. Many springs have poor water quality in terms of major and/or trace element chemistry based on EPA standards. Dissolved gases in springs are dominated by CO₂, with lesser amounts of N₂, H₂S, O₂, H₂, He, Ar, and CH₄. The d¹³C of CO₂ bubbling from springs ranges from -18.2 to -1.0 ‰ (vs. PDB). The d¹⁵N of dissolved N₂ in the springs ranges from -0.9 to +2.2 ‰ (vs. Air). Helium isotope (³He/⁴He) ratios range from 0.1 – 0.6 R_A, indicating crustal sources of volatiles to several percent mantle contribution. Based on these natural tracers, the sources of CO₂ include metamorphic, organic and magmatic devolitilization. These may also be useful tracers to identify leaks in a deep carbon repository.

Estimates of carbon loss range from 100’s to 1000’s of kg/yr for individual springs along faults. If these point sources are scaled up to tectonic regions (e.g., the southern Tibetan Plateau) for diffuse loss along faults from the deep crust to shallow aquifers and ultimately the atmosphere, the estimates can grow to the order of 0.01 gigatons of carbon per year. This is approximately 1% of the annual anthropogenic addition to the atmosphere.