New Model Improves Understanding of the Transport of Carbon Isotopes in the Unsaturated Zone

USGS scientist collecting gas samples for analysis of Carbon-14 gases from a deep test hole at the Amargosa Desert Research Site, Nevada

U.S. Geological Survey (USGS) scientists have developed and applied a new computer model that simulates the transport and reaction of gases in the unsaturated zone. The model is useful because it accounts for the transport of each isotopic species of carbon dioxide (CO₂). Isotopes are different versions of the same element that have the same atomic number but different atomic masses. Some isotopes are unstable (radioactive), such as carbon 14 (¹⁴C). USGS scientists studying the transport of contaminants (including radioactive carbon and tritium) at the Amargosa Desert Research Site, Nevada, applied the new model to an uncontaminated area to help understand the transport of gases in the site's thick dry zone between the land surface and the water table (the unsaturated zone). The biggest source of CO₂ in the uncontaminated subsurface is root and microbial respiration in the shallow root zone, 0-1 meter deep; however, the scientists discovered a small source of CO₂ at the water table, 110 meters deep, that is generated in part by the precipitation of calcareous deposits. Despite its small size relative to root-zone respiration, the deep source strongly controls the distribution of carbon isotopes throughout most of the unsaturated zone. Model results and measurements give scientists a greater understanding of the transport of carbon isotopes, including radioactive ¹⁴C, in desert environments. Knowledge of the transport and sources of carbon isotopes in uncontaminated systems will help to:

• Identify, interpret, and manage contaminated systems in which radioactive and other gases (such as chlorinated volatile organic compounds) may be moving away from disposal areas.
• Determine the likelihood that radioactive gases, such as ¹⁴C-containing CO₂, will reach ground water through overlying unsaturated zones.
• Assess the potential impact of changes in water-table depth caused by ground-water pumping or climate change on regional-scale CO₂ production rates.
• Determine if calcite precipitation at the water table creates barriers to ground-water flow and gaseous transport in the subsurface.

The new model simulates the physical and chemical interactions among solid, liquid, and gaseous phases that control carbon-dioxide concentration and isotopic distributions at depth. The new model gives scientists, water-resource managers, and hazardous waste clean-up professionals a useful tool to better understand the occurrence, transport, and fate of carbon isotopes in desert environments.

References

Walvoord, M.A., Striegl, R.G., Prudic, D.E., and Stonestrom, D.A., 2005, CO₂ dynamics in the Amargosa Desert--Fluxes and isotopic speciation in a deep unsaturated zone: Water Resources Research, v. 41, no. 2, W02006, doi:10.1029/2004WR003599.

Thorstenson, D.C., and Parkhurst, D.L., 2004, Calculation of individual isotope equilibrium constants for geochemical reactions: Geochimica et Cosmochimica Acta, v. 68, no. 11, p. 2449-2465, doi:10.1016/j.gca.2003.11.027.

Thorstenson, D.C., and Parkhurst, D.L., 2002, Calculation of individual isotope equilibrium constants for implementation in geochemical models: U.S. Geological Survey Water-Resources Investigations Report 02-4172, 129 p.

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