New Model Allows for More Accurate Simulation of Tritium Movement in the Unsaturated Zone

Modeled tritium migration in unsaturated-zone sediments from an idealized representation of a radioactive-waste disposal trench. Contours (wavy blue lines) show concentrations in tritium units after 40 years of coupled gas-phase and liquid-phase transport. The new model accounts for waste heating by decay of radioisotopes and organic matter, pressurization by gas generation, and sediment properties that account for preferential transport parallel to sedimentary layering. (Click on photo for larger version)

USGS scientists and their colleagues have developed a new computer model that simulates the movement of tritium in the unsaturated zone more accurately than previous models. Tritium is the common name for hydrogen-3 (³H), which is a radioactive isotope of hydrogen. Tritium comprises a large proportion of the contaminants in low-level radioactive waste, and it can be transported in the environment as part water molecules in both liquid and vapor form.

At the Amargosa Desert Research Site, Nevada, USGS is studying tritium transport at a closed low-level radioactive waste disposal facility. They found that tritium and other compounds at the site move much more readily along layers of sediment deposition than across these layers (see figure). The new model accounts for the effects of sediment layering on tritium transport within the unsaturated zone, that portion of the subsurface above a regional water table. The model also accounts for heat and pressure generated by organic and inorganic wastes decaying in disposal trenches. By embodying more of the processes that control transport, the model reduces discrepancies between theoretical predictions of tritium migration and field observations. Models that simulate subsurface contaminant transport can provide useful information to environmental professionals designing waste-isolation facilities and associated monitoring networks.

References

Mayers, C.J., Andraski, B.J., Cooper, C.A., Wheatcraft, S.W., Stonestrom, D.A., and Michel, R.L., 2005, Modeling tritium transport through a deep unsaturated zone in an arid environment: Vadose Zone Journal, v. 4, p. 967-976, doi:10.2136/vzj2004.0179.

More Information

• Waste Disposal and Contaminant Migration in the Arid Southwest, Amargosa Desert Research Site, Nevada
• Environmental Tracers of Surface-Water/Ground-Water Interactions

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