Assessment of microbial processes on gas production at radioactive low-level waste disposal sites

Abstract

Factors controlling gaseous emanations from low level radioactive waste disposal sites are assessed. Importance of gaseous fluxes of methane, carbon dioxide, and possible hydrogen from the site, stems from the inclusion of tritium and/or carbon-14 into the elemental composition of these compounds. In that the primary source of these gases is the biodegradation of organic components of the waste material, primary emphasis of the study involved an examination of the biochemical pathways producing methane, carbon dioxide, and hydrogen, and the environmental parameters controlling the activity of the microbial community involved. Initial examination of the data indicates that the ecosystem is anaerobic. As the result of the complexity of the pathway leading to methane production, factors such as substrate availability, which limit the initial reaction in the sequence, greatly affect the overall rate of methane evolution. Biochemical transformations of methane, hydrogen and carbon dioxide as they pass through the soil profile above the trench are discussed. Results of gas studies performed at three commercial low level radioactive waste disposal sites are reviewed. Methods used to obtain trench and soil gas samples are discussed. Estimates of rates of gas production and amounts released into the atmosphere (by the GASFLOW model) are evaluated.more » Tritium and carbon-14 gaseous compounds have been measured in these studies; tritiated methane is the major radionuclide species in all disposal trenches studied. The concentration of methane in a typical trench increases with the age of the trench, whereas the concentration of carbon dioxide is similar in all trenches.« less