Hot VOCs and Tritium Transport — A Conundrum Solved?

Commercial low-level radioactive waste (LLRW) was buried in 2-15-m deep unlined trenches at a disposal facility in the Amargosa Desert, Nevada, between 1962 and 1992. The majority of the disposed radioactivity (0.34 of 0.59 million curies) came from tritium-bearing compounds. A portion of the disposed tritium is migrating from the trenches through a 110-m thick unsaturated zone. Observed transport rates are difficult to reconcile with models that assume that: (1) migration takes place as tritiated water molecules diffusing and advecting through interconnected liquid and gas phases in the massive and largely unfractured alluvial sequences making up the desert floor, and (2) the migrating water remains in isotopic equilibrium with pre-existing unsaturated-zone pore water. This holds true even when multiphase mass-and-energy transport is driven by continuously elevated temperatures and gas pressures in the waste trenches. Vertical profiles of gas composition in the deep unsaturated zone 33 and 100 m outside the disposal area show that stratigraphically controlled methane bulges (up to ~20 ppm) and carbon dioxide bulges (up to ~2 %) are roughly coincident with the elevated tritium concentrations. Efforts are underway to isolate and analyze methane as a possible tritium-bearing compound. To test the hypothesis that non-methane VOCs — used for nuclear decontamination, liquid scintillation, etc. — are not tritiated, approximately 1,000 L of soil gas from each of three high-tritium concentration locations was passed through a series of cryogenic and sorbent traps that removed first water, then non-methane VOCs from the gas stream for analysis. Non-methane VOCs proved highly radioactive relative to condensed water. This unexpected finding helps account for the anomalously rapid and long-range transport of tritium occurring at this disposal facility and presumably other burial facilities receiving similar waste streams.