Migration of technetium-99 in the alluvial aquifer at the Nevada test site, Nevada

The Cambric Experiment, under the auspices of the Hydrology/Radionuclide Migration Project, is measuring the migration of radionuclides from the site of an old underground nuclear test. Ten years after the 1965 test, a re-entry well (RNM-1) was drilled into the remnant of the explosion cavity to obtain core and water samples. Pumping water from a satellite well (RNM-2S) 91 m from the cavity subsequently induced an artificial gradient that has allowed soluble radionuclides to migrate from the cavity. Tritium (HTO) has been observed in the RNM-2S water; its elution has been well characterized. Other radionuclides have also been monitored in water from RNM-2S: 36Cl, 85Kr, 129I, and 106Ru. We have recently measured 99Tc at the 10-20 fg/l level in RNM-2S water. In contrast to the 3H source term, which is essentially entirely available to transport, these results indicate that only ∝ 0.01% of the 99Tc has escaped from the vitrified rock matrix into the water. The technetium in solution appears to be migrating more slowly than the 36Cl is. Although 99Tc's initial breakthrough is similar to that for 3H, the migration rate of the 99Tc center of mass appears to slightly exceed that of 3H, perhaps as a result of anion exclusion effects. All measured 99Tc concentrations are considerably below limits established for public drinking water. (orig.)