MIGRATION OF TC-99 IN THE ALLUVIAL AQUIFER AT THE NEVADA TEST SITE, NEVADA

The Cambric Experiment, under the auspices of the Hydrology/Radionucli de 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 exp losion cavity to obtain core and water samples. Pumping water from a s atellite well (RNM-2S) 91 m from the cavity subsequently induced an ar tificial gradient that has allowed soluble radionuclides to migrate fr om the cavity. Tritium (HTO) has been observed in the RNM-2S water; it s elution has been well characterized. Other radionuclides have also b een monitored in water from RNM-2S:Cl-36, Kr-85, I-129, and Ru-106. We have recently measured Tc-99 at the 10-20 fg/l level in RNM-2S water. In contrast to the H-3 source term, which is essentially entirely ava ilable to transport, these results indicate that only approximately 0. 01% of the Tc-99 has escaped from the vitrified rock matrix into the w ater. The technetium in solution appears to be migrating more slowly t han the Cl-36 is. Although Tc-99's initial breakthrough is similar to that for H-3, the migration rate of the Tc-99 center of mass appears t o slightly exceed that of H-3, perhaps as a result of anion exclusion effects. All measured Tc-99 concentrations are considerably below limi ts established for public drinking water.