99Tc, 236U, and 237Np in the Snake River Plain aquifer at the Idaho National Engineering and Environmental Laboratory, Idaho Falls, Idaho

The Idaho National Engineering and Environmental Laboratory (INEEL) is located on the eastern Snake River Plain in southeastern Idaho; it is a multipurpose complex operated by the U.S. Department of Energy. Among its installations is the Idaho Chemical Processing Plant (ICPP), a facility designed principally to recover highly enriched uranium (greater than or equal to 93% U-235) from different nuclear fuel types used in naval propulsion, research, and test reactors. Starting in 1952 and continuing until 1984, low-level radioactive waste was discharged from the ICPP directly to the Snake River Plain aquifer by means of an injection well and seepage ponds. Over time, a suite of radionuclides have been measured in the aquifer including H-3, Cl-36, Sr-90, Cs-137, I-129, and Pu isotopes. Reported here are the first measurements of the long-lived radionuclides Tc-99, U-236, and Np-237 in the aquifer and their downgradient concentration changes during water transport through fractured basalt. Like Cl-36, 99Tc behaves conservatively during transport while I-129, U-236, and Np-237 indicate retardation.