Roy C. Bartholomay Neil V. Maimer 2019 <p>From 1953 to 1988, approximately 0.941 curies of iodine-129 (¹²⁹I) were contained in wastewater generated at the Idaho National Laboratory, with almost all of it discharged at or near the Idaho Nuclear Technology and Engineering Center (INTEC). Until 1984, most of the wastewater was discharged directly into the eastern Snake River Plain (ESRP) aquifer through a deep disposal well; however, some wastewater was also discharged into unlined infiltration ponds or leaked from distribution systems below the INTEC.</p><p>During 2017–18, the U.S. Geological Survey, in cooperation with the U.S. Department of Energy, collected samples for ¹²⁹I from 30 wells that monitor the ESRP aquifer to track concentrations and changes of the carcinogenic radionuclide that has a 15.7 million-year half-life. Concentrations of ¹²⁹I in the aquifer ranged from 0.000016 ± 0.000001 to 0.88+/- 0.03 picocuries per liter (pCi/L), and concentrations generally decreased in wells near the INTEC as compared with previously collected samples. The average concentration of 15 wells sampled during 5 different sample periods decreased from 1.15 pCi/L in 1990–91 to 0.168 pCi/L in 2017–18, but average concentrations were similar to 2011–12 within analytical uncertainty. All but four wells within a 3-mile radius of the INTEC showed decreases in concentration, and all samples had concentrations less than the U.S. Environmental Protection Agency’s maximum contaminant level of 1 pCi/L. These decreases are attributed to the discontinuation of disposal of ¹²⁹I in wastewater and to dilution and dispersion in the aquifer. Some wells southeast of INTEC showed increasing trends; these increases were attributed to variable transmissivity.</p><p>Although wells near INTEC sampled in 2017–18 showed decreases in concentrations compared with data collected previously, some wells south of the INL boundary showed small increases. These increases are attributed to historical variable discharge rates of wastewater that eventually moved to these well locations as a pulse of water from a particular disposal period.</p> application/pdf 10.3133/sir20195133 en U.S. Geological Survey Iodine-129 in the Eastern Snake River Plain aquifer at and near the Idaho National Laboratory, Idaho, 2017–18 reports