From 1953 to 1988, approximately 0.941 curies of iodine-129 (¹²⁹I) were contained in wastewater generated at the Idaho National Laboratory (INL) with almost all of this wastewater discharged at or near the Idaho Nuclear Technology and Engineering Center (INTEC). Most of the wastewater containing ¹²⁹I was discharged directly into the eastern Snake River Plain (ESRP) aquifer through a deep disposal well until 1984; lesser quantities also were discharged into unlined infiltration ponds or leaked from distribution systems below the INTEC. During 2010–12, the U.S. Geological Survey in cooperation with the U.S. Department of Energy collected groundwater samples for ¹²⁹I from 62 wells in the ESRP aquifer to track concentration trends and changes for the carcinogenic radionuclide that has a 15.7 million-year half-life. Concentrations of ¹²⁹I in the aquifer ranged from 0.0000013±0.0000005 to 1.02±0.04 picocuries per liter (pCi/L), and generally decreased in wells near the INTEC, relative to previous sampling events. The average concentration of ¹²⁹I in groundwater from 15 wells sampled during four different sample periods decreased from 1.15 pCi/L in 1990–91 to 0.173 pCi/L in 2011–12. All but two wells within a 3-mile radius of the INTEC showed decreases in concentration, and all but one sample had concentrations less than the U.S. Environmental Protection Agency 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. The decreases in ¹²⁹I concentrations, in areas around INTEC where concentrations increased between 2003 and 2007, were attributed to less recharge near INTEC either from less flow in the Big Lost River or from less local snowmelt and anthropogenic sources. Although wells near INTEC sampled in 2011–12 showed decreases in ¹²⁹I concentrations compared with previously collected data, some wells south and east of the Central Facilities Area, near the site boundary, and south of the INL showed small increases. These slight increases are attributed to variable discharge rates of wastewater that eventually moved to these well locations as a pulse of water from a particular disposal period. Wells sampled for the first time around the Naval Reactors Facility had ¹²⁹I concentrations slightly greater than background concentrations in the ESRP aquifer. These concentrations are attributed to possible leakage from landfills at the Naval Reactors Facility or seepage from air emission deposits from INTEC, or both. In 2012, the U.S. Geological Survey collected discrete groundwater samples from 25 zones in 11 wells equipped with multilevel monitoring systems to help define the vertical distribution of ¹²⁹I in the aquifer. Concentrations ranged from 0.000006±0.000004 to 0.082±0.003 pCi/L. Two new wells completed in 2012 showed variability of up to one order of magnitude of concentrations of ¹²⁹I among various zones. Two other wells showed similar concentrations of ¹²⁹I in all three zones sampled. Concentrations were well less than the maximum contaminant level in all zones.