T.M. Beasley L. J. Mann 1994 From 1953 to 1990, an estimated 0.56 to 1.18 curies of iodine-129 were contained in wastewater generated by the Idaho Chemical Processing Plant (ICPP) at the Idaho National Engineering Laboratory. The waste- water was discharged directly to the Snake River Plain aquifer through a deep disposal well prior to February 1984 and through unlined disposal ponds in 1984-90. The wastewater did not contain measurable concentrations of iodine-129 in 1989-90. Samples were collected from 51 wells that obtain water from the Snake River Plain aquifer and 1 well that obtains water from a perched ground-water zone. The samples were analyzed for iodine-129 using an accelerator mass spectrometer which is two to six orders of magnitude more sensitive than neutron- activation methods. Therefore, iodine-129 was detectable in samples from a larger number of wells distributed over a larger area than previously was possible. Ground-water flow velocities calculated using iodine-129 data are estimated to be at least 6 feet per day. These velocities compare favorably with those of 4 to 10 feet per day calculated from tritium data and tracer studies at wells down- gradient from the ICPP. In 1990-91, concentrations of iodine-129 in water samples from wells that obtain water from the Snake River Plain aquifer ranged from less than 0.0000006+0.0000002 to 3.82.+0.19 picocuries per liter (pCi/L). The mean concentration in water from 18 wells was 0.81+0.19 pCi/L as compared with 1.30+0.26 pCi/L in 1986. The decrease in the iodine-l29 concentrations from 1986 to 1990-91 chiefly was the result of a decrease in the amount of iodine-129 disposed of annually, and changes in disposal techniques. application/pdf 10.3133/wri944053 en U.S. Geological Survey ; USGS Earth Science Information Center, Open-File Reports Section [distributor], Iodine-129 in the Snake River Plain aquifer at and near the Idaho National Engineering Laboratory, Idaho, 1990-91 reports