IODINE-129 IN THE SNAKE RIVER PLAIN AQUIFER AT THE IDAHO NATIONAL ENGINEERING LABORATORY, IDAHO

From 1953 to 1983, an estimated 0.01 to 0.136 Ci (curies) per year of iodine-129 were contained in wastewater generated by the ICPP (Idaho Chemical Processing Plant) at the Idaho National Engineering Laboratory. The wastewater was directly discharged to the Snake River Plain aquifer through a deep disposal well until February 9, 1984, when use of the well was discontinued and the well was replaced by an unlined infiltration pond; a second pond was put into use on October 17, 1985. For 1984-86, the annual amount of iodine-129 in wastewater discharged to the ponds ranged from 0.0064 to 0.039 Ci. In August 1986, iodine-129 concentrations in water from 35 wells near the ICPP ranged from less than the reporting level to 3.6-0.4 pCi/L (picocuries per liter). By comparison, in April 1977, the water from 20 wells contained a maximum of 27-1 pCi/L of iodine-129; in 1981, the maximum concentration in water from 32 wells was 41-2 pCi/L. The average concentration of iodine-129 in water from 18 wells that were sampled in 1977, 1981, and 1986 was 4.0, 6.7 and 1.3 pCi/L, respectively. The marked decrease in the iodine-129 concentrations from 1981 to 1986 was the result of three factors: (1) The amount of iodine-129 disposed annually; (2) a change from the routine use of the disposal well to the infiltration ponds; and (3) dilution of the iodine-129 in the aquifer by an increase in recharge from the Big Lost River.