Between 1952 and 1984, low‐level radioactive waste was introduced directly into the Snake River Plain aquifer at the Idaho National Engineering Laboratory (INEL), Idaho Falls, Idaho. These wastes were generated, principally, at the nuclear fuel reprocessing facility on the site. Our measurements of 36C1 in monitoring and production well waters, downgradient from disposal wells and seepage ponds, found easily detectable, nonhazardous concentrations of this radionuclide from the point of injection to the INEL southern site boundary. Comparisons are made between 3H and 36Cl concentrations in aquifer water and the advantages of 36C1 as a tracer of subsurface‐water dynamics at the site are discussed.

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 (g93% 235 U) 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 3 H, 36 Cl, 90 Sr, 137 Cs, 129 I, and Pu isotopes. Reported here are the first measurements of the long-lived radionuclides 99 Tc, 236 U, and 237 Np in the aquifer and their downgradient concentration changes during water transport through fractured basalt. Like 36 Cl, 99 Tc behaves conservatively during transport while 129 I, 236 U, and 237 Np indicate retardation.

Groundwater samples from 76 wells and 1 hot spring at or near the Idaho National Engineering Laboratory were analyzed for 36 purgeable organic compounds during 1990-91. The samples were collected and analyzed as a continuation of a water-quality program initiated in 1987, and as part of studies conducted by the U.S. Geological Survey. Most of the wells obtain water from the Snake River Plain aquifer. Samples were collected from these wells using dedicated or portable pumps. Water samples from 31 wells completed in the Snake River Plain aquifer contained detectable concentrations of at least 1 of 14 purgeable organic compounds. Most commonly detected were carbon tetrachloride, 1,1,1-trichloroethane, and trichloroethylene. The maximum concentrations of specific compounds in ground water were 5.0 micrograms per liter (ug/L) or less; the concentrations of most compounds were less than the reporting level of 0.2 ug/L. In addition, water from three wells contained detectable concentrations of one of two tentatively identified organic compounds, trimethylbenzene and isopropylbenzene.

Selected purgeable organic compounds, such as total xylene and methylene chloride, were detected in some groundwater samples and some blank samples consisting of boiled deionized water. Their presence in the blank samples suggests the compounds could have been inadvertently introduced into the groundwater samples during or subsequent to collection.

From 1952 to 1988, approximately 30,900 curies of tritium were contained in wastewater generated by the ICPP (Idaho Chemical Processing Plant) and the TRA (Test Reactor Area) at the Idaho National Engineering Laboratory. The wastewater at the ICPP was discharged directly to the Snake River Plain aquifer through a disposal well until February 9, 1984, when routine use of the well was discontinued and an unlined infiltration pond was put into use. A second pond was put into use on October 17, 1985. Wastewater disposed at the TRA has been discharged to one to three infiltration ponds since 1952. The average annual concentration of tritium in water from 26 selected wells at the INEL decreased from 250 pCi/mL (picocuries per milliliter) in 1961 to 18 pCi/mL in 1988, a decrease of 93 percent. The maximum tritium concentration was 844±5 pCi/mL in 1961 and 61.6±1.1 pCi/mL in 1988. Four factors are responsible for this decrease in tritium concentration: (1) a decrease in the amount of tritium disposed annually to ponds and wells from 1961 to 1988; (2) the change from the use of a disposal well to infiltration ponds at the ICPP; (3) radioactive decay; and (4) dilution from recharge.