title: Identifying the sources of subsurface contamination at the Hanford Site in Washington using high-precision uranium isotopic measurements.
abstract: In the mid-1990s, a groundwater plume of uranium (U) was detected in monitoring wells in the B-BX-BY Waste Management Area at the Hanford Site in Washington. This area has been used since the late 1940s to store high-level radioactive waste and other products of U fuel-rod processing. Using multiple-collector ICP source magnetic sector mass spectrometry, high-precision uranium isotopic analyses were conducted of samples of vadose zone contamination and of groundwater. The isotope ratios 236U/238U, 234U/238U, and 238U/235U are used to distinguish contaminant sources. On the basis of the isotopic data, the source of the groundwater contamination appears to be related to a 1951 overflow event at tank BX-102 that spilled high-level U waste into the vadose zone. The U isotopic variation of the groundwater plume is a result of mixing between contaminant U from this spill and natural background U. Vadose zone U contamination at tank B-110 likely predates the recorded tank leak and can be ruled out as a significant source of groundwater contamination, based on the U isotopic composition. The locus of vadose zone contamination is displaced from the initial locus of groundwater contamination, indicating that lateral migration in the vadose zone was at least 8 times greater than vertical migration. The time evolution of the groundwater plume suggests an average U migration rate of approximately 0.7-0.8 m/day showing slight retardation relative to a groundwater flow of approximately 1 m/day.
authors.affiliation: Center for Isotope Geochemistry, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 70A4418, Berkeley, California 94720, USA. jnchristensen@lbl.gov
authors.forename: John N
authors.initials: JN
authors.lastname: Christensen
authors.forename: P Evan
authors.initials: PE
authors.lastname: Dresel
authors.forename: Mark E
authors.initials: ME
authors.lastname: Conrad
authors.forename: Kate
authors.initials: K
authors.lastname: Maher
authors.forename: Donald J
authors.initials: DJ
authors.lastname: DePaolo
mesh_terms: D004784:Environmental Monitoring; D011850:Radioactive Waste; D012990:Soil Pollutants, Radioactive / Q000032:analysis*; D014501:Uranium / Q000032:analysis*; D014861:Washington; D014872:Water Movements; D014875:Water Pollutants, Radioactive / Q000032:analysis*
chemical_list: D011850:Radioactive Waste; D012990:Soil Pollutants, Radioactive; D014875:Water Pollutants, Radioactive; D014501:Uranium