High-precision uranium isotopic analysis for environmental forensics using MC-ICPMS: Demonstration studies at the Hanford Site, Washington
The contrasts in isotopic composition between natural and anthropogenic uranium and the wide variation in the composition of different processed uranium sources, promotes the measurement of uranium isotopic composition as a fingerprint and tracer of uranium contamination in the environment. Previous studies mainly have focused on the use of only one of the isotope ratios of U, e.g., 234U/238U, 238U/235U or 236U/238U. We measure all three of these ratios in environmental samples in order to better distinguish, characterize, identify and apportion U sources. For our U isotopic measurements, we employ an IsoProbe (GV Instr. Inc.) multiple-collector ICP source magnetic sector mass spectrometer. U isotopic compositions are measured simultaneously using a combination of Faraday cups (for 235U and 238U) and a Daly photomultiplier ion counting system (for 234U and 236U in two separate analyses). U is separated from samples (e.g., vadose zone pore water, groundwater, rock/soil samples) prior to introduction to the MC-ICPMS via a desolvation system. At 7E-11 amps of 238U ions, a single analysis of a 20ppb U solution uses ~10ng of sample U. For correction of instrumental mass fractionation, we use bracketing analyses of a natural secular equilibrium U standard. This allows us to avoid the use of a double 233U-236Uspike for mass fractionation correction that would compromise our ability to measure 236U/238U. We also use the standard analyses for Daly/Faraday gain and for peak-tail correction of the 236U analyses. Typical precision for 238U/235U is ?0.05% 2s, while for 234U/238U it is ?0.15% 2s. Precision for 236U/238U is ?0.15% 2s down to the 10-7 range where precision degrades by a factor of ten. The limit for 236U/238U measurement is about 2x10-8, only ~five times higher than accelerator MS. For 1ppb U, this represents 5x107 atoms 236U per liter water.We will present three ongoing studies at the Hanford site as a demonstration of our techniques: (1) investigation of groundwater and vadose zone contamination in the B-BX-BY WMA (Christensen et al. (2004) Env. Sci. Tech., in press) (2) signatures of vadose zone contamination in waste cribs near U-Plant and (3) investigation of U contamination of the Columbia River.