Tracing Young and Old Groundwater Using Atom Trap Trace Analysis

Due to its simple production and transport in the terrestrial environment, 81Kr (half-life = 230,000 yr) is an ideal tracer for old water and ice with mean residence times in the range of 105-106 years. Likewise, the anthropogenic isotope 85Kr (half-life = 10.8 yrs), which is predominantly introduced into the atmopshere through nuclear fuel reprocessing activities, is a valuable tracer of respectively young groundwater. In recent years, the ultra-trace analysis of both isotopes has been made available to the earth science community at large thanks to the development of Atom Trap Trace Analysis (ATTA), an efficient and selective laser-based atom counting method. Further upgrades and improvements to the ATTA instrument at Argonne National Laboratory have recently allowed us to measure Kr isotope ratios with relative uncertainties at the level of 1% while also significantly increasing sample throughput and reducing sample size requirements. As examples of recent achievements, we have placed a 1.4% limit on the anthropogenic contribution to 81Kr relative to its natural abundance by precisely comparing the 81Kr abundance in a 1940 athmospheric sample to that of modern air, we are now able to handle samples as small as 1µL of Kr gas (STP), and we can measure 85Kr isotope abundances in as little as four hours of measurement time enabling us to analyze six or more samples in 24 hrs. This work is supported by Department of Energy, Office of Nuclear Physics, under Contract No. DEAC02-06CH11357.