The age of groundwater is one of the most elusive geologic parameters to quantify, despite its crucial significance for water resources, waste management, subsurface reactive transport, and paleoclimate studies. It has long been recognized that 81 Kr is a nearly ideal tracer for determining residence times of old groundwater in the 5 10 4 10 6 year range, yet because of its low abundance ( 81 Kr/Kr 10 -12 ) it has been nearly impossible to measure. However, a new technique for measurement of 81 Kr/Kr ratios in small quantities of Kr (as little as 50 micromoles) was recently reported (Atom-Trap Trace Analysis, or ATTA; Chen et al., 1999; Du et al., 2003), and its application to old groundwater was demonstrated successfully by the PI and colleagues in their NSF-funded study of the Nubian Aquifer, Egypt (Sturchio et al., 2003). That study established the feasibility of using the ATTA technique for routine 81 Kr/Kr measurement in hydrologic investigations of systems having long groundwater residence times. The ATTA technique, which can also be used for measuring 85 Kr/Kr, is thus an ideal tool with which to address a range of fundamental questions in hydrology. This proposal requests support for a graduate student to work with the PI and colleagues (Z. T. Lu's group at Argonne National Laboratory) in developing further applications of the trace Kr isotopes ( 81 Kr and 85 Kr) in hydrologic investigations. The intellectal merit of this research is that it will lead to an enhanced understanding of the water cycle within the Earth's crust. Specific investigations that we propose are: (1) to determine the residence time of saline groundwaters in the central U.S.A., which on the basis of stable isotope data are believed to have traveled more than 1,000 km from a recharge area in the Rocky Mountains (Musgrove and Banner, 1993), and which should therefore have a residence time on the order of 10 6 years; (2) to constrain the residence time of hydrothermal waters at Yellowstone National Park, for which little information is available (Pearson and Truesdell, 1978); (3) to determine the residence time of groundwater in the Milk River aquifer of southern Alberta, which was the subject of an earlier multi-tracer study sponsored by the IAEA that was focused on dating very old groundwater (Ivanovich, Frohlich, and Hendry, 1991); and to investigate residence times of young (<50 yr) groundwaters where other tracers such as tritium-helium and CFC's do not provide coherent results. These investigations are representative of the broad range of potential applications of trace Kr isotopes in groundwater hydrology. The broader impact of the proposed research is that it will provide opportunities for research, training, and education at the University of Illinois at Chicago (UIC), which has the nation's fifth most diverse student population; it will establish enhanced local Chicago-area partnership for routine application of trace Kr isotopes in hydrology, because the Kr separations will henceforth be performed in a laboratory at UIC (rather than in Switzerland as in the previous NSF-funded study) with ATTA measurements performed nearby at Argonne National Laboratory; it will help establish the new subdiscipline of Kr isotope hydrology, which may have significant impact on hydrologic investigations and water resources management worldwide.
