Residence Time of Nubian Aquifer Ground Water Determined

University Of Illinois At Chicago, Chicago IL

Investigators

Abstract

126297 Sturchio Knowledge of the subsurface residence time of groundwater has a multitude of important applications, both scientific and utilitarian. The determination of groundwater residence time is fraught with complexity because of uncertainties in aquifer properties, initial conditions, and hydrodynamic history, as well as possible subsurface production or water-rock interactions involving the isotopic or chemical tracers used for residence time determination. Although there are a number of well-developed tools for dating relatively young groundwaters, there is a paucity of tools for dating relatively old groundwaters. The most commonly used tool for dating old groundwater is accelerator-based mass spectrometry of 36Cl, yet this approach is compromised by the temporal and geographic variability of the initial 36Cl/Cl ratio as well as the locally variable magnitude of subsurface 36CI production. A more ideal tool than 36Cl,, yet one which has been neglected because of analytical difficulties, is 81Kr Resent developments in magnetooptical atom-trapping methods have made accurate, precise 81Kr analysis possible by using a benchtop-scale apparatus. We propose to use the prototype instrument for atom-trap trace analysis (developed at Argonne National Laboratory by Z. T. Lu and colleagues) to perform the first 81Kr measurements of very old groundwater from the Nubian Aquifer of the Westem Desert of Egypt. The Kr will be extracted and purified from samples of 8,000-10,000 liters each by B. Lehmann and colleagues of the University of Bem who have developed large-sample noble gas extraction methods for 239Ar analysis of ground waters. These 81Kr data will be obtained in conjunction with a suite of other chemical and isotopic data (including solute ion and dissolved gas analyses; stable isotopes of H, C, N, 0, S, and Cl; and tritium, radiocarbon, and radiochlorine) and used to address fundamental questions about groundwater dating and the history of the Nubian aquifer.