Krypton-81 case study: the Nubian aquifer, Egypt. Chapter 14

The Nubian aquifer of north-east Africa is one of the world's largest fresh groundwater resources (∼150 000 km3). An area in the Western Desert of Egypt, for which a number of earlier studies provided sufficient background data on the Nubian aquifer, was selected for the first application of the atom trap trace analysis (ATTA) method (Section 5.4.3) for measurement of 81Kr in groundwater. This area has relatively simple geology, potential for containing very old groundwater, and favourable characteristics for comparison of the 81Kr and 36Cl methods (Sturchio et al. (2004)). Before this work, little evidence regarding absolute groundwater residence time in the N ubian aquifer was available. Estimates of groundwater age in the range of 0.54 × 106–1.58 × 106 a had been obtained from He/Ar ratios (Himida (1967) [729]). A large number of 14C data had been obtained for the Nubian aquifer waters in Egypt (Fröhlich et al. (2007) [730]; Sonntag et al. (1979) [731]). Chlorine-36 data indicated possible residence times of up to ∼9 × 105 a (unpublished data).