  Measurements of 81Kr/Kr in deep groundwater from the Nubian Aquifer (Egypt)
were performed by a new laser-based atom-counting method. 81Kr ages range from
\~2x10^5 to ~1x10^6 yr, correlate with 36Cl/Cl ratios, and are consistent with
lateral flow of groundwater from a recharge area near the Uweinat Uplift in SW
Egypt. Low delta-2H values of the 81Kr-dated groundwater reveal a recurrent
Atlantic moisture source during Pleistocene pluvial periods. These results
indicate that the 81Kr method for dating old groundwater is robust and such
measurements can now be applied to a wide range of hydrologic problems.
