One million year old groundwater in the Sahara revealed by krypton-81   and chlorine-36
  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. 
N.C. Sturchio, X. Du, R. Purtschert, B.E. Lehmann, M. Sultan, L.J.   Patterson, Z.-T. Lu, P. Mueller, T. Bigler, K. Bailey, ...