One million year old groundwater in the Sahara revealed by krypton‐81 and chlorine‐36

Measurements of 81 Kr/Kr in deep groundwater from the Nubian Aquifer (Egypt) were performed by a new laser‐based atom‐counting method. 81 Kr ages range from ∼2 × 10 5 to ∼1 × 10 6 yr, correlate with 36 Cl/Cl ratios, and are consistent with lateral flow of groundwater from a recharge area near the Uweinat Uplift in SW Egypt. Low δ 2 H values of the 81 Kr‐dated groundwater reveal a recurrent Atlantic moisture source during Pleistocene pluvial periods. These results indicate that the 81 Kr method for dating old groundwater is robust and such measurements can now be applied to a wide range of hydrologic problems.