Cosmogenic, radiogenic, and stable isotopic constraints on groundwater residence time in the Nubian Aquifer, Western Desert of Egypt

Measurements of radiochlorine ( 36 Cl), radiogenic noble gases ( 4 He and 40 Ar), and stable chlorine isotope ratios were obtained to assess the residence time of groundwater in the Nubian Aquifer of the Western Desert of Egypt. Measured 36 Cl/Cl ratios yield apparent residence times from ∼0.2 to 1.2 × 10 6 years in the deep (600–1200 m) groundwater (assuming constant Cl) and ≤0.16 × 10 6 years in the shallow (<600 m) groundwater. Values of δ 37 Cl in the groundwater strengthen the application of the 36 Cl dating method by constraining Cl sources and identifying groundwater mixing. Dissolved gases were measured in some of the deep groundwater samples. Measured 4 He concentrations indicate accumulation of radiogenic 4 He that is qualitatively consistent with the age progression indicated by the 36 Cl/Cl ratios, but the flux of external 4 He from the underlying crust has not been quantified and is not constant throughout the aquifer. Concentrations of 40 Ar range from 3.3 to 6.7 × 10 −4 ccSTP/g and indicate excess air incorporation at recharge. Measured 40 Ar/ 36 Ar ratios do not exceed the atmospheric ratio. A two‐dimensional numerical hydrodynamic transect of the aquifer was modeled from the area of the Uweinat Uplift to the northern Bahariya Oasis. Predicted groundwater velocities in the deep portion of the aquifer are 0.5–3.5 m/yr with groundwater residence times up to 9 × 10 5 years; residence times up to 1.3 × 10 6 years are predicted in the confining shale. Aquifer properties are estimated by using the model to fit the measured 36 Cl/Cl ratios. Under these conditions, hydrodynamic residence times are within about 30% of those calculated from 36 Cl when mixing of Cl − is accounted for in the highest‐Cl − deep groundwaters. By mutually calibrating multiple methods (hydrodynamic, 36 Cl, and 4 He), a consistent picture of the Nubian Aquifer has emerged in which lateral flow from a southern recharge area dominates the deep horizons, while shallow horizons contain younger, autochthonous recharge.