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

Abstract

Measurements of radiochlorine (36Cl), radiogenic noble gases (4He and 40Ar), 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 36Cl/Cl ratios yield apparent residence times from ∼0.2 to 1.2 × 106 years in the deep (600–1200 m) groundwater (assuming constant Cl) and ≤0.16 × 106 years in the shallow (<600 m) groundwater. Values of δ37Cl in the groundwater strengthen the application of the 36Cl dating method by constraining Cl sources and identifying groundwater mixing. Dissolved gases were measured in some of the deep groundwater samples. Measured 4He concentrations indicate accumulation of radiogenic 4He that is qualitatively consistent with the age progression indicated by the 36Cl/Cl ratios, but the flux of external 4He from the underlying crust has not been quantified and is not constant throughout the aquifer. Concentrations of 40Ar range from 3.3 to 6.7 × 10−4 ccSTP/g and indicate excess air incorporation at recharge. Measured 40Ar/36Ar 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 × 105 years; residence times up to 1.3 × 106 years are predicted in the confining shale. Aquifer properties are estimated by using the model to fit the measured 36Cl/Cl ratios. Under these conditions, hydrodynamic residence times are within about 30% of those calculated from 36Cl when mixing of Cl− is accounted for in the highest‐Cl− deep groundwaters. By mutually calibrating multiple methods (hydrodynamic, 36Cl, and 4He), 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.