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

[1] 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 × 10 ⁶ 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 indica e excess air incorpora ion a recharge. Measured ⁴⁰Ar/³⁶Ar ra ios do no exceed he a mospheric 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.