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

[ 1] 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 similar to 0.2 to 1.2 x 10(6) years in the deep ( 600 - 1200 m) groundwater ( assuming constant Cl) and less than or equal to 0.16 x 10(6) years in the shallow (< 600 m) groundwater. Values of delta(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 x 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 x 10(5) years; residence times up to 1.3 x 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.