Mantle-derived fluids in the continental-scale Nubian aquifer

The Nubian aquifer of northeastern Africa is one of the largest confined fossil-water aquifer systems in the world. It is central for ongoing water needs and future development goals of Egypt, Libya, Sudan and Chad. Groundwater extraction and consumptive use over the past 40 years have led to a continuous decline of water level and to the reduction and/or cessation of natural flow in artesian wells and springs. This study introduces new noble gas and stable isotope geochemical data from deep groundwater supply wells of the main oases of the Western Desert of Egypt (Bahariya, Farafra, Dahkla, and Kharga) and places these data in the context of a regional compilation of published hydrological and hydrochemical data. Helium isotopes, stable isotope compositions for water (δ2H; δ18O) and dissolved inorganic carbon (δ13C), salt compositions, and [CO2] suggest deeply derived fluid inputs. Highly variable groundwater temperatures (20–56 °C) also suggest the presence of geothermal inputs that mix with aquifer water. Nubian water displays high PCO2 (up to three orders of magnitude higher than atmosphere) and hydrochemical analysis using δ13C suggests up to ~30% of the CO2 is derived from deep geologic sources. Helium isotope (3He/4He) ratios of samples obtained from the Nubian aquifer range from crustal values of 0.02RA (where RA reflects the 3He/4He in air or 1.384 × 10−6) to values of up to 0.36RA (2.5% mantle helium) in Dahkla Oasis of Egypt and up to 1.4RA (17% mantle helium) in Libya. Correlations among excess heat, increases in [CO2], δ13C, and 3He/4He indicate that small volumes of deeply sourced fluids carrying mantle-derived volatiles are leaking into the Nubian aquifer. Geothermal water apparently ascend along faults and shear zone conduits and mix with regional groundwater. The origin of these volatiles may be partial melts in low velocity upper mantle beneath the aquifer and young magmatism surrounding the Nubian aquifer. Fluid-rock interactions along flowpaths can explain variable associations and correlations among the tracers. Variability of composition and temperature suggest complex fluid mixing within the Nubian aquifer, the existence of fault-bounded hydrologic sub-basins, and a connection between tectonics and water quality. Better understanding of the sources and admixtures of deep geothermal fluid inputs is needed for informed water management.