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

Abstract

[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 $0.2 to 1.2 Â 10 6 years in the deep (600-1200 m) groundwater (assuming constant Cl) and 0.16 Â 10 6 years in the shallow (<600 m) groundwater. Values of d 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 Â 10 À4 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 Â 10 5 years; residence times up to 1.3 Â 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 À is accounted for in the highest-Cl À 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 G 3

The isotope hydrology of quaternary climate change☆

“…As Sturchio et al state, "The age of groundwater is one of the most elusive geologic parameters to quantify…" (2004: 1). Nevertheless the results from that paper and a related one by Patterson et al (2005) show reasonably good agreement between the 36 Cl and 81 Kr techniques (Figure 8c; study location in Figure 3). The figure also shows a considerable difference between isotope ratios of modern and palaeo-recharge.…”

“…The figure also shows a considerable difference between isotope ratios of modern and palaeo-recharge. While this was already known from waters of radiocarbon age in the general area (e.g., Gat and Issar, 1974;Sonntag et al, 1978;Darling et al, 1987;Vengosh et al, 2007) and attributed to changes in the ITCZ, the Sturchio et al (2004) and Patterson et al (2005) studies suggest a recurring predominant Atlantic source of rainfall for at least the past million years.…”

“…18 O and noble gas (NG) recharge temperature versus 14 C age for waters from the Uitenhage aquifer, South Africa (based on data of Heaton et al, 1986). Lehmann et al, 2003), (b) the Continental Intercalaire of Tunisia and northern Algeria (data from Guendouz and Michelot, 2006), and (c) the Nubian sandstone of western Egypt (data from Sturchio et al, 2004;Patterson et al, 2005). …”

The climatic and physiographic controls of the eastern Mediterranean over the late Pleistocene climates in the southern Levant and its neighboring deserts

“…In turn, this interpreted wetter southern Negev and Sinai was used to infer a shift of the climatic zones farther south (e.g., Issar and Bruins, 1983), toward geographical settings far south of the current wet climatic zone. Recently, the residence ages of the water in the Nubian aquifer in the Western Desert, previously thought of as the result of a similar late Pleistocene recharge and were partially the basis for accepting the latest Pleistocene ages of the Negev's Nubian aquifer water, were constrained by Patterson et al (2005) to N200 ka and probably ∼1 ma. Hence, the use of the proposed radiocarbon activity as age of a latest Pleistocene recharge event in reconstructing the regional paleoclimate is suspect (e.g., Phillips and Castro, 2005, p. 464), as is the interpretation of a very large latitudinal shift of the Mediterranean climate into the heart of the hyperarid desert.…”

Groundwater Conditions and the Geoenvironmental Impacts of the Recent Development in the South Eastern Part of the Western Desert of Egypt

“…Using environmental isotopes and Hydrogeochemical techniques they determined that the main bulk of water is "fossil" and that contributions from recent recharge are insignificant. For more detailed study of the Nubian aquifer system in the western desert, are available in [15]- [21]. Based upon the lithology of the drilled wells in the area, four hydrogeological cross-sections were made ( Figure 6).…”