Measurements of 81Kr/Kr in deep groundwater from the Nubian Aquifer (Egypt) were performed by a new laser‐based atom‐counting method. 81Kr ages range from ∼2 × 105 to ∼1 × 106 yr, correlate with 36Cl/Cl ratios, and are consistent with lateral flow of groundwater from a recharge area near the Uweinat Uplift in SW Egypt. Low δ2H values of the 81Kr‐dated groundwater reveal a recurrent Atlantic moisture source during Pleistocene pluvial periods. These results indicate that the 81Kr method for dating old groundwater is robust and such measurements can now be applied to a wide range of hydrologic problems.
[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
Coastal marine sediment samples were collected from 31 sampling stations along the Egyptian Mediterranean Sea coast. All sediment samples were analyzed to determine aliphatic and polycyclic aromatic hydrocarbons (PAHs) as well as total organic carbon (TOC) contents and grain size analysis. Total concentrations of 16 EPA-PAHs in the sediments were varied from 88 to 6338 ng g(-1) with an average value of 154 ng g(-1) (dry weight). However, the concentrations of total aliphatic were varied from 1.3 to 69.9 ng g(-1) with an average value of 15.6 ng g(-1) (dry weight). The highest contents of PAHs were found in the Eastern harbor (6338 ng g(-1)), Manzala (5206 ng g(-1)) and El-Jamil East (4895 ng g(-1)) locations. Good correlations observed between a certain numbers of PAH concentrations allowed to identify its origin. The average total organic carbon (TOC) percent was varied from 0.91 to 4.54%. Higher concentration of total pyrolytic hydrocarbons ( summation operatorCOMB) than total fossil hydrocarbons ( summation operatorPHE) declared that atmospheric fall-out is the significant source of PAHs to marine sediments of the Egyptian Mediterranean coast. The selected marked compounds, a principal component analysis (PCA) and special PAHs compound ratios (phenanthrene/anthracene vs fluoranthene/pyrene; summation operatorCOMB/ summation operatorEPA-PAHs) suggest the pyrogenic origins, especially traffic exhausts, are the dominant sources of PAHs in most locations. Interferences of rather petrogenic and pyrolytic PAH contaminations were noticed in the harbors due to petroleum products deliveries and fuel combustion emissions from the ships staying alongside the quays.
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