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Sixteen groundwater samples collected from production wells tapping Lower Cretaceous Nubian Sandstone and fractured basement aquifers in Sinai were analyzed for their stable isotopic compositions, dissolved noble gas concentrations (recharge temperatures), tritium activities, and C-14 abundances. Results define two groups of samples: Group I has older ages, lower recharge temperatures, and depleted isotopic compositions (adjusted C-14 model age: 24,000-31,000 yr BP; delta O-18: -9.59 parts per thousand. to -6.53 parts per thousand; delta H-2: -72.9 parts per thousand to -42.9 parts per thousand.; <1 TU; and recharge T: 17.5-22.0 degrees C) compared to Group H (adjusted C-14 model age: 700-4700 yr BP; delta O-18: -5.89 parts per thousand. to -4.84 parts per thousand.; delta H-2: -34.5 parts per thousand. to -24.1 parts per thousand; <1 to 2.78 TO; and recharge T: 20.6-26.2 degrees C). Group II samples have isotopic compositions similar to those of average modern rainfall, with larger d-excess values than Group I waters, and locally measurable tritium activity (up to 2.8 TO). These observations are consistent with (1) the Nubian Aquifer being largely recharged prior to and/or during the Last Glacial Maximum (represented by Group I), possibly through the intensification of paleowesterlies; and (2) continued sporadic recharge during the relatively dry and warmer interglacial period (represented by Group II) under conditions similar to those of the present. (C) 2013 University of Washington. Published by Elsevier Inc All rights reserved.