2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 12
Presentation Time: 4:15 PM


PATTERSON, Leslie J.1, STURCHIO, Neil C.1, SULTAN, Mohamed2, LU, Z.-T.3, PURTSCHERT, R.4, LEHMANN, B.E.5, EL ALFY, Z.6, EL KALIOUBY, B.7, DAWOOD, Yehia H.8 and ABDALLAH, A.M.A.9, (1)Earth and Environmental Sciences, Univ of Illinois at Chicago, 845 West Taylor St., MC-186, Chicago, IL 60607, (2)Geology Department, Univ at Buffalo, 876 NSC, Buffalo, NY 14260, (3)Physics Division, Argonne National Lab, 9700 South Cass Ave., PHY-203, Argonne, IL 60439, (4)Physics Institute, Univ of Bern, Sidlerstrasse 5, Bern, 3012, Switzerland, (5)Physics Institute, Univ of Bern, Sidlerstrasse 5, Bern, 3012, (6)Remote Sensing Laboratory, Egyptian Geol Survey and Mining Authority, Abbassia, Egypt, (7)Geology Department, Ain Shams Univ, Cairo, Egypt, (8)Geology Department, Faculty of Science, Ain Shams Univ, Cairo, 11566, Egypt, (9)Geology Department, Ain Shams Univ, Cairo, lpatte2@uic.edu

The Nubian Aquifer in the Dakhla Basin of the Western Desert of Egypt underlies the hyperarid northeast corner of the Sahara Desert.  It contains ~50,000 km3 of groundwater in a thick (up to 3,000 m) sandstone formation with thin interbedded shales.  Residence times from 2 ´ 104 – 106 a are suggested by aquifer hydrodynamics.  The hydrology is complex and provides a good test for the robustness of the 36Cl method of groundwater dating.   In this study, about 30 groundwater samples from relatively deep (200 – 1200 m) wells in major oasis areas, and shallow (<200 m) wells in the Lake Nasser and Toshka Canal areas were collected and analyzed for chemistry, d37Cl, 36Cl, and other stable and radiogenic isotopes.  Chloride concentrations range from 21 to 394 ppm and decrease with increasing depth; this is typical for arid environments.  Values of d37Cl (SMOC) range from -1.16 to +0.51, indicating at least two chloride sources.  Processes capable of producing variable d37Cl are considered, including diffusion of Cl from saline pore fluids within aquicludes and halite precipitation-dissolution in the vadose zone.  36Cl/Cl ratios range from 16 – 228 ´ 10-15.  The data indicate that water from the deeper portions of the aquifer has a single, meteoric source of Cl and can be dated accurately by 36Cl showing a systematic increase in residence time of 4 ´ 105 a down gradient.  Samples from shallow wells have higher Cl- concentrations, variable d37Cl values, and generally higher 36Cl activities than those from deeper wells, indicating transient local recharge of Cl-enriched surface waters.  Numerical simulations of groundwater flow and chloride transport (including 36Cl) in the aquifer indicate that baseflow from remote recharge areas occupies the deeper portions of the aquifer and follows the contour of the piezometric surface, whereas transient local recharge events influence the chemistry and isotopic characteristics of the shallower portions of the aquifer.