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CHLORINE ISOTOPES, CHLORIDE SOURCES, AND RESIDENCE TIMES OF GROUNDWATERS IN THE NUBIAN AQUIFER, EGYPT
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.