Uranium isotopic study of artesian and pluvial contributions to the Nubian Aquifer, Western Desert, Egypt

Abstract

The groundwater systems of the Nubian Aquifer beneath six major oases (Baris, Kharga, Dakhla, Farafra, Bahariya, and Siwa) in the Western Desert of Egypt have been studied to establish their sources and mixing volumes using uranium isotopes. One hundred six groundwater samples from different depths of the Nubian Aquifer have been analyzed for uranium content and 234U/ 238U activity ratio (AR). The aquifer under the Western Desert is known to have more than one source of water. At Bahariya and Farafra Oases, the Nubian artesian water migrating from the south has been augmented by local recharge during pluvial times. At Siwa Oasis in the northwestern desert, a shallower aquifer component migrating from the north or west is also present. At Dakhla, Kharga and Baris Oases in the southwestern desert, the main source is the Nubian artesian water migrating from southeast Uweinat Upland and northwest Sudan. These water masses have distinctive uranium isotopic signatures. The Nubian Aquifer water is characterized by very low U concentrations (<0.05 ppb) and a relatively high 234U/ 238U AR (>1.5). The shallow northwest aquifer water also has a high AR but a much higher U concentration. The locally recharged pluvial waters have high U concentrations (>0.1 ppb) but low ARs, near unity. A diagnostic derivative parameter is excess 234U content. The deep Nubian Aquifer water is characterized by a relatively low excess 234U (<0.02 ppb equivalent), while the shallow Siwa water has a very high excess 234U [(AR−1)(U conc.)]. The Bahariya and Farafara waters are also high in this component, probably because of pluvial water percolation through phosphate rich strata. At all oases, U isotopic mixing diagrams show that the deep aquifer sources predominate; however, the pluvial contributions are significant, ranging from about 5% at remote Baris Oasis to about 26% at the more northerly Farafra Oasis. The observed lowering of potentiometric surfaces in the Western Desert is caused not only by pumping at a rate greater than inflow from the aquifer systems, but also by the withdrawal of pluvial water which in modern times is not being replaced at all.