Authors
Ram, Roi; Kip Solomon, D.; Yokochi, Reika; Burg, Avihu ; Purtschert, Roland; Seltzer, Alan; Yechieli, Yoseph; Zappala, Jake; Lu, Zheng-Tian; Jiang, Wei; Mueller, Peter; Adar, Eilon
Abstract
Illuminating past hydrological and climatological conditions in arid regions may provide insights into futuretrends in groundwater availability. The main objective of this study is to explore paleorecharge processes in thedeep regional Nubian Sandstone Aquifer (NSA), which stretches below the hyperarid deserts of the SinaiPeninsula (Egypt) and the Negev (Israel), using dissolved noble gas data. Extremely high amounts of dissolvedexcess air (Ne concentrations up to 4 solubility equilibrium) were observed in the ancient (81Kr-depleted)groundwater that was recharged during past pluvial epochs in the basin. The observed unique excess air signaland the clear spatiotemporal structure of the noble gas compositions in the aquifer are hypothesized to reflecttwo major characteristics of the groundwater system: (a) large-scale, long-term rises in the water table thatfacilitate the entrapment and dissolution of air bubbles under increased hydrostatic pressure, and (b) thegeological settings in the southern recharge area in Sinai, including an almost horizontal position of the layersand intercalation of low-permeability rock formations within the aquifer, which inefficiently reject air bubblesfrom groundwater, thereby allowing for substantial entrapment of bubbles that dissolve as the water table rises.Enhanced dissolution of entrapped air was also observed in other paleo-groundwater-containing regionalsandstone aquifers across North Africa, which, together with the recent findings from the NSA, suggest that largescale water table fluctuations have likely extensively occurred during past pluvial periods over these (andpossibly other) arid regions. A preliminary assessment of noble gas recharge temperatures (NGTs) indicates anapparent decoupling of surface and water table temperatures in the case of deep aquifers with a thick (hundredsof m) unsaturated zone. This observed decoupling calls for a re-evaluation of previously obtained NGT recordsand a need for future work to consider the modification of NGTs due to soil air fractionation and geothermalheating of the underlying deep unsaturated zones.