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Timing of recharge, and the origin, evolution and distribution of solutes in a hyperarid aquifer system
W.W. Wood, , A.S. Alsharhan
Published in
2003
Volume: 50
   
Issue: C
Pages: 295 - 312
Abstract
Examination of an aquifer system in the Liwa Crescent/Bu Hasa area of the Emirate of Abu Dhabi on the southeastern edge of the Rub al Khali, Arabian sub-continent, provides insight into the timing of ground-water recharge and the origin and evolution of solutes in a representative hyperarid area. Ground-water flow in the aquifer system is radially outward from the center of two ground-water mounds, corresponding to two 110 m-thick sand deposits. The isotopic data from ancient ground waters from the Liwa Oasis with its unusual "d" (deuterium excess) of approximately -15 indicate that Holocene moisture derived from previously evaporated water on the surface of the Indian Ocean rather than from the Mediterranean/Arabian Gulf. Such a source is consistent with a summer monsoonal circulation. Hydraulic heads in underlying aquifers are lower than those of the Liwa/Bu Hasa aquifer; thus, there is no advective water or solute input from these lower units. Solute diffusion from underlying aquifers provides a small solute flux, but it is inadequate to account for the observed solute mass, nor is the ionic ratio consistent with this source. There are no laterally adjacent aquifers; thus, there is no influx of solutes from these sources. Dissolution of the aquifer framework provides for only a minor fraction of most of the observed solutes. Most solutes in this aquifer system are derived from atmospheric precipitation. Salts contained in rain were stored on the surface and in the unsaturated zone during the hyperarid time interval between the ends of the Pleistocene recharge event (26,000 years BP) and the beginning of the Holocene recharge (9000 years BP). During the Holocene recharge era (9000 to 6000 years BP), these stored salts were mobilized and transported to the ground water. The initial solute distribution has been slightly modified as the solutes and water were transported along the flow path. As ground water moves away from the apex of the mound, it encounters a series of interdunal sabkhat. Water is evaporated from the sabkhat leaving soluble chloride and nitrate minerals on the surface and retrograde carbonate and sulfate minerals in the unsaturated zone. When recharge occurs through the interdunal sabkhat, which are the only areas of recharge in the aquifer, soluble salts on the surface, modern 14C, and tritium are added to the aquifer, aquifer because the density of the recharged water is greater than the density of the ground water in this nearly homogenous and isotropic aquifer. Relatively insoluble carbonate and sulfate minerals are retained in the unsaturated zone of the interdunal sabkhat, causing a change in the solute ratios downgradient. Only the portion of the aquifer not associated with interdunal sabkhat retains the original solute composition determined by mobilization of stored salts. These findings, although specific to this aquifer, provide a useful model by which to evaluate other aquifer systems in hyperarid environments. © 2003 Elsevier B.V. All rights reserved.
About the journal
JournalDevelopments in Water Science
ISSN1675648
Open AccessNo