Timing of Recharge, and the Origin, Evolution and Distribution of Solutes in a Hyperarid Aquifer System

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 th