Farouk Soliman

Red Sea rifting controls on aquifer distribution: Constraints from geochemical, geophysical, and remote sensing data

Highly productive wells in the Central Eastern Desert of Egypt are tapping groundwater in subsided blocks of Jurassic to Cretaceous sandstone (Taref Formation of the Nubian Sandstone Group) and Oligocene to Miocene sandstone (Nakheil Formation), now occurring beneath the Red Sea coastal plain and within the proximal basement complex. Aquifer development is related to Red Sea rifting: (1) rifting was accommodated by vertical extensional displacement on preexisting NW-SE– to N-S–trending faults forming a complex array of half-grabens and asymmetric horsts; and (2) subsided blocks escaped erosion accompanying the Red Sea–related uplift. Subsided blocks were identifi ed and verifi ed using satellite data, geologic maps, and fi eld and geophysical investigations. Interpretations of very low frequency (VLF) measurements suggest the faults acted as conduits for ascending groundwater from the subsided aquifers. Stable isotopic compositions (δD: –19.3‰ to –53.9‰; δ 18 O: –2.7‰ to –7.1‰) of groundwater samples from these aquifers are interpreted as mixtures of fossil (up to 70%) and modern (up to 65%) precipitation. Groundwater volumes in subsided blocks are large; within the Central Eastern Desert basement complex alone, they are estimated at 3 × 10 9 m 3 and 10 × 10 9 m 3 for the

Integrated solutions for hydrologic investigations in arid lands

Hydrological assessment studies across vast regions of the arid world are often hindered by the inaccessibility of these areas and the paucity of data sets, as well as the high expenses and diffi culties entailed in acquiring these data sets, their unpublished nature, and their varying scales, projections, and datum. Using the Eastern Desert (ED) of Egypt (225,000 km 2 ) and the Sinai Peninsula (61,000 km 2 ) as test sites, we demonstrate practical and cost-effective integrated (geochemistry, geophysics, and modeling) solutions that utilize web-based geographic information system (GIS) (http://www.esrs .wmich.edu/webmap) technologies and take advantage of readily available global remote sensing data sets. Adopted methodologies allowed: (1) development of conceptual models for hydrogeologic settings conducive to groundwater entrapment and augmentation, including groundwater in fractured basement aquifers, groundwater impounded by dike swarms crosscutting alluvial aquifers, and groundwater residing in alluvial aquifers associated with ascending deep-seated fossil waters; (2) selection of criteria to identify and validate the preferred distribution of each of these aquifer types and usage of the selected criteria and observations from the GIS data sets to identify, test, and refi ne potential well locations; and (3) construction and calibration of hydrologic models to estimate average annual recharge over the major watersheds in the Sinai (463 ◊ 10 6 m 3 /yr) and ED (171 ◊ 10 6 m 3 /yr) and the average modern contributions to Nubian fossil aquifers (Sinai: 13 ◊ 10 6 m 3 /yr), and to model the partitioning of precipitation as a function of precipitation amounts. The successful application of the integrated and cost-effective methodologies developed for the study areas should invite similar applications in arid regions elsewhere.