GEOCHEMICAL AND ISOTOPIC CONSTRAINTS ON THE ORIGIN OF THE RUB AL KHALI GROUNDWATERS, SAUDI ARABIA

Chemical and stable isotopic compositions of coastal and inland aquifers from the Rub Al Khali, the largest desert in the world located in the southeastern part of Saudi Arabia were analyzed to decipher the origin of the extensive groundwater reservoir(s) of the hyper-arid deserts of Arabia. Samples were collected from a variety of wells along the perimeter of the eastern half of the Rub Al Khali, well depths ranging from 1.5 up to 800 meters, including flowing artesian wells, pumped wells (formerly artesian), and shallow hand-dug wells including areas covered by sabkha areas. Preliminary data analysis indicates that the water from the artesian and formerly artesian wells is likely to represent the contents of confined aquifers. Such water (Group 1) is isotopically depleted (δ²H values ranging from –60‰ to –35‰), but has total dissolved solids concentrations ranging from ~1,300 to ~76,000 mg/L, indicating that much of the salinity is acquired in the subsurface. These waters represent either high-elevation recharge from mountainous areas to the west of the Rub Al Khali, and/or recharge largely formed of paleowater precipitated during moist climate intervals of the late Pleistocene. Water from shallow hand-dug wells including those in sabhka areas (Group 2) has experienced significant evaporation (δ²H values ranging from –34‰ to +19‰) as well as salinization (TDS as high as ~92,000 mg/L) by dissolution of salts including halite and gypsum from sabhkas and/or salt diapirs. Because many of the Group 2 samples plot along an evaporation trend line that extends upward from the Group1 samples, they are here interpreted as Group 1 samples that ascended through structural discontinuities and were affected by substantial amounts of evaporation as well as dissolution of salts. A suggestion supported by the presence of sabkhas and salt diapirs in areas of Group 2 samples and by the analysis of geochemical and isotopic data for Group 1 and Group 2 samples that indicates that seawater intrusion does not play a major role in salinization of groundwater at the sampled locations. Further integrated studies (numerical modeling, chemical/isotopic studies, remote sensing investigations, geophysical studies, and GIS applications) are underway to evaluate the extent and recharge rate of groundwater reservoirs and the groundwater potentialities of the Rub Al Khali.