Geochemical, isotopic, and remote sensing constraints on the origin and evolution of the Rub Al Khali aquifer system, Arabian Peninsula

SummaryChemical and stable isotopic compositions of groundwater samples from the Rub Al Khali (RAK) in southern Saudi Arabia were analyzed. Samples were collected from wells of variable depth (1.5–800 m) along the perimeter of the eastern half of the RAK including flowing artesian wells, pumped wells (formerly artesian), and shallow hand-dug wells encompassing those in sabkha areas. Data indicate that the water from the artesian and formerly artesian wells represents the contents of confined aquifers. Such water (Group 1) is isotopically depleted (δ2H values ranging from −60‰ to −35‰), and has total dissolved solids (TDS) concentrations ranging from 1300 to 76,000 mg/L, indicating that much of the salinity is acquired in the subsurface. Water from shallow hand-dug wells including those in sabkha 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 sabkha salts including halite and gypsum. Stable isotope data for the Group 2 water samples define an evaporation trend line originating from the Group 1 water samples. This relationship indicates that the Group 2-type water evolved from Group 1-type water by ascending through structural discontinuities, dissolving evaporative salts, and undergoing substantial near-surface evaporation in groundwater discharge zones (sabkhas) characterized by shallow groundwater levels (<2 m). This interpretation is supported by the relatively unradiogenic Sr isotope ratios of groundwater samples (Sr87/Sr86 = 0.70771–0.70874) that are inconsistent with that of modern seawater (87Sr/86Sr = 0.70932). The RAK aquifer water represents either high-elevation recharge from the Red Sea Hills, and/or recharge largely formed of paleo-water precipitated during moist climate intervals of the late Pleistocene recharging aquifers cropping out at the foothills of the Red Sea mountains. This inference is supported by a progressive decrease in hydraulic head and increase in groundwater salinity from west to east, substantial precipitation over the Red Sea Hills, and a major E–W trending channel network that channels precipitation from the Red Sea Hills toward recharge areas. Analysis of 3-hourly TRMM (Tropical Rainfall Measuring Mission: 1998–2006) precipitation data and digital elevation data shows that 27% of the average annual precipitation (150 × 109 m3) over the Arabian Peninsula is channeled toward the recharge zone of the RAK aquifer system, of which an estimated 4 × 109 m3 a−1 to 10 × 109 m3 a−1 of this water is partitioned as recharge to the RAK aquifer system. Additional integrated studies on recharge rates, sustainability, and water quality issues for the RAK aquifers could demonstrate that the RAK is one of the most promising sites for groundwater exploration in the Arabian Peninsula. Results highlight the importance of investigating the potential for sustainable exploitation of similar large aquifer systems that were largely recharged in previous wet climatic periods yet are still receiving modest modern meteoric contributions.