Understanding the paleohydrological setting of the Arabian Peninsula: An integrated approach

Abstract:

The Rub Al Khali (RAK), encompasses one of the World's largest (area: 650,000 km²) aquifer systems, the Rub Al Khali Aquifer System (RAKAS) of the Arabian Peninsula. In this study, we develop and apply an integrated and cost-effective approach, involving geophysical, geochemical, isotopic, field, and remote sensing investigations to understand the origin, evolution, and the factors controlling the paleo and current distribution of the RAKAS groundwater. Paleohydrological setting of the RAK is investigated by exploring the spatial relations between plaeo-drainage, paleo-lakes, faults, and sapping features. The following investigations were accomplished: (1) groundwater samples were collected from wells in the western portion of the Rub Al Khali and analyzed for their solute chemistry, stable isotopic composition (O, H), and Cl-36 and C-14 ages; (2) paleo-topography (prior to dune encroachment) was extracted using elevations within inter-dunal areas; (3) drainage systems were extracted using standard stream delineation techniques; (4) a regional mosaic of dual polarized (HH,HV) ALOS PALSAR data (10 m resolution) was created covering entire Arabian Peninsula; (5) paleo-drainage systems across the RAK were mapped using PALSAR data and SRTM-derived networks; and (6) Landsat 8 imagery and Digital Elevation Models (DEM) were used to map the distribution of paleo-lakes and to delineate sapping features. Findings include: (1) presence of relatively fresh (TDS ranging from 800 - 2,800 mg/L in deeper wells) suitable for drinking and/or irrigation; (2) major ions are Na, Ca, Cl, SO₄ and HCO₃; (3) radiocarbon model ages range from 1,400 to 30,600 years before present, but conservative Cl-36 model ages indicate much higher residence time (up to 750,000 years or more); (4) Cl-36 ages and stable isotopic compositions indicate that groundwater recharge in previous wet climatic periods; (5) features with stubby looking geometry, theater-like heads, U-shaped profile, flat floors and structurally controlled patterns indicative of sapping processes were mapped along scarps; (6) landscape analysis indicate onset of stream networks from mapped sapping features along escarpments suggesting groundwater discharge; and (7) large paleodrainage patterns indicate a general runoff direction from west to east.