Isotopic Tracers (Ra, Sr, O, H) for Elucidating the Sources of High Naturally Occurring Radioactive Groundwater from the Disi Aquifer, Jordan

The Nubian sandstones basins of the Middle East and northeastern Africa host several important aquifers with high-quality, yet fossil, groundwater that is becoming an essential water source in the region. In Jordan, fossil groundwater from the Paleozoic Disi (Saq) aquifer is part of a new large-scale water project in which water will be transferred to the capital Amman to compensate water shortage in Jordan. Here, we investigate the geochemical (major and trace elements), radium, strontium, oxygen, and hydrogen isotope compositions of groundwater from the Disi aquifer in southern Jordan. We have identified three groundwater types: (1) low-saline from the unconfined Rum Group; (2) low-saline from the confined Rum Group; and (3) saline from the Khreim Group. The Khreim saline water has a distinguished high ⁸⁷Sr/⁸⁶Sr (0.70117), low Ra content, and low δ¹⁸O values relative to the Rum aquifer (0.70854). This suggests negligible leakage between the aquifer systems. The Ra activities in the low-saline groundwater from the Rum Group are high and largely exceed international drinking water standards. The ²²⁸Ra/²²⁶Ra ratios in groundwater from the unconfined (1.8) and confined zones (2.8) are higher than sandstone rocks (~1.5), while their ²²⁴Ra/²²³Ra ratios (32-25) are consistent with theoretical ²²⁴Ra/²²³Ra ratios ratios emitted by recoil from sandstone rocks. The high ratios of the short-lived ²²⁴Ra and ²²³Ra to the long-lived ²²⁶Ra and ²²⁸Ra isotopes indicate that Ra mobilization is controlled by recoil process from the aquifer solids combined with rapid Ra adsorption. We propose that contents of efficient radium scavenging minerals (e.g., clays, Fe- and Mn-oxides) in sandstone aquifers would determine the Ra retardation and thus the Ra activity levels in groundwater. These findings could have implications for high radium anomalies in low-saline groundwater from similar Nubian Sandstone basins in the region and exacerbate the water crisis in the Middle East.