From water-rock interactions to flow rates: insights from 234U/238U activity ratios and REE patterns of the ancient Kurnub Group Aquifer

• A novel single-column method for separation of U and REE from brackish groundwater. • Observed high 234 U/ 238 U activity ratios show systematic decrease downflow. • Radioactive decay downflow enables the estimation of long-term average flow rate to 0.29 m yr −1 . • Preferential leaching during fluid interactions with fractured rocks led to 234 U enrichment. Measurements of uranium isotopes and Rare Earth Element (REE) compositions were used to evaluate water–rock interaction processes, mixing, and flow rates in the Kurnub Group Aquifer (KGA), a deep confined regional sandstone aquifer extending from the Sinai Peninsula (Egypt) to the Negev Desert (Israel). The study involved developing a methodology that allowed to separate both U and REE from the brackish groundwater using a single column. Groundwater was sampled along two main flow paths of the KGA: East (n = 9) and North (n = 8), and from the overlying Judea Group Aquifer (JGA; n = 2). Overall, samples along the two KGA flow paths show consistent low REE concentrations (ppt levels) and patterns, with low LREE and HREE and high MREE. A systematic decrease in 234 U/ 238 U Activity Ratio (AR) downflow is observed along two main trend lines, from 8.37 upstream to 1.41 (non-linear) and to 3.72 (linear), with remarkable correlation to the distance from the Dead Sea outlets (R 2 = 0.98). These trends suggest that uranium tracer is controlled by processes during the travel time downflow with potential relation to residence time and flow rates. Assuming minimal water–rock interactions downflow and a decrease in AR solely due to radioactive decay, long-term average flow rate of 0.29 m·yr −1 is assessed, similar to recently obtained 81 Kr-based age estimates. The exceptionally high AR is observed near a major fault zone, suggesting that the highly enriched 234 U may originate from this deep-seated active deformation zone, whereby fluid interactions with fractured rocks lead to the preferential leaching of the 234 U isotope.