From water-rock interactions to flow rates: insights from234U/238U activity ratios and REE patterns of the ancient Kurnub Group Aquifer

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 234U/238U 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 (R2 = 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 81Kr-based age estimates. The exceptionally high AR is observed near a major fault zone, suggesting that the highly enriched 234U may originate from this deep-seated active deformation zone, whereby fluid interactions with fractured rocks lead to the preferential leaching of the 234U isotope.