Identifying recharge processes into a vast “fossil” aquifer based on dynamic groundwater ⁸¹Kr age evolution

Water in deep aquifers in arid regions is often considered to be “fossil” when modern recharge rates are negligible relative to the reservoir capacity. Over the past five decades, the Nubian Sandstone Aquifer (NSA) in the arid region of the Sinai Peninsula (Egypt) and the Negev Desert (Israel) has been considered to contain fossil water based on ¹⁴C dating, which revealed ¹⁴C ages of about 30 kyr over most of the aquifer. However, this relatively homogeneous age distribution contradicts the expected increase in groundwater age in the direction of decreasing piezometric head along the flow trajectories. Here, dating results with the longer-lived ⁸¹Kr radioisotope (t_1/2 = 229 ± 11 kyr) are presented, highlighting a wide age range of 40 kyr to 630 kyr in the confined sections of the aquifer, all with very low ¹⁴C activity (<1 pmC). Elevated ⁸¹Kr and ¹⁴C activities were only observed within or close to the system's recharge areas. These findings support a new perception of groundwater replenishment during different epochs from the early mid-Pleistocene to the Holocene. By tracking the downstream age evolution, rejuvenation was identified in places where the confinement had been breached. At other locations, the existence of an older groundwater body contributing to the aquifer was detected by means of strongly depleted ⁸¹Kr activity. High spatial heterogeneity in groundwater ages close to the discharge zone of the system is attributed to pronounced age stratification with depth. Calculated ages in the more isolated sections of the system were used to assess regional flow velocity, hydraulic conductivity, and their agreement with present recharge rates. We conclude that groundwater ages should be reevaluated with ⁸¹Kr in regional aquifers where low ¹⁴C activities prevail. With an effective age range beyond one million years, this may enable the reconstruction of recharge history well into the Pleistocene and provide crucial information for the management of groundwater resources.