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

•81Kr dating provides new insight on recharge dynamics and groundwater flow pattern.•81Kr negates the former 14C-dating misconception of homogeneous age distribution.•Active recharge windows are identified in a predominantly confined desert aquifer.•Age stratification is demonstrated in confined and phreatic sections of the aquifer.•Combining 14C and 81Kr data enables the reconstruction of paleo and recent recharge.