Old water, rapid responses: Process-based hydrogeological domains reveal hydraulic–age decoupling and implications for recharge assessment in a complex aquifer system of the Heuningnes Catchment, South Africa

Groundwater-level fluctuations are commonly used as recharge indicators because groundwater-level rises are often assumed to represent additions of recently recharged water to aquifer storage. In heterogeneous, multilayer aquifer systems, this assumption remains uncertain because groundwater-level variations may also reflect hydraulic connectivity, pressure transmission, storage redistribution, and groundwater mixing. Misinterpretation of groundwater-level behavior may consequently bias recharge estimates and groundwater-resource assessments. This study evaluated relationships among groundwater-level behavior, groundwater residence times, and hydrogeological process domains within the Heuningnes Catchment, South Africa. Groundwater hydrographs, tritium-derived groundwater ages, and a domain-based conceptual framework were integrated to investigate spatial variability in groundwater flow and recharge processes. Piston-flow (PF) apparent ages ranged from 0 to 60 years, whereas exponential–piston flow model (EPM)-derived mean transit times (MTTs) ranged from 2 to 80–100 years across hydrogeological domains. Recharge-dominated systems showed strong seasonal groundwater-level responses despite the coexistence of young and broad groundwater-age distributions and highly variable hydrograph responses, reflecting mixed recharge and storage influences. Structurally controlled systems showed rapid hydraulic responses despite prolonged groundwater residence times, suggesting pressure transmission independent of groundwater transport times. Residence-time-dominated systems were characterized by older groundwater and subdued hydrograph responses associated with prolonged storage. Groundwater-level response rate showed a weak and statistically non-significant relationship with MTT (Pearson's r = − 0.307, p = 0.460), demonstrating hydraulic–age decoupling and indicating that recharge assessments based solely on groundwater-level fluctuations may substantially misrepresent groundwater recharge processes in structurally complex multilayer aquifer systems.