Identification of groundwater flow systems under intensive anthropogenic disturbances using ³H–¹⁴C constrained mixing ratios and hydrochemical clustering

Study area The study was conducted in the Sanjiang Plain of northeastern China, a large porous aquifer basin that has undergone long-term intensive pumping. Study focus This study aims to improve groundwater flow system delineation in pumping-affected aquifers, where strong vertical mixing between shallow modern water and deep paleowater obscures natural flow-system boundaries. To address this problem, 3 H- 14 C constrained mixing ratios were integrated with hierarchical hydrochemical clustering and applied to the Sanjiang Plain to delineate its hierarchical groundwater flow systems. New hydrological insights for the region The integrated framework reveals that the Sanjiang Plain groundwater system has been restructured from a naturally stratified regime into a vertically mixed one due to intensive pumping. Specifically, modern groundwater has been driven to significantly deeper depths, expanding the vertical extent of local flow systems and blurring their original boundaries with intermediate systems within groundwater depression cones. Although deep aquifers remain dominated by paleowater, localized modern-water intrusion is now detectable in some pumped zones. These findings provide the quantitative evidence that long-term pumping can fundamentally alter the hierarchical architecture of flow systems in a large porous basin, shifting the system toward enhanced vertical connectivity. Consequently, shallow local systems, while still rapidly renewed, are now more vulnerable to downward contaminant transport, whereas deeper intermediate and regional systems, acting as strategic reserves, require stricter protection against induced modern-water intrusion.