Groundwater flow and storage properties in weathered/fractured hard rock aquifers vary greatly, affecting groundwater pathways and residence times. By analyzing clay mineralogy and geophysical data, researchers found that these properties differ significantly laterally and with depth. Models incorporating this heterogeneity showed deeper, longer groundwater flow paths and higher age mixing, supported by tritium data. Areas with extensive weathering/fracturing act as drainage structures for younger groundwater, impacting groundwater resilience to climate extremes and pollution.