Revisiting Mt Fuji's groundwater origins with helium, vanadium and environmental DNA tracers

Known locally as the water mountain, for millennia Japan's iconic Mt Fuji has provided safe drinking water to millions via a vast network of groundwater and springs. The prevailing conceptual model assumes simple, largely laminar groundwater flow through three basaltic aquifers with negligible vertical exchange. This view contrasts with Fuji’s tectonic instability at a unique continental triple junction, complex geology and unusual microbial spring communities. Using a unique combination of microbial environmental DNA (eDNA), vanadium (V) and helium (He) tracers, the study reveals prevailing deep circulation and a previously unknown deep groundwater contribution to Fuji's freshwater springs, with the strongest deep upwelling along the tectonically active Fujikawa-kako Fault Zone (FKFZ). Findings broaden hydrogeological understanding of Fuji and demonstrate the potential of combining eDNA, on-site noble gas and trace element analyses for groundwater science.