Environment: Mount Fuji’s springs flow from deep underground

Mount Fuji’s vast network of groundwater and freshwater springs, which has provided drinking water for millennia, is fed by deep aquifers, according to a study in Nature Water. These findings come from new hydrological tracer techniques and may aid our understanding of declining water quality from the mountain.

Japan’s Mount Fuji has provided water to millions of people, earning it the local title “water mountain”. Fuji’s springs were believed to be fed exclusively from shallow groundwater aquifers close to the Earth’s surface. However, this model failed to explain Fuji’s complex hydrogeology and a recent decline in water quality, believed to be linked to groundwater pollution.

Conventional methods for groundwater level monitoring and classic hydrological tracers (substances added to — or naturally present in — groundwater to observe flows and mixing) could not detect vertical mixing of groundwater from different depths in Mt Fuji. To investigate potential vertical mixing, Oliver Schilling and colleagues used three unconventional natural tracers: helium, vanadium and environmental DNA (eDNA). Using a combination of these tracers, the authors present evidence of deep groundwater contributions. The authors propose that the Fujikawa-kako Fault Zone, Japan’s tectonically most active structure, may provide a pathway for vertical water flow, according to helium concentrations found in springs. Elevated vanadium concentrations in springs may be explained by upwelling of deep groundwater with a long flow time, they suggest. The authors indicate that the presence of microbial eDNA in Fuji’s springs confirms deep groundwater contributions, as the environmental conditions that allow microbes with this particular DNA to develop have so far only been found at great depths in Mount Fuji.

The findings demonstrate the contribution of deep groundwater to Fuji’s springs. Understanding these pathways and flows can inform prevention and management of groundwater and spring contamination, the authors conclude.