Why has the water quality of Mt. Fuji declined? New strategy for understanding and managing these world 'water towers'

　　Why has the water quality of Mt. Fuji declined?

New strategy for understanding and managing these world 'water towers'

　　Mountains are known as the "water towers" of the world, and mountain rivers are vital water resources for both local residents and downstream populations.

At present, groundwater in mountainous areas has been recognized as an important water resource.

One of the most iconic mountains in the world, including Japan's Mount Fuji, was inscribed on the UNESCO World Heritage List in June 2013.

　　Mount Fuji is known locally as the "Mountain of Water".

For thousands of years, groundwater and icy groundwater springs gushing from the volcano have provided millions of people with safe drinking water, fueled tourism and supplied agricultural production.

In the high-altitude parts of the mountain, abundant precipitation replenishes the aquifers made of rocky basalt on the sides of the mountain, eventually creating numerous freshwater springs in the foothills of Mount Fuji.

　　Current conceptual models consider Mount Fuji to be a simple laminar groundwater flow system with little vertical exchange between the three aquifers.

That is, the spring water of Mt. Fuji is considered to be supplied entirely by shallow underground aquifers near the surface.

However, this model fails to account for Mt. Fuji's complex hydrogeology, as well as the recent decline in water quality, which is thought to be related to groundwater pollution.

　　A recent study published in the international journal Nature-Water by Oliver Schilling of the University of Basel in Switzerland and colleagues challenges this model.

They propose that Mount Fuji's vast network of groundwater and springs that have provided drinking water for thousands of years are fed by deep aquifers.

The findings come from a new hydrological tracing technique, which the researchers believe may help us understand the decline in Mt. Fuji's water quality.

　　In their study, they point out that conventional groundwater level monitoring methods and classical hydrological tracers cannot detect the vertical mixing of groundwater at different depths in Mount Fuji.

To investigate possible vertical mixing, Schilling and colleagues used three unconventional natural tracers, helium, vanadium and environmental DNA (eDNA).

Using these tracer combinations, they found evidence of deep groundwater injection.

　　Schilling et al. propose that the Fujikawa-kako Fault Zone—the most tectonically active structure in Japan—may provide a channel for vertical water flow, based on the helium concentrations found in the spring water.

They suggest that the upwelling of deep groundwater, which has been flowing for a long time, could explain the high vanadium concentrations in the springs.

　　Schilling et al. also showed that the presence of microbial eDNA in Mt. Fuji springs confirmed its deep groundwater origin, because the environmental conditions that allow the growth of microorganisms with this specific DNA have only been found at the extreme depths of Mt. Fuji.

　　Taken together, these findings suggest that deep groundwater fed Mt. Fuji's springs.

　　Lauren Somers of the Department of Civil and Resource Engineering at Dalhousie University in Canada notes in an accompanying News Opinion article that all three tracers in this study have been used before, but, as Schilling and colleagues did, will Together they are new.

　　Somers noted that if each tracer was used in isolation, it would limit the interpretation of its results.

But the helium, vanadium, and eDNA results correlate well at the Mt. Fuji survey site, providing confidence in the authors' interpretation that some atmospheric groundwater gushes from the depths, passing through low-permeability layers between aquifers, contributing to the springs flow.

Somers also pointed out that these findings also demonstrate the shortcomings of traditional hydrogeological methods in some cases.

　　Schilling et al. conclude that understanding these channels and flows in Mt. Fuji can inform the prevention and management of groundwater and spring pollution.

　　Somers agrees that millions of people around the world depend on mountain hydrological systems altered by climate change, but in many regions the inner workings of mountain groundwater systems remain unclear, hampering effective groundwater management.

"Schilling and colleagues' research provides insights and tools to address this challenge."

　　The Paper reporter He Liping