Abstract. A huge amount of groundwater is stored in the subsurface environment of Mt. Fuji, the largest volcanic mountain in Japan. Based on the concept of piston flow transport, residence time of stored groundwater at Mt. Fuji was estimated at ~ 15–30 years by the ³⁶Cl / Cl ratio (Tosaki et al., 2011). This range, however, represents the average residence time of groundwater that was mixed before it flushed out. To elucidate the route of groundwater in a given system, we determined signatures of direct impacts of rainfall on groundwater, using microbial, and stable isotopic (delta ¹⁸O), and chemical analyses (concentration of silica). Chemical analysis of the groundwater gave an average value of the water, which was already mixed with waters from various sources and routes in the subsurface environment. The microbial analysis suggested locations of water origin and paths.

In situ observation during four rainfall events revealed that the stable oxygen isotopic signature of spring water and shallow groundwater obtained at 726 m a.s.l. (site G1), where the average recharge height from rainfall was 1500–1800 m, became greater than values observed prior to a torrential rain producing more than 300 mm of precipitation. The concentration of silica decreased after this event. In addition, the density of Bacteria in spring water increased, suggesting the influence of the heavy rain. Such changes did not appear when rainfall was less than 100 mm per event. The above findings indicate a rapid flow of rain through the shallow part of the aquifer, which appeared within a few weeks of the torrential rain in the studied geologic setting. Interestingly, we found that after the torrential rain, the density of Archaea increased in the deep groundwater at site G3, ~ 12 km downstream of G1. However, chemical parameters did not show any change after the event. This suggests that strengthened piston flow caused by the heavy rain transported archaeal particles from the geologic layer along the groundwater route. This finding was supported by changes in constituents of Archaea, dominated by Halobacteriales and Methanobacteriales, which were not seen from other observations. Those two groups of Archaea are believed to be relatively tightly embedded in the geologic layer and were extracted from the environment to the examined groundwater through enforced piston flow. Microbial DNA can thus give information about the groundwater route, which is never shown by analysis of chemical materials dissolved in the groundwater.