Water isotope evidence for inter-catchment groundwater flow across drainage divides in faulted and volcanic terrains of central Japan

Catchments are commonly treated as hydrologically closed systems; however, subsurface flow across topographic watershed boundaries can occur in tectonically complex regions. In this study, tracer methods including stable water isotopes were used to investigate potential inter-catchment groundwater contributions in faulted and volcanic terrains of central Japan.

The study area covers the Miya River and Kamanashi River catchments, where drainage divides are ambiguous and major fault zones run subparallel to river channels. River water and spring water were sampled, and water chemistry and oxygen/hydrogen stable isotope ratios were analyzed.

A local meteoric water line was made using precipitation collected for 5 years, providing a regional isotopic reference. All samples plotted near the meteoric water line, indicating a meteoric origin; however, systematic spatial differences were observed. Overall, isotope ratios increased in the order of spring water, Kamanashi River, and Miya River. Tributaries of the Miya River showed distinct isotopic clustering depending on their source mountains: tributaries originating from the Akaishi Mountains plotted along the meteoric water line, whereas those from the Yatsugatake volcanic area formed a linear trend slightly offset from the meteoric water line. In contrast, both the main stream and tributaries of the Kamanashi River consistently plotted on the meteoric water line, regardless of source area.

Along the Miya River main stream, upstream sites reflected isotopic signatures of local tributaries, while downstream sites showed a shift toward meteoric-line values that cannot be explained solely by mixing of Miya River tributaries. This interpretation is supported by total dissolved solids (TDS) data: tributaries from the Yatsugatake area exhibited higher TDS, whereas the Miya River main stream showed lower values than expected from tributary contributions alone. Given that the Kamanashi River catchment is characterized by generally lower TDS, these combined isotopic and geochemical patterns suggest subsurface water contributions from the Kamanashi River catchment to the Miya River across the drainage divide.