Dissolved helium distribution in deep groundwaters from the Tono area, central Japan: a tool for tracing groundwater flow in fractured granite

Abstract

The helium isotopic composition of groundwater collected from boreholes was measured in the Tono area from up to 840 m underground. The 3He/4He and 4He/20Ne ratios ranged from 9.1 × 10−8 to 1.48 × 10−6 and from 0.22 to 190, respectively, implying that the groundwater accumulates significant amounts of radiogenic 4He as it flows. In groundwater from sedimentary rocks, the levels of accumulated helium increase with depth, whereas those in the samples from fractured granite vary from borehole to borehole. Considering the hydrogeological and topographical model together, dissolved He concentrations in groundwater indicate the groundwater flow direction. Although the groundwater flow in fractured granite is generally thought to be controlled by the fracture pattern, the overall flow of the Tono area inferred from helium isotopes is also controlled by topography.