Vertical distribution of groundwater age in and around the freshwater–saltwater transition zone determined using noble gases and carbon isotopes

Abstract

The distribution of groundwater ages in coastal aquifers are complicated because of the heterogeneity of the hydrogeology and the mixing of terrestrial and marine water sources. Therefore, multiple studies verifying the groundwater age distribution via borehole surveys are required to better understand groundwater flow in coastal aquifers. This study examined groundwater age distributions in coastal aquifers adjacent to a high-elevation volcano (Mt. Fuji, Japan) by analyzing geochemical and isotopic tracers retrieved from drilling 350- and 160-m-deep boreholes. Brackish groundwater samples collected from the boreholes had high helium isotope ratios, indicating a large contribution of mantle-derived helium. Groundwater ages determined from carbon and helium isotopes indicate that terrestrial groundwater recharged at high elevations flows from the volcano to the coast, mixing with seawater in a transition zone. The groundwater age at the freshwater–saltwater transition zone and the upper part of the saltwater zone is older than that of the underlying saline water. The distributions of the groundwater age in the study area confirm that the conventional groundwater flow system in coastal aquifers is applicable even in deep aquifers. The complex patterns of groundwater age and mixing derived from geochemical and isotopic tracers can guide sustainable use of groundwater resources and the subsurface.