Groundwater flow system as determined by multi-tracer approach in Tokyo Metropolitan area, Japan

We performed an intensive monitoring and sampling of groundwater in Tokyo Metropolitan area to investigate the groundwater flow system from upland to lowland areas. We took 83 groundwater samples at 39 locations, 25 stream water samples, 7 spring water samples from August 2018 through June 2021. We also observed the spatial distribution of hydraulic head in the groundwater. The stable isotopic composition of oxygen 18 and deuterium, SF₆ concentration, and solute ions concentrations were determined on all water samples.

The SF₆ age of groundwater in the upland area ranges from a few years to 40 years, whereas that in the lowland area ranges from 40 years to more than 80 years. The solute concentrations are characterized by Ca-HCO3 type in the upland, whereas that is categorized in Na-HCO3 or Na-Cl type. In addition, d¹⁸O of the groundwater in the upland ranges from -10.4 per mil to -8.8 per mil, whereas that in the lowland ranges from -9 per mil to -8 per mil.

The hydraulic head distribution shows that the unconfined groundwater flows from west to east directions in parallel with the topographical surface, and the confined groundwater flows from south-west toward north-east directions in parallel with the bedrock surface topography.

The results show that the groundwater flows from west toward east directions across the border of the upland and the lowland, and it flows across the boundary of the aquifers, meaning the unconfined groundwater recharges the confined groundwater in an area where a certain amount of unconfined groundwater is pumped up.