Basin-wide groundwater flow study in a volcanic low permeability bedrock aquifer with coastal submarine groundwater discharge

The purpose of this study is to reveal the basin-wide groundwater flow system in a Quaternary pyroclastic bedrock aquifer by using several hydrological methods: catchment hydrometric observations, environmental isotope study of the spring water and observation borehole levels including inland, on-shore and offshore boreholes, basin-wide groundwater potential monitoring, geophysical methods to understand the aquifer distribution, water balance for the representative paired catchments in the study basin including micro-meteorological evapotranspiration measurements, direct submarine groundwater discharge (SGD) measurements by automatic seepage meters, and three-dimensional groundwater flow simulations based on observed hydrological data. The results clearly show that topographically driven groundwater flow systems with different flow dynamics and residence times exist in the study catchments and strongly support the hydrological characteristics of local springs and a river discharge system including coastal SGD. Also, a stagnant fresh groundwater system exists under the present sea bed which is completely separate from the land-based groundwater flow systems, and is thought to be a kind of remnant palaeo groundwater recharged during the previous regression era. This has no direct relation to the present SGD in the area.