Groundwater flow system study in volcanic low permeability bedrock basin

In Japan, the groundwater aquifer for pumping purposes are mostly in alluvium and diluvium deposits and the general concept and methodology for those aquifers has been already established. However, little study has been conducted for the groundwater flow system in relatively low permeability igneous and sedimentary rocks and it was not yet well understood. Although the groundwater of those aquifers is thought to have relatively small flow system in Japan, it should be particularly important for the mountainous small residents as their water resources. Japan's humid climate and high topographical gradient must create active groundwater flow system even in those low permeability aquifers. In this study, a basin scale groundwater flow region including the mountainous ridge to the coastal area within one river-water catchment basin, which is geologically composed by the volcanic lava and tuff-breccia bedrock, was selected to clarify their groundwater flow system. The methods used are both the (environmental isotopes 2H,18O,3H,14C) in the water as a flow tracer and the groundwater potential (long-term monitoring at several observation wells depth of 20-200m). The three-dimensional groundwater flow characteristics in the basin have been studied by using their seasonal changes. Also the obtained hydrological data was compared with the relating regional geological information from the borehole cores and observation of outcrops. As a result, the flow system in this basin has been revealed as the conventional topographical driven system (local flow system, the intermediate flow system and the regional deep flow system). Local flow system is appeared in mountainous head water area and independent from main groundwater body to exist as perched groundwater. The regional deep flow system has a longest residence time. For instance, the one that seeps out in the coastal area in the basin as a subsurface groundwater discharge could be applicable to this system. The intermediate flow system has medium residence time between previous two flow systems, and springs in Furuyashiki area in the mid basin area could be as this system. After the Furuyashiki borehole groundwater potential measurement, it shows the clear evidence of two different groundwater fluxes, one is ascending flux to the shallower aquifer toward the natural springs and another is deeper sub-horizontal flux to the deeper aquifer of the regional scale movement. This flow system trend is in consistent with fissure patterns observed in the borehole cores.