Observations on Groundwater Level Change at Depths below 300 m in a Borehole Penetrating through the Futagawa Fault

<p>The main borehole of Futagawa fault scientific drilling （FDB borehole） penetrated through the strata older than the deep groundwater aquifer called second aquifer in the Kumamoto area. In this study, to reveal the great-depth groundwater dynamics deeper than the second aquifer, we report results from observations on groundwater level change at depths below 300 m in the FDB borehole for more than 3 years. The observed groundwater level was affected by rainfall throughout the year, and indicated seasonal changes with greatly different patterns depend on the seasons. Observations on the groundwater level change and precipitation revealed that the groundwater level change in the borehole and groundwater supply from the Aso caldera rim mountains which are located in upper reaches of main recharge area in the Kumamoto area are related, suggesting the existence of response of groundwater level to rainfall at depths below 300 m. The patterns of groundwater level change in the FDB borehole and pre-existing observation wells in the plateau at the western foot of the Aso caldera rim mountains for the second aquifer located along the Futagawa fault are very similar, and no clear time lag was observed. These results indicate that the second aquifer, which is mainly composed of Aso-1 to Aso-3 pyroclastic flow deposits widely distributed in the Kumamoto area, may continue to older sedimentary rocks strata and pre-Aso volcanic rocks at depths below 300 m. Taking existing information of core descriptions and image logs into consideration, we suggested that the Futagawa fault damage zone contributes for downward extension of the second aquifer, and for the active groundwater flow systems in the Kumamoto area.</p>