Subsurface thermal environment and groundwater flow around Tokyo Bay,Japan

Previous studies and borehole temperature measurements suggest that subsurface temperature distribution on the west side of Tokyo Bay (from Tokyo to Yokohama) is higher than that of the east side (Chiba side). To understand the groundwater flow and other factors which may contribute to the subsurface temperature discrepancy such as geological setting in the study area, groundwater temperature profiles were measured in 119 boreholes around the Tokyo Bay from 2002 to 2007. The data were analyzed and compared with previous studies. Horizontal distribution of subsurface temperature at the depths of −50 and −100 m was made to show the distribution of thermal regime. A cross-section across the bay of Tokyo was made to see the isothermal lines and distribution of hydraulic heads in a vertical perspective. These results show that the highest subsurface temperature zone is in the Tokyo area, along the river valley. Subsurface temperature at the depth of 50- and 100-m below sea level in the western part of the bay is comparatively higher than its eastern side at the same elevation and distance from the bay. This fact suggests that there is a regional groundwater flow system in the area and it is strongly affected by the geological structure, particularly buried valley systems of the bay during the Paleo-Tokyo River and the topographical driving force which is the result of the different elevation of recharge areas. Groundwater discharge is concentrated along the buried valley of Paleo-Tokyo River.