Urbanization and human activities in the Tokyo metropolitan area have induced various changes of groundwater environment, such as groundwater flow regime and subsurface temperature distribution. Additionally, subsurface temperatures have been affected by surface warming caused by not only urbanization but also global warming. Thus, it is essentially required to clarify the mechanism and process of changes of subsurface thermal distribution and to evaluate the present situation of that in this area for sustainable management of groundwater environment.
We have conducted repeated measurements of temperature-depth profiles at observation wells which are for groundwater levels and land subsidence to grasp the three-dimensional distribution of the subsurface temperature and its secular change since 2000s. We also have carried out subsurface temperature monitoring using high resolution and accuracy temperature loggers (resolution: 0.001K) for two or more years to find the subsurface temperature change in detail. In addition, the groundwater flow regime has been evaluated based on distribution of hydraulic heads obtained by the observation wells.
Secular subsurface warming was found at shallow depths in the whole study area, that is, not only in urban area but also suburban areas. Warming trends in the urban area which were estimated by repeated measurements of temperature-depth profiles were higher than that in the suburban areas. The warming trends obtained at each well were decreased in accordance with depth. These tendencies suggest that the subsurface warming has been caused by the effects of both urbanization and global warming. As for the effect of urbanization, subsurface warming has been induced by the heat from not only the ground surface but also the subsurface constructions such as basements and subway.
Subsurface temperature beneath the urban center was particularly high even in the deep part. On the other hand, in the suburban areas, the subsurface warming in groundwater development areas were recognized at the relatively deep part, even though the effects of surface warming were relatively small. The results of the monitoring showed continuous warming tendency beneath both the urban center and the suburbs. However, the warming at 40m deep at the urban center showed fluctuations in the interval of few months. This result was considered to be induced by heat supply from the subsurface constructions. Differences of the tendencies between the urban center and the suburbs suggest the possibility to separate the effects of the urbanization and the climate change due to global warming in the Tokyo metropolitan area.