[A-HW27] Water Environment and Geology in Urban Areas

[AHW27-P05] Long-term subsurface temperature changes in an urban coastal area: Observed subsurface warming in the Tokyo waterfront landfill area

*Akinobu Miyakoshi¹, Takeshi Hayashi² (1.Geological Survey of Japan, AIST, 2.Faculty of Education and Human Studies, Akita University)

Keywords:subsurface temperature, groundwater flow, urban coastal area, Tokyo waterfront landfill area, subsurface warming

Subsurface warming has been recognized in urban and suburban areas, in the Kanto Plain, where the Tokyo metropolitan area is located (Miyakoshi et al., 2019). To understand subsurface warming and their formation mechanisms based on analysis of observation data, researchers have repeatedly measured subsurface temperature profiles at land subsidence and groundwater observation wells managed mainly by local governments in the Tokyo metropolitan area since 2000. Additionally, subsurface temperature monitoring at observation wells in Tokyo and Saitama prefecture have been conducted since 2009. These observations indicate that the subsurface temperature has been continuously increasing at many sites, mainly in the shallow depressions of the subsurface. There were clear regional differences in the distribution of subsurface temperature and its temporal change. The temperature in the urban area was considerably higher, and the warming rate was higher than that in the suburban area. Subsurface warming in the Tokyo metropolitan area is thought to be caused by the combined effects of land use changes, especially the temperature increases at the surface and shallow subsurface due to urbanization, regional groundwater flow affected by groundwater development, and global warming (Miyakoshi et al., 2019).
The Port of Tokyo and its surrounding waterfront area, located deep within the Tokyo Bay area, is suitable for evaluating the effects of surface warming because the area has been reclaimed for development, and groundwater pumping is strictly regulated. In this study, we discuss the subsurface warming observed in the long-term data and the estimated formation factors at the observation well in the Odaiba area in the Tokyo waterfront landfill area. The temperature-depth profile shows a peculiar variation with the highest value at a depth of 18 m. The temperatures decreased with increasing depth to a depth of 60m at the bottom of the well. Since 2009, a continuous temperature increase has been observed at every depth, except for the seasonal changes in the shallow subsurface. However, the warming rate is not uniform, and the trend is greater at depths shallower than 40 m. A warming rate, at approximately 18 m deep was especially significant, with the temperature increasing by more than 1.5 ℃ from 2009 to 2021.
Subsurface warming was mainly observed in the clay layers of the alluvium. At 18 m, the highest temperature in the profile corresponds to the boundary between the sandy layer in the alluvium and the clay layer which is thick. The groundwater flow is relatively slow, and heat advection is small in this clay layer. Therefore, subsurface warming is thought to be caused by heat conduction, and waste heat from underground structures acts as a heat source. Contrarily, the subsurface temperature change of the sand and gravel layer in the Tokyo Formation at a depth of approximately 40 m shows the effects of large-scale underground construction in the surrounding area. The results of this study demonstrate that subsurface warming in urban areas can be greatly affected by global warming and warming brought about by subsurface development for urbanization such as the construction of underground structures.