Subsurface temperature profiles measured in boreholes can be analyzed to infer depths of thermal disturbance and associated surface warming due to combined global warming and urbanization (heat island effects). Asian cities are extremely vulnerable because of rapid increases in population. Average subsurface temperature profiles in four Asian cities (Tokyo, Osaka, Seoul, and Bangkok) were compared and analyzed to evaluate the effects of surface warming. The magnitude of surface warming is largest in Tokyo (2.8°C), followed by Seoul (2.5°C), Osaka (2.2°C), and Bangkok (1.8°C). Comparisons between analytical solutions and observations show that the mean depth of deviation from the regional geothermal gradient in each urban area may be one of the indicators of the history of urbanization in each city. The mean depth of deviation from the steady thermal gradient, which is approximately 140 m in Tokyo, 80 m in Osaka, and 50 m in Seoul and Bangkok, indicates the time from the start of the additional heat from urbanization. These results agree qualitatively with air temperature records in the cities during the last 100 yr. The heat island effect on subsurface temperature is an important global groundwater quality issue because it may alter the groundwater systems chemically and microbiologically. Measurement of subsurface temperature data provides important information for understanding the joint effects of urbanization and global warming on groundwater systems.

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