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[AHW31-P05] Quick response of urban shallow groundwater level to a rainfall: Focusing on urban groundwater recharge process
Keywords:Urban area, Shallow groundwater, Water level , Rainfall
We are currently conducting a study of shallow groundwater in the Kita-Shinagawa and Minami-Shinagawa areas of Shinagawa Ward, Tokyo, to elucidate the origin, water chemistry evolution, and abundance of urban shallow groundwater. As a part of this research, we have installed data loggers for water level measurement at two wells in the Kita-Shinagawa area and one well in the Minami-Shinagawa area since June 2024, and have been conducting continuous observation of groundwater levels in order to understand the seasonal changes in shallow groundwater levels and their response to rainfall. This report describes the results obtained mainly at Well N2 in the Kita-Shinagawa area.
Well N2 is a tube well with a depth of 9.1 m, a diameter of 12 cm, and an unknown screen depth. The geology consists of sand, silty sand, and sandy silt. A data logger was installed in the groundwater at a depth of 3.0 m below the ground surface, and observations were made at 10-minute intervals. The results of the six-month observation revealed that the shallow groundwater table at Well N2 responded quickly to rainfall. For example, on August 16, 2024, 1.0 mm of rain fell between 10:50 and 11:00 a.m., followed by 9.0 mm of rain between 11:00 and 11:10 a.m. In response to this rainfall, the groundwater table rose by 0.45 m from 1.16 m below the ground surface before the rainfall (10:50 a.m.) to 0.71 m at 11:10 a.m. The time lag between the rainfall peak and the groundwater level was less than 10 minutes. Similar phenomena were observed at Site N2 during another rainfall events with higher rainfall intensities.
At this time, we estimate that this rapid rise in the groundwater table may be caused by underground leakage of rainwater from drainage pipes. Rainwater flows into the sewer pipes (located 0.9-1.0 m below the ground surface around Well N2) via drainage pipes, but these drainage pipes are likely to be damaged due to aging. It is considered that the groundwater table rises quickly in response to rainfall because the rainwater component that flows out from the damaged parts of the drainage pipes directly recharges the groundwater. Considering the high surface coverage rate of Shinagawa Ward (82.2% in 2022) and the low permeability of the surface layer due to compaction, it is unlikely that rainwater infiltrating and percolating vertically through the ground surface would recharge groundwater in such a short period of time.
This study was supported by the Sasakawa Scientific Research Grant from the Japan Science Society.
Well N2 is a tube well with a depth of 9.1 m, a diameter of 12 cm, and an unknown screen depth. The geology consists of sand, silty sand, and sandy silt. A data logger was installed in the groundwater at a depth of 3.0 m below the ground surface, and observations were made at 10-minute intervals. The results of the six-month observation revealed that the shallow groundwater table at Well N2 responded quickly to rainfall. For example, on August 16, 2024, 1.0 mm of rain fell between 10:50 and 11:00 a.m., followed by 9.0 mm of rain between 11:00 and 11:10 a.m. In response to this rainfall, the groundwater table rose by 0.45 m from 1.16 m below the ground surface before the rainfall (10:50 a.m.) to 0.71 m at 11:10 a.m. The time lag between the rainfall peak and the groundwater level was less than 10 minutes. Similar phenomena were observed at Site N2 during another rainfall events with higher rainfall intensities.
At this time, we estimate that this rapid rise in the groundwater table may be caused by underground leakage of rainwater from drainage pipes. Rainwater flows into the sewer pipes (located 0.9-1.0 m below the ground surface around Well N2) via drainage pipes, but these drainage pipes are likely to be damaged due to aging. It is considered that the groundwater table rises quickly in response to rainfall because the rainwater component that flows out from the damaged parts of the drainage pipes directly recharges the groundwater. Considering the high surface coverage rate of Shinagawa Ward (82.2% in 2022) and the low permeability of the surface layer due to compaction, it is unlikely that rainwater infiltrating and percolating vertically through the ground surface would recharge groundwater in such a short period of time.
This study was supported by the Sasakawa Scientific Research Grant from the Japan Science Society.