[A-HW23] Isotope Hydrology 2021

[AHW23-P08] An isotopic study on the role of groundwater in streamflow generation in the concrete-lined Nomi-gawa river, Ohta Ward, Tokyo, Japan

*Masaya Yasuhara¹, Seongwon Lee¹, Yuki Itoh², Akihiko Inamura³, Noritoshi Morikawa³, Takashi Nakamura⁴ (1.Rissho University, 2.Saitama Agricultural Cooperatives, 3.Geological Survey of Japan, AIST, 4.University of Yamanashi)

Keywords: urban river, concrete-lined channel, groundwater seepage, streamflow generation, oxygen isotope of water

A role of groundwater in streamflow generation in a concrete-lined urban river was studied for the days of no rainfall from July 2015 to September 2019 in the Nomi-gawa river, Ohta Ward, Tokyo, Japan. Although the entire surface of side walls and riverbed is covered with concrete, groundwater seeps out through discharge pipes on the concrete walls and gaps in the riverbed concrete plates, making a considerable contribution in the river runoff formation. A two-component mixing analysis on the basis of oxygen isotope ratio of water made it clear that this kind of groundwater seepage accounts for in the range between 20% and 30% for the wet season and about 10% for the dry season, of the discharge in the Nomi-gawa river. These estimates are in accordance with those in our previous report (Fujioka et al., 2018) based on electric conductivity of water. The difference of groundwater contribution in the formation of river runoff can be attributed to the different groundwater levels in the vicinity of the river for the wet summer and dry winter seasons.