[A-HW26] Isotope Hydrology 2022

[AHW26-P01] Changes of concentration and sources of nitrate in shallow groundwater in the highly-urbanized Shinagawa area, central Tokyo, Japan.

*Yuki Itoh¹, Masaya Yasuhara², Seongwon Lee³, Takashi Nakamura⁴, Kazuyoshi Asai⁵, Akihiko Inamura⁶ (1.Rissho University graduate School of Geo-environmental Science, 2. Rissho Univ., 3.Department of Environmental Systems, Faculty of Geo-Environmental Science, RISSHO University, 4.International Research Center for River Basin Environment, University of YAMANASHI, 5.Geo Science Laboratory, 6.Geological Survey of Japan, AIST)

Keywords:urban, shallow groundwater, nitrate, sewage leakage, denitrification, mixing analysis

Urban shallow groundwater samples in the highly-urbanized Shinagawa area (Kita-Shinagawa and Minami-Shinagawa) , central Tokyo were collected from 10 wells of less than 10 m deep in February (cold dry season) and July (hot wet season) 2019. For seven groundwater samples from Kita-Shinagawa in February and July, the concentration/ratio is 1.6-34.1 mg/L and 0.0-34.8 mg/L for NO₃^-, 17.4-31.9 mg/L and 15.7-42.3 mg/L for Cl^-, 14.7-24.7 ‰ and 12.3-21.8 ‰ for δ¹⁵N-NO₃^-, and -2.3-12.6 ‰ and 0.8-19.9 ‰ for δ¹⁸O-NO₃^-, respectively. It has been estimated by the authors that, on the basis of a ternary mixing analysis, shallow groundwater with elevated NO₃^-and Cl^-concentrations is contaminated with sewage leaking from damaged sewers. Although the wells are located in close proximity in a small area of about 100 m (E-W) and about 60 m (N-S), shallow groundwater in Kita-Shinagawa showed a wide range of chemical concentrations and stable isotopic ratios, indicating sewage leakage is likely to be a point source of pollution.
On the other hand, for the groundwater samples collected once every two months in Kita-Shinagawa from January 2019 to February 2020, those δ¹⁵N-NO₃^-and δ¹⁸O-NO₃^- values generally plot along the trendline with a slope of 0.5. It is suggested that denitrification occurs and proceeds extensively in the local groundwater environment, although its development is subject to change depending on the season and relative location of each well. A mixing proportion among three sources, that is 1) rainfall infiltration (natural recharge), 2) water supply leakage, and 3) sewage leakage, and the subsequent denitrification is estimated to be responsible for the NO₃^- concentration and its seasonal change in shallow groundwater of the study area.