5:15 PM - 6:30 PM
[AHW23-P09] Inorganic nitrogen concentrations and nitrogen and oxygen isotope ratios of nitrate in shallow groundwater in an urban area, Shinagawa, Tokyo, Japan
Keywords:densely-populated urban area, shallow groundwater, inorganic nitrogen concentration, nitrogen and oxygen isotopes, mixing, denitrification
A geochemical study on shallow groundwater of the densely-populated Kitashinagawa district in Shinagawa City, in the heart of Tokyo was carried out from January 2019 through February 2020, when water samples were collected from 7 shallow wells once every two months. In Kitashinagawa district, despite quite a small area of about 60 m (N-S) by 100 m (E-W), its shallow groundwater represents a wide range of nitrate (NO3- ) concentration: for example1.6-34.1 mg/L NO3- in February 2019 (dry season) and 0.0-34.8 mg/L NO3- in July 2019 (wet season). Nitrogen (d15N) and oxygen (d18O) isotope ratios of nitrate also show a large range of values, about 10-25‰ and 2-20‰ for d15N and d18O, respectively. For many wells, the ratio of enrichment of oxygen to nitrogen is found close to 1:2 for d18O and d15N plots, and on d15N vs. ln [NO3-N] plots they yield a straight line, which can be explained using the Rayleigh equation and is indicative of the occurrence and progress of denitrification. In contrast, some wells fall on a straight line for d15N vs. 1/[NO3-N] plots, indicating mixtures of two or three sources (water bodies). An overall conclusion is then that nitrate in shallow groundwater in the study area is primarily of sewage leakage origin (Itoh et al., 2020), but its concentration is subject to alternation to a large extent by the denitrification and/or by the mixing with other water bodies such as rainfall infiltration (natural recharge) and water supply leakage with different nitrogen characteristics.