Groundwater Contamination by Nitrate Ions in the Highly Urbanized Shinagawa Area, Central Tokyo, Japan

Shallow groundwater in the densely populated Shinagawa area (Kita-Shinagawa and MinamiShinagawa), central Tokyo, was sampled from 10 shallow wells (less than 12 m deep) in February (cold dry season) and July (hot wet season) 2019. The concentrations in seven groundwater samples from Kita-Shinagawa in February and July were 1.6-34.1 mg/L and not detected -34.8 mg/L for NO3-, and 17.4-31.9 mg/L and 15.7-42.3 mg/L for Cl-. The measured isotopic ratios were 11.9-23.8 parts per thousand and 12.3-21.8 parts per thousand for delta 15N-NO3-, and 5.1-11.8 parts per thousand and 0.8-19.9 parts per thousand for delta 18O-NO3-, respectively. Shallow groundwater with elevated NO3- and Cl- concentrations was probably contaminated by sewage leaking from damaged sewers. Although the wells are near each other 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 as a source of groundwater contamination. Among groundwater samples collected once every two months in Kita-Shinagawa from January 2019 to February 2020, delta 15N-NO3- and delta 18ONO3- values generally plot along the trendline with a slope of 0.5. This indicates that the study area had suitable conditions for denitrification to occur, although the degree of denitrification depended on the season and relative location of each well in the study area. The observed NO3concentration and seasonal variability in the shallow groundwater were attributed to the mixing of three groundwater sources: 1) rainfall infiltration (natural recharge), 2) water-supply leakage, and 3) sewage leakage, with subsequent denitrification.