[A-HW27] Water Environment and Geology in Urban Areas

[AHW27-P02] Urban groundwater quality in Kitakarasuyama, Setagaya, Tokyo, Japan

Fukumi Mano¹, *Masaya Yasuhara², Seongwon Lee² (1.Atox, Co., Ltd., 2.Rissho Univ.)

Keywords:urban shallow groundwater, perched water, nitrate ion, chloride ion, sulphate ion, nitrogen isotope

In September 2020, a groundwater survey which targets a perched water aquifer was carried out in the highly-urbanized Kitakarasuyama area, Setagaya, Tokyo to assess the status quo of groundwater quality and hydrogeochemical processes responsible for both secular and spatial changes in water chemistry. Seventeen shallow wells of less than about 5-6 m deep are located in close proximity in a small area of 400 m (N-S) and 500 m (E-W) . Groundwater is characterized by the unique water quality of Na-SO₄·NO₃ type, Ca ·Mg-SO₄·NO₃ type and Ca-SO₄ type and its concentration showed a wide range of values in 2.9-13.7 mg/L for Cl^-, 0.6-68.9 mg/L for NO₃^- and 2.7-38.7 mg/L for SO₄^2-. These results strongly suggest that leakage from wastewater systems is a possible source of the local groundwater contamination. Taking account of high SO₄^2-concentration of Tokyo tap water of up to 40 mg/L, leaky water supply systems are likely to be responsible for an elevated SO₄^2-concentration of perched water in the study area. Furthermore, the presence of residual fertilizer, which was once used in agricultural field and has still been stored in the present-day surface soils, may also be a potential contributor of NO₃^- and SO₄^2- to groundwater.