10:10 AM - 10:25 AM
[AHW23-09] Nitrate ion concentration and nitrogen isotope ratio of shallow groundwater in Kitakarasuyama, Setagaya, Tokyo
Keywords:Urban shallow groundwater, Musashino Plateau, Nitrate ion, Nitrogen isotope, Origins of groundwater
A study is underway in Kitakarasuyama of Setagaya, Tokyo, to understand the current status of urban shallow groundwater quality, the water quality evolution process, and the origin of groundwater. This report presents the results of a survey conducted in March 2022 (dry season) and October 2022 (wet season) at 17 shallow wells located within an area measuring approximately 500 m east-west and 400 m north-south in Kitakarasuyama.
In the four wells where water-level measurements were possible, groundwater table was 1.7-2.4 m and 1.0-1.3 m below the ground surface in March and October 2022, respectively. Almost all of the groundwater compositions were of the Na (Mg, Ca)-SO4-NO3 or Ca-SO4 type for both seasons. In March 2022, during the dry season, Cl-, NO3-, and SO42- were 3.2-7.6 mg/L, 0.7-28.6 mg/L, and 2.8-38.5 mg/L, respectively, while in October 2022, during the wet season, they were 3.4-29.6 mg/L, 0.8-156.9 mg/L, and 156.9 mg/L, respectively. The pH showed a clear decrease from 6.1-7.7 in March to 3.9-6.5 in October.
The role of sewage leakage as the origin of urban shallow groundwater is recently drawing much attention (e.g., Ito et al., 2023). Based on the above-mentioned groundwater chemistry, nitrogen isotopic ratio (e.g., 6.3-17.7 ‰ in March) and oxygen isotopic ratio (e.g., -2.3-20.1 ‰ in March) of NO3-, and the relationship between NO3- concentration and nitrogen isotopic ratio with water supply leakage, rainfall infiltration, and sewage leakage as three end members, the considerable contribution of sewage leakage is evident for groundwater in some wells in Kitakarasuyama. In addition, the occurrence and progression of denitrification are suggested at some locations. In contrast, the contribution of water supply leakage and rainfall infiltration as the origin of groundwater is predominant at many sites. In other words, the relative roles of water supply leakage, rainfall infiltration, and sewage leakage in groundwater recharge, and the occurrence and progression of denitrification are considered to determine the groundwater quality at each site. We plan to further elucidate the shallow groundwater system in the area based on multiple tracers, including measurement of sulfur isotope ratios of SO42- and groundwater residence time based on CFC concentrations.
In the four wells where water-level measurements were possible, groundwater table was 1.7-2.4 m and 1.0-1.3 m below the ground surface in March and October 2022, respectively. Almost all of the groundwater compositions were of the Na (Mg, Ca)-SO4-NO3 or Ca-SO4 type for both seasons. In March 2022, during the dry season, Cl-, NO3-, and SO42- were 3.2-7.6 mg/L, 0.7-28.6 mg/L, and 2.8-38.5 mg/L, respectively, while in October 2022, during the wet season, they were 3.4-29.6 mg/L, 0.8-156.9 mg/L, and 156.9 mg/L, respectively. The pH showed a clear decrease from 6.1-7.7 in March to 3.9-6.5 in October.
The role of sewage leakage as the origin of urban shallow groundwater is recently drawing much attention (e.g., Ito et al., 2023). Based on the above-mentioned groundwater chemistry, nitrogen isotopic ratio (e.g., 6.3-17.7 ‰ in March) and oxygen isotopic ratio (e.g., -2.3-20.1 ‰ in March) of NO3-, and the relationship between NO3- concentration and nitrogen isotopic ratio with water supply leakage, rainfall infiltration, and sewage leakage as three end members, the considerable contribution of sewage leakage is evident for groundwater in some wells in Kitakarasuyama. In addition, the occurrence and progression of denitrification are suggested at some locations. In contrast, the contribution of water supply leakage and rainfall infiltration as the origin of groundwater is predominant at many sites. In other words, the relative roles of water supply leakage, rainfall infiltration, and sewage leakage in groundwater recharge, and the occurrence and progression of denitrification are considered to determine the groundwater quality at each site. We plan to further elucidate the shallow groundwater system in the area based on multiple tracers, including measurement of sulfur isotope ratios of SO42- and groundwater residence time based on CFC concentrations.