10:05 AM - 10:20 AM
[AHW30-04] Evolution of shallow urban groundwater chemistry in Meguro-dai, southeastern Musashino Upland, downtown Tokyo, Japan
Keywords:Urban area, Shallow groundwater chemistry, Musashino Upland
A study was conducted to elucidate the evolution of urban shallow groundwater chemistry on the highly urbanized Meguro-dai area (M1b surface by Endo et al., 2019), located in the southeastern part of the Musashino Upland, Shinagawa Ward, Tokyo. The shallow groundwater surveyed in 24 wells of about 10-15 m depth exists in the Musashino Gravel Layer below the less permeable Itabashi Clay Layer. Field investigation and water sampling were conducted from November to December 2023, and water chemistry, δ18O-H2O, δ15N-NO3-, δ18O-NO3- and δ34S were analyzed.
A ternary mixing analysis was carried out based on Cl- concentration and δ18O to estimate the contribution of three sources of water, that is 1) rain water, 2) water-supply leakage, and 3) sewage leakage to shallow groundwater recharge of the study area. The results show that the contribution of sewage leakage varied widely among sites, ranging from 3 to 45%. The relationship between δ15N-NO3- and δ18O-NO3- indicates that denitrification is well underway at most wells, while the relationship between SO42- concentration and δ34S indicates sulfate reduction is occurring at some wells characterized by much lower redox potential. Overall, the water chemistry of urban shallow groundwater in the Meguro-dai area of Shinagawa Ward is determined primarily by the degree of sewage leakage contribution, but secondary reactions such as denitrification and sulfate reduction further change its water chemistry.
A ternary mixing analysis was carried out based on Cl- concentration and δ18O to estimate the contribution of three sources of water, that is 1) rain water, 2) water-supply leakage, and 3) sewage leakage to shallow groundwater recharge of the study area. The results show that the contribution of sewage leakage varied widely among sites, ranging from 3 to 45%. The relationship between δ15N-NO3- and δ18O-NO3- indicates that denitrification is well underway at most wells, while the relationship between SO42- concentration and δ34S indicates sulfate reduction is occurring at some wells characterized by much lower redox potential. Overall, the water chemistry of urban shallow groundwater in the Meguro-dai area of Shinagawa Ward is determined primarily by the degree of sewage leakage contribution, but secondary reactions such as denitrification and sulfate reduction further change its water chemistry.