[A-HW26_1PO1] Groundwater and environmental geology in urban areas

[AHW26-P02] Source of nitrate in shallow groundwater in the Shakujii river catchment, central Tokyo, Japan

*Takashi NAKAMURA¹, Takeshi HAYASHI², Masaya YASUHARA³ (1.ICRE, University of Yamanashi, 2.Akita University, 3.Geological survey of Japan, AIST)

Keywords:tokyo, urban, groundwater, nitrate nitrogen and oxygen isotopes

Water chemistry of shallow groundwater in the Shakujii river catchment in the downtown Tokyo is discussed with special reference to its nitrate and chloride concentrations. The catchment is divided into the highly urbanized lower reaches (Toshima, Kita and Itabashi Wards) and the upper reaches which have been urbanized to a lesser extent (Nerima Ward, and Nishi-Tokyo and Kodaira Cities). Shallow groundwater samples were collected from 28 wells of less than 10m deep at October 2012 and October 2013. Groundwater aquifer is in the Kanto loam layer and/or underlying stream terrace gravels. The nitrate-nitrogen concentration had wide ranges (from 0.1 to 13.6mg/l). The total coliform was detected from all shallow groundwater samples. The nitrate nitrogen isotope ranges from 5.6 to 12.3 permil, which overlaps fertilized soil and wastewater nitrogen. Moreover, End-member mixing analysis using hydrogen and oxygen isotope values revealed spatial distribution in the contribution ratios of the local precipitation and domestic water (sewage and tap). The concentration of nitrate nitrogen and total coliform was increasing along with contribution ratios of precipitation in shallow groundwater, except some samples that has high nitrogen isotope and chloride concentration. This trend suggests that the nitrate source in this area is not only from sewage leakage. It also needs to consider the loading of the nitrogen fertilizer to shallow groundwater by the precipitation infiltration.