[A-HW26] Isotope Hydrology 2022

[AHW26-P03] A geochemical study on the shallow groundwater system in the Arakawa alluvial fan, Saitama, Japan

Fuyuki Ishizawa¹, *Masaya Yasuhara², Seongwon Lee², Yuki Itoh³, Takashi Nakamura⁴ (1.Fuji Yakuhin Co., Ltd., 2.Rissho Univ., 3.Graduate School, Rissho Univ., 4.Yamanashi Univ.)

Keywords:alluvial fan , shallow groundwater, nitrogen isotope, livestock industry, chemical fertilizer, pickle manufacturing industry

In May and from July to September 2021, a groundwater survey which targets a shallow aquifer was carried out in the Kushibiki terrace in the Arakawa alluvial fan of about 10 km long and 8 km wide, Saitama, to assess the status quo of groundwater quality and hydrogeochemical processes responsible for spatial changes in water chemistry. Groundwater samples from fifteen wells of 5-10 m deep and two springs were analyzed for water chemistry and δ¹⁵N-NO₃ and δ¹⁸O-NO₃. Groundwater in agricultural field including horticulture is rich in NO₃^- and SO₄^2-, which is estimated to be of chemical fertilizer origin (ammonium sulfate). Significant nitrate contamination of groundwater was observed in the middle parts of the fan where intensive livestock farming is being carried out. In the former case, groundwater is characterized by lower δ¹⁵N-NO₃ of 7-8‰ or less_, whereas the latter by higher values of more than 10‰. It was also found that many pickle manufacturing factories are responsible for Na⁺ and Cl^- rich groundwater in the middle parts of the fan. Groundwater in the urban center of Fukaya City, sprawled in the lower parts of the fan, is likely to be affected by sewage leakage, which is supported by the relatively high NO₃^- and Cl^- concentrations and higher δ¹⁵N-NO₃.