[A-HW17] 水循環・水環境

[AHW17-P03] N₂Oの空間・時系列変化およびその大気寄与の評価

*有冨 大樹¹、小野寺 真一¹、齋藤 光代²、大西 晃輝³、丸山 豊¹ (1.広島大学大学院総合科学研究科、2.岡山大学大学院環境生命科学研究科、3.（株）復建調査設計)

In order to evaluate the spatial and temporal variation of N₂O and associated flux into the air in a granite unconfined aquifer of Ikuchi Island, water samples were collected from 9 observation wells with different depths and 6 observation wells in the groundwater discharge area from 2013 to 2015 and analyzed for N₂O, NO₃^--N and Cl^-. The results showed that the concentrations of dissolved N₂O changed with water depth, which can be attributed to the C/N ratio. When the C/N ratio ≤5, high concentrations of dissolved N₂O occur. In addition, the N₂O concentrations increased with the redox condition of water changes from oxidation to reductive. However, when water was in strong reductive conditions such as ORP ≤-200mV, the relative low concentrations of N₂O took place, since N₂O can change into N₂ due to the complete denitrification. Dissolved N₂O concentrations also increased in dry seasons, when most of observation wells being in a reductive state. In the groundwater discharge area, dissolved N₂O and NO₃^--N concentrations decreased along the groundwater flow pathway, which results from the dilution of seawater and denitrification. The flux of N₂O into air was estimated to be 49gha^-1year^-1, the same level as seawater emission rate. Whereas, in a high precipitation event(precipitation ≤ 30mm/day), 5gha^-1day^-1 (about 10 percent of annual N₂O emission) would emission into air.