5:15 PM - 6:30 PM
[ACG40-P03] Spatial variation of ²²²Rn concentration in river water and groundwater in the coastal area of Tokyo Bay
Keywords:²²²Rn, groundwater, coastal area
It is important to correctly assess the source and amount of freshwater-derived nitrogen and phosphorus that enter from the land to shallow coastal areas. However, it is not possible to distinguish the contributions of groundwater and river water based only on salinity changes, and the impact of groundwater inflow on shallow coastal ecosystems has not been evaluated in the Tokyo Bay. Therefore, we aimed to (1) investigate the spatial distribution of 222Rn in groundwater and river water in the coastal area of Tokyo Bay, and to explore the factors causing its variation and (2) confirm the significant difference in 222Rn concentration between river water and groundwater in the coastal area, and to show the effectiveness of 222Rn as an indicator of groundwater discharge to Tokyo Bay. Groundwater, spring water, and river water were collected from September 2020 to January 2021 in the coastal area of Tokyo Bay from Miura Peninsula to Boso Peninsula. 222Rn concentrations were analyzed using an radon detector based on electrostatic collection of alpha-emitters (RAD7), and general water quality was analyzed using ion chromatography. 222Rn concentration in groundwater (dpm/L) was 43-477 in the innermost part of the bay, 9-1480 in the western part of the bay, and 134-386 in the eastern part of the bay. The concentrations of 222Rn in river water (dpm/L) were 24-30 in the innermost part of the bay, 10-62 in the western part of the bay, and 21-50 in the eastern part of the bay. In all areas, the difference in ²²²Rn concentrations between groundwater and river water was about one to two orders. The factor causing the difference in 222Rn concentration in each location will be discussed in the viewpoint of geology, topography, groundwater residence time, soil porosity/crack, and underground redox state.