9:30 AM - 9:45 AM
[ACG40-03] Characteristics of submarine groundwater discharge (SGD) in inner Tokyo Bay using radioisotopes
Keywords:Submerged Groundwater Discharge, Tokyo Bay, Radioisotopes, Residenve time
Introduction.
Submarine Groundwater Discharge (SGD) has been suggested to be essential for transporting terrestrial materials to the ocean. SGD is divided into two main categories: fresh submarine groundwater discharge (FSGD) and recirculated submarine groundwater discharge (RSGD), which is salty water discharged into the water column after intruding the seafloor sediments by tidal pumping and so on. It is difficult to evaluate the ratio of SGD to freshwater discharge because it has large spatio-temporal variations that cannot be visually detected due to its upwelling below the surface of the ocean. Even in the Tokyo Bay, the contribution of groundwater to freshwater inflow has not been clarified. In this study, 222Rn (t1/2 = 3.84 days), which is 10-1000 times more abundant in groundwater (regardless of freshwater or seawater) than in seawater, and Ra isotopes (223Ra; t1/2 = 11.4 days, 224Ra; t1/2 = 3.66 days, and 226Ra; t1/2 = 1600 years), the objective of this study was to evaluate the spatial distribution of SGD in the inner part of Tokyo Bay and the residence time according to the location.
Materials and Methods
Seawater samplings were conducted six times from December 2021 to November 2022 (December 2021, February, May, August, September, and November 2022) at four sites (off the mouth of Tama River: F3, off the mouth of Edo River: CB, off the mouth of Arakawa River: AR, in front of Aomi container pier: AO) to measure Rn and Ra concentrations. In addition, Rn was analyzed along the coastal line from Miura Peninsula to Cape Futtsu in November 2022.
Results & Discussion
Plotting 222Rn concentration and salinity in surface seawater at each study site and comparing the y-intercept value of the approximation straight line with representative 222Rn values of nearby river water and groundwater indicated that the influence of river water was dominant off the mouth of the Tama River. The influence of river water and FSGD was also suggested off the mouth of the Arakawa River, and the presence of FSGD was also suggested in the bottom layer off the mouth of the Edogawa River. Assuming that salinity and Ra isotope concentrations were determined by simple mixing of offshore seawater and river water, there was no excess Ra (i.e., no presence of RSGD) off the mouth of the Tama River. On the other hand, excess Ra was observed off the mouth of the Edogawa River and the Arakawa River, suggesting the presence of RSGD. At our presentation, we will discuss how regional differences in freshwater runoff characteristics, including differences in seawater residence time inferred from 224Ra/223Ra and 226Ra, affect the marine ecosystem in the inner part of Tokyo Bay.
Submarine Groundwater Discharge (SGD) has been suggested to be essential for transporting terrestrial materials to the ocean. SGD is divided into two main categories: fresh submarine groundwater discharge (FSGD) and recirculated submarine groundwater discharge (RSGD), which is salty water discharged into the water column after intruding the seafloor sediments by tidal pumping and so on. It is difficult to evaluate the ratio of SGD to freshwater discharge because it has large spatio-temporal variations that cannot be visually detected due to its upwelling below the surface of the ocean. Even in the Tokyo Bay, the contribution of groundwater to freshwater inflow has not been clarified. In this study, 222Rn (t1/2 = 3.84 days), which is 10-1000 times more abundant in groundwater (regardless of freshwater or seawater) than in seawater, and Ra isotopes (223Ra; t1/2 = 11.4 days, 224Ra; t1/2 = 3.66 days, and 226Ra; t1/2 = 1600 years), the objective of this study was to evaluate the spatial distribution of SGD in the inner part of Tokyo Bay and the residence time according to the location.
Materials and Methods
Seawater samplings were conducted six times from December 2021 to November 2022 (December 2021, February, May, August, September, and November 2022) at four sites (off the mouth of Tama River: F3, off the mouth of Edo River: CB, off the mouth of Arakawa River: AR, in front of Aomi container pier: AO) to measure Rn and Ra concentrations. In addition, Rn was analyzed along the coastal line from Miura Peninsula to Cape Futtsu in November 2022.
Results & Discussion
Plotting 222Rn concentration and salinity in surface seawater at each study site and comparing the y-intercept value of the approximation straight line with representative 222Rn values of nearby river water and groundwater indicated that the influence of river water was dominant off the mouth of the Tama River. The influence of river water and FSGD was also suggested off the mouth of the Arakawa River, and the presence of FSGD was also suggested in the bottom layer off the mouth of the Edogawa River. Assuming that salinity and Ra isotope concentrations were determined by simple mixing of offshore seawater and river water, there was no excess Ra (i.e., no presence of RSGD) off the mouth of the Tama River. On the other hand, excess Ra was observed off the mouth of the Edogawa River and the Arakawa River, suggesting the presence of RSGD. At our presentation, we will discuss how regional differences in freshwater runoff characteristics, including differences in seawater residence time inferred from 224Ra/223Ra and 226Ra, affect the marine ecosystem in the inner part of Tokyo Bay.