9:15 AM - 9:30 AM
[ACG53-02] Evaluation of recirculated submarine groundwater discharge using short-lived Ra in Tokyo Bay
Keywords:recirculated saline groundwater discharge, radium, coastal seas
Recent studies have reported that submarine groundwater discharge (SGD) supplies water and nutrients to the ocean in quantities comparable to or even exceeding those of rivers globally. Among the components of SGD, the saline fraction, known as recirculated saline groundwater discharge (RSGD), primarily delivers nutrients regenerated from organic matter within sediments to the water column. In Japan, research along the Sanriku Coast has shown that RSGD plays a significant role in nutrient transport from land to the ocean. However, in Tokyo Bay, studies on terrestrial nutrient transport have primarily focused on riverine inputs, with a few reports on fresh submarine groundwater discharge (FSGD) based on radon (Rn) and radium (Ra) concentrations limited to the innermost part of the bay. This study aims to identify RSGD discharge locations in Tokyo Bay by analyzing the distribution of Ra concentrations, which serve as an effective tracer for RSGD, to better understand its contribution.
From October to December 2024, surface water samples were collected from 11 locations in the inner bay and 2 locations in the outer bay of Tokyo Bay. Additionally, bottom water samples were collected from 2 locations in the innermost part of the bay. At all sampling sites, salinity and water temperature were measured simultaneously with water collection. The collected samples were filtered into Mn-fibers and analyzed for Ra concentrations in the laboratory.
In the surface layer of the Edogawa River estuary, the measured 224Ra values were close to those expected from the simple mixing of river water and seawater (hereafter referred to as "simple river-seawater mixing"). However, in the bottom layer, the values were significantly higher than those predicted by simple river-seawater mixing, consistent with previous studies. Although previous research suggested the possibility of RSGD discharge in this area, weather and ocean conditions may influence the extent of RSGD and its spread to the surface layer. A decrease in salinity was observed in the surface layer off the Tama River estuary at a depth of about 26 m, and the 224Ra values closely matched those estimated from simple river-seawater mixing. Similar to previous studies, the results suggest that river water strongly influences this region. Near Futtsu, a water mass with high 224Ra values was identified, that distinctly differed from the water masses in the outer bay, which had f high-salinity and low-224Ra, and river water, which had low-salinity and low-224Ra concentrations. Since the bottom sediment near Futtsu is highly permeable sand, the possibility of RSGD was suggested.
From October to December 2024, surface water samples were collected from 11 locations in the inner bay and 2 locations in the outer bay of Tokyo Bay. Additionally, bottom water samples were collected from 2 locations in the innermost part of the bay. At all sampling sites, salinity and water temperature were measured simultaneously with water collection. The collected samples were filtered into Mn-fibers and analyzed for Ra concentrations in the laboratory.
In the surface layer of the Edogawa River estuary, the measured 224Ra values were close to those expected from the simple mixing of river water and seawater (hereafter referred to as "simple river-seawater mixing"). However, in the bottom layer, the values were significantly higher than those predicted by simple river-seawater mixing, consistent with previous studies. Although previous research suggested the possibility of RSGD discharge in this area, weather and ocean conditions may influence the extent of RSGD and its spread to the surface layer. A decrease in salinity was observed in the surface layer off the Tama River estuary at a depth of about 26 m, and the 224Ra values closely matched those estimated from simple river-seawater mixing. Similar to previous studies, the results suggest that river water strongly influences this region. Near Futtsu, a water mass with high 224Ra values was identified, that distinctly differed from the water masses in the outer bay, which had f high-salinity and low-224Ra, and river water, which had low-salinity and low-224Ra concentrations. Since the bottom sediment near Futtsu is highly permeable sand, the possibility of RSGD was suggested.