Presentation Information
[AOS26-P12]Evaluation of Submarine Groundwater Discharge in Sagami Bay Using 222Rn and Ra Isotopes
*Kodai Miyano1, Yu Umezawa1, Kaori Harashima1, Ryoutarou Ueba1, Kei Yamamoto1, Yuji Takahashi1, Keisuke Umebayashi1, Shinji Shimode2, Yoshiki Takayama2, Kagami Maiko2 (1.Tokyo University of Agriculture and Technology, 2.Yokohama National University)
Keywords:
Submarine Groundwater Discharge,radon,radium,coastal seas
Recent studies have demonstrated that submarine groundwater discharge (SGD) supplies water and nutrients to the ocean in amounts comparable to or exceeding those delivered by rivers. SGD is generally classified into fresh submarine groundwater discharge (FSGD) and recirculated submarine groundwater discharge (RSGD). In Japan, SGD has been investigated in several semi-enclosed bays, including Tokyo Bay, Osaka Bay, Ise Bay, and Obama Bay. In this study, we aim to identify potential SGD in Sagami Bay, characterized by an open coastal system and steep nearshore bathymetry, using the spatio-temporal distributions of Rn and Ra isotopes, which serve as effective tracers of SGD. Field observations were conducted every month from April 2025 to January 2026 (excluding May, July, and November). Surface water samples were collected at six stations along the transect line extending from the mouth of the Sakawa River, eleven coastal stations, and four river stations. In addition, groundwater samples were collected at four sites, and sandy beach porewater samples were collected at three sites. Salinity and temperature were measured simultaneously at all stations. For 222Rn analysis, 1.5 L of seawater and 250 mL of freshwater were collected and analyzed using a radon detector (RAD8). For 223Ra and 224Ra analyses, 100 L of water was filtered through Mn-fiber and measured using a radium delayed coincidence counter (RaDeCC). Nutrient samples were analyzed colorimetrically using an automated nutrient analyzer (AACS II). Analysis of salinity and 222Rn concentration distributions in Sagami Bay indicated that while river water influence was significant in most coastal areas, FSGD contribution was suggested in the northwestern part of Sagami Bay, characterized by extensive mountainous terrain with higher precipitation. This finding was supported by the similarity between the nutrient composition in this area and that of groundwater. Furthermore, analysis of salinity and 224Ra concentration distributions in Sagami Bay suggested the possibility of RSGD contributions in shallow waters. However, compared to Tokyo Bay, the surface seawater in Sagami Bay, which is deeper and an open system, exhibited lower 224Ra concentrations and higher salinity, suggesting that the open ocean strongly influences it. Furthermore, the 223Ra/224Ra ratio in surface seawater was not constant in September, suggesting the possibility of multiple Ra sources, including inflow from Tokyo Bay.