[A-OS16] 沿岸域の海洋循環と物質循環

[AOS16-P07] 利根川沖陸棚域への地下水・間隙水起源物質の輸送

*中島 壽視¹、古市 尚基²、増永 英治³、杉本 亮⁴、梅澤 有⁵、乙坂 重嘉¹、伊藤 幸彦¹ (1.東京大学大気海洋研究所、2.水産研究・教育機構 水産技術研究所、3.茨城大学、4.福井県立大学、5.東京農工大学)

キーワード：地下水流入、陸棚域、ラジウム同位体、砂浜海岸、利根川

Advective water transport processes across the sediment-water interface, such as fresh groundwater discharge and seawater circulation in sediments, have been increasingly recognized as a substantial source of nutrients to the coastal seas with a focus on nearshore ecosystems. Although these processes may also play an important role as a pathway of nutrients to the shelf environments, the contribution of groundwater on water and nutrient dynamics in shelf regions has not well been ascertained. The region off the Tone River, facing the Pacific Ocean, is characterized by the inflow from the Tone River and extensive open sandy beaches extending to the north and south. Because the seawater circulation process is mainly driven by tides and waves, the seawater circulation process might be promoted in the nearshore sediment of the open coasts and may be an important source of nutrients in the region off the Tone River. However, the contribution of groundwater and/or porewater on the dynamics of water and nutrients in the region has not been evaluated. Here, we carried out two oceanic observations aboard the R/V Taka-maru in July and September 2023. Samples of surface seawater were taken for analysis of Ra isotopes (as a tracer of saline groundwater) and nutrient concentrations. Our observation stations radially extend offshore from the Tone River mouth, with distances of approximately 1-21 km from the shore. Groundwater and river water samples as endmembers were also taken in the intertidal zone of sandy beaches (Kashinamada and Kujukuri Coasts) and two rivers (the Tone and Hitachi-Tone Rivers), respectively.
²²³Ra (t_1/2 = 11.4 d) and ²²⁴Ra (t_1/2 = 3.66 d) concentrations of seawater ranged from 0.01 to 0.92 dpm 100 L⁻¹, and from 0.21 to 12.67 dpm 100 L⁻¹, respectively. Both Ra concentrations showed high values near the shore and decreased with distance to the offshore, indicating that Ra isotopes were supplied from nearshore sediments and these concentrations decreased through decay and mixing with offshore seawater. The mixing diagram of salinity and each Ra concentration showed that Ra concentrations of seawaters were higher than the conservative mixing line between river water and offshore seawater. Considering that the Ra concentration of groundwater (4.1–13.2 dpm 100 L⁻¹ for ²²³Ra and 66.9–219.9 dpm 100 L⁻¹ for ²²⁴Ra) was 1-2 orders of magnitude higher than that of seawaters, this result suggests that groundwater and porewater inputs into the region off the Tone River. The presentation will show the effects of groundwater/porewater inputs on water and nutrient dynamics.