[A-CG39] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG39-02] Evaluation of submarine groundwater discharge using ²²²Rn in Tokyo Bay

*Hitomi Matsumoto¹, Yu Umezawa¹, Toshimi Nakajima², Mitsuhiro Ishii³, Tsuyoshi Yoshida⁴, Ryo Sugimoto⁵ (1.Tokyo University of Agriculture and Technology, 2.Atmosphere and Ocean Research Institute, The University of Tokyo, 3.Chiba Prefectural Fisheries Research Center, 4.Chiba Prefectural Environmental Research Center, 5.Fukui Prefectural University)

Keywords:submarine groundwater discharge, radon, coastal seas, seaweed bed

It is important to correctly assess the nutrient source and their fluxes to the ocean in order to evaluate the primary production supporting the food web in coastal waters and to consider factors that cause environmental problems such as red and green tides. Submarine groundwater discharge (SGD) has a variety of impacts as a terrestrial material transport pathway to coastal marine ecosystems. In Tokyo Bay, where groundwater storage in the watershed has been increasing in recent decades, groundwater discharge to the ocean may be increasing, but no SGD studies have been reported. In a previous study, ²²²Rn concentration in groundwater in the coastal area of Tokyo Bay was significantly higher than in river water, confirming that ²²²Rn can be used as a groundwater marker. This study aims to evaluate the distribution of SGD in Tokyo Bay and the factors contributing to its spatio-temporal variability using ²²²Rn.
Surveys were conducted in May and November to measure ²²²Rn concentration, salinity, and temperature in surface water in Tokyo Bay. At the same time, surface water samplings for nutrient analyses were conducted at 17 sites. In Tokyo Bay, the distribution of ²²²Rn concentration differed depending on the season. Assuming that the mixing between offshore water and freshwater (i.e., river water with low ²²²Rn concentration and groundwater with high ²²²Rn concentration) determines the water quality at the monitoring locations, the plot of ²²²Rn concentration against salinity suggested that the contribution of river water was more significant in May and that of groundwater in November. The mud Alluvium layer extends across the seafloor of Tokyo Bay, and the sandy Shimousa layer, which is an aquifer, is present below the Alluvium. The distribution of sand and gravel in the surface sediments of Tokyo Bay coincided with the locations where SGD was suggested, indicating that SGD from the fractures in the Alluvium occurred. Since the characteristics of groundwater differ significantly from river water in the timing of discharge from precipitation, nutrient composition, and concentration, and the amount of suspended solids, the ecological effects of groundwater, such as the distribution of seagrass beds and fish fauna in coastal areas, are the focus of attention.