Investigating spatio-temporal variation in submarine groundwater discharge (SGD) on an intertidal beach scale, temperate coastal area

Submarine groundwater discharge (SGD) is defined as subsurface water flow from the seabed to the coastal sea and ocean. Because groundwater often contains higher nutrients than river water, SGD delivers comparatively large quantities of nutrients to coastal ecosystems. However, there are few studies to examine the effect of extreme climatic event such as flooding on the SGD pattern as well as the spatial relation between SGD and coastal ecosystem such as seagrasses and seaweeds. In the present research, we aimed to investigate the spatio-temporal variation in SGD on an intertidal beach scale in a temperate coastal island, western Japan.

The study areas are located on Ikuchijima Island in Seto Inland Sea, southern Japan. The regional climate is mild, with an annual mean precipitation of 1,100 mm and temperature of 15.6 °C. The whole island is characterized by steep slopes and is widely covered by citrus farms with more than 40% of the island. Large part of intertidal beach area are covered by seagrass and seaweed meadows. The previous researches confirmed that SGD can rival or even exceed surface runoff in this island. However, the spatial and temporal variation in SGD on a beach scale was not well examined.

We installed piezometers with about 50 cm deep in a beach located on the south coast of the island, and measured water pressure, water temperature, electric conductivity (EC), and radon (²²²Rn) concentration in pore water in July 2014, July and December 2018, and July 2019. Before the field campaign in July 2018, western Japan including the study area had an extreme rainfall event (> 500mm). The beach has steep backland topography without surface water inflow during the base flow period. Main bottom material is sand, however, the western part of the beach is covered by coarse gravels. Large spatial variation in hydraulic potential, water temperature, EC and ²²²Rn concentration in pore water were confirmed. Lower EC and higher ²²²Rn in pore water suggest that higher rate of fresh-SGD in the western part. In addition, significant lower EC and higher ²²²Rn concentration were confirmed after the rainfall event in July 2018 than that in July 2014. This result suggests fresh-SGD increased after the extreme flood.