The Relationship between a Distribution of Submarine Groundwater Discharge and a Local-scale Coastal Geology and Topography in Northern Japan

Abstract:

Submarine groundwater discharge (SGD) has been recognized as an important pathway for material transport to the marine environment. Submarine fresh groundwater discharge will occur wherever an aquifer is hydraulically connected with the sea and the water table is above sea level. The driving force behind this process is the hydraulic gradient from the upland region of a watershed to the surface water discharge location at the coast. Permeability also affects the rate of recharge into an aquifer and discharge into a sea. In the present study, we thus evaluated the SGD impact on the two locations in eastern (Yuza) and western (Otsuchi) side of the northern Japan to clarify the relationship between the coastal distribution of SGD and the local-scale coastal geology and topography. We applied ²²²Rn monitoring survey with the dual-loop system (Dimova et al. 2009) to assess the local-scale impact of SGD. In the Yuza area, abundant spring discharges are present around the coast at the terminus of volcanic lava flows. We conducted the continuous ²²²Rn monitoring at boat speeds of < 2 knots along the coast for a few kilometers scale assessment. Moreover, at the two contrastive specific sites (Kamaiso beach and Shonai dune) where are different properties of geology and topography, the continuous ²²²Rn monitoring using a rubber boat at speeds of 1 m per 20 seconds was done for a few hundred meters scale assessment. As a result, ²²²Rn activity clearly showed the higher impact of SGD on the Kamaiso beach compared to the Shonai dune. In the Otsuchi area, on the other hand, ²²²Rn monitoring survey was also done at boat speeds of < 2 knots in Otsuchi Bay and Funakoshi Bay in same tidal conditions (2 hours before lowest tide), because there are negligible influence of tidal changes on ²²²Rn concentrations in the Pacific coast of Japan. ²²²Rn concentrations in Otsuchi Bay were significantly higher than those in Funakoshi Bay. Although the Otsuchi Bay and Funakoshi Bay is adjacent embayment, larger watershed and steeper hydraulic gradient in Otsuchi Bay would be induced the stronger influence of SGD compared to Funakoshi Bay.