What are the main sources of Fukushima-derived radionuclides to the ocean off Japan five years later?

Sanial, V.*; Buesseler, K. O.*; Charette, M.*; Casacuberta, N.*; Castrillejo, M.*; Henderson, P.*; Juan Diaz, X.*; Kanda, Jota*; Masque, P.*; Nagao, Seiya*; Nishikawa, Jun*; Otosaka, Shigeyoshi  ; Pike, S.*

Radiocesium activities in the coastal ocean off Fukushima dropped by orders of magnitude within one year after the accident of Fukushima Dai-ichi Nuclear Power Plant (FDNPP), but have remained relatively constant over the past 5 years exceeding background values. We investigated distribution of radiocesium in surface seawater and submarine groundwater along the Fukushima coast in October 2015. Our highest radiocesium activities were not found in the ocean, but in groundwater samples underlying coastal beaches 35 km south from the FDNPP. This may be due to sorption of the extremely contaminated waters on to beach sands/clays early after the accident and subsequent desorption back in to the ocean. Submarine groundwater discharge, which is widely recognized to be an important vector for the transport of chemicals from land to ocean, is thus a non-negligible path for transport of Fukushima-derived radionuclides to the ocean.