Radiocesium transfer from hillslopes to the Pacific Ocean after the Fukushima Nuclear Power Plant accident: A review

Abstract

The devastating tsunami triggered by the Great East Japan Earthquake on March 11, 2011 inundated the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) resulting in a loss of cooling and a series of explosions releasing the largest quantity of radioactive material into the atmosphere since the Chernobyl nuclear accident. Although 80% of the radionuclides from this accidental release were transported over the Pacific Ocean, 20% were deposited over Japanese coastal catchments that are subject to frequent typhoons. Among the radioisotopes released during the FDNPP accident, radiocesium ((134)Cs and (137)Cs) is considered the most serious current and future health risk for the local population. The goal of this review is to synthesize research relevant to the transfer of FDNPP derived radiocesium from hillslopes to the Pacific Ocean. After radiocesium fallout deposition on vegetation and soils, the contamination may remain stored in forest canopies, in vegetative litter on the ground, or in the soil. Once radiocesium contacts soil, it is quickly and almost irreversibly bound to fine soil particles. The kinetic energy of raindrops instigates the displacement of soil particles, and their bound radiocesium, which may be mobilized and transported with overland flow. Soil erosion is one of the main processes transferring particle-bound radiocesium from hillslopes through rivers and streams, and ultimately to the Pacific Ocean. Accordingly this review will summarize results regarding the fundamental processes and dynamics that govern radiocesium transfer from hillslopes to the Pacific Ocean published in the literature within the first four years after the FDNPP accident. The majority of radiocesium is reported to be transported in the particulate fraction, attached to fine particles. The contribution of the dissolved fraction to radiocesium migration is only relevant in base flows and is hypothesized to decline over time. Owing to the hydro-meteorological context of the Fukushima region, the most significant transfer of particulate-bound radiocesium occurs during major rainfall and runoff events (e.g. typhoons and spring snowmelt). There may be radiocesium storage within catchments in forests, floodplains and even within hillslopes that may be remobilized and contaminate downstream areas, even areas that did not receive fallout or may have been decontaminated. Overall this review demonstrates that characterizing the different mechanisms and factors driving radiocesium transfer is important. In particular, the review determined that quantifying the remaining catchment radiocesium inventory allows for a relative comparison of radiocesium transfer research from hillslope to catchment scales. Further, owing to the variety of mechanisms and factors, a transdisciplinary approach is required involving geomorphologists, hydrologists, soil and forestry scientists, and mathematical modellers to comprehensively quantify radiocesium transfers and dynamics. Characterizing radiocesium transfers from hillslopes to the Pacific Ocean is necess...

Unexpected source of Fukushima-derived radiocesium to the coastal ocean of Japan

“…Therefore, assuming that the flushing rate of the harbor has remained constant, the present-day FDNPP harbor flux should be ∼0.6 TBq·y −1 . Another ongoing source of FDNPP-derived 137 Cs is from rivers, with release estimates for total 137 Cs ranging from 2 to 12 TBq·y −1 (28)(29)(30)(31). Typhoons and heavy rain events have been shown to increase the river runoff flux; the 137 Cs input from this source is largely in the particulate phase (32), of which only a small amount is capable of entering the dissolved phase via desorption in the estuarine mixing zone (19).…”

Fukushima Daiichi–Derived Radionuclides in the Ocean: Transport, Fate, and Impacts

“…Radiocesium inputs to Japanese coastal waters also occur via riverine sources and surface water runoff (Chartin et al 2013, Nagao et al 2013, Evrard et al 2015. Cs has a high affinity for particles in freshwater, and thus its transport and delivery via rivers are associated largely with high sediment loads that occur during heavy rains and flood events.…”

“…In a review of Cs transfer from land to the ocean, Evrard et al (2015) summarized modeling simulations indicating that up to 10-12 TBq of particle-associated 137 Cs was transferred from land to the ocean after the initial release, specifically during the first year. This would correspond to less than 1-2% of the total inventory deposited on land.…”

“…Zheng et al (2013) summarized studies related to the release of Pu from the FDNPPs. Ratios of FDNPP-derived 137 Cs to 239,240 Pu are 1 million or greater, based on an examination of leaf litter and soils in which isotopic ratios of Pu were used to distinguish weapons-testing-derived Pu from FDNPP-derived Pu (Zheng et al 2012, Evrard et al 2015. In total, only a relatively small amount of Pu, on the order of 1.0-2.4 × 10 9 Bq of 239,240 Pu, was released into the environment from the damaged FDNPP reactors (Zheng et al 2012).…”

Do forests represent a long-term source of contaminated particulate matter in the Fukushima Prefecture?

“…The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident deposited a significant quantity of radionuclides over Japanese soils (Chino et al, 2011;Evrard et al, 2015;Gro€ ell et al, 2014). Of these radionuclides, radiocesium represents the most serious threat for the foreseeable future (Kitamura et al, 2014;Saito and Onda, 2015).…”