Geophysical Research Abstracts
Vol. 18, EGU2016-10650-1, 2016
EGU General Assembly 2016
© Author(s) 2016. CC Attribution 3.0 License.

Three-dimensional modeling of oceanic dispersal of land-derived
multi-class suspended radionuclides after the Fukushima Dai-ichi accident
Yusuke Uchiyama (1), Takafumi Yamanishi (1), Daisuke Tsumune (2), Kazuhiro Misumi (2), and Yuichi Onda (3)(1) Kobe University, Japan (uchiyama@harbor.kobe-u.ac.jp), (2) Central Research Institute of Electric Power Industry, Japan,(3) University of Tsukuba, Japan

Several oceanic dispersal modeling have been conducted on dissolved radionuclides leaked from the Fukushima
Dai-ichi Nuclear Power Plant (FNPP) where the direct release of radionuclides from the FNPP and atmosphericdeposition are major sources. In the present study, we view freshwater discharge from the rivers as a missing piece
for the inventory of the radionuclides in the ocean. The land-derived input introduces a time lag behind the direct
release through hydrological process because these radionuclides mostly are attached to suspended particles thatare transported quite differently to the dissolved matter. We thus develop a multi-class, non-cohesive sedimenttransport model based on ROMS along with a wave-enhanced bed boundary layer model of Soulsby (1995) anda stratigraphy submodel proposed by Blaas et al. (2007). Suspended 137 Cs attached to the sediment is evaluatedwith an empirical power law proposed by Onda et al. (2014) as a function of specific surface area of the suspended
particles. A 128 x 256 km domain with the grid resolution of dx = 250 m centered at the FNPP is configured asa test bed nested in the existing double nested ROMS domain with dx = 1 km (Uchiyama et al., 2012, 2013).
The wave field is computed with an operational wave model SWAN (Booij et al., 1999) embedded in the JMA
GVP-CWM wave reanalysis. A total of 20 rivers in the domain are configured as point sources of freshwater andsediments by exploiting a surface runoff model HYDREEMS (Toyoda et al., 2009) and an empirical discharge
to sediment flux relation (Takekawa and Nihei, 2013). Fractions of three sediment classes, viz., fine sand, silt andclay, in the riverine discharge are determined empirically based on the outcome of a USLE-based river sedimentmodeling conducted by JAEA.
The developed model successfully reproduces the dispersal of the land-derived sediments and their recirculation
processes associated with resuspension and deposition in the Fukushima coast for 4 months after the accident.
The discharged sediments can be transported about 50 km from the shore with prominent patchiness of depositionand erosion near the mouth of each river. For instance, the offshore region of the mouth of the Niida River isevaluated to be erosion dominated, consistent with the measurement. Misumi et al. (2014) estimated sedimentary
137
Cs in the bed with a model accounting for static adsorption and desorption of 137 Cs by using dissolved 137 Cs
model result of Tsumune et al. (2012). The inferred bed 137 Cs agrees well with the observation in the shallow area,
whereas substantially underestimated in the offshore area at depth deeper than 200 m. They attributed the reason of
the underestimate to the 137 Cs due to sediment transport that is omitted in their model. We carefully diagnose ourmodel results and find that although the clay-class sediments reach the deeper area, the time-integrated deposition
has a minor contribution. It is therefore suggested that detritus and debris of organic matters rather than landderived minerals likely cause the offshore 137 Cs deposition.