10:45 AM - 12:15 PM
[AHW18-P12] Spatial and depth distribution of radiocesium in near-coastal sediment cores of Fukushima Prefecture
Keywords:Sediment tracing, Heavy rainfall event, Coastal Pacific Ocean
The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident trigged by the Great East Japan Earthquake and subsequent tsunami in March 2011 released significant quantities of radionuclides in terrestrial and marine environments of Fukushima Prefecture. Although radiocesium (i.e. 134Cs and 137Cs) activity in these environments has decreased since the accident, the secondary inputs via the rivers draining and eroding the main terrestrial radioactive plume were shown to sustain high levels of 137Cs in riverine and coastal sediments, which are likely deposited off the coast of the Prefecture. Accordingly, identifying the sources of sediment is required to elucidate the links between terrestrial and marine radiocesium dynamics and to anticipate the fate of persistent radionuclides in the environment.
Our project aims to develop an original fingerprinting technique to quantify the catchment source contributions to coastal sediment and associated radionuclides deposited off the coast of Fukushima Prefecture. Target coastal sediment cores (n=6) with a length comprised between 20 and 60cm depth were collected during cruise campaigns between July and September 2022 at the Ota (n=2), Niida (n=1) and Ukedo (n=3) river mouths. Prior to gamma spectrometry measurements, sediment cores were opened and cut into 2 cm increments, oven-dried at 50°C for at least 48 hours, ground and passed through a 2-mm sieve.
Preliminary results regarding the spatial and depth distribution of radiocesium in these samples show a strong heterogeneity, with highest radiocesium levels (up to 134 ± 2 and 4882 ± 11 Bq kg-1 for 134Cs and 137Cs in 2022, respectively) found in coastal sediment cores located at the Ukedo river mouth. On the opposite, no traces or low levels of Fukushima-derived radiocesium were found in the Niida and in one sediment core of the Ota River mouths. Three sediment cores were therefore selected to implement the fingerprinting technique. Additional measurements will be conducted to determine the physico-chemical properties of sediment (particle size distribution, elemental geochemistry, organic matter and stable isotopes) in order to select the optimal combination of tracers. These tracers will then be introduced into un-mixing models to quantify the source contributions to coastal sediment. This increase knowledge will undoubtedly be useful for watershed and coastal management in the FDNPP post-accidental context.
Our project aims to develop an original fingerprinting technique to quantify the catchment source contributions to coastal sediment and associated radionuclides deposited off the coast of Fukushima Prefecture. Target coastal sediment cores (n=6) with a length comprised between 20 and 60cm depth were collected during cruise campaigns between July and September 2022 at the Ota (n=2), Niida (n=1) and Ukedo (n=3) river mouths. Prior to gamma spectrometry measurements, sediment cores were opened and cut into 2 cm increments, oven-dried at 50°C for at least 48 hours, ground and passed through a 2-mm sieve.
Preliminary results regarding the spatial and depth distribution of radiocesium in these samples show a strong heterogeneity, with highest radiocesium levels (up to 134 ± 2 and 4882 ± 11 Bq kg-1 for 134Cs and 137Cs in 2022, respectively) found in coastal sediment cores located at the Ukedo river mouth. On the opposite, no traces or low levels of Fukushima-derived radiocesium were found in the Niida and in one sediment core of the Ota River mouths. Three sediment cores were therefore selected to implement the fingerprinting technique. Additional measurements will be conducted to determine the physico-chemical properties of sediment (particle size distribution, elemental geochemistry, organic matter and stable isotopes) in order to select the optimal combination of tracers. These tracers will then be introduced into un-mixing models to quantify the source contributions to coastal sediment. This increase knowledge will undoubtedly be useful for watershed and coastal management in the FDNPP post-accidental context.