17:15 〜 18:45
[AHW22-P18] Quantifying the catchment sources of sediment and associated radiocesium deposited off the coast of Fukushima Prefecture
キーワード：North Pacific Ocean, Sediment Tracing, Heavy rainfall event, Energy dispersive X-ray fluorescence
The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) incident, triggered by the Great East Japan Earthquake and subsequent tsunami in March 2011, resulted in the release of significant quantities of radionuclides into the terrestrial and marine environments of Fukushima Prefecture. While the activity of radiocesium (i.e., 134Cs and 137Cs) in these environments has decreased since the accident, continuous inputs through rivers draining and eroding the main terrestrial radioactive plume have been measured, sustaining elevated levels of 137Cs in riverine and coastal sediment off the Prefecture's coast. Consequently, 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 study aims to implement the fingerprinting technique (or sediment tracing) for quantifying catchment source contributions to coastal sediment and the associated radionuclides deposited off the coast of Fukushima Prefecture.
Six targeted coastal sediment cores, ranging from 20 to 60cm in 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 and analyzed by gamma spectrometry. Preliminary findings concerning the spatial and depth distribution of radiocesium in these samples reveal a strong heterogeneity, with the highest radiocesium levels (up to 134 ± 2 and 4882 ± 11 Bq kg-1 for 134Cs and 137Cs, respectively) observed in coastal sediment cores situated at the Ukedo river mouth. A strong relationship is observed between the particle size measured in these sediment cores and radiocesium activities, with higher levels observed in finer sediment. Conversely, no traces or low levels of Fukushima-derived radiocesium were detected in the Niida and in one sediment core of the Ota River mouths. Features of the seafloor topography were shown to be significant factors controlling this spatial and depth distribution of contaminated sediment. Additional measurements are currently conducted to determine the physico-chemical properties of this sediment (XRF core scanner, organic matter properties) in order to identify the optimal combination of tracers (conservatives and discriminants) to quantify the relative contribution of the three catchment sources considered (cropland, forest, subsoil). Preliminary results and perspectives regarding the adaptation of a sediment tracing method originally developed for terrestrial environments to the marine environment will be presented and discussed. This increased knowledge will undoubtedly be useful for understanding 137Cs transfer processes between continental and marine environments, as well as for watershed and coastal management in the post-accidental context of the FDNPP.
Six targeted coastal sediment cores, ranging from 20 to 60cm in 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 and analyzed by gamma spectrometry. Preliminary findings concerning the spatial and depth distribution of radiocesium in these samples reveal a strong heterogeneity, with the highest radiocesium levels (up to 134 ± 2 and 4882 ± 11 Bq kg-1 for 134Cs and 137Cs, respectively) observed in coastal sediment cores situated at the Ukedo river mouth. A strong relationship is observed between the particle size measured in these sediment cores and radiocesium activities, with higher levels observed in finer sediment. Conversely, no traces or low levels of Fukushima-derived radiocesium were detected in the Niida and in one sediment core of the Ota River mouths. Features of the seafloor topography were shown to be significant factors controlling this spatial and depth distribution of contaminated sediment. Additional measurements are currently conducted to determine the physico-chemical properties of this sediment (XRF core scanner, organic matter properties) in order to identify the optimal combination of tracers (conservatives and discriminants) to quantify the relative contribution of the three catchment sources considered (cropland, forest, subsoil). Preliminary results and perspectives regarding the adaptation of a sediment tracing method originally developed for terrestrial environments to the marine environment will be presented and discussed. This increased knowledge will undoubtedly be useful for understanding 137Cs transfer processes between continental and marine environments, as well as for watershed and coastal management in the post-accidental context of the FDNPP.