Source dynamics of radiocesium-contaminated particulate matter deposited in an agricultural water reservoir after the Fukushima nuclear accident

Abstract

The Fukushima nuclear accident in Japan resulted in the deposition of radiocesium over forested and rural landscapes northwest of the power plant. Although there have been several investigations into the dynamics of contaminated river sediment, less attention has been paid to the sources of deposited particulate matter in dams and reservoirs. In the Fukushima Prefecture, there are 10 significant dams and over a 1000 reservoirs for both agricultural and surface water management. These reservoirs may have trapped a significant volume of radiocesium-contaminated sediment. Therefore, characterizing the sources of contaminated particulate matter is important for the ongoing management of contamination in the region. Accordingly, the composition of particulate matter deposited in the Mano Dam reservoir, approximately 40km northwest of the power plant, was investigated with the analyses and modelling of carbon and nitrogen stable isotope ratios (δC and δN), total organic carbon (TOC) and total nitrogen (TN) concentrations. Four sediment cores, with lengths ranging 29-41cm, were sampled in the Mano Dam. Source samples from 46 forest soils, 28 cultivated soils and 25 subsoils were used to determine the source contributions of particulate matter. Carbon and nitrogen parameters were analyzed on all samples and a concentration-dependent distribution modelling approach was used to apportion source contributions. Three of the four cores sampled in the Mano Dam reservoir had distinct radiocesium peaks representative of the initial post-accident wash-off phase. Cultivated sources were responsible for 48±7% of the deposited fine particulate matter whereas forests were modelled to contribute 27±6% and subsoil sources 25±4%. Ongoing decontamination of cultivated sources in the Fukushima region should result in a decrease of contaminated matter deposition in reservoirs.

Effectiveness of landscape decontamination following the Fukushima nuclear accident: a review

“…The analysis of deposited particulate matter collected in three fallout-contaminated coastal catchments between November 2012 and November 2014 demonstrated that forest sources supplied a mean of 17 % (standard deviation -SD -10 %) of the sediment transiting these river systems . Huon et al (2018) obtained similar results through the analysis of sediment cores collected between November 2014 and April 2015 in a dam reservoir draining fallout-impacted cultivated and forested landscapes, with the latter supplying a mean of 27 % (SD 6 %) of the material deposited in the lake. These conclusions were validated through an analysis of a larger number of sediment samples (n = 400) collected in coastal river systems in the Fukushima region over a longer time period (from November 2011 to November 2017), where a mean of 24 % (SD 21 %) of the mate-rial transiting these systems was modelled to be derived from forested landscapes .…”

Strategies and effectiveness of land decontamination in the region affected by radioactive fallout from the Fukushima nuclear accident: A review

“…The analysis of deposited particulate matter collected in three fallout-contaminated coastal catchments between November 2012 and November 2014 demonstrated that forest sources supplied a mean of 17 % (standard deviation, SD, 10 %) of the sediment transiting these river systems . Huon et al (2018) obtained similar results through the analysis of sediment cores collected between November 2014 and April 2015 in a dam reservoir draining fallout-impacted cultivated and forested landscapes, with the latter supplying a mean of 27% (SD, 6%) of the material deposited in the lake. These conclusions were validated through an analysis of a larger number of sediment samples (n=400) collected in coastal river systems in the https://doi.org/10.5194/soil-2019-43 Preprint.…”

Reconstruction of uranium and plutonium isotopic signatures in sediment accumulated in the Mano Dam reservoir, Japan, before and after the Fukushima nuclear accident

“…(2015) measured the radiocesium concentration in bottom sediment and in lake water samples in 2012 and 2013, and they observed respective concentration decreases of 57% and 43% between the successive sampling campaigns. Huon et a/. (2018) measured the vertical distribution of 137Cs activities in four sediment cores sampled across the lake.…”

“…Three of these four cores showed a distinct peak likely corresponding to the initial radiocesium wash-off and migration phase following the FDNPP accident. In the current study, one of these sediment cores (referred to as 'DD1') located in the downstream section of the lake and composed of fine sediment material (Huon et al, 2018) was selected to characterise variations of Pu and U isotope ratios with depth. More details on the core sampling and the radiocesium analyses are provided in Huon et a/.(2018).…”

“…In the current study, one of these sediment cores (referred to as 'DD1') located in the downstream section of the lake and composed of fine sediment material (Huon et al, 2018) was selected to characterise variations of Pu and U isotope ratios with depth. More details on the core sampling and the radiocesium analyses are provided in Huon et a/.(2018). In brief, the sediment core was collected on April 24, 2015 (37°43'25.75"N, 140°49'49.84"E) using a gravity core sampler, under approximately 45 m of water.…”