Using reservoir sediment deposits to determine the longer-term fate of chernobyl-derived 137Cs fallout in the fluvial system

Applicability of reconstruction of temporal changes in 137Cs concentrations in pond water based on the vertical distribution of bottom sediments: a case in three ponds in the Fukushima evacuation zone

Long-term changes in particulate and dissolved 137Cs concentrations in rivers and dam reservoirs have been reconstructed using 137Cs depth distributions in bottom sediments near Chornobyl and Fukushima. However, few studies have applied this method to pond water. This study tested the applicability of this reconstruction method in ponds within the Fukushima evacuation zone. Bottom sediments in ponds were collected in November 2019. Our reconstruction of a shallow closed pond disagreed with our observations. Even in ponds where the reconstruction agreed well with observations, underestimations may have occurred because the time constant for the decrease in 137Cs concentration was lower than those previously reported. We conclude that the present reconstruction method may be applied to deeper Fukushima ponds with inflow and outflow, but underestimations of initial 137Cs concentrations in water may lead to uncertainties. For realistic reconstruction, attention should be paid to pond characteristics, catchment conditions, and sediment core selection.

Spatial and vertical distribution of 137Cs activity concentrations in lake sediments of Turawa Lake (Poland)

The main objective of this research was to study the spatial and vertical distribution of 137Cs activity concentrations in the bottom sediments of Turawa Lake 32 years after the Chernobyl fallout to investigate possible factors responsible for the post-fallout migration and accumulation of 137Cs in the selected reservoir. The results demonstrated a strong relationship between the increasing 137Cs and 40K activity concentrations and the decreasing grain size of sediments. Significant amounts of 137Cs were detected in the bottom sediments deposited in the deeper parts of the reservoir (especially near the dam). Therefore, this research showed that Turawa Lake can be an important trap for sediments polluted with 137Cs. Moreover, disturbed vertical distribution of 137Cs activity concentrations in the sediment columns collected from the littoral zone of this lake was observed, which is probably related to the bottom erosion intensified by wind-wave action, bioturbations, and water-level fluctuations. In the profundal zone, the vertical distribution of 137Cs activity concentrations was undisturbed, which indicates stable sedimentation conditions in this part of Turawa Lake.

Heavy Metal Content and Pollution Assessment in Typical Check Dam Sediment in a Watershed of Loess Plateau, China

To understand historical trends and assess the ecological risk associated with heavy metal pollution, the concentration of eight species of heavy metals (vanadium (V), chromium (Cr), manganese (Mn), cobalt (Co), nickel (Ni), zinc (Zn), copper (Cu), and arsenic (As)) in typical silt dam sediments on the Loess Plateau were analyzed. The radionuclide 137Cs was used to quantify rates of erosion, deposition, and heavy metal contamination in the soils of a watershed that supplies a check dam. The sediment record revealed three time periods distinguished by trends in erosion and pollutant accumulation (1960–1967, 1968–1981, and 1985–1991). Heavy metal concentrations were highest but exhibited significant fluctuation in the first two periods (1960–1967 and 1968–1981). From 1985 to 1991, heavy metal pollution showed a downward trend and tended to be stable. The potential risks of heavy metals in silt dam sediments were explored by applying the geo-accumulation index and the potential ecological risk index. The results indicated medium risk associated with Cu and As accumulation, especially in 1963, 1971, and 1986 when the assessed values increased significantly from previous levels. Agricultural practices and high rates of slope erosion may be responsible for the enrichment of As and Cu in soil and the accompanying increase in risk. Land use optimization and the careful use of fertilizers could be used to control or intercept heavy metal pollutants in dammed lands. The results provide the basis for evaluating the current status and ecological risk of heavy metal contamination in dam sediments and for predicting possible heavy metal pollution in the future.

Reconstruction of time changes in radiocesium concentrations in the river of the Fukushima Dai-ichi NPP contaminated area based on its depth distribution in dam reservoir's bottom sediments

Radionuclide depth distribution in bottom sediments in deep-water zones of dam reservoirs, where no sediment mixing occurs, can be used to reconstruct time changes in particulate activity concentrations of radionuclides strongly bound to bottom sediments. This approach was used to analyze the ¹³⁷Cs concentration profile in a bottom sediment core collected from Ogaki dam reservoir on the Ukedo River in the Fukushima Dai-ichi nuclear power plant contaminated zone in October 2019. The derived ¹³⁷Cs particulate concentrations provided a basis for estimating the dissolved concentration and its temporal trend in the Ukedo River, using the mean value of the apparent ¹³⁷Cs distribution coefficient. The reconstructed particulate and dissolved ¹³⁷Cs concentrations and their temporal trends are consistent with monitoring data. The annual mean particulate and dissolved ¹³⁷Cs wash-off ratios were also calculated for the period of eight years after the accident. Interestingly, the particulate ¹³⁷Cs wash-off ratios for the Ukedo River at Ogaki dam were found to be similar to those for the Pripyat River at Chernobyl in the same time period after the accident, while the dissolved ¹³⁷Cs wash-off ratios in the Ukedo River were an order of magnitude lower than the corresponding values in the Pripyat River. Both the particulate and dissolved ¹³⁷Cs wash-off ratios in the Ukedo River declined faster during the first eight years after the FDNPP accident than predicted by the diffusional model, most likely, due to greater natural attenuation and, to some extent, remediation measures implemented on the catchments in Fukushima.

Accumulation capability for cesium differs among bacterial species: A comprehensive study using bacteria isolated from freshwater and coastal sediment

The fate and behavior of radioactive cesium (Cs) in the water environment are of great concern. The involvement of bacteria regarding their accumulation capability for this element is the most fundamental factor that needs to be clarified even for exploring the interactions between many environmental factors that involve together in governing the transport and distribution of Cs. As the first systematical study that aimed to evaluate the accumulation capability of environmental bacteria for Cs, bacteria in the sediment of a freshwater reservoir and coastal water environment were isolated and multiplied for contact experiment with Cs under different temperature conditions (5, 25, and 35 °C). The accumulation concentration of Cs in bacteria from freshwater sediment varied in 3.95 × 10^-6 to 5.68 × 10^-4ng-Cs/cell, and that from coastal sediment in 1.52 × 10^-6 to 7.41 × 10^-4ng-Cs/cell, indicating obvious differences among bacterial species. Bacteria of coastal sediment possessed higher accumulation capability for Cs than bacteria from freshwater sediment, and temperature dependency was confirmed for bacteria from coastal sediment. The findings of this study have great reference value for better understanding and controlling the fate and behavior of radioactive Cs associated with bacteria in the water environment.