Use of natural and artificial radionuclides to determine the sedimentation rates in two North Caucasus lakes

Source, preservation and re-suspension of 239,240Pu in a well dated peat core collected from northwest China

Owning to the high radiotoxicity in high concentrations, as well as the irreplaceability in quantifying soil erosion rates, demarcating the Anthropocene, and dating of sediment, anthropogenic 239,240Pu have drawn high attention. However, the source in specific areas, preservation characteristics in different environment media, and re-distribution process after the cessation of atmospheric nuclear weapons tests, have not been fully understood, which obscures the exact start year, temporal variation, and deposition flux of 239,240Pu in sedimentary records, and hinders the wide application of 239,240Pu in environment study. A sediment core from the Yiwu peat bog with dominance of atmospheric deposition in the source material, simple sedimentary environment, and high dust deposition flux, was collected to examine the 239,240Pu, and explore the source, preservation, and re-distribution process. The double peaks of 239,240Pu in 1952 CE and 1963 CE, as well as 240Pu/239Pu ratios of 0.163-0.190 with an average of 0.177 ± 0.010 confirmed that the 239,240Pu source originated predominantly from global fallout. The minimal vertical migration of plutonium in the Yiwu peat core was attributed to the near-neutral pH condition. The high inventory of 128 ± 7 Bq m-2, as well as the atypical negative correlation between 239,240Pu concentrations and organic matter content (r = - 0.79, P < 0.01), was attributed to the contribution of 239,240Pu re-suspended with dust from the neighboring Gobi Desert, particularly in the cold and dry years. The total re-suspended 239,240Pu was estimated to be 77 Bq m-2, exceeding the direct fallout level of 51 Bq m-2 during 1945-2016 CE. In this study, the specified deposition pattern of 239,240Pu after the cessation of atmospheric nuclear weapons was established, providing an important standard for multiple environmental studies, and the re-suspended amount of 239,240Pu in a typical arid area was quantified for the first time.

The spatiotemporal variation, background, and baseline activities of radionuclides in the soil of Aragats Massif (Armenia)

The present study was performed to reveal the distribution patterns and spatiotemporal changes of radionuclides in the soil of the highest mountain of Armenia: Aragats Massif. In this regard, two surveys were implemented in 2016-2018 and 2021 with an altitudinal sampling strategy. The activities of radionuclides were determined by gamma spectrometry system with HPGe detector (CANBERRA). Correlation and linear regression analysis were applied to determine the dependence of radionuclides' distribution from altitude. Classical and robust statistical methods were used to assess the local background and baseline values. In two sampling profiles, the spatiotemporal variation of radionuclides was studied. A significant correlation was revealed between ¹³⁷Cs and altitude pointing to global atmospheric migration as a primary source of ¹³⁷Cs in Armenian environment. The predicted values of regression model revealed a 0.08-Bq/kg and 0.03-Bq/kg increase of ¹³⁷Cs in each m on average, for the old and new survey, respectively. The assessment of background activities of NOR (naturally occurring radionuclides) enables setting the local background for ²²⁶Ra, ²³²Th, and ⁴⁰ K in soils of Aragats Massif: 831.3 ± 20.2 Bq/kg and 540.6 ± 18.3 Bq/kg for ⁴⁰ K, 85.5 ± 3.1 Bq/kg and 27.7 ± 2.6 Bq/kg for ²²⁶Ra, and 66.8 ± 3.2 and 46.4 ± 3.0 Bq/kg for ²³²Th, respectively, for the years of 2016-2018 and 2021. ¹³⁷Cs baseline activity was estimated by altitude: 350 ± 3.7 Bq/kg and 108 ± 2.5 Bq/kg, respectively, for the years of 2016-2018 and 2021.

Bottom sediment radioactivity of the six Caucasus lakes located in different altitude zones

Natural and artificial radioactivity of bottom sediment in the six lakes of the Western and Central Caucasus have been evaluated. It allowed to define the variation of sedimentation rate during the last 100-150 years using technogenic (¹³⁷Cs) and natural (²¹⁰Pb, ²²⁶Ra) radionuclides as a chronomarkers. The studied lakes are located in the contrasting geographic conditions, different orographic positions, and have different origin. The average annual precipitation in the area of each of the lakes has been detected to stay relatively constant during the ¹³⁷Cs fallout period, while the air temperature has markedly increased during the last decades. The detected sedimentation rates are the indirect indicator of climate change in the mountains. They are slightly decreasing owing to the increased protection of soil by vegetation cover in the lower altitude zone; in the upper zones, they are growing due to accelerated glacier retreat. The radioecological situation is estimated as normal. High levels of ¹³⁷Cs (33 kBq m^-2) and ²⁴¹Am (0.1 kBq m^-2) in bottom sediments are attributed to the region-specific geographical characteristics.

Sediment organic carbon dynamics response to land use change in diverse watershed anthropogenic activities

Sediment organic carbon (SOC) is a precious archive that synthesizes anthropogenic processes that remove geochemical fluxes from watersheds. However, the scarcity of inspection about the dynamic mechanisms of anthropogenic activities on SOC limits understanding into how key human factors drive carbon dynamics. Here, four typical basins with similar natural but significantly diverse human contexts (high-moderate-low disturbance: XJ-ZS and YJ-LS) were selected to reconstruct sedimentation rates (SR) and SOC dynamics nearly a century based on 200-cm corers. A partial least squares path model (PLS-PM) was used to establish successive (70 years) and multiple anthropogenic data (population, agriculture, land use, etc.) quantification methods for SOC. Intensified anthropogenic disturbances shifted all SR from pre-stable to post-1960s fluctuating increases (total coefficient: high: 0.63 < low: 0.47 < medium: 0.45). Although land use change was co-critical driver of SOC variations, their trend and extent differed under the dams and other disturbances (SOC mutated in high-moderate but stable in low). For high basin, land use changes increased (0.12) but dams reduced (-0.10) the downstream SOC. Furthermore, SOC mutation corresponded to soil erosion due to urbanization in both periods A and B. For moderate, SOC was reversed with the increase in afforestation and cropland (-0.19) due to the forest excitation effect and deep ploughing, which corresponded to the drought in phase B and the anthropogenic ecological project in A. For low, the increase in SOC corresponded to the Great Leap Forward deforestation in period B and the reed sweep in A, which suggested the minor land change substantially affected (0.16) SOC in fragile environments. Overall, SOC dynamics revealed that anthropogenic activities affected terrestrial and aquatic ecosystems for near the centenary, especially land use. This is constructive for agroforestry management and reservoir construction, consistent with expectations like upstream carbon sequestration and downstream carbon stabilization.

The character of radionuclide contamination of natural lakes at the territory of the Semipalatinsk test site

The paper presents data on the research of the radionuclide contamination redistribution in the ecosystems of natural lakes at the Semipalatinsk test site and adjacent territories. Natural lakes, which are the research objects, are located on the territory of the test sites, in the zone of their influence, in the zone impacted by radioactive fallout or in the "background" territories. Sampling of water, bottom sediments and plants was carried out, in which the content of radionuclides 241Am, 137Cs, 90Sr and 239+240Pu was determined. Based on the obtained quantitative data, distribution coefficients (Kd) for sediments and water, as well as concentration ratio (FV) for plants and sediments were calculated.

Determination of 241Am in Environmental Samples: A Review

The determination of 241Am in the environment is of importance in monitoring its release and assessing its environmental impact and radiological risk. This paper aims to give an overview about the recent developments and the state-of-art analytical methods for 241Am determination in environmental samples. Thorough discussions are given in this paper covering a wide range of aspects, including sample pre-treatment and pre-concentration methods, chemical separation techniques, source preparation, radiometric and mass spectrometric measurement techniques, speciation analyses, and tracer applications. The paper focuses on some hyphenated separation methods based on different chromatographic resins, which have been developed to achieve high analytical efficiency and sample throughput for the determination of 241Am. The performances of different radiometric and mass spectrometric measurement techniques for 241Am are evaluated and compared. Tracer applications of 241Am in the environment, including speciation analyses of 241Am, and applications in nuclear forensics are also discussed.

The anthropogenic fallout radionuclides in soils of Mount Khuko (the Western Caucasus) and their application for determination of sediment redistribution

The purposes of this study are to determine the content and origin of anthropogenic fallout radionuclides (FRN) in soils of Mount Khuko, located in the western sector of the Caucasus Mountains and to assess the possibility to use them for evaluation of sediment redistribution for the alpine grasslands,. The field study was carried out in August 2019 near the top of Mount Khuko, located in the western part of the main Caucasus Mountain Ridge. Integral and incremental soil samples were collected from the different morphological units of the studied area. The content of ¹³⁷Cs and ²⁴¹Am in soil samples was evaluated using laboratory gamma-spectrometry. A part of samples was selected for Pu isotopes extraction and then alpha-spectrometric analysis. It was established that the ¹³⁷Cs contamination of soils in the studied area has at least two sources of origin. The first source is the ¹³⁷Cs bomb-derived fallout after the bomb tests in 1950-60th, which is widespread across the globe. The second source is ¹³⁷Cs Chernobyl-derived fallout High random variability (Cv = 25-42%) was found within reference sites, located at the undisturbed areas on the local flat interfluves due to high variability of soil characteristics (grain size, density, organic matter content etc.). However minimum spatial variability (range 12,2-14,3 kBq/m²) was identified for the mean value of ¹³⁷Cs inventories for all 5 reference sites located in the different parts of the studied area. It is difficult to separate individual peaks of the bomb-derived and Chernobyl-derived ¹³⁷Cs falloutin sediment sinks with low sedimentation rates. Application ^239,240Pu as an additional chronological marker allows to identify the origin of above mention peaks in the soils of alpine grasslands and of dry lake bottom.

A comparative study of biota and sediments as monitors of plutonium in the Yenisei River (Siberia, Russia)

MOX-fuel production and spent nuclear fuel reprocessing started recently at the Mining-and-Chemical Combine (MCC, Zheleznogorsk, Russia) have caused an increase in controlled releases of plutonium to the Yenisei River. In this study, we analyzed time-dependent trends of plutonium (^239,240Pu and ²³⁸Pu) in biota and bottom sediments of the Yenisei during 2008-2019, to estimate comparatively the potential of abundant representatives of biota as bio-monitors of contamination of the Yenisei by plutonium. Gamma-emitting radionuclides (⁴⁰ K; ⁶⁰Co; ¹³⁷Cs; ¹⁵²Eu; ²⁴¹Am) were measured in environmental samples of the Yenisei as well. Samples of bottom sediments, water moss (Fontinalis antipyretica), shining pondweed (Potamogeton lucens), caddisfly larvae with casings (Apatania crymophila), and amphipods (Eulimnogammarus viridis and Palaseopsis cancelloides) were collected downstream and upstream of the radioactive discharge site. Environmental samples of the Yenisei collected downstream of the radioactive discharge site differed considerably in activity concentrations of plutonium but were similar in time-dependent trends of plutonium, reflecting the trends of annual discharges of plutonium. In 2018, the year of a sharp increase in controlled discharge of plutonium, the concentration of ^239,240Pu in water moss (26 Bq kg^-1 d.w.) was higher than in sediments (14 Bq kg^-1 d.w.). In other years, the highest activity concentration of ^2391,240Pu was observed in bottom sediments. In view of the higher magnitude of increase in plutonium concentration, water moss and shining pondweed can be considered as more sensitive indicators of increased fresh releases of plutonium than bottom sediments. Taking into account the food-related mechanism of plutonium uptake by amphipods, this representative of biota can be regarded as a sensitive monitor of bioavailable plutonium in the Yenisei.

Distinctive distribution and migration of global fallout plutonium isotopes in an alpine lake and its implications for sediment dating

This work investigated plutonium (Pu) isotopes in sediment cores collected from an alpine lake (Lake Heinongpo with 3779 m above sea level) in Southwestern China. ²⁴⁰Pu/²³⁹Pu atom ratios in all sediment samples showed the typical global fallout values of ∼0.18 without any influences from other Pu contaminant sources. ^239+240Pu activities with surface and subsurface maximums followed by exponential decline with sediment depth were respectively observed in the two sediment cores. The distinctive depth distributions of ^239,240Pu in the lake sediments was attributed to the very slow sediment deposition rate due to the lack of terrestrial sediment input, while the alpine snowmelt input was the primary source of Pu in the lake sediments in addition to the direct atmospheric deposition. The total Pu inventory was estimated to be 56.3 ± 1.4 and 63.9 ± 0.8 Bq/m² respectively in the two sediment cores. The generally higher Pu inventory in the Lake Heinongpo compared with other reported lakes in similar latitude should be mainly attributed to their different Pu input passages. The advection-diffusion equation was further applied to evaluate the downward migration of Pu isotopes in the sediment cores and predict the future evolution of Pu distribution in the sediment cores. The fitted results indicated that the diffusion effect controlled the downward migration of Pu in the sediments, but this diffusive migration will not prevent the peak of global fallout Pu in undisturbed sediment cores from being a valuable time marker for sediment dating.

Contemporary Long-Term Trends in Water Discharge, Suspended Sediment Load, and Erosion Intensity in River Basins of the North Caucasus Region, SW Russia

For the first time, contemporary trends in water discharge, suspended sediment load, and the intensity of overall erosion in the river basins of the North Caucasus region, as one of Russia’s most agriculturally developed geographic areas, were identified. The study was carried out using monitoring data of the Federal Service for Hydrometeorology and Environmental Monitoring of the country for 21 rivers by comparing two periods: 1963–1980 and 2008–2017. According to the study’s results, trends of an increase in the mean annual water discharge (by 2–97%) and the essential reduction in its intra-annual variability have been found in most of the studied rivers. On the contrary, the trends of reduction in annual suspended sediment load and the intensity of erosion in the river basins were identified in most of the study region. Their most essential and statistically significant decreases (by 47–94%) were recorded within the Stavropol Upland, which several decades ago was considered one of the most erosion-dangerous territories of the entire country, as well as in some river basins of the central part of the Greater Caucasus’s northern slope (by 17–94%). The changes in climate (reducing the depth of soil freezing and meltwater runoff on the soil) and land use/cover (reduction of acreage and load (pressure) of agricultural machinery on the soil, reducing livestock on pastures, and the transfer of water from the neighboring, more full-flowing rivers) are considered the leading causes of the aforementioned trends. The findings will contribute to solving some economic and environmental problems of both the region and adjacent territories and water areas.