Impact of soil organic matter on Pu migration in five Lithuanian surface soils

Pu distribution coefficient K_d variation was experimentally determined and examined in natural soil samples considering the type of soil, particle size, pH, the concentration of macroelements and organic matter content. This research was carried out with sand, silty sand, peat, clayey sand and clayey loam samples by applying ²³⁶Pu tracer in flow-through column tests. Due to relatively short contact time of 0.5-40 h the tests are considered as have not reached equilibrium state and represent the fast-moving contaminants retardation processes closer to field conditions. Every soil sample was fractionated into two particle size fractions: ≤0.25 mm and 0.25 ÷ 0.5 mm. Analysis revealed that K_d of Pu is higher for the smaller soil particle fraction (≤0.25 mm). The experimental study with 1.6, 4, 6 and 9 pH tracer solution revealed a tendency of elevated K_d when 4 pH and 6 pH solutions were applied, but obtained K_d values were not correlated with initial soil pH due to high buffering capacity of soils. This study shows a very significant influence (r = 0.98) of organic matter content on the Pu distribution coefficient. The K_d of Pu for the fine fraction of peat soil with high organic matter content (67%) reached maximum values of 6597 L/kg and 6200 L/kg when tracer solution was applied of pH = 4 and pH = 6, respectively. In comparison, the minimum K_d value of 3.9 L/kg was obtained for the coarse silty sand fraction with the lowest organic matter content of 1.3% at tracer pH = 1.6. A statistically reliable high correlations of r = 0.95 and 0.94 were also observed between K_d and specific soil elements Mg and Pb content in soils, respectively. The content of Fe in soils was significantly correlated (r = 0.67) with the K_d values of plutonium as well. However, the organic matter content in soils appeared to be the governing factor determining good correlations and causing the highest K_d of Pu values.

Experimental and modelling studies of radiocesium sorption/desorption processes in the fixed-bed moss column

The experimental and modelling studies of the fixed-bed column filled with inanimate mosses Ptilium crista-castrensis containing background levels of ¹³⁷Cs, flushed with the distilled water are presented. Fitting of the experimental results by Langmuir and Freundlich sorption models gave significant deviations. A more sophisticated two kinetic site model created for the case of non-equilibrium transport of the radiocesium solute through porous media was more suitable to describe the experimental data. It has been shown that theoretically the two site sorption model can be simplified to the conventional one kinetic site model with time dependent parameters. The radiocesium transport equation included the terms taking into account advection, dispersion and decay. General analytical solution was derived for two types of the border conditions: 1) when a constant flux of the material on the surface layer of the absorbent is known (i.e. a Cauchy type border condition); 2) a border condition with constant initial material concentrations. The mathematical solution was derived using the Laplace transformation method. In such a way, the obtained simulated profile of the radionuclide concentration distribution within the fixed-bed column with inanimate mosses showed the best-fitting to the experimental results.

137Cs and plutonium isotopes accumulation/retention in bottom sediments and soil in Lithuania: A case study of the activity concentration of anthropogenic radionuclides and their provenance before the start of operation of the Belarusian Nuclear Power Plant (NPP)

Knowledge of the background activity concentrations of anthropogenic radionuclides before the start of operations of the new nuclear facilities in Belarus is of great value worldwide. Inland water bodies in Lithuania (specifically the Neris River, the Nemunas River and the Curonian Lagoon) are near the site of the Belarusian NPP under construction and, for this reason, sediments and flooded soils from these sensitive areas were analysed for radiocesium and plutonium isotopes (macrophytes were analysed only for ¹³⁷Cs) in 2011-2012. The ¹³⁷Cs and ^239+240Pu activity concentrations in bottom sediments from the Nemunas River, sampled in 1995-1996 and re-calculated to the year 2016, were compared with those of 2011-2012. The obtained activity of ¹³⁷Cs in bottom sediments of the Nemunas River and Curonian Lagoon varied from 1 Bq/kg to 47.0 Bq/kg. The activity of ¹³⁷Cs in the tested soils ranged from 5.3 B g/kg to 32.9 Bq/kg. The ^239+240Pu activity in bottom sediments of the studied sampling sites varied between 0.016 and 0.34 Bq/kg and in flooded soils from 0.064 to 0.55 Bq/kg. The ²³⁸Pu activity values were very low or lower than the detection limit. The activity of ¹³⁷Cs in macrophytes varied from values lower than the detection limit to 6 Bq/kg. A strong positive linear correlation for bottom sediments was calculated between: ^239+240Pu and total organic carbon (TOC), r = 0.86, p-value 0.01; ^239+240Pu and silt, r = 0.80, p-value 0.029; ¹³⁷Cs and silt, r = 0.78, p-value 0.04; and ¹³⁷Cs and TOC, r = 0.85, p-value 0.015. The similar peculiarities of ¹³⁷Cs and ^239+240Pu accumulation in bottom sediments and flooded soil allow us to assume that ¹³⁷Cs can be used as a tracer for ^239+240Pu in the initial stage of searching for radionuclide accumulation zones. A remaining impact of the Chernobyl fallout in average comprised: in the Lower Nemunas River and Curonian Lagoon sediments - 51%, in the Middle Nemunas River -90% and in the floodplains of the Nemunas River - 59%, while the provenance of plutonium in studied bottom sediments and flooded soil was the global fallout.

Study of the formation of the primary 137Cs vertical profile in the organic matter-rich sediments

A laboratory study of the formation of the primary vertical profile of the ¹³⁷Cs activity concentration in the anaerobic organic matter-rich sediments was carried out. The incubation was performed using sediment baths method (initial thickness of the water layer over the sediments ∼40 cm, initial radiocesium activity concentration in water ∼14.53 Bq·L^-1). An exponential profile (R²∼0.999) with the half-height width of ∼0.65 cm was formed after ∼32 days of the incubation. A further course of the experiment (sediments were sampled after 64, 130 and 293 days of the incubation) revealed deviations in the form of the radiocesium activity concentration profile related to the appearance of the additional Gauss factor with the increasing half-height width of ∼1.2, ∼1.3 and ∼3 cm, respectively. In spite of the evident growth of the half-height width, the radiocesium migration rate decreased from 0.02 cm·day^-1 for the first 32 days of incubation down to 0.009 cm·day^-1 at the end of the incubation experiment. Such a time-dependent decrease in the radiocesium migration is possibly related to the beginning of the second radiocesium migration phase depending on the retardation factor. Besides diffusion, the formation of vertical profiles of radiocesium activity concentrations in sediments was additionally induced by effects of buoyancy of the upper sediment layer due to the seepage of the above lying water into sediments.

Accumulation and translocation peculiarities of (137)Cs and (40)K in the soil--plant system

Long-term investigations (1996-2008) were conducted into the (137)Cs and (40)K in the soil of forests, swamps and meadows in different regions of Lithuania, as well as in the plants growing in these media. The (137)Cs and (40)K activity concentrations, the (137)Cs/(40)K activity concentration ratio and accumulation, and translocation in the system, i.e. from the soil to plant roots to above-ground plant part of these radionuclides, were evaluated after gamma-spectrometric measurements using a high purity germanium (HPGe) detector. Based on the obtained data, it can be asserted that in the tested plant species, the (137)Cs and (40)K accumulation, the transfer from soil to roots and translocation within the plants depend on the plant species and environmental ecological conditions. The (137)Cs/(40)K activity concentration ratios in the same plant species in different regions of Lithuania are different and this ratio depends on the biotope (forest, swamp or meadow) in which the plant grows and on the location of the growing region. Based on the determined trends of statistically reliable inverse dependence between the activity concentrations in both soil and plants, it can be stated that the exchange of (137)Cs and (40)K in plants and soil is different. Different accumulations and translocations of investigated radionuclides in the same plant species indicate diverse biological metabolism of (137)Cs and its chemical analogue (40)K in plants. A competitive relationship exists between (137)Cs and (40)K in plants as well as in the soil.

Anthropogenic radionuclide fluxes and distribution in bottom sediments of the cooling basin of the Ignalina Nuclear Power Plant

Based on γ-ray emitting artificial radionuclide spectrometric measurements, an assessment of areal and vertical distribution of (137)Cs, (60)Co and (54)Mn activity concentrations in bottom sediments of Lake Drūkšiai was performed. Samples of bottom sediments from seven monitoring stations within the cooling basin were collected in 1988-1996 and 2007-2010 (in July-August). For radionuclide areal distribution analysis, samples from the surface 0-5 cm layer were used. Multi sample cores sliced 2 cm, 3 cm or 5 cm thick were used to study the vertical distribution of radionuclides. The lowest (137)Cs activity concentrations were obtained for two stations that were situated close to channels with radionuclide discharges, but with sediments that had a significantly smaller fraction of organic matter related to finest particles and consequently smaller radionuclide retention potential. The (137)Cs activity concentration was distributed quite evenly in the bottom sediments from other investigated monitoring stations. The highest (137)Cs activity concentrations in the bottom sediments of Lake Drūkšiai were measured in the period of 1988-1989; in 1990, the (137)Cs activity concentrations slightly decreased and they varied insignificantly over the investigation period. The obtained (238)Pu/(239,240)Pu activity ratio values in the bottom sediments of Lake Drūkšiai represented radioactive pollution with plutonium from nuclear weapon tests. Higher (60)Co and (54)Mn activity concentrations were observed in the monitoring stations that were close to the impact zones of the technical water outlet channel and industrial rain drainage system channel. (60)Co and (54)Mn activity concentrations in the bottom sediments of Lake Drūkšiai significantly decreased when operations at both INPP reactor units were stopped. The vertical distribution of radionuclides in bottom sediments revealed complicated sedimentation features, which may have been affected by a number of natural and anthropogenic factors resulting in mixing, resuspension and remobilization of sediments and radionuclides. The associated with particles (137)Cs flux was 129 Bq/(m(2) year). The (137)Cs transfer rate from water into bottom sediments was 14.3 year(-1) (or, the removal time was 25 days). The Kd value for (137)Cs in situ estimated from trap material was 80 m(3)/kg. The associated with particles (60)Co flux was 21 Bq/(m(2) year), when (60)Co activity concentration in sediment trap particles was 15.7 ± 5 Bq/kg. (60)Co activity concentration in soluble form was less than the minimum detectable activity (MDA = 1.3 Bq/m(3)). Then, the conservatively derived Kd value for (60)Co was >90 m(3)/kg.

Spatial patterns and ratios of ¹³⁷Cs, ⁹⁰Sr, and Pu isotopes in the top layer of undisturbed meadow soils as indicators for contamination origin

Spatial distribution of activity concentrations of (137)Cs, (90)Sr, and (239,240)Pu in the top layer of undisturbed meadow soils was compared between two regional transects across Lithuania: one in the SW region, more affected by the Chernobyl radioactive fallout, and the other in the NE region. Radiochemical, γ-, α-, β-, and mass spectrometric methods were used to determine the radionuclide activity. Our results validate that higher activity concentrations in the top soil layer were present in the SW region, despite the fact that sampling was performed after 22 years of the Chernobyl Nuclear Power Plant (NPP) accident. Using the activity concentration ratio (137)Cs/(239,240)Puglobal, the contribution of the Chernobyl NPP accident to the total radiocesium activity concentrations in these meadow soils was evaluated and found to be in the range of 6.5-59.1%. Meanwhile, the activity concentration ratio (238)Pu/(239,240)Pu showed that Chernobyl-derived Pu occurred at almost half of the sampling sites. The locations with maximal values of 47% of Chernobyl-derived Pu material were close to northeastern Poland, where deposition of most of non-volatile radioisotopes from the Chernobyl plume was determined.

Accumulation and transfer of 137Cs and 90Sr in the plants of the forest ecosystem near the Ignalina Nuclear Power Plant

The radioecological state of the forest ecosystem in the vicinity of the Ignalina Power Plant prior to decommissioning was analysed with specific emphasis on (137)Cs and (90)Sr activity concentrations in plant species growing in two reference sampling sites (Tilze and Grikiniskes). In the period of 1996-2008 the mean contamination of plants with (137)Cs was from 45 to 119 Bq/kg and with (90)Sr - from 3 to 42 Bq/kg. Measured (137)Cs TF values for soil-root transfer mainly ranged between 1.0-1.4, except for Calamagrostis arundinacea which had a TF value of 0.1. On average, the (137)Cs TF value from root to shoot was 1.7 fold higher than for soil to root transfer. (90)Sr TF values (soil-root) were in the range of 1.2-1.8 but for Calluna vulgaris it was 0.2. The mean root to shoot TF value for (90)Sr was 7.7 fold higher. These results indicate the higher (90)Sr bioavailability than that of (137)Cs in the forested area. The Grikiniskes reference site is located nearby the Ignalina NPP, specifically the heated water outlet channel, which results in altered microclimatic conditions. These specific microclimatic conditions result in relationships between (137)Cs TF (soil-root) values and pH, moisture and organic matter content in the soil at Grikiniskes which appear to be different to those at the Tilze reference sampling site.

Distribution of artificial gamma-ray emitting radionuclide activity concentration in the top soil in the vicinity of the Ignalina Nuclear Power Plant and other regions in Lithuania

The impact of the operating Ignalina Nuclear Power Plant (INPP) on the contamination of top soil layer with artificial radionuclides has been studied. Results of the investigation of artificial gamma-ray emitting radionuclide distribution in soil in the vicinity of the INPP and distant regions in Lithuania in 1996-2008 (INPP operational period) show that nowadays (137)Cs remains the most important artificial gamma-ray emitting radionuclide in the upper soil layer. Mean (137)Cs activity concentrations in the top soil layer in the vicinity of the INPP were found to be significantly lower compared to those in remote regions of Varėna and Plungė (~300 km from INPP). In 1996 and 1998 mean (137)Cs activity concentrations were in the range of 28-45 Bq/kg in the nearest vicinity to the INPP, 103 Bq/kg in Varėna and 340 Bq/kg in Plungė region. (137)Cs activity concentrations were 5-20 times lower in meadow soil (4-14 Bq/kg) compared to swamp and forest soil. (60)Co, the INPP origin radionuclide, was detected in samples only in 1996 and 2000, and the activity concentration of (60)Co was found to be in the range from 0.4 to 7.0 Bq/kg at the sampling ground nearest to the INPP. Average annual activity concentrations of the INPP origin (137)Cs and (60)Co in the air and depositions in the INPP region were modeled using Pasquill-Gifford equations. The modeling results of (137)Cs and (60)Co depositional load in the INPP vicinity agree with the experimentally obtained values. Our results provide the evidence that the operation of INPP did not cause any significant contamination in soil surface.