Environmental behaviour of radioactive particles from chernobyl.

Long-term environmental behaviour of radioactive particles released during the Chernobyl accident and deposited in sandy topsoil in Ivankiv district of Kyiv Region (Ukraine), in radioactive trench waste materials from the Red forest, and in bottom sediments from the Cooling pond has been assessed. The efficiency of the models describing the dissolution/weathering rates of U fuel particles developed 15-20 years ago was tested, and their predictions for the dynamics of remobilization, mobility and plants uptake of 90Sr were confirmed. It was found that at present in the topsoil and in radioactive trench waste material, total dissolution of fuel particles of low chemical stability (UO2+x) has occurred and about half of the non-oxidized chemically stable fuel particles (UO2) has also dissolved, indicating radiological stabilization of the environment and that the mobile fraction of radionuclides would be reduced in the future. The biological availability of 90Sr in topsoil due to fuel particles dissolution has reached maximum values and further decrease is expected. The presence of chemically extra-stable fuel particles (U-Zry-Ox) in environments should be taken into account when the total radionuclides activity concentrations are assessed during radioactive materials management. It was shown that nearly half of the 90Sr activity remained as part of the non-dissolved UO2 fuel particles at the time of the study. Taking into consideration that 31 ±â€¯4% of the radionuclide activities were still associated with non-dissolved chemically extra-stable particles (U-Zry-Ox) in radioactive trench waste materials from the Red forest, increased dissolution should not be expected in the near future. The physico-chemical form of radionuclides in air exposed sediments from the Cooling pond were determined, and results showed that about 70-80% of total 90Sr, 241Am and plutonium isotopes activity were associated with U fuel particles. The low dissolution rate of radionuclides from the pond sediments is attributed to prolonged slightly alkaline pH in the medium due to zebra mussel residues. According to new data, the emission value of 238Pu associated with fuel particles released during the Chernobyl accident amounted to 1.8 × 1013Bq (1.2% of the activity in the reactor) and 90Sr amounted to 2.6 × 1015Bq (1.5% of the activity in the reactor).

Analysis of the relationship binding in situ gamma count rates and soil sample activities: Implication on radionuclide inventory and uncertainty estimates due to spatial variability.

The paper strives to identify through geostatistical simulations the parameters which build up a correlation between radionuclide activity concentrations measured on core samples and corresponding in situ total gamma count rates measured into boreholes drilled within the contaminated soil. This numerical exercise demonstrates that a linear relationship should exist between logarithmic values of in situ count rates and logarithmic values of activity concentrations when the contamination is strongly structured through space. A sensitivity analysis to some parameters (geostatistical range of the contamination structure, core sampling method, soil water content, multiple gamma-emitter contamination, etc.) is undertaken to identify which situations may impede the use of such a correlation. Then this approach is applied on Chernobyl measurements undertaken in 2015 and compared to the co-kriging method which considers the localization of the measurements and the additional measurements. It appears that co-kriging is a better estimator than linear regression, but the latter remains an acceptable way of estimating activity from gamma emitters and presents better results than lognormal regression. Therefore, total gamma logging measurements performed into boreholes of porous media contaminated by gamma-emitting radionuclides can be used for characterizing contamination and dealing with its spatial variability with the use of co-kriging.

Estimation of sedimentation rates based on the excess of radium 228 in granitic reservoir sediments.

Knowledge of sedimentation rates in lakes is required to understand and quantify the geochemical processes involved in scavenging and remobilization of contaminants at the Sediment-Water Interface (SWI). The well-known 210Pb excess (210Pbex) method cannot be used for quantifying sedimentation rates in uranium-enriched catchments, as large amounts of 210Pb produced by weathering and human activities may dilute the atmospheric 210Pb. As an alternative dating method in these cases, we propose an original method based on 232Th decay series nuclides. This study focuses on an artificial lake located in a granitic catchment downstream from a former uranium mine site. The exponential decay of 228Ra excess (228Raex) with depth in two long cores yields sedimentation rates of 2.4 and 5.2 cm yr-1 respectively. These sedimentation rates lead to the attribution of the 137Cs activity peak observed at depth to the Chernobyl fallout event of 1986. The 228Raex method was also applied to two short cores which did not display the 137Cs peak, and mean sedimentation rates of 2.1 and 4.0 cm y-1 were deduced. The proposed method may replace the classical radiochronological methods (210Pbex, 137Cs) to determine sedimentation rates in granitic catchments.

Complementing approaches to demonstrate chlorinated solvent biodegradation in a complex pollution plume: Mass balance, PCR and compound-specific stable isotope analysis.

This work describes the use of different complementing methods (mass balance, polymerase chain reaction assays and compound-specific stable isotope analysis) to demonstrate the existence and effectiveness of biodegradation of chlorinated solvents in an alluvial aquifer. The solvent-contaminated site is an old chemical factory located in an alluvial plain in France. As most of the chlorinated contaminants currently found in the groundwater at this site were produced by local industries at various times in the past, it is not enough to analyze chlorinated solvent concentrations along a flow path to convincingly demonstrate biodegradation. Moreover, only a few data were initially available to characterize the geochemical conditions at this site, which were apparently complex at the source zone due to (i) the presence of a steady oxygen supply to the groundwater by irrigation canal losses and river infiltration and (ii) an alkaline pH higher than 10 due to former underground lime disposal. A demonstration of the existence of biodegradation processes was however required by the regulatory authority within a timeframe that did not allow a full geochemical characterization of such a complex site. Thus a combination of different fast methods was used to obtain a proof of the biodegradation occurrence. First, a mass balance analysis was performed which revealed the existence of a strong natural attenuation process (biodegradation, volatilization or dilution), despite the huge uncertainty on these calculations. Second, a good agreement was found between carbon isotopic measurements and PCR assays (based on 16S RNA gene sequences and functional genes), which clearly indicated reductive dechlorination of different hydrocarbons (Tetrachloroethene--PCE-, Trichloroethene--TCE-, 1,2-cisDichloroethene--cis-1,2-DCE-, 1,2-transDichloroethene-trans--1,2-DCE-, 1,1-Dichloroethene--1,1-DCE-, and Vinyl Chloride--VC) to ethene. According to these carbon isotope measurements, although TCE biodegradation seems to occur only in the upgradient part of the studied zone, DCE and VC dechlorination (originating from the initial TCE dechlorination) occurs along the entire flowpath. TCE reductase was not detected among the Dehalococcoides bacteria identified by quantitative PCR (qPCR), while DCE and VC reductases were present in the majority of the population. Reverse transcriptase PCR assays (rt-PCR) also indicated that bacteria and their DCE and VC reductases were active. Mass balance calculations showed moreover that 1,1-DCE was the predominant DCE isomer produced by TCE dechlorination in the upgradient part of the site. Consequently, coupling rt-PCR assays with isotope measurements removes the uncertainties inherent in a simple mass balance approach, so that when the three methods are used jointly, they allow the identification and quantification of natural biodegradation, even under apparently complex geochemical and hydraulic conditions.