Evaluation of DGT and DGT-PROFS modeling approach to estimate desorption kinetics of Cs in soils.

The aim of this paper is to assess the suitability of DGT to extract kinetic rates of desorption of cesium (Cs) from soils. For this purpose, laboratory experiments with a natural soil spiked with Cs were carried out under three different contamination conditions, reflecting either an increase in Cs contamination level or an ageing of the contamination within the soil. The experimental results, i.e. the Cs accumulation kinetics onto DGT probes were interpreted by the DGT-PROFS model. The latter calculates the partitioning of Cs between two particulate pools, describing weak and strong interactions respectively, as well as kinetic rates describing exchange reactions. Experimental conditions did not show any major impact on desorption rates, suggesting that desorption kinetics were not significantly affected by contamination level and ageing. Instead, the distribution of Cs among weak and strong sites was shown to be the predominant factor governing the differences observed in the remobilization of Cs to porewater among experimental conditions. The DGT technique combined with the DGT-PROFS modelling approach was proved to be efficient in estimating desorption kinetic rates of Cs in soils.

Validation of a fuel particle dissolution model with samples from the Red Forest within the Chernobyl exclusion zone.

The contamination in the near exclusion zone of the Chernobyl nuclear power plant (ChNPP) with 90Sr, 238-240Pu and 241Am is associated with irradiated nuclear fuel particles. Fit for purpose models enabling long term prediction of mobility and bioavailability of particle-associated radionuclides are crucial for radiation protection of humans and the environment, as well as for planning of remediation measures and future land use. In the present work, a dynamic fuel particle dissolution model developed in 1999-2002 is described and validated using data based on sampling in 2015. The model is based on the characterization of the radionuclide source term of the waste in a shallow sub-surface radioactive waste storage, trench #22, in the Chernobyl Pilot Site (CPS) located in the Red Forest, 2.5 km west of the ChNPP, as well as the description of physical and chemical properties of the fuel particles and subsequent radionuclide leaching into the soil solution. Transformation rate constants of the fuel particle dissolution model related to topsoil, radioactive waste trench and submerged materials, and drained cooling pond sediments, should largely control the mobility and bioavailability of radionuclides (e.g., solubility in the soil, migration to groundwater and transfer to plants). The predicting power of the Chernobyl fuel particle dissolution model with respect to radionuclide leaching dynamics was validated using samples from the same experimental site, showing that predicted particle leaching and subsequent mobility and bioavailability were within 46 ± 3% of the observed data. Therefore, linking source- and release-scenario dependent characteristics of radioactive particles to their potential weathering can provide information that can be implemented in impact assessments models for existing contaminated areas as well as for future events.

Millimeter-scale alkalinity measurement in marine sediment using DET probes and colorimetric determination.

Constrained DET (Diffusive Equilibration in Thin films) probes equipped with 75 sampling layers of agarose gel (DGT Research(©)) were used to sample bottom and pore waters in marine sediment with a 2 mm vertical resolution. After retrieval, each piece of hydrogel, corresponding to 25 µL, was introduced into 1 mL of colorimetric reagent (CR) solution consisting of formic acid and bromophenol blue. After the elution/reaction time, absorbance of the latter mixture was read at 590 nm and compared to a calibration curve obtained with the same protocol applied to mini DET probes soaked in sodium hydrogen carbonate standard solutions. This method allows rapid alkalinity determinations for the small volumes of anoxic pore water entrapped into the gel. The method was assessed on organic-rich coastal marine sediments from Thau lagoon (France). Alkalinity values in the overlying waters were in agreement with data obtained by classical sampling techniques. Pore water data showed a progressive increase of alkalinity in the sediment from 2 to 10 mmol kg(-1), corresponding to anaerobic respiration in organic-rich sediments. Moreover, replicates of high-resolution DET profiles showed important lateral heterogeneity at a decimeter scale. This underlines the importance of high-resolution spatial methods for alkalinity profiling in coastal marine systems.