Nonlinear transfer of elements from soil to plants: impact on radioecological modeling

Distribution and in situ bioaccumulation test of radioecologically relevant metals in boreal freshwater sediments

Sediments act as important sinks for metals and their radionuclides in aquatic environments and play a crucial role in their transfer and uptake to aquatic organisms. Traditional radioecological models use radionuclide concentrations in water to predict concentrations in aquatic organisms. In this study, we investigated the distribution of radioecologically important metals (Ba, Co, Ni, Sr, U) among sediment, porewater and hypolimnion over seasons. We also studied the uptake of these metals to benthic organisms and importance of sediment as an uptake source by conducting a 28-day in situ bioaccumulation experiment with oligochaete worms (Lumbriculus variegatus). The studied metals were chosen based on common occurrence of their radioactive isotopes in nuclear fuel cycle. Measurements of total elemental concentration were used as proxies to study the behavior of specific radionuclides. Sediment and water samples were collected from two small lakes connected to a former uranium mine in Eastern Finland, and from a nearby reference lake connected to a different drainage area. Environmental characteristics and concentrations measured from sediment, porewater and overlying water indicated only minor changes between seasons. Measured metals were highly associated with sediment particles, rather than porewater or hypolimnion. Both the distribution of metals and in situ experiment indicated the importance of sediment as the main source of bioaccumulation. Significant differences in Ba, Ni and U concentrations between treatments containing contaminated sediment and reference sediment were noted, regardless of water concentrations. Additionally, as U contaminated lakes lacked seasonal overturn during our monitoring period, metal distribution and environmental conditions remained unchanged in deeper parts of those lakes. Lastly, the results of this in situ bioaccumulation experiment are in line with the findings of our previous laboratory study using sediments from these same lakes.

Plant functional type and peat properties determine elemental transfer in boreal mire vegetation

Uptake of elements into plants is an integral part of many environmental impact assessments. Typically, the plant uptake is determined using an empirical soil-to-plant transfer factor (CR). The elemental concentrations in plants are expected to vary with plant species and plant functional type (PFT), but also according to soil and element properties. Specifically, the uptake of essential elements is regulated, and likely less related to soil concentrations than the uptake of non-essential elements. In this study, the impact of PFT, species and environmental factors on the CR of mire plants was tested. The plants included in the study were four common boreal peatland species (Andromeda polifolia, Vaccinium oxycoccus, Eriophorum vaginatum and Carex rostrata) sampled from 40 minerogenic mires along an age gradient. The results show that while plant species and PFT (heathers and sedges) are the main determinants of the CR value, also environmental factors, such as peat C:N ratio, are important. Further, concentrations of essential elements in plants were only weakly correlated to peat concentrations, whereas the correlation was stronger for non-essential elements and elements utilized at trace amounts. The results of this study verify that CR values may vary substantially between peatland plant species and PFTs. Further, the results suggest that it is relevant to include effects of PFTs on CR and among-species variation in environmental impact assessments. This is because the PFT may have a large impact on the exposure pathways to humans, which could, for example, be berries or animal feed, and also due to the uncertainties of the composition of the future vegetation communities. Since CR varies systematically with several soil properties, there may be potential for adjusting the CR values for expected environmental changes, and thereby reduce the uncertainties of empirical CR values determined from a broad range of environmental conditions.

Transfer of elements from soil to earthworms and ground beetles in boreal forest

Data on the transfer of elements (such as heavy metals) and their radionuclides into organisms is needed for assessing environmental risks. The current data on many elements, species and environments is limited, but more information can be obtained both from field studies and experimental laboratory studies. However, it is essential to evaluate whether experimental studies adequately predict transfer in natural conditions. Moreover, because of the sparsity of species-specific empirical data, it is a common practice in current radioecological modelling to use data available for related species under the assumption that transfer into organisms is similar within broader taxonomic groups. Earthworms and ground beetles are examples of important invertebrates living near soil surface in terrestrial ecosystems. In this study, the transfer of 34 elements from soil to these organisms was studied in a field study conducted in boreal forest. The earthworm concentrations were compared to the values obtained in an experimental mesocosm study using soil from the field site and were found to be highly correlated. This indicates that the results of mesocosm studies can be used for predicting the transfer of elements from soil to fauna in natural conditions. Furthermore, concentrations in individual earthworm and beetle species were found to be similar to those observed in broader groups of related species, indicating that the generic approach used in current radioecological models may be useful for predicting uptake of elements into single species.

Transfer of elements into boreal forest ants at a former uranium mining site

Ants can influence ecological processes, such as the transfer of elements or radionuclides, in several ways. For example, they redistribute materials while foraging and maintaining their nests and have an important role in terrestrial food webs. Quantitative data of the transfer of elements into ants is needed, e.g., for developing improved radioecological models. In this study, samples of red wood ants (genus Formica), nest material, litter and soil were collected from a former uranium mining site in Eastern Finland. Concentrations of 33 elements were analyzed by Inductively Coupled Plasma-Mass Spectroscopy/Optical Emission Spectroscopy. Estimated element concentrations in spruce needles were used as a proxy for studying the transfer of elements into ants via aphids because spruces host the most important aphid farms in boreal forests. Empirically determined organism/medium concentration ratios (CRs) are commonly used in radioecological models. Ant/soil CRs were calculated and the validity of the fundamental assumption behind the of use of CRs (linear transfer) was evaluated. Elements that accumulated in ants in comparison to other compartments were cadmium, potassium, phosphorus, sulfur, and zinc. Ant uranium concentrations were low in comparison to soil, litter, or nest material but slightly elevated in comparison to spruce needles. Ant element concentrations were quite constant regardless of the soil concentrations. Non-linear transfer models could therefore describe the soil-to-ant transfer better than conventional CRs.

Analysis of 238U, 226Ra, and 210Pb transfer factors from soil to the leaves of broadleaf tree species

This analysis of 238U, 226Ra and 210Pb transfer factors from the soil to the leaves of different native broadleaf trees at sites previously modified by uranium presence and at the site of background radioactivity levels, was conducted using data from a few available studies from the literature. The broadleaf tree species Quercus ilex, Quercus suber, Eucalyptus camaldulensis, Quercus pyrenaica, Quercus ilex rotundifolia, Populus sp. and Eucalyptus botryoides Sm. at the affected sites and Tilia spp. and Aesculus hippocastanum L. at the back ground site were in cluded in the study regardless of the deciduous or evergreen origins of the leaves. In the papers cited here, data about basic soil parameters: pH, total Ca [gkg-1], sand [%], and silt + clay [%] fractions were also available. All the collected data of activity concentration [Bqkg-1] dry weight in the soil (n=14) which was in the range: 22-6606 for 238U, 38-7700 for 226Ra, and 37-7500 for 210Pb, and the tree leaves in the range: 2.7-137.6 for 238U (n=10), 2.6-134.2 for 226Ra (n=14), and 27-77.2 for 210Pb (n=14), indicated that it was normally distributed after log-transformation. The present study was conducted under the hypothesis that biological differences between the examined broadleaf tree species have a lesser influence on the transfer factors of the investigated radionuclides from soil to tree leaves compared to the impact of the soil parameters and radionuclides activity concentrations in the soil. Consequently, it was examined whether 238U, 226Ra, and 210Pb soil-to-leaves transfer factor values for average broadleaf species could be predicted statistically in the first approximation based on their activity concentration in the soil and at least one basic soil parameter using multiple linear regression.

Transfer of elements relevant to radioactive waste into chironomids and fish in boreal freshwater bodies

Information on transfer of elements and their radionuclides is essential for radioecological modeling. In the present study, we investigated the transfer of Cl, Co, Mo, Ni, Se, Sr, U and Zn in a boreal freshwater food chain. These elements were selected on the basis that they have important radionuclides that might be released into the biosphere from various stages of the nuclear fuel cycle. Water, sediment, chironomid larvae (Chironomus sp.), roach (Rutilus rutilus) and perch (Perca fluviatilis) were sampled from two ponds near a former uranium mine and one reference pond located further away from the mining area. Concentrations measured in water, sediment and the three animal species indicated the importance of sediment as a source of uptake for most of the elements (but not Cl). This should be considered in radioecological models, which conventionally predict concentration in aquatic organisms from concentration in water. The results also show that the assumption of linear transfer (constant concentration ratio) may not be valid for elements into fish. The results of this study show that further basic research is needed to understand the fundamental processes involved in transfer of elements into freshwater organisms in order to develop radioecological models.

Substratum influences uptake of radium-226 by plants

Radium-226, an alpha emitter with half-life 1600 years, is ubiquitous in natural environments. Present in rocks and soils, it is also absorbed by vegetation. The efficiency of ²²⁶Ra uptake by plants from the soil is important to assess for the study of heavy metals uptake by plants, monitoring of radioactive pollution, and the biogeochemical cycle of radium in the Critical Zone. Using a thoroughly validated measurement method of effective ²²⁶Ra concentration (EC_Ra) in the laboratory, we compare EC_Ra values of the plant to that of the closest soil, and we infer the ²²⁶Ra soil-to-plant transfer ratio, R_SP, for a total of 108 plant samples collected in various locations in France. EC_Ra values of plants range over five orders of magnitude with mean (min-max) of 1.66 ± 0.03 (0.020-113) Bq kg^-1. Inferred R_SP values range over four orders of magnitude with mean (min-max) of 0.0188 ± 0.0004 (0.00069-0.37). The mean R_SP value of plants in granitic and metamorphic context (0.073 ± 0.002; n = 50) is significantly higher (12 ± 1 times) than that of plants in calcareous and sedimentary context (0.0058 ± 0.0002; n = 58). This difference, which cannot be attributed to a systematic difference in emanation coefficient, is likely due to the competition between calcium and radium. In a given substratum context, the compartments of a given plant species show coherent and decreasing R_SP values in the following order (acropetal gradient): roots > bark > branches and stems ≈ leaves. Oak trees (Quercus genus) concentrate ²²⁶Ra more than other trees and plants in this set. While this study clearly demonstrates the influence of substratum on the ²²⁶Ra uptake by plants in non-contaminated areas, our measurement method appears as a promising practical tool to use for (phyto)remediation and its monitoring in uranium- and radium-contaminated areas.

Approaches to modelling radioactive contaminations in forests - Overview and guidance

Modelling the radionuclide cycle in forests is important in case of contamination due to acute or chronic releases to the atmosphere and from underground waste repositories. This article describes the most important aspects to consider in forest model development. It intends to give an overview of the modelling approaches available and to provide guidance on how to address the quantification of radionuclide transport in forests. Furthermore, the most important gaps in modelling the radionuclide cycle in forests are discussed and suggestions are presented to address the variability of forest sites.