Radionuclides on the Move: Insights into Water-Bound Soil Transport Processes

In Germany, radionuclide exposure is calculated for various scenarios according to derived international standards (AVV-Tätigkeiten 2020). The radiation protection law and the relevant administrative regulations (StrlSchG, StrlSchV) define exposure limits and the implementation of the calculations. These administrative regulations are used in authorisation procedures and in the prospective determination of the expected exposure of individuals in the population. These regulatory measures are intended to ensure the protection of the population from additional radioactive radiation within the framework of the 10 µSv criterion of the IAEA (IAEA 1988 Safety Guides No. 89). Understanding the underlying mechanisms of radionuclide transport in defined ecosystems is pivotal to achieve the best-estimated scenarios.

Contamination of the soil can occur through air or water discharge. Discharge via air can result in dry or wet deposition. In the case of discharge via water, contamination can occur through artificial irrigation, sediment deposition from irrigated fields or contamination of soils in floodplains. All these pathways are considered relevant for assessing radionuclide fate and transport through soil.

In the current project, the transport processes of these contamination pathways on cropland and pasture are analysed in more detail aiming to identify the driving parameters and review all processes that have not yet been included in the equations in the calculation bases. Soil properties and hydrogeological characteristics play an essential role in water-bound transport in the soil. Soils are divided into different soil horizons (O, A, B, C and R horizons) with different properties, which are subject to seasonal changes as well as changes caused by cropland use and pasture. For dissolved substances, chemical processes such as speciation and complex formation, solubility and solution kinetics, retention by sorption and redox reactions play an important role, often strongly dependent on pH values. In addition to chemical processes, physical processes such as advection, diffusion, dispersion, and capillary rise are also relevant. At the root-soil interface, biological processes such as root exudation, mycorrhizal symbiosis (fungal activity), root-hair interactions and plant-controlled water movements within the plant are increasingly important. In terms of plant species, a distinction is made between leafy vegetables, root vegetables and grains.

The aim of the work is to take an interdisciplinary and holistic look at possible contamination pathways in soils by means of a system analysis. The BIOMASS process involving the definition of a FEP (Features, Events and Processes) list and visualizing it in an interaction matrix for the analysis. This conceptual model will be used to implement the appropriate mathematical models for each interaction between compartments. The concentration in the soil will then be calculated using a compartment model or numerical groundwater models. Different methods and models will be analysed and applied in individual cases. The key challenges are the different scales of the area to be analysed, the heterogeneity of the soil and the general uncertainty in the data.