Impact of land use change on the migration of historical radionuclide and heavy metal contamination in riparian soils of the Grote Nete Valley

In the Grote Nete Valley, certain riparian areas are contaminated with radionuclides (RN) and heavy metals (HM) originating from historical contamination into the tributaries Grote Laak and Molse Nete, which occurred until the 1980’s. Currently the radiological, environmental and health risks of this soil contamination are negligible, but recent plans were proposed to change the land use of these contaminated areas (Sigmaplan). In particular, the installation of floodplains by breaking or natural degradation of river dykes might affect the mobility of the present contaminants. The periodic or permanent flooding of the soil is expected to lower the redox potential and alter the speciation of specific RN and HM. In addition, the larger fluxes of water induced by flooding will enhance the migration of soluble RN and HM. Clearly, the combination of these chemical and physical processes must be strongly considered to assess the risks of the HM and RN upon the proposed land use change. This PhD study will consist of an exploratory approach combined with geochemical modelling in order to identify the processes involved in the (im)mobilisation of RN/HM upon waterlogging and to assess which processes or soil factors are key in determining the net trend. The analytical approach will involve dedicated experiments to understand as well the current status as the future controlled flooding conditions of the soils. As research novelties, the PhD will include special attention to the effects of colloidal transport of some RN and HM. Additionally, improvements for some analytical techniques involving colloidal detection of these elements will be proposed. Numerical modelling based on state-of-the-art databases and implementing sorption and speciation processes (with different geochemical models) is intended to elucidate the changes in the contaminant behaviour in these systems. Most of these models have been already validated for oxic soils. Nonetheless, the implementation of redox processes in a combined numerical model for waterlogged soils is still a challenge.