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. In the current situation the radiological, environmental and health risks of this contamination are negligible, but recently plans were proposed to change the land use of these contaminated areas. In particular, the installation of floodplains by breaking or natural degradation of the dykes will affect the mobility of the present contaminants. The periodic flooding of the soil will lower the redox potential and alter the speciation of the 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 the chemical and physical processes must be taken into account to assess the risks of the HM and RN upon the proposed land use change.
It is well established that a lowering of redox potential my increase the mobility of some elements (cesium, arsenic) while other elements are immobilised under anaerobic conditions (chromium). For several elements such as zinc and cadmium, waterlogging causes contrasting trends on metal mobility with the net effect defined by site-specific conditions (soil and soil solution), but also by the duration of the waterlogging. This PhD proposal is set-up to address the physico-chemical processes affecting the mobility of the HM and RN in this specific area, thereby allowing a better appraisal of risks associated with the proposed land use change. The novelty of the approach is that all contaminants are studied in the same experiments (mixed contamination approach) and that geochemical modelling of the speciation is included as a comprehensive framework.
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, an exploratory approach will be combined with geochemical modelling. This exploratory approach consists of setting up dedicated experiments with controlled flooding of soils. This will involve a series of samples of the contaminated area, taken in different soil horizons up to 1 m depth along a transect with a hydrologic gradient and with different levels of contamination. This set-up, which comprises both batch and column experiments, is combined with in-depth characterisation of the changing physico-chemical conditions, in order to couple observed contaminant (im)mobilisation with changing environmental parameters. Numerical modelling based on state of the art databases and implementing all important sorption and speciation processes (with different geochemical models) will aid in elucidating the change in contaminant behaviour in these systems. In oxic soils, these kind of models have been validated yet, but implementation of the redox processes in a combined numerical model for waterlogged soils is still a challenge.