Dynamic Modelling of Soil-Vegetation-Atmosphere Transfer (SVAT) in perennial ecosystems (PhD)

Long-lived radionuclides released from an underground nuclear waste repository may reach the surface ecosystem via the groundwater and would thereby constitute a potential risk for humans and biota through different pathways. Due to their perennial character, forest vegetation or perennial vegetation systems in general, may affect radionuclide dispersion and distribution in the biosphere in the long-term.

The biogeochemical cycling and storage of elements in an ecosystem is dependent on a large number of variables, such as climate, geological heterogeneity, flux of water and vegetation.  The interactions between elements and the living and non-living components of an ecosystem play an important role in defining the fluxes and storage of elements in the ecosystem. By describing the distribution of the elements in an ecosystem, it is possible to describe the potential pathways to humans and biota. However, in most cases it is not possible to study the actual radionuclides themselves. As a result, the distribution patterns of naturally occurring radionuclides or their stable isotopes is used to study the long-term behaviour of the radionuclides that may originate from nuclear waste. The chemical behaviour of some radionuclides is similar to other elements, such as macronutrients or trace elements, and these analogues may thus be utilised for modelling purposes as well.

An important pathway for radionuclide distribution in the biosphere is through the water fluxes between the different biosphere compartments, like upper groundwater layers, soil and vegetation. Once assimilated into the vegetation tissues, the radionuclides most likely follow the path of the organic matter; i.e. growth of plant organs and litterfall. A comprehensive descriptive ecosystem model, describing pools and fluxes of organic matter and water, may therefore be useful to describe and quantify accumulation and transfer of radionuclides.

The research objective is to make a long-term assessment of the influence of a forest vegetation system on the radionuclide dispersion and accumulation in upper-soil layers, and the subsequent exposure to humans and biota. To assess the transport and accumulation of bio-available contaminants in a perennial ecosystem, this study will make use of a Soil-Vegetation-Atmosphere Transfer (SVAT) model.

SVAT models explicitly consider the role of vegetation in affecting water, energy and carbon balance by taking into account its physiological properties. These models are often process-based and therefore suitable to simulate  the water, carbon fluxes in natural and managed ecosystem under different environmental conditions, including climate change conditions. The rational for using the carbon, energy and water fluxes is that they can be used as a proxy for the rates at which contaminants will be partioned in the environment.

The descriptions of pools and fluxes of matter will provide insight into where the largest flux of organic matter, and thus bio-available radionuclides, are to be found. It will therefore help to identify the magnitude of the potential assimilation and accumulation of bio-available radionuclides and may work as a basis for more detailed transport  and exposure modelling at longer timescales.