The prediction of the fate of contaminants is agricultural and natural soils is highly dependent on numerical models. Yet, current models can be highly unstable when the spatial variability in the soil properties are, rightfully, considered. This leads to unreliable prediction of contaminant levels and of the dynamic of reactions, which affects, for instance, the evaluation of green gas emission for soils. This represents the main issues that the project aimed to address.
Today, most of the world food supply depends on the use of agrochemicals. The massive transformation of atmospheric free nitrogen (N) into a reactive form for agricultural use and its transfer into water systems led to a threatening disturbance of the N-cycle. First, an excessive amount of nitrate (NO3-) is leaching from agricultural lands, which have led to the long-term deterioration of groundwater quality in many basins putting at risk the health of exposed human populations and of dependent ecosystems.
Moreover, nitrate in the subsurface is in many cases consumed by microorganisms under anaerobic conditions. A denitrification process occurs, which leads to the reduction of NO3- levels, but also to the production of nitrous oxide (N2O), a gas with a greenhouse effect about 300 times greater than carbon dioxide.
Improving our representation of the subsurface component of the N-cycle is, therefore, of immediate importance to secure our access to fresh water and to better assess all sources of climate change.
In this context, the project aimed to contribute to two vital challenges in Denmark, the EU, and worldwide: (1) Protect groundwater quality by providing reliable and versatile numerical solutions for complex reactive systems in complex soils; and (2) better understand the anthropogenic emanation of nitrous oxide by accurately simulating processes involved in subsurface nitrogen transformation.
After its completion, we can state that the project allowed the successful development of numerical models that remains efficient and stable under complex conditions representative of natural soils. These models were used to identify the main processes controlling transport and reactions in heterogeneous soils. Among other, we concluded that reactive systems, such as those involved in the bio-reduction of nitrate, are dependent on the heterogeneity in the soil physical parameters, which is itself dependent on the infiltration-induced water fluxes and the spatial variability in the diffusion process.
Main conclusions of the action:
- The newly developed Lagrangian methods allow to simulate conservative and reactive transport in unsaturated soils in a more reliable way than previously used modeling approaches;
- Diffusion is a key transport process that has to be well described to properly assess contaminant fate in soils;
- Reactions such as denitrification in soils are highly dynamic and are in great part controlled by the spatial variability of the soil physical parameters, which forms hot spots and hot moments;
- Upscaling transport and reactions in soils required to account for the dynamic and combined effect of heterogeneity, advection, and diffusion.
