The increase of inputs caused by the intensive agriculture in Brittany led to nitrogen excess compared to plants needs. This has resulted in nitrate pollution within aquifers and hydrological systems causing environmental and water consumption issues. One consequence is the proliferation of green algae on the coast.
In this context, modelling studies were developed on the most impacted catchments using the TNT2 model developed by INRAE (Patrick Durand, UMR SAS). This model allows to simulate water and nitrogen budgets at fine resolution taking into account agricultural practices. However, nitrate behavior within aquifers is still uncertain due to the complexity of this media. Aquifers properties (volume, permeability, denitrification potential) are difficult to estimate and highly heterogeneous. Nevertheless, because of their huge volume, aquifers strongly controls the long-term response to agricultural practices change.
This study published in Science of The Total Environment in October 2021, driven by Luc Aquilina, was financed by the Région Bretagne, the Loire-Bretagne Water Agency, and with the support of the Creseb. It consisted to complete available observed data (river streamflow, nitrate concentrations in rivers) with measurements of groundwater residence times. Then, a parsimonious modelling approach was conducted. Results allowed to:
• reproduce the main processes driving water and nitrate transit time within catchments
• define the main aquifer properties
• interpret these properties within a geological framework
• test two extreme scenarios of future agricultural practices.
The scheme illustrates the main processes included in the model. Representing these processes in a simple way but with a physical basis allowed to reproduce different observed data: rivers streamflow, groundwater residence time (sampled thanks to the MORAQUI project with Aurélie Guillou, Camille Vautier and Virginie Vergnaud, OSUR) and nitrate concentrations in rivers. A modelling approach was developed by Luca Guillaumot, Jean-Raynald de Dreuzy, Jean Marçais, Rémi Dupas and Camille Bouchez. Each catchment (~30 km²) was represented by an equivalent hillslope (average slope of 6% and average length of 1 km). Water transfer in the aquifer follows the groundwater flow equation and reproduces natural groundwater transit time stratification. This study showed that groundwater transit time stratification controls the long-term temporal variability of observed river concentrations. Along the flow path, denitrification occurs in function of transit time. Finally, the aquifer overflows in the downstream part of the hillslope reproducing thus groundwater flow toward springs and rivers. When groundwater recharge occurs (usually between November and April), aquifer storage increases as well as the downstream saturated area feeding springs and rivers. Recharge rates, and associated nitrate concentrations, were obtained by simulations from the hydrological and nitrogen TNT2 model realized by Patrick Durand (INRAE, SAS).
On the three studied catchments, mean water transit time varies between 10 and 32 years and is associated with a limited denitrification within the aquifer (10-20 %). For the Ris catchment, 60% of nitrate infiltrated between 1955 and 2010 reached the river. In average, water transit time was around 21 years, but 10% of the water reached the river in less than 1 year. So, one part of the catchment response is fast. On the same period, 9% of nitrate was removed by denitrification while 31 % of the nitrate mass was still stored in the aquifer in 2010.
Two extreme scenarios were tested (until 2050):
• a business as usual scenario where future nutrient inputs follow the trend observed during the last years of the simulation
• a scenario following the same behavior until 2025, then nutrient inputs are removed.
Results from the first scenario highlights that nitrate concentrations in rivers are not decreasing significantly anymore since several years. More efforts will be necessary to reduce again river concentrations. As illustrated by second scenario, an important change in agricultural practices will cause a fast reduction of nitrate concentration in rivers, 20-30 % of the final reduction (equivalent to the change in the recharge concentration) will occur during the first 5 years. Then, the long water transit time and the denitrification will involve a slow decrease of the river concentration, 60-80% of the final reduction will be accomplished in 25 years. This response time will be similar whatever the amplitude of the change in agricultural practices.
This study constitutes an insight on the modelling approach and on the functioning of aquifers as well as their relation with rivers. Results are also important from a practical point of view. They allow to quantify nitrate removal potential in each catchment and to demonstrate that a change in agricultural practices will involve a fast answer (~1yr) and a slow answer (from 20 to 80% of the decrease will occur during the next 5 to 15 years). The last part of the aquifer response will be the slowest and will require decades. As we inherit during 10 or 20 years of past practices, what we do today will control future trajectories.
Luca Guillaumot, Jean Marçais, Camille Vautier, Aurélie Guillou, Virginie Vergnaud, Camille Bouchez, Rémi Dupas, Patrick Durand, Jean-Raynald de Dreuzy, Luc Aquilina, A hillslope-scale aquifer-model to determine past agricultural legacy and future nitrate concentrations in rivers, Science of The Total Environment, Vol 800, 2021, 149216, doi:10.1016/j.scitotenv.2021.149216
 Topography-based Nitrogen Transfert and Transformations : modèle agro-hydrologique distribué maillé, basé sur le couplage d’un modèle hydrologique et d'un modèle agronomique
Beaujouan, V., Durand, P., Ruiz, L., Aurousseau, P., & Cotteret, G. (2002). A hydrological model dedicated to topography‐based simulation of nitrogen transfer and transformation: rationale and application to the geomorphology–denitrification relationship. Hydrological Processes, 16(2), 493-507. doi : 10.1002/hyp.327