Le programme CORMORAN-INRA : de l'importance du milieu physique dans la régulation biogéochimique de la teneur en nitrate des eaux superficielles

The CORMORAN-INRA programme: the role of physiography on the biogeochemical control of nitrate level in surface water

¹ INRA-ENSA Laboratoire de recherche de la chaire de science du sol 65, route de Saint-Brieuc, 35042 Rennes Cedex
² Géosciences, Université de Rennes 1

Résumé

Le programme CORMORAN (Caractérisation, ObseRvation et MOdélisation des tRansferts en milieu Agricole iNtensif) a pour objet de contribuer à la connaissance et à la maîtrise de la pollution agricole diffuse. Il est centré sur le bassin versant du Coët-Dan à Naizin, bassin de référence inclus dans le réseau des bassins de référence du MRES. Des études intensives sont menées par l'INRA depuis 1992, sur les plans géomorphologique, pédologique, hydrologique, géochimique et agricole.

Les principaux résultats portent sur :

- le rôle de la topographie dans le contrôle des facteurs du milieu. L'intérêt de cette approche, basée sur une variable facile d'accès - l'altitude -, est de permettre la transposition et l'extrapolation aisées des résultats ;

- la définition des "zones actives", par rapport à différents processus, et notamment le transfert de polluants ;

- l'importance des zones de sol hydromorphes dans le contrôle de la pollution azotée et les conséquences en terme d'aménagement.

On peut ainsi proposer un modèle topographique de la distribution des sols, de la genèse des flux hydriques, du contrôle des flux géochimiques. Enfin, des résultats récents se rapportant au contrôle de la pollution azotée par les zones de sols hydromorphes sont développés.

Abstract

The CORMORAN programme (characterization, observation and modelling of water and pollutant movement in an intensive farming zone) is focused on the study and control of non point source agriculture pollution. The study site is the catchment of Coët-Dan, which is a part of the european network of representative and experimental basins. The investigations carried out by the INRA since 1992 span over various disciplines: geomorphology, soil science, hydrology, geochemistry and agronomy.

The main fields of research are the following:

- the role of the topography on the soil distribution and on the hydrology. This approach, based on easy-to-measure parameters, allows the transposition and the extrapolation of the results;

- the identification of active zones, regarding different processes, and specifically the movement of pollutants;

- the role of hydric soils in the control of nitrogen pollution, and the consequences for the management of farmland catchments.

The more significant results obtained so far in these fields are presented.

The soil distribution is tightly controlled by the topography. The soils of the catchment can be described as a spatial arrangement of a limited number of horizon types with genetic relationships. Regarding their hydrodynamic properties (permeability and sorptivity), these horizons have been classified into "building blocks": this will allow to define physically based parameters for spatially distributed hydrological modelling. These horizons types are widely represented in Brittany. The soil moisture distribution have also been related to topography via DTM analysis, radar and field measurements. In summary, the catchment shows two domains with contrasting drainage regime: the hillslopes with homogeneous soil moisture and well drained soils, and the bottom lands with variable water content and hydromorphic soils.

Classically, the bottom lands are supposed to reduce nitrate concentrations in water via denitrification and vegetation uptake. In this study, the denitrification potential rate of hydromorphic soils have been found very high (10 to 50 kgN.ha^-1.day^-1). The redox conditions in soil water are highly variable in time and space, with complex geochemical controls. The importance of these wetland areas has been highlighted by a significant and negative correlation between the percentage of bottom lands and the nitrate concentrations of surface waters in different subcatchments. However, the analysis of water pathways using natural tracers during storm events shows that a significant proportion of the fluxes contributing to runoff "by-passes" the bottom lands. This is partly due to the network of anthropogenic structures such as roads, ditches, etc...

It is concluded that the bottom wetlands play an important role in the control of nitrogen pollution, although their epuration potential is not fully used. Beside the necessary reduction of agriculture inputs, an adequate landscape management taking into account the actual functionning of these areas may well mitigate significantly the adverse effects of intensive farmland.