Motivations et présentation du programme « Bassins versants et transmission des pollutions au littoral »

As for its physical layout, Brittany is characterised by a very dense water system, catchment areas that are mostly of small size (a few dozen square kilometres) and a very significant line of coasts with regard to its surface area. These environments, very close and interdependent, are rich with life, and yet have been affected for many years by dramatic incidents of contamination: the proliferation of marine green algae and the degradation of the quality of the surface fresh waters are the most visible manifestation of this. Among the incidents of contamination that affect these environments—many of them are brought mainly and crucially by continental waters—are those originating from agriculture. Over the last 30 years, Brittany has experienced very significant growth in its agricultural production. This growth has been made possible by a very significant increase in the inputs used (fertilisers, animal feed) and an escalation of production methods. The overall imbalance between the contributions to the fields and their needs is now well known, just as is the concept of SSZs: Structural Surplus Zones. This growth has caused damage to the environment: eutrophication of coastal waters by nitrates, of continental waters by phosphorus, deterioration in fresh water quality from nitrates and pesticides... Of course, contaminants from agricultural sources are not the only cause of the environmental damage observed. The difficulty of recovering the quality of the environment is explained in part by their being spread throughout the territory. Actions taken against other sources, particularly industrial and household pollution, even if they have to go on, give rapid results that are often immediately noticeable on the water quality downstream from discharge points. The 80s and 90s witnessed an awareness of these issues. The 1994-1998 State-Region Planning Contract led to coordinating the research activities conducted on this very theme of transmission of contamination from the catchment area to the shoreline by public research organisations: BRGM, CEMAGREF, ENSP, IFREMER, INRA, ISAMOR. This programme's main objectives were to understand the mechanisms involved in the transfer of the contaminants to the waterways and through the waterways to the shoreline. Within this framework, the programme aimed to identify the variables and parameters that influence these transfers, whether they are meteorological, geological, hydrological, or agricultural... and to quantify and rank their influences. Also studied at the same time were the possibilities of modifying or reducing the transfers, or of improving the processes of natural decontamination (damp areas, groundwater...). Moreover, research has been done on measuring the effectiveness of a shoreline marsh in reducing green tides on beaches located downstream. An inventory of sites suitable for fitting out such marshes appropriate for the shoreline of Brittany has been done. This research was carried out on two catchment areas: Coët-Dan's at Naizin in the Morbihan, followed for more than 20 years now by CEMAGREF, and Kerharo's, a tributary of Duarnenez Bay in Finistère. In the respective shoreline areas, the flows of nitrogen brought to the sea by the streams have been monitored, as well as their relationship to the size of the green tides that develop there every year. The research focused on nitrates, due to the deterioration of the quality of water meant for supplying drinking water and to its direct influence on the growth of green algae, as well as on phosphorus, responsible for the eutrophication of fresh water. Research was done on pesticides and bacteria of sanitary interest from farms, subject little studied before that time. This programme did not intend to be exhaustive. It focused on the parameters whose influence on environmental quality had been brought to light.