Résumé
It has been known publicly since the 1960s and the publication of a controversial book Silent Spring by Rachel Carson that agricultural practices have a direct impact on water ecosystems. Despite the fact that strict regulation applies to the making, selling and use of phytopharmaceutical products - PPP's or pesticides - contamination of rivers and underground water still occurs frequently. France is amongst the top three pesticide users in the world, along with the USA and Brazil, despite having a much smaller geographical surface dedicated to agriculture (but with a higher productivity). Scientific monitoring shows that the concentration of these substances in rivers and groundwater often surpasses the threshold permitted by national and European law. It tends to be the same in drinkable water. Even though the effect on the entire water ecosystems and human health have not been clearly proven yet because of the complex biochemical processes operating, the problems raised by the use of pesticides have finally been put on the political agenda. Whistle-blowing and growing pressures from campaigning groups are now triggering government action. In 2005, the French Ministry of Agriculture and the Ministry of Environment financed an important research study of the subject, the result of which was the launch of the Inter-ministerial Programme for the Reduction of Pesticides which aims uppermost at the reduction of inputs (a target of 50% of reduction was announced and is now strictly applicable since summer, 2008). It's very unlikely that a definite ban will be declared on pesticides even though alternatives are being encouraged. Firstly, what the effects on crops would be is not clearly known because industrialisation has made agriculture very dependent on PPPs; and second, agrochemical professionals will certainly lobby to sell their molecules. To reduce pesticide intrants in agriculture is one way to deal with middle term expected effects caused by the remanence of the products. Scientists think that the molecules of tomorrow will be mostly undetectable which will negate the effects of a PPPs reducing plan. This is why some of them have been trying for a few years to promote any disposition that would facilitate the dissipation of PPPs and would operate whether the molecule is detectable or not. Buffer zones such as grass and wood strip are a well known solution and widely installed but limitations are reported, especially for waterlogged regions, which cover one third of the total arable land in France. To overcome this problem other techniques or devices such as artificial wetlands -which are already used to treat sewage and mitigate pollution - are currently being tested. Several experiments are being carried out to determine which solutions under which conditions would the most efficient. Thanks to recent research, there is better understanding on how pesticides are mobilised and transported in streaming down to the nearby rivers. Better design and management strategies for processing pollution in these artificial wetlands could be thought-up. While these bio-attenuation devices are still at the R&D level, water agencies and local governments have been showing a lot of interest in them. However, these devices should not be viewed as perfect black boxes. Their performance should be assessed in order to understand better what exactly is causing the deterioration of pesticides. Scientists insist on the fact that research and development should go hand in hand. The aim of this paper is to present the analysis of the different steps which led to the development of such a device in a small catchment (400ha) in the North-East of France, a zone of subsurface artificial drainage and intensive farming (pesticides use included) . The TRUSTEA project it describes started in 2006 when a local association in charge of the protection of the 2 600 km2 local groundwater contacted the Cemagref for scientific and technical support. In fact, several cities had to ask for an exemption from the law to supply drinkable water to their inhabitants. Indeed, the soil is made of limestone and locally covered by a thin impervious clay layer; about forty natural sinkhole in karstical context have been identified where ditches flow in totality and connect the surface polluted water to groundwater. In view of the reinforcement of legislation due to the EWF, the local association hired agronomists and hydrobiologists to fulfil its difficult mission (making visible a problem that was not visible before and raising consciousness about groundwater and drinkable water quality). Since then, it regularly sends out consultants into the fields to increase farmers' awareness about the environmental problems generated by pesticides. It also helps them reduce their phyto-phamaceutical inputs. It provides an agronomic follow up service, substituting in this role from the standard and institutional one delivered by professional organisations. The construction of artificial wetlands for limiting the contamination was a complementary solution the association wanted to investigate.