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Maxime Berthelot
Published on
Oct. 4, 2024 at 6:00 p.m.
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Is a solution for the future about to be found to combat groundwater pollution resulting from agricultural activities ? In Seine-et-Marne, a territory which has more than 2,400 farms distributed over 340,000 hectares of culturesthe issue is taken very seriously. For almost 15 years, an experiment is carried out in the heart of Brie by researchers from the National Research Institute for Agriculture, Food and the Environment (INRAE) and l’association Aqui’Brie. In collaboration with the Water Agencythe mixed union of the Four Valleys of Briethe Nangissian Brie et around ten farmersfour artificial wetland buffer zones (ZTHA) were thus created in 2010 in Rampillon, over the 4 km2 from the slope of ru des Gouffres.
Their goal? Improve water quality of the Champigny tablecloth. The latter is polluted speak runoff from agricultural crops which rush into it via natural infiltration holes present in certain ditches, and bring part of it drainage water agricultural which include of the nitrates and phytosanitary products.
« He built three small ponds et a larger pond of 0.5 ha up to one meter deep, explains Julien Tournebize, research engineer at Inrea. The latter is powered by a diversion system allowingintercept these polluted waters to store them, decontaminate them naturally, before letting them leave. »
Depolluting water through the combined effect of the sun, plants and bacteria
To do this, the objective is to promote contact time between runoff water and the microbiota of the wetland. The longer the water is stored, the more nitrates will be able to be degraded under the combined effect of the sun, the temperature of the water, the plants and the bacteria that develop there.
If plants are going to have the capacity to take the nitrogentheir development depends on the seasons. Bacteria, on the other hand, are present all year round in sediments. They work the hardest by depriving the nitrates of their oxygen.
” More efficiency remains seasonal and subject to periods of spreading. For example, we have already noticed the presence of 0% nitrate at the exit of the wet zone in spring and summer. If rainfall remains normal, 80% of the pollution may even have disappeared after five days, observes Julien Tournebize. This is less the case in autumn or winter, where lower temperatures and greater precipitation reduce the effectiveness of the natural filtering provided by ZTHA. »
On average, the results are encouraging. On the Rampillon ZTHA, researchers were able to observe a overall drop in nitrate levels of 15%. “In terms of value, that represents 40 to 45 mg of nitrates per liter at the exit of the wet zone, compared to 55 to 60 mg/l at the entrance,” specifies Julien Tournebize.
Pesticide rates down 37%, but…
For the phytosanitary productson the other hand, the battle is tougher, because more complex. If the results of multi-year studies conclude that an average reduction of 37% between incoming and outgoing flows, the effectiveness of the ZTHA changes completely when we break down the analysis product by product.
« Environ 100 different molecules are used, around forty of which are found in water, specifies the INRAE researcher. For some, ZTHA will be 100% effective, for others, it will be useless. »
As with nitrates, degradation by the action of bacteria is one of the keys. But given the complexity of phytosanitary molecules, the latter is sometimes only partial: “The bacteria will be able to cut a hair from the molecule, but not an arm”, illustrates Julien Tournebize.
For example, the average reduction in the rate of growth regulators is 100%, while that observed on THE fungicides is 57%, that of herbicides by 30%, and that of insecticides by only 10%. “We therefore encourage farmers to use molecules that tend to degrade more easily, which is not always easy,” recognizes the researcher.
Another solution: l’absorptionmore effective: “As the flow rate is slower in the ZTHA, certain molecules will attach to the particles in the wet zone. They remain trapped there, but the downside lies in the fact that we are not yet able to know what happens to them once stored in the ground, because the quantities recorded are insufficient. On the 600,000 m3 of water passing through the experimental site annually, only 500 g of pesticides were detected. »
Pesticides trapped in the ground, but what impacts on wildlife?
The INRAE teams therefore looked into the potential impact of pesticide storage in the sediments of the ZTHA, particularly on wildlife reproduction aquatic. “We find there eight species of amphibians of the 17 present in Île-de-France, details Julien Tournebize. However, in some, we noticed the presence ofan enzyme representative of toxic stressnotably green frogs which reproduce in May, at a time when phytosanitary measures may be more important. This is something that is not found in a healthy pond. »
Conversely, and this is clear proof that the activity of decontaminating the wetland produces positive effectsTHE freshwater shrimp located at the exit of the site seem to be in better health than their congeners located at the entrance: “They eat better et move more easilyconfides the specialist, who also highlights the refreshing action of ZTHA. In time of heat wavewe noted up to 5 degrees cooler within a perimeter of 100 m around the site. This is an avenue worth exploring. »
Whether it is improving water quality or from environmental protectionartificial wetland buffer zones are therefore promising, but studies must continue to avoid potential harmful side effects on biodiversity. In Rampillon, the work of scientists will continue at least until 2028while two similar developments are planned in Brie Nangissienne.
“An area is being studied in Saint-Méry, in the Val d’Ancœur, and another in the communes of Nangis and Fontains,‘Buisson aux Lièvres Pond. The construction of these two sites should take place at horizon 2025-2026“, announces Julien Tournebize.
Before concluding: “Even if ZTHAs are effective, they will not be sufficient. They must be accompanied by a global reflection on the changes in agricultural practices towards less use of phytosanitary products, the cause of groundwater pollution. »
The artificial wetland buffer zones (ZTHA) of Rampillon can be discovered on October 5 during a hike offered as part of Pink October. Information: 01 64 08 06 17.
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