On the beach of Vallais de Saint-Brieuc, covered with toxic green algae, a sign invites you to protect yourself from the gases they emit, on July 10, 2019. LOIC VENANCE / AFP
A report published in early February by the Observatory of the environment in Brittany reveals the almost omnipresence of chemicals in its waters.
An exorbitant figure, which confirms the dynamic initiated for years. The critical state of the waterways of Brittany is detailed in the report of the Observatory of the environment of the region updated on February 3, “Key figures for water in Brittany“. The structure, co-created in 2007 by the State and the Region, has the task of drawing up regular assessments of the regional situation, with a view to improving public policies in environmental matters. It reports on the poor condition of water in all its forms: small and large rivers, ponds, groundwater… Almost all of them contain pesticide residues, in particular because of intensive agriculture.
Of the 220 stations where the samples were taken, more than 99% of the analyzes show the presence of different types of pesticides. They show traces of at least one herbicide, or a metabolite (product resulting from the degradation of pesticides in the natural environment, editor’s note). These results are reaching records in some circles: “83 different pesticide substances were detected simultaneously on the same monitoring station“, can we read in particular.
At the top of the ranking of the most present substances, we find glyphosate. First herbicide sold in Brittany, it was detected in no less than 35% of sampling stations. Its degraded form – or metabolite – was detected in 65% of the stations evaluated. A finding that is all the more worrying, when we know that the metabolites of certain substances persist longer than their parent cell.
Green Algae Tides
One of the main consequences of the spillage of pesticides in the waters is eutrophication, recognizable by the tides of green algae. In aquatic ecosystems, algae proliferate in the event of excessive nutrient supply, leading to a degradation of the natural environment. They are potentially toxic and causenegative consequences on the uses of drinking, industrial and recreational water“. The cause: phosphorus, which is abused by intensive agriculture in this region: the report indicates that “phosphorus levels remain high in the majority of courtyards.»
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Revealing the complexity of this inventory: nitrate appears as “the limiting factor to be favored in order to limit the eutrophication of surface waters and, in particular, the proliferation of green algae.If the proportion of this substance is to be monitored, a balancing act is necessary to temper the excesses of phosphorus while reducing the nitrate content, which also involves risks. Generated by the spreading of nitrogenous fertilizers and slurry, nitrates have seen their concentration drop by 17% from 1995 to 2020 in rivers, and even by 27% in groundwater. “The result of the efforts of the entire profession“, according to the EPO. “The concern is that the average levels stagnate around 32 mg per litre, which is still high“, analyzes Aurélie Mestres for 20 minutes. «40% of the Breton population has received water that does not meet the quality threshold for pesticides“, Details the EPO.
In light of these findings, the Observatory concludes that only 32% of watercourses can be said to be “in good ecological condition“. An average which conceals significant disparities, since it is only 2% in Ille-et-Vilaine against 61% in Finistère who deserve this qualifier. This proportion can be explained, among other things, by very low rainfall, the development of waterways and the intensification of agriculture south of Rennes.