2024-01-03 12:40:07
After the temporary ban on the marketing of oysters from the Arcachon basin and a few “limited” sites in Manche and Calvados, producers point to rainwater management “failing” which would have led to the contamination of the marine environment with norovirus, the pathogen causing gastroenteritis.
The sales ban followed “several cases of collective food poisoning” to norovirus noted by health authorities and occurred just before the New Year holidays, which normally represent one of the busiest economic periods for oyster farmers.
Oysters are animals that filter seawater for food and reproduction, and are therefore very sensitive to the quality of the surrounding water. However, in the event of heavy precipitation, the runoff water which flows into the watershed can be loaded with various pollutants and pathogens, which contaminate the marine environment.
Human origin
“As with most cases of maritime pollution, norovirus comes from land-based activities and is indeed of human origin”summarizes Thierry Lafon, president of the Arcachon Basin Water Defense Association (Adeba) and himself an oyster producer in Gujan-Mestras (Gironde).
In the Arcachon basin, the sanitation network operates according to a so-called “separative” system, with a first system for wastewater and a second for rainwater. The latter flow directly into the natural environment, in particular through watercourses or ditches, kinds of dug and permeable trenches which allow surface water to infiltrate into the soil.
But during exceptional rainfall like that recorded in the Arcachon basin in recent months, rainwater comes out of these natural beds and floods the ” agricultural areas, private plots, roads, etc., and thus takes care of all our human footprints, explains Sabine Jeandenand, general director of services of the intercommunal union of the Arcachon basin (Siba). Water that remains on the surface can also enter the wastewater network, and thus come into contact with human defections.which in turn can carry the norovirus.
By saturating the pipes, the mixture of rainwater and wastewater can then escape into the environment, particularly at manhole covers or retention areas.
A centennial event
“The precipitation that we were able to see at the beginning of November corresponds to a centennial event, that is to say with a probability of occurrence every hundred years, insists Sabine Jeandenand. In just eight days we recorded 200 mm of rainfall, even 250 mm in certain areas, compared to an annual average of 800 mm. » The consequences of this event have only recently been felt due to the flow time in this enormous watershed of 4,000 square kilometers.
“We cannot blame everything on this exceptional rainfall, retorts Thierry Lafon. There were also management faults, development and infrastructure choices made by municipalities without taking into account the morphology of the territory. » The oyster farmer points out in particular the flush water tables, that is to say very close to the surface, which quickly saturate in the event of heavy rain, and the transformation of ditches into narrow pipes, with a more limited flow and capacity, both of which promote flooding.
According to Thierry Lafon, norovirus can survive for around fourteen days in the environment before degrading naturally. As a precautionary measure, the temporary ban should last twenty-eight days from the last contamination noted. “As molluscs filter the surrounding water, the stocks should automatically decontaminate themselves once the water quality is restored”he notes.
Oyster farmers in the Arcachon basin are demanding compensation for the losses caused by the temporary sales ban, estimated at around 8 million euros.
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