The argument comes up regularly in the mouth of the Minister for Ecological Transition: the future basins of Deux-Sèvres will have “positive effects” on groundwater and waterways, according to Christophe Béchu. He hammered it out on Saturday March 25 in the 8:30 am interview with franceinfo, and mentioned it again two days later on France Inter.
>> “Mega-basins” in Deux-Sèvres: but, by the way, what is a basin?
Each time, the minister relies on a report from the Geological and Mining Research Bureau (BRGM). On franceinfo, he claimed that the protocol for installing water reserves had “has been validated by the BRGM which says: it will improve the low water flow during the summer, it is generally virtuous”.
But each time, the minister is contradicted by an opponent of these basins. Thus, the environmentalist deputy for Vienne Lisa Belluco replied to him on Sunday March 26 on franceinfo, assuring that “the BRGM [avait] taken a position publicly by saying stop quoting our study because there are a number of blind spots and in particular no consideration of climate change”.
Who is telling the truth? There is truth and at least truth on both sides, but the balance leans more towards the member for Vienne.
“Positive effects” according to a simulation…
The BRGM has indeed produced a technical report on the consequences of the 16 future substitution reserves which must be dug in Deux-Sèvres. These reserves, called “basins”, will take water from groundwater during the winter, store it and make it available to a few farmers so that they can water their fields during the summer. The office submitted its report in the summer of 2022 and it does indeed conclude, as the minister asserts, that the basins could have generally positive effects on groundwater, watercourses and wetlands.
“The 2021 scenario for the implementation of substitution reservoirs proposed by the Deux-Sèvres Water Cooperative would allow an overall improvement in the level of the water tables”, writes the BRGM, predicting that “the interactions between rivers, wetlands and piezometries [autrement dit, le niveau des nappes souterraines] being strong in the sector studied, any piezometric improvement will have repercussions on the other environments”.
The BRGM also observes “a positive effect in spring/summer” on the waterways “around + 6% flow gain at the basin outlet for the month of July”. This would even rise to +40% for the Mignon watercourse. Other watercourses could be less dry in the summer. In winter, the withdrawal of water from the aquifers would only have an impact “weak and negligible” on rivers, which would fall by only 1%.
As for wetlands such as the Marais poitevin, which depend a lot on the filling of groundwater, these basins “would allow better water retention of wetlands at the bottom of valleys and would improve the flows entering the wet marsh, either by overflow (springs, seepage), or via the interconnection aquifers/border canals and by increasing flows rivers”.
… but the BRGM does not foresee the future
All these observations seem to prove the Minister of Ecological Transition right, yet this is not quite the case, because he uses them in a misleading way to predict the future when this is not what the BRGM does.
The office issued a press release in February 2023 to re-explain its report and give it some context. He does not say, as the affirMrs. Lisa Belluco, not to use it any more, but he says in effect, expressly, that it “is not an in-depth study, nor an impact study of all the possible consequences of the planned water withdrawals. It does not nor is it a scientific research article subject to evaluation by the scientific community”he says.
His report “allows us to assess what would have happened if the replacement reserves had been put in place during the years 2000-2011”. It is therefore a simulation in the past. “Strictly speaking, this reference period does not take into account recent and even less future weather conditions”continues the office.
Empty basins because of global warming?
The BRGM acknowledges that it did not take into account the risks of water evaporation from the reserves – which opponents of basins criticize it for – nor climate change and that it would be “important” to do. He is updating his data to eventually be able to do new simulations that may include the years 2010 to 2020 and climate change.
The office points to another issue with these basins: they cannot be filled in winter if the groundwater level is too low, except that “the recurrence of winter drought periods could repeatedly lead to groundwater levels below regulatory thresholds, compromising the filling of reserves in certain years”.
In Vienne, a department next door, where 30 basins are also to be built, a report predicts that the groundwater level will regularly be too low and that the basins will not be able to take water from it two years out of ten. Finally, the BRGM simulations do not respond to the central argument of those opposed to basins: the question of water monopolization by a few farmers.