Lafarge acknowledged the spillage but insisted it had been the result of sabotage, rather than an intentional act by the company.
The contaminated liquid is composed of a mixture of cement, wastewater treatment liquid and plastic microfibres and comes from a Lafarge site in the east of the city, Europe 1 radio said.
“It's a complete environmental scandal, as we have been working with our partners for a long time to improve the quality of the River Seine,” Paris Mayor Anne Hidalgo said on Twitter.
C’est un véritable scandale écologique, alors que nous travaillons avec nos partenaires depuis de nombreuses années pour améliorer la qualité du fleuve. La Ville de Paris va saisir le procureur de la République pour ces faits graves qui portent atteinte à notre environnement. https://t.co/B7Aij6wPrX
— Anne Hidalgo (@Anne_Hidalgo) September 1, 2020
The city government “will ask prosecutors to open a case for these grave acts that harm our environment,” she added.
Paris prosecutors told AFP they had opened an investigation into pollution of the river with harmful products on August 27th.
In a statement, Lafarge said it was the “victim” and said the spillage was provoked by a “malicious act”.
The cement giant said the leakage has now been stopped and controls reinforced.
Deputy mayor Emmanuel Grégoire said the town hall was “very angry” with the firm, which should have informed the city government of the problem.
This incident “will not be without consequences on our overall attitude on these kinds of sites,” Gregoire added.
Since Tuesday, city-dwellers have been informing Paris authorities of harmful practices of a range of cement-making companies, such as toxic waste outpourings into the river, the official said.
Sanctions are not heavy or dissuasive enough, he added.
Environmental activists have previously accused Lafarge of contaminating the Seine and in February staged demonstrations against the Paris site.
J’ai demandé aux inspecteurs de l’environnement du ministère @Ecologie_Gouv de se rendre sur place. Comme le prévoit la loi, les responsables seront poursuivis et répareront les dégâts.https://t.co/063TbHtEVx
— Barbara Pompili (@barbarapompili) September 1, 2020
Environment Minister Barbara Pompili said on Twitter that a team from her ministry would be going to the scene and those responsible would face justice.
In April 2019, France's Vinci firm was accused of discharging cement wastewater in the River Seine. The company was subsequently fined €50,000.