Post-Mining 2005, November 16-17, Nancy, France 1 THE POST-MINING CONTEXT AT DECAZEVILLE-FIRMI CONCESSION (AVEYRON, FRANCE): ANALYSIS OF IMPACTS RESULTING FROM THE CESSATION OF PUMPING AT THE CENTRAL SHAFT. SURVEY OF VARIOUS SCENARIOS RELATED TO THE WATER LEVEL OF THE PIT LAKE IN THE GRANDE DECOUVERTE. COJEAN Roger1, FRANCO Nicolas2, LAZAREWICZ Jean-Claude3, BLACHERE Agnès4, LEFORT Dorothée4, SORGI Claudia5 1 ECOLE des MINES de PARIS, Institut des Géosciences, Centre de Géologie de l’Ingénieur, UMLV, Bât. IFI, 5 Bd Descartes, Champs-sur-Marne, 77454 Marne-la-Vallée, cedex 2.
[email protected] 2 CHARBONNAGES de FRANCE, Direction Technique Nationale, 4 square François Margand, BP 534, 42007 Saint- Etienne, Cedex 1.
[email protected] 3 CHARBONNAGES de FRANCE, Direction Technique Nationale, BP1, 13590 Meyreuil. jean-
[email protected] 4 Bureau d’Etudes CESAME, ZA du Parc, Secteur Gampille, 42490 Fraisses.
[email protected] 5 INERIS, Parc Technologique ALATA, BP 2, 60550 Verneuil-en-Halatte.
[email protected] ABSTRACT: Within the frame of the Survey related to the cessation of mine workings in Decazeville- Firmi concession, various impacts resulting from the cessation of pumping at the Central Shaft were assessed. Mainly these impacts are related to groundwater behaviour in the abandoned underground coalmines, hydrochemistry of waters discharged to the environment, ground stability concerns and coal gas emanations. This analysis allowed the choice of the most appropriate elevation of the pit lake level in the Grande Découverte, with the necessity to continue the pumping. Two main objectives were reached. The elected elevation is high enough to result in a permanent chemical stratification, which allows the pumping of the superficial waters and its discharge to natural watercourses without any treatment.