Mine water management in the former Gardanne lignite mine: towards sustainable uses of the mine aquifer
Résumé
The Gardanne mine, extending 17 km E‐W and 10 km N‐S to the south of Aix‐en‐Provence in France, operated seams of lignite interlayered with Fuvelian karstic limestone (Upper Cretaceous) from the seventeenth century up to 2003. The mine is in two main parts: one part lies in autochthonous terrain and the other part lies in a thrust sheet, the Lambeau Charrié. To work in the dry, the water was pumped out and discharged into the Mediterranean (at Marseille) through a 14 km tunnel, the Galerie de la Mer, since 1905. The water came from two sources: the underlying Jurassic limestone aquifer, which has greater hydraulic head pressure than the mine and is in contact with it via the mine workings, and the rainwater infiltrating through the Fuvelian outcrops (autochthonous and thrust zones). When mining ceased in 2003, the decision was made to let the water rise into the mine reservoir up to a certain level, at which point it would be maintained by pumping the excess water into the Gérard mineshaft on the Lambeau Charrié, to prevent mineralised mine waters from flooding into the natural environment. The water is drained first through the Galerie de la Mer, then through inclined boreholes to discharge diffusers at a depth of 30 m offshore from Marseille, to prevent the iron oxides from colouring the harbour water. In 2006, BRGM (French Geological Survey) was mandated by the state to manage the former mining concession at Gardanne. By August 2010, the water level had risen from -1,100 to -14 meters above sea level (MASL), and pumping began at a rate of 600 m3/h. This proved too slow to keep the water level under control. The rate was increased to 1,000 m3/h but water continued to rise (reaching -4 MASL in March 2013). Therefore, in 2012, the functional concept map of the mine reservoir was reviewed with the aid of a 3D representation of the mine and hydrogeological simulations. These showed that the eastern, autochthonous, part of the mine, which had been worked by room‐and‐pillar methods with no backfilling, could be used to store a large volume of water under more hydraulic pressure than in the Gérard mineshaft. The study also showed that inflows of Jurassic water could be larger than anticipated when dewatering ceased near Gardanne. The interpretation of two years of pumping data concurred with the simulation results: changing the pumping facilities to increase their capacity therefore had to be considered. In time, iron and sulphate concentrations are expected to drop, which would allow direct gravity discharge of the water into the port of Marseille. Potentially, the water could also be used as a low‐enthalpy geothermal resource and/or a strategic drinking water resource.
Domaines
Géologie appliquée
Origine : Accord explicite pour ce dépôt
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