Abstract: The Reocín mine is located in Cantabrian region, in northern Spain. Its exploitation ended in 2003 due to the exhaustion of its reserves. In November 2004, the controlled flooding of the openpit began. During this process, both the qualities of stored water and piezometric levels have been monitored to control the possible water detraction from the Saja-Besaya hydrographic system. This paper deals with the water chemistry analysis of the pit lake surface, as well as the different conditions of the area. Geological and hydrogeological contexts play an important role in the lake water chemistry. The lake water quality continues improving. The sulphate content and zinc concentrations are already below the permitted pouring limits. Three factors are distinguished: the washing of the mine shafts is completed; the water supply from the aquifer contributes to the dissolution of the salt content and the bedrock, and dolomite, which neutralizes acid waters and improves the water quality during the flooding process with a pH value of 8. Owing to these conditions, the stored water meets the necessary conditions for discharge and provides the opportunities to use it.

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