Hydrochemical stratigraphic analysis of the filling of the Meirama mine shaft I: monitoring and filling

The application of an old open-pit lignite mine as an auxiliary reservoir for the supply of the city of La Coruña has involved a detailed analysis of the chemical quality of the vertical profile. The study has shown that a stratified lake is formed where the shape of the filling (first with groundwater and later surface waters from the nearby rivers were added) has influenced the formation of the lake profile. The Meirama mine is an open-pit brown lignite mine located in NW Spain. Exploitation ended in March 2008, leaving a pit of approximately 146 hm³ with a maximum depth of 205 m. The rehabilitation of the pit consisted of the creation of a lake that will serve as an auxiliary water reservoir for the city of La Coruña. The filling process lasted approximately 8 years (March 2008–May 2016) with subterranean and surface contributions from the environment. The formation of an invariant chemocline and a seasonal thermocline was observed. Therefore, in this work, (the first of a series of two articles), an analysis of the evolution of various physicochemical parameters, elements, and compounds at certain heights (the surface, the first 2 m (2 mbs), at 59 masl, and at the bottom (2 masl)). In this first article, the physicochemical parameters of pH, dissolved oxygen, temperature, and some metallic elements (iron and manganese) are analysed. From the analyzed parameters it can be seen the creation of a surface thermocline and a chemocline at a depth of 59 masl with the beginning of the formation of anoxic conditions of the bottom of the pit through the consumption of oxygen.