Hydrochemical stratigraphic analysis of the filling of the Meirama open pit mine II: parameters and elements

In the first article, entitled “Hydrochemical stratigraphic analysis of the filling of the Meirama open pit mine I: Monitoring and filling” (Juncosa et al. Environ Sci Pollut Res 20(11):7520–7533, 2013), the filling process of the old Meirama mining pit (NW Spain), as well as the methodology used in the sample collection and analysis, was described. Likewise, the evolution of the temperature, pH, dissolved oxygen, iron, and manganese in the filling and postfilling processes are shown. This second article presents the temporal evolution of other major components and nutrients during the filling period (2008–2016) and postfilling period (2016–2019). The continuation of the analysis initiated in the aforementioned article is done at certain heights of the vertical profiles monitored at the midpoint of the lake (the surface, the first 2 m of depth with respect to the surface (2 mbs), at 59 masl, and at the bottom (2 masl)). As explained in the filling process, an invariant chemocline and a seasonal thermocline near the water surface are formed. Therefore, the analysis encompasses not only the bottom and surface of the lake but also includes an intermediate point where the chemocline is found. Based on the analysis carried out, it has been possible to verify that the most superficial waters (80 m) are in line with the geological and fluvial environment of the basin, so that the stored waters do not need a special physicochemical treatment. However, at deeper levels, anoxization processes are developing, a step prior to the methanization of the lake bottom.