The old lignite mines of As Pontes and Meirama are located in the northwest of Spain, an approximate distance of 80 km between them. Both ended their extractive activity at the end of 2007; the respective mining pits began to be filled at the beginning of 2008. The filling process, enabled by the deviation of nearby rivers, differed between the two cases, taking approximately 4 and a half years to completely fill the 547-hm3 hole at As Pontes and 8 years and 2 months to fill the 146-hm3 hole at Meirama. In both lakes, a steady-state hydrological regime has been established, in which the volume of water in the filled holes remains constant.Despite the difference in filling time, very similar hydrochemical behavior would be expected for both water masses, as the geological, climatological, and agroforestry environment is similar at both sites and the maximum depth reached by the water is also the same for both (205 m).Thus, once the filling of both holes was completed, a comparative hydrochemical study was carried out on the state of the waters of both lakes, analyzing the hydrochemical profiles of the most representative elements and compounds of the system, as well as the chemical stratification generated. In this study, the hydrochemical results are shown. The early completion of filling in As Pontes with respect to Meirama provides significant information to know the future behavior of Meirama due to the similarity that both mine open pit present.
Keywords: Hydrochemical; Hydrology; Mining lakes; Water quality.