Analysis of the reduction processes at the bottom of Lake Meirama: a singular case of lake formation.

The formation of natural lakes is a process that takes place over thousands of years, although the volumetric formation depends on hydrological and climatological phenomena, reaching a stationary hydraulic regime, the evolution of hydrochemistry is more complex and obeys not only phenomena of stoichiometry and chemical kinetics but also diffusion processes. Depending on the depth of the lakes, the anoxization process originating from the bottom is the first phase of the lake's methanogenesis. For this, the course of many thousands of years is necessary, so the studies carried out in the lakes are limited to the current knowledge of the state in which they are, without being able to have real information in this process of methanogenesis. There are no data available on the generation process of a natural lake in its primary stages. In this case, taking advantage of the rehabilitation of the old open-pit mining of Meirama (Northwest Spain), consisting of the controlled flooding of the hole by groundwater, by stopping the perimeter pumping, and the derivation of the nearby streams, whose contribution was the majority with respect to the subterranean contribution, there has been the opportunity to physically and chemically monitor the complete filling of the said hole. The present study focuses on the analysis of the evolution of the different processes initiated in the methanogenesis of the lake bottom identified in the well-known Redox ladder: obtaining oxygen from the reduction of nitrogenous compounds and metallic oxides, from the reduction of the sulfate and the generation of methane from carbon compounds, the latter phase without reaching. Although the methanization process is very slow, it has had the opportunity to know the formation of a lake at its origin, from the hydrochemical point of view. It has been possible to verify that the methanization processes at the bottom, given the anoxia conditions, are in a very primitive phase with the reduction of nitrate and nitrite to ammonium and beginning a reduction of metal oxides and sulfate.

Comparative hydrochemical analysis of the formation of the mining lakes of As Pontes and Meirama (Spain).

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.

WITHDRAWN: Non-isothermal multiphase flow and reactive transport modelling of compacted bentonite simultaneously heated and hydrated.

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