Incidencia de procesos AMD en la hidroquímica de embalses afectados en la Faja Pirítica Ibérica

Abstract

In Europe, one of the main problems of metals pollution in rivers and reservoirs is promoted by acid mine drainage (AMD), occurring in the Iberian Pyrite Belt (IPB) in the SW of Europe, throughout the provinces of Huelva, Seville, and in the south of Portugal. The intense mining activity in this metallogenic region left a legacy of 86 abandoned mines, with about 200 million m3 of wastes dispersed by innumerous tailings, mining dams and waste dumps, which constitute nowadays sources of pollution, affecting the waters and sediments of the receiving rivers. The problem becomes more important when these AMD polluted streams reach the water reservoirs, reducing their uses. The Spanish Society of Dams and Reservoirs (SEPREM), officially recognizes the public and private ownership, and used for agricultural, industrial or urban supply. Many of these reservoirs are located in channels affected by acid mine drainage, which arrives at the reservoirs with low pH and high metal and sulfate loads. The main objective of this thesis is to characterize the pollutant loads transported to the reservoirs by acid mine drainage and to model the global process. This study gives a real vision of the hydrochemical state of the reservoirs, with special attention to the less desirable contaminants. In order to achieve this central objective, several water sampling campaigns were performed at different temporal and spatial scales. A sampling campaign in all AMD affected reservoirs was carried out during a hydrological year. Additionally, other campaigns were performed in a second hydrological year, covering several points established in a particular and representative reservoir. Also, a specific sampling was performed in an affected stream in order to achieve complete characterization of a representative AMD generating source. Finally, a sampling campaign devoted to sediment geochemistry was also conducted in a clogged mining reservoir. The application of several statistical graphic treatments as well as fuzzy logic and data mining techniques highlighted that all the reservoirs suffer from AMD influence, each one of them showing a different range of contamination, not being clear any overall pattern of behavior. Also, it is noted a high contrast between extreme values of pH and concentration of metals and sulfates for the different reservoirs. This individual pattern may be the result of the coexistence of very different mineral paragenesls throughout the IPB, as well as the diversity of size and nature of the watersheds. Other factors, such as the affected area, type, intensity and duration of the mining processes also contribute to the development of individual hydrochemical patterns. Detailed studies, with samples from two different points of a representative reservoir, in Sancho reservoir, indicated that most parameters are closely related, and presenting high correlation coefficients between the two points. This study revealed the existence of attenuation processes as the contaminants enter the reservoir and move along it. So, dilution promoted by the contact with a large volume of water as well as iron hydroxysulfates precipitating as a consequence of the rise in the pH are among the main attenuation phenomena. This is incremented by the subsequent sorption processes that occur on the surfaces of these solid phases. At the same representative reservoir, a study of vertical distribution of the typical AMD parameters indicated a stratification of the water column at different depth intervals that determine the occurrence of oxidation-reduction processes and precipitation. Another representative reservoir, Marismillas reservoir, which is clogged by AMD precipitates, was studied for sediments geochemistry. The spatial distribution of elements along three vertical profiles reflects the fluctuations throughout former mining activity on the Tinto river basin. Finally, the development of a risk map for the entire Iberian Pyrite Belt has allowed the visualization of the whole metallogenic province concerning the contamination degree by AMD of the reservoirs located in its network. The scientific production resulting from this study is consisted of: Cerón etaL, 2013a-b; Grande et a/., 2012- 2013b-2014a-b; Santisteban etaL, 2012-2013, de la Torre etaL, 2014a-b-c; Santisteban etal., 2012-2013- 2014a-b-c; Valenteetal., 2012b-2014.