Factores condicionantes de la acidificación de embalses por lixiviados mineros en la Faja Pirítica Ibérica

Abstract

Los ríos que drenan la Faja Pirítica Ibérica (FPI) presentan un grave problema de contaminación por aguas ácidas procedentes de minas de sulfuros abandonadas. La construcción de la presa de Alcolea, localizada en el río Odiel, se empezó en 2014 pero se paralizó en 2017, aunque actualmente hay una fuerte presión de los regantes para que se retome. Debido a la acidez que presenta el río Odiel existe una gran polémica sobre la calidad del agua de este embalse. En este trabajo se investiga la calidad del agua en los grandes embalses de la FPI a partir de la información analítica de la red oficial de control de calidad. Los embalses no afectados por aguas ácidas (Jarrama y Corumbel) tienen valores de pH próximos a neutros y muy baja concentración de sulfatos (<12 mg/L). Los embalses de Andévalo y Olivargas tienen un nivel de afección intermedio, presentan también un pH neutro aunque se tienen concentraciones más elevadas de sulfatos (~60 mg/L) y de algunos metales. El embalse del Sancho presenta un nivel de afección mucho mayor y, como consecuencia, tiene un pH medio de 3,6 y elevadas concentraciones de sulfatos (media de 184 mg/L) y metales tóxicos. Se estima que, si no se adoptan medidas de restauración, las condiciones en el embalse de Alcolea serán similares o peores a las del embalse del Sancho.

The rivers of the Iberian Pyrite Belt (IPB) have a very serious problem of contamination by acid waters from abandoned sulphide mines, intensely exploited in the past. The most extreme and well-known case is the Tinto river, but this problem affects a much greater length of fluvial courses in the Odiel river basin. Other rivers in the IPB that also receive acid leachates, although with less intensity, are the Rivera del Chanza and the Guadiamar. In 2014, the Alcolea dam began to be built on the Odiel River, mainly for agricultural use. This construction was stopped in 2017 but currently there is a strong pressure from the irrigators to resume it. Due to the conditions of the Odiel River in this area, with pH values below 4 and high concentrations of toxic metals, there is great uncertainty about the quality of the water that this reservoir will store. In this work, the quality of the water in the large reservoirs of the IPB is investigated from the analytical information of the official quality control network (period 2008 to 2021). The reservoirs not affected by acid mine waters (Jarrama and Corumbel) have pH values close to circumneutral and very low concentrations of sulphates (<12 mg/L). In the reservoirs that have intermediate levels of contamination (Andévalo and Olivargas) the pH is slightly lower, although above 7, and the sulphate content is significantly higher (~60 mg/L). However, in the case of Olivargas, the concentrations of some divalent metals from mining are relatively high and the values of Cd and Zn cause that its condition is classified as ‘worse than good’. The Sancho reservoir has a much higher level of pollution and, as a consequence, presents an average pH of 3.6 and high concentrations of sulphates (average 184 mg/L) and toxic metals (Al, Cd, Cu, Fe, Zn, etc.). Depending on the concentration of sulphates, it is estimated that the conditions in the Alcolea reservoir will be similar to or worse than those in the Sancho reservoir. To ensure a good water quality in the Alcolea reservoir, it would be necessary to eliminate 70% of the acidic discharges in the Odiel River basin.