The Spontaneous Combustion of Coal-Mine Waste and Stream Effects in the El Bierzo Coalfield, Spain

Abstract The El Bierzo is the largest and westernmost Stephanian coalfield in the Cantabrian mountain range (northwest of Spain), and mining activity has generated waste dumping over vast areas of the original soil. The mining waste deposits have been exposed to natural alterations that have caused the production of mining acid drainage and the spontaneous combustion of coal waste piles (CWP), which in turn have an environmental impact on the air, water, soil, and vegetal and animal life of the surrounding area. The identification and characterization of the effects caused by coal waste deposits in the riverine system of the region of Fabero of the El Bierzo coalfield are the main goals of this study. Water samples from the Cua River and its tributaries in the region (Galladas, Mourin, and Lamocos tributaries) were collected in two different periods of the year (wet and dry seasons). For the physicochemical characterization of water samples the following analyses were performed: pH and ionic conductivity measurements; the determination of the carbon concentration by the catalytic oxidation combustion; the evaluation of the chloride, nitrates, and sulfate anion concentrations by ion chromatography; and determination of the trace element concentrations by means of inductively coupled plasma mass spectrometry. Some physicochemical parameters of the water samples evidenced the production of acid mine drainage as the result of the circulation of waters through the coal waste deposits, which promoted the dissolution of metals. The results indicate that the physicochemical characteristics of the waters collected in the areas upstream of the coal waste deposits and old mining infrastructures are less affected by the circulation of waters through those materials. On the other hand, the samples collected downstream of the coal waste deposits and old mining infrastructures present the highest trace element concentration, a lower pH, a higher conductivity, and a higher sulfate concentration, as the result of the circulation of waters through these materials. To investigate the quality of the water streams in the region of Fabero, the chemical composition of the studied waters was compared with previous studies on the quality of groundwater in the El Bierzo area and other information about acid mining drainage waters in other coal mining regions, as well as guideline values from water quality legislation. These comparisons point out the presence of certain metallic species (i.e., Cd, Co, Cu, Fe, Mn, Ni, Pb, and Zn) and their likely environmental impact on the riverine system in the Fabero mining area. To identify the probable source of the enriched elements detected in the studied water samples, their chemical composition was compared with the chemical composition of the solid materials from the CWP that had percolated into the studied tributaries. The comparison indicates that the percolation of waters through the Arroyo Galladas CWP caused the dissolution of toxic metals (specifically Mn, Ni, Zn) under acidic conditions, leading to high concentrations of these metallic species in the water samples.