The Valborraz tungsten mine: Description of a potential source of arsenic pollution
Résumé
Soil contamination levels were studied in the land affected by an abandoned tungsten mine in Valdeorras (NW Spain). The mine worked mainly in the first half of the twentieth century, and it was definitely closed by 1953. Soil samples were taken in thirteen points within the mine (including both natural soils and soils developed over mine wastes), and in two natural soils close to the mine area. Total heavy metals (Fe, Zn, Ni and Cu) and As were analyzed by X-ray fluorescence in sieved (< 2 mm) ground samples. Extremely high As levels (>1000 mg kg-1) were found in all the samples of the mine area, whereas for the heavy metals analyzed no abnormally high concentrations were observed. Although high As background levels are common in soils developed over slates in the region, the concentrations found for the natural soils within the mine were too high to have a geological origin, and they are probably due to contamination from the close mine dumps. The As concentrations in the mine soils were higher than in the natural soils, ranging from 0.5-1% approximately. The highest As concentrations (near 4%) were found in iron oxyhydroxide deposits, which precipitate directly from water in some places in the area. This high As concentrations represent a certain threat to environmental health which has not been quantified yet. Although additional research is needed to determine the potential ecological and human health risks associated with the mine, our results suggest that this location could act as a dangerous As source.