Arsenic is associated with Hg in the mercury deposits of the Asturias region of northern Spain. Until 1974, the mines of La Peña and El Terronal exploited an ore deposit located in the valley of the San Tirso River (Mieres district). At this site, a great volume of waste material from mining and metallurgical works have been stock-piled for years, constituting a spoil heap of about 20 000 m2 in area. Total As concentrations in representative samples of waste materials on the surface of the spoil heap reach a maximum value of 72 153 mg kg−1 and an average value of 15 967 mg kg−1; analysis of deeper samples (trenches on the spoil heap) exceed 99 999 mg kg−1. The total flow of polluted water from its circulation through the spoil heap is estimated at 3465 m3 a−1. Weathering processes facilitate the dissolution and leaching of As and heavy metals in the ore and waste material. Arsenic contents in surface waters (San Tirso River) immediately downstream of the spoil heap reach 7.9 mg l−1. The impact on the area, reflected by elevated concentrations of As in adjacent soils, biota and surface/underground waters, is discussed.