The abandoned mercury mining works of "Los Rueldos" are located 20 km from Oviedo, along the northwestern border of the Asturian Central Coal Basin, in an area with intense tectonization. Hg mainly appears as cinnabar, but occasionally metacinnabar and native Hg are present; associated with Hg ore appear As-rich minerals (arsenopyrite, As-rich pyrite, realgar and scorodite). In the spoil heap, Hg content ranges from 14 to 2224 mg kg(-1), and As from 4746 to 62,196 mg kg(-1). Mine drainage and spoil heap leachates show acidic conditions (pH: 2.43-2.50), 2900-4600 mg l(-1) sulphate, 1.4-9.2 mg l(-1) As, 0.03-0.48 mg l(-1) Pb, and 3.6-14 microg l(-1) Hg. According to the analytical data and characteristics of the site, the application of corrective measurements to avoid the dispersion of contaminants in the environment must be considered.