Concentration and spatial variability of mercury and other heavy metals in surface soil samples of periurban waste mine tailing along a transect in the Almadén mining district (Spain)

Abstract

Mercury (Hg) is one of the elements with increasing environmental significance. A total of 22 samples (soils, rocks and gels) were collected along a 6km transect around the Valdeazogues River valley in the southwest of the Iberian Peninsula (Almaden, Spain). Samples were characterized by different soil types of depositional sequences associated with mining tailings, type and system tracts: 15 surface soil samples included in the transect; 3 of a Haploxerept soil profile developed on slates; 2 of quartzite and slates rocks (reference rocks in the area). Moreover, two of a gel substance (in the lower tract of the river). Soil samples were analyzed for Hg, Cu, Ni, Cr, V, Pb, Cd and As, as well as for organic matter, pH abrasion and calcium carbonate content. All samples were collected from the Almaden mining district. The level of occurrence of the elements (especially Hg) and the effect of some properties on its concentration distributions were investigated. The total mercury contents varied in the range 7,315–3.44mgkg−1. The mean concentration of total mercury in soils and rocks was 477.03mgkg−1dry mass. This value is very high compared to the regional background value of other areas. Only rarely is it higher than 1%: in one sample (7,315mgkg−1) it was almost eight times in comparison with the affected zones, with a high value of 1,000mgkg−1. Significant differences between samples were found in the total content of mercury. A large percentage of the samples registered detectable levels of V, Cr, Ni, Pb, As and Cu. Cd readings were below the detectable range for all samples tested. Cr mean concentration was 216.95mgkg−1 (minimun concentration 86, maximun 358); V mean concentration was 119.09mgkg−1 (minimun concentration 69, maximun 1,209); As mean concentration was 51.24mgkg−1 (minimun 13.3 and maximun 319.4); Ni mean concentration was 45.64mgkg−1 (minimun concentration 21.2 and maximun 125.6); Cu mean concentration was 33.25mgkg−1 (minimun concentration 19.3 and maximun 135); Pb mean concentration was 15.19mgkg−1 (minimun 1.12 and maximun 1013). Metal distribution generally showed spatial variability ascribed to significant anthropogenic perturbation by mining tailing type. Hg showed vertical profile characterized by surface enrichment, with concentrations in the upper layer (93.7–82.2mgkg−1 in front of 3.4 of the rock value) exceeding, in several occasions, the background value. The results obtained denote a potential toxicity of some heavy metals in some of the studied samples. Water-soluble mercury could enter the aquatic system and accumulate in sediments. Mercury and other heavy metals contamination depended on the duration and intensity of mining activities.