Inconspicuous waste heaps left by historical Zn-Pb mining are hot spots of soil contamination

This study discovers and surveys sites left by former Zn-Pb mining in western Malopolska (S Poland), focusing on old heaps of mining waste rock located in agricultural land and suburban wasteland. Topsoil samples were taken from 73 heaps and described using many parameters, including heavy metal contamination (the content of total, EDTA-extractable, BaCl2-extractable and water-extractable metals), macronutrient content, pH and texture. At five sites, a short transect was delineated from heaps towards their surroundings to estimate the impact of the heaps on the adjacent agricultural soil. The total heavy metal concentration in the heap soil varied greatly across sites, ranging from 5 to 522 mg Cd kg(-1), from 94 to 23,006 mg Pb kg(-1), from 6 to 51 mg Tl kg(-1) and from 394 to 70,435 mg Zn kg(-1). These values were very high compared to that measured in the soil of the control areas: 2-5 mg Cd kg(-1), 13-67 mg Pb kg(-1), 1-17 mg Tl kg(-1) and 63-476 mg Zn kg(-1). The extractability of heavy metals from heap soil decreased in the following order: Cd > Pb > Zn > Tl (EDTA) and Cd > Zn > Pb similar to TI (BaCl2). For the most mobile metal (Cd), the extractability averaged 43% and 5% for the EDTA and BaCl2 extractants, respectively. A factor analysis reduced 33 soil physicochemical parameters to five factors that explained 72% of the variance in the data. Factor 1 represented heavy metal contamination (concentrations of both total and mobile Cd, Pb and Zn), factor 2 represented organic matter accumulation, reflecting the most likely age of a heap, factor 3 represented the total concentration of Ca, Mg and Tl derived from weathering of the waste material - mostly dolomite and calcite, factors 4 and 5 represented soil particles of different sizes. In the transect study, the amount of total and EDTA-extractable heavy metals in soil generally decreased when increasing the distance from the heaps, but still remained high in agricultural soil sampled 10 m from the foot of the heaps. The results of this study suggest that remnants of the historical Zn-Pb ore mining are hot spots of persistent soil contamination and may pose an environmental problem, especially those located in the inhabited areas; their status should be monitored by the local authorities. (C) 2014 Elsevier B.V. All rights reserved.