Distribution and behaviour of selected elements in soil developed over a historical Pb–Ag mining site at Sv. Jakob, Croatia

Abstract

The purpose of this study was a geochemical assessment of soils developed on a historical Pb–Ag mining site (galena with approx. 500 mg/kg of Ag) situated near the chapel of Sv. Jakob on the crest of Mt. Medvednica near Zagreb. A total of 100 brown forest (0–25 cm) soil samples were collected from the regular grid covering an area of approximately 75,000 m 2. Ten samples were selected for detailed chemical and mineralogical analyses. Pb, Zn, Cu, Hg values were obtained by analysis of aqua regia extracts, while residence sites of metals in soils were identified by sequential extraction analysis. Shallow soils developed above the historic mining site `Sv. Jakob' contain very high concentrations of Pb, Zn and Cd due to oxidation of primary sulphides in the near-surface weathering environment. The aqua regia extractable Pb values varied from 9 to 18,188 mg/kg, Zn from 12 to 9125 mg/kg, Cd from <0.25 to 189 mg/kg and total Hg from 0.06 to 1829 mg/kg. Distribution patterns of Pb, Zn, Cd and Hg outline both the surface extent of mineralization and the area affected by historical mining activities (tailings, exploration pits and trenches) which are today almost indistinguishable from the natural landscape. Pb and Cd are dominantly enriched in carbonate and Fe–Mn oxide fractions while Zn exhibits enrichment in organic-sulphide and Fe–Mn fractions. This distribution is probably the result of solubility controls for Cd and Pb in carbonates, affinity of Zn for organic material and coprecipitation of these elements with iron oxides. Fe, Cu and Ni are enriched in the organic-sulphide fraction, indicating that a part of pyrite has probably not been altered completely (or was recently incorporated in soil as a result of downslope movement and colluviation), and in the residual fraction. For the sequential extraction procedure used in this study, mobility and biological availability are assumed to decrease in the order of the metal extraction sequence. On this basis, the apparent mobility and potential bioavailability of the metals for contaminated soils are: Cd > Pb > Zn > Cu > Ni. Spatial distributions and high contents of Pb, Hg, and Zn in topsoils of the northern lowland residential parts of Zagreb were formerly attributed only to fossil fuel consumption and the flooding of the river Sava. The high contents of these elements in topsoils covering areas of historical mining sites situated within the Mt. Medvednica Protected Park of Nature are probably also sources of observed pollution. The streams that drain these areas are characterised by violent mudflows that flood northwestern parts of the city at least once every three years and deposit high quantities of mud. Studies that aim to determine this link are currently in their initial phases but evidently materials with high bioavailable contents of potentially toxic elements are being regularly supplied to the gardens and orchards of northwestern parts of Zagreb.