Geoavailability of lithogenic trace elements of environmental concern and supergene enrichment in soils of the Sierra de Aracena Natural Park (SW Spain)

- Carbonate-hosted sulphides are of major significance as lithogenic source of geoavailable metal(oid)s.
- Abundance and distribution of metal(oid)s reveal the influence of local geology on soil geochemistry.
- Concentration factor and geo-accumulation index point to strong geogenic contamination.
- The soils could eventually turn from a sink to a source of harmful trace elements.

The chestnut grove soils of the Sierra de Aracena Natural Park (SW Spain) are a striking example of geoavailability and pedochemical enrichment with geologically-sourced trace elements, notably Zn, Pb, Ag and Cd, but also As, Sb, Cu and Tl. The data from this study provide evidence that such metal(loid)s were originally bound to reactive sulphides (pyrite, sphalerite and galena) and Ag-Sb-(Pb) sulfosalts residing in the soil parent rock. At the early stages of weathering, these geoavailable elements were easily liberated from the mineralised bedrock through oxidative dissolution reactions, and then transferred to the soil, thus reaching anomalously high concentrations. The most extreme amounts of Zn (12-26 g kg− 1), Pb (> 5 g kg− 1), As (346 mg kg− 1), Cd (319 mg kg− 1), Cu (197 mg kg− 1), Ag (154 mg kg− 1), Sb (109 mg kg− 1), and Tl (82 mg kg− 1) were measured in soils developed over carbonate-hosted sulphide occurrences and their related gossan outcrops. When compared to the regional pedogeochemical baseline, the soils are largely enriched with all the elements analysed, except Co, Cr and Ni, yielding concentration factors particularly high for Zn (50.8), Cd (49.3), Ag (30.6), Sb (22.5), and Pb (20.8), and geo-accumulation indices that consistently point to strong geogenic contamination. The pedochemical enrichment may be reasonably attributed to the combined result of precipitation as a solid phase (e.g., Zn-vermiculite, hemimorphite, hydrozincite, and plumbogummite) and specific adsorption by soil constituents, mainly organic matter and iron oxy-hydroxides. The survey area can be considered as a hot spot in terms of both valuable metal prospecting and environmental concern. Although the soil acted as a natural sink for lithogenic trace elements, it could eventually turn into a source of potentially hazardous contaminants in response to future land-use changes and biogeochemical alterations.