Enriquecimiento supergénico, fraccionamiento geoquímico y destino ambiental de elementos traza potencialmente tóxicos en suelos del Parque Natural Sierra de Aracena y Picos de Aroche (Huelva)

Zusammenfassung

Soil developed on mineralized bedrock areas of the Sierra de Aracena and Picos de Aroche Natural Park (SW Spain) is a striking example of geoavaiiability and pedochemical enrichment with geologically-sourced trace elements, notably Zn, Pb, Ag and Cd, but also As, Sb, Cu and Tf. The data from this study provide evidence that such metal(loid)s were originally bound to reactive sulfides (pyrite, sphalerite and galena) and Ag-Sb-(Pb) sulfosaits residing in carbonated-hosted exhalative sedimentary deposits. At the early stages of weathering, these geoavailable elements were easily liberated from the mineralized 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 (más de 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), y Ti (82 mg kg*1) were measured in soii developed over carbonate-hosted sulfide occurrences, When compared to the regional pedogeochemical baseline, the soil was found to be largely enriched with all the fourteen elements analyzed, 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-accumuiation indices (I5fi0= 4-5) that consistently point to strong geogenic contamination. The pedochemical enrichment of Zn can be reasonably attributed to the result of precipitation as solid phases (Zn-vermicuiite, hemimorphite, hydrozincite). A combination of experimental methods (XRD, EPMA and selective chemical extractions) and DFT-based computational calculations supported the conclusion that Zn is structurally bound in vermiculite and resides in octahedral sheets (up to 0.52 apfu) rather than in interiayer exchange sites. To assess environmental availability, chemical speciation and potential health risk of trace elements, soil samples were subjected to one-step extraction procedures (0,01 M CaCI2, i M MgCI2, 1 M NH4OAc, and 0.05 M EDTA), aqueous speciation modeling, and site-specific risk analysis. Metal fraction elements, soil samples were subjected to one-step extraction procedures (0,01 M CaCI2, i M MgCI2, 1 M NH4OAc, and 0.05 M EDTA), aqueous speciation modeling, and site-specific risk analysis. Metal fraction available for plant uptake or leaching to groundwater was found to be negligible (< 1%) due to the low activity of dissolved and exchangeable ions in soil solution, as predicted from the CaCI2 extracts. Based on modeling calculations, free metal Ions, primarily Cd2+ and Zn2+, were the dominant species in solution over the soil pH range of 5.8 to 7.8. For most metals, the EDTA-extractable pool generally accounted for less than 5%, except Cd (23%) and Pb (20%), showing that a limited reservoir of trace elements could be potentially available. Trace elements of concern were also analytically partitioned into residual and labile fractions, using a five-step sequential extraction procedure optimized for Fe-rich soils, notably crystalline iron oxy-hydroxides (goethite and hematite). The findings conclusively indicate that the majority of heavy metals are tightly bonded to residual or reducible phases, so that day minerals and iron oxy- hydroxides, respectively, played an important role In the metal accumulation. However, significant proportions of Cd (22%) and Zn (7%) are bound to carbonates, and locally considerable amounts of Ag (60%) and Cu (32%) seem to be retained in the oxidizable fraction, suggesting association with secondary sulfides in the remnants of a supergene enriched zone. The results of the health risk assessment for ingestion exposure to groundwater affected by soil leaching revealed that the hazard quotients of heavy metals, calculated using the RBCA software, are all within the acceptable risk level. The cumulative hazard index (HI= 0.55) fell below the regulatory threshold value of 1.0, even in the worst-case scenario, leading to the conclusion that no toxic effects are expected to humans under present-day environmental conditions. The increasing abandonment of traditional chestnut groves by their owners due to unprofitability constitutes a driving force for environmental changes that might affect the geochemical status of the trace elements stored in the soil. Results from the geochemically-based sequential extractions allowed us to simulate the effects of changing environmental conditions. Upon reducing conditions induced either by waterlogging processes or by structural degradation, the dissolution of iron oxy-hydroxides could release large amounts (45-60%) of adsorbed and occluded trace elements, particularly As, Pb and Cd, in a worst (but unlikely) case scenario of gleying.