Soil quality changes in an Iberian pyrite mine site 15 years after land reclamation

Reclamation of highly degraded mine lands to a sustainable environmental quality has become a major policy concern in many countries with a long mining tradition. This paper reports the soil quality status of a historical mine site fifteen years after its reclamation, using indicators of chemical reactivity, soil fertility and health, and discusses the progress and effectiveness of the implemented measures by a comparative analysis between pre- and post-reclamation conditions. Twenty composite surface samples (0-20 cm depth) were obtained for physical and chemical characterization from the open pit mine and waste disposal area, using a stratified random sampling approach. Additionally seven sites were selected to collect topsoil samples for microbiological testing. Results showed that soil quality changed noticeably over the study period (2003-2018), with overall improvements in key properties, such as structural stability, cation exchange capacity, degree of base saturation, soil organic carbon, and available phosphorus. Prior to reclamation, the mine soil was unable to support vegetation due to hyperacidity coupled with elevated levels of toxic metals and nutrient deficiencies. The combined use of sugar beet lime and composted biosolids as a low-cost, locally available, soil amendment was effective in neutralizing both active and exchangeable acidity, and reducing the mobility, plant uptake and human bio-accessibility of trace elements. The amendment addition also enhanced soil fertility, carbon storage, nutrient availability and microbial biomass (bacteria and fungi). Revegetation with Pinus pinea and Nerium oleander has proven to be a successful strategy to create a vegetative cover aesthetically pleasant and environmentally compatible with the surrounding undisturbed landscape, although further efforts should be made to monitor over time the phytotoxic and bioaccessible levels of residual metals, notably Cd, Cu and Zn. The insights gained from this land reclamation experience provide success criteria for assisting natural attenuation in other abandoned mining sites worldwide.