The fate of arsenic in soils adjacent to an old mine site (Bustarviejo, Spain): mobility and transfer to native flora

Abstract

The mobility of arsenic in soils and its transfer to other environmental components present significant environmental risks. The management of polluted land is determined by the availability, mobility, and transfer of inorganic pollutants to different ecosystem compartments. In this paper, the fate of arsenic at this mining site has been evaluated to determine future management practises to minimise such risk. In a field study carried out in the area adjacent to a mining site at Bustarviejo (North Madrid, Spain), samples of soils, plants, and water were collected from areas adjacent to the core of the former mining activity. The following parameters were investigated in soil samples: pH, organic matter, pseudo-total As, P, and Fe, and labile As and P, and a sequential extraction procedure was performed to investigate As speciation in soil. Plant materials were analysed for both As and P. Arsenic concentrations in water samples (surface and soil pore water collected in the field) were also measured. Results are considered in tandem with previous data on metal concentrations in soils and plants from this site. Despite high As concentrations in soils impacted by former mining activities (spoil accumulation and drainage from spoil heaps resulted in concentrations of up to 3,000 mg kg−1), it was not present in a labile form. Sequential extraction revealed that arsenic was mainly retained by Al- and Fe-(oxihydr)oxides (up to 80%). Therefore, only a small proportion of the total soil pool was potentially available for plant uptake (0.3% and 7% extracted by (NH4)2SO4 and NH4H2PO4, respectively). There was very limited transfer of arsenic from soil to plants, and concentrations of arsenic in shoot tissues were relatively low (<8 μg g−1). There was no evidence of phytotoxic effects in the flora that had colonised this site, indicating that a sustainable ecosystem had been established. High levels of arsenic occur at this site, but arsenic mobility appears to be primarily controlled by the presence of amorphous and crystalline Fe and Al hydrous oxides. Although a low labile As fraction was extracted, concentrations of arsenic in both surface and soil pore water are of concern. The risk of arsenic remobilisation by plant uptake or transfer to the food chain via plant consumption is relatively low in these soils. Large amounts of metals and arsenic remain at the site, and potential risks need to be monitored. Some possible remediation strategies that take into account the presence of both arsenic and heavy metals will be suggested. Natural attenuation and phytostabilisation processes are taking place in several parts of the study area. These natural processes could be enhanced by application of both compost and a suitable Fe-based amendment. This augmentation of the re-vegetation of the affected area could act to promote both arsenic and metal stabilisation in mine tailings with additional benefits for further vegetation establishment.