Assessment of biopiles treatment on polluted soils by the use of Eisenia andrei bioassay

A long-term case of residual pollution is studied after 20 years since the largest mining accident in Spain (the Aznalcóllar spill) happened. This pollution is manifested through a surface zoning consisting of bare soils (B0), sparsely vegetated soils (B1), and densely vegetated and recovered soils (B2). A biopiles treatment with a mixture of contaminated soils (B0 and B1) with recovered soils (B2) at 50% (w/w), and vermicompost addition (50 tons ha-1) was evaluated. To assess the effectiveness of treatments, total, water-soluble, and bioavailable fractions of the most polluting elements in the zone (Cu, Zn, As, Pb, Cd, and Sb) was analyzed. To evaluate the potential risk of contamination for the ecosystem, a bioassay with earthworm Eisenia andrei was carried out. Twenty years after the accident, there are still soils where total As and Pb exceed the regulatory levels and water-soluble Zn and As exceed the toxicity guidelines. According to toxicity bioassay, weight variation and juvenile production of earthworms showed an improvement after biopiles treatment, with values that trend to be similar to those of recovered soils. The only bioaccumulated element in earthworms was Cd (BAF>1), both in polluted as in treated soils, which indicates the possible existence of exclusion mechanisms of the other pollutants by earthworms. The comparison between biopiles and polluted soils showed no significant differences for the bioaccumulation factor of trace elements, with the exception of Zn and Cu, which slightly increased after treatment. According to our results, biopiles treatment combined with vermicompost addition is a good technique for the recovery of residual contaminated areas, by the improvement of soil properties and the reduction of the potential toxicity; anyway, monitoring of soils and organisms is needed to prevent the increase of bioavailability of some potentially pollutant elements over time.