Assessment of the Bioavailability and Speciation of Heavy Metal(loid)s and Hydrocarbons for Risk-Based Soil Remediation

For the assessment of the environmental and sanitary risks deriving from contamination of agricultural soils, it is crucial to identify and characterize the contaminants and study the soil chemical properties influencing their mobility and bioavailability. This information is essential for the selection of the best site remediation and securing strategy. The study site of this work is agricultural land of 6 ha in the province of Naples (Italy) subject to the past illegal burial of industrial wastes, principally from tanneries. With the aim of identifying the contaminants and assessing their mobility and bioavailability, the soil of the site was characterized for the main chemical and physical properties and for the concentration of potentially toxic elements and hydrocarbons. The readily and potentially bioavailable fractions of the main metal contaminants and their distribution in the soil geochemical fractions were determined by extraction in 1 M of NH4NO3, 0.05 M of ethylenediaminetetraacetic acid (EDTA) pH 7, and European Community Bureau of Reference (EU-BCR) sequential fractionation. Further, the speciation of heavy hydrocarbons and chromium was carried out. The agricultural soil was widely contaminated by chromium, zinc, and heavy hydrocarbons (up to 4487, 1846, and 1800 mg/kg, respectively). In some sub-areas, contaminations by cadmium, lead, and copper (up to 283, 417, and 1183 mg/kg, respectively) were also observed. The chromium was found to be scarcely mobile and bioavailable and was mainly associated with the oxidizable, residual, and reducible fractions of the soil (on average 56%, 25%, and 19% of the total, respectively). However, chromium speciation revealed the presence of a significant amount of highly toxic Cr(VI) (above the legal threshold of 2 mg/kg), despite the low oxidizing power of the soil. Zinc was more mobile and bioavailable than chromium and was mainly distributed among the acetic acid-extractable and reducible fractions of the soil (on average 28% and 47% of the total, respectively). Cadmium was found to be very mobile and bioavailable, and was mainly distributed in the acetic acid-extractable and reducible fractions of the soil (on average 40% and 45% of the total). The speciation of heavy hydrocarbons showed that they consist almost entirely of long-chain aliphatic hydrocarbons, considered not very toxic and immobile. These results suggest that the use of not-edible plant coverage might be the best securing and remediation action for the study site, with the potential to preserve the soil ecosystem services, contain the risk of soil erosion and particle dispersion, and phytoextract the bioavailable metals.