Microbial community and ecotoxicity analysis of bioremediated, weathered hydrocarbon-contaminated soil

Bioremediated soils are usually disposed of after meeting legislated guidelines defined by chemical and ecotoxicity tests. In many countries including Australia, ecotoxicity tests are not yet mandatory safety requirements. This study investigated the biotreatment of weathered hydrocarbon-contaminated soils in 12-week laboratory-based microcosms. Monitored natural attenuation resulted in ∼43% reduction of total petroleum hydrocarbon contaminant to 5503mg/kg (C₁₆C₃₅), making the soil suitable for disposal as waste under current guidelines (pesticide and metal contents within safe limits). 16S rDNA (universal and AlkB) and ITS-based DGGE fingerprints showed stable and adapted microbial communities throughout the experimental period. However, ecotoxicology assays showed 100% mortality of earthworms (Eisena fetida) in potting soils containing <50% (<2751mg/kg, legally safe in situ concentrations) contaminated soil over 14 days. Up to 70% reduction in radish (Raphanus sativus) seed germination was observed in potting soils containing <10% contaminated soil (<550mg/kg, legally safe ex situ concentrations for soil disposal into residential areas). The results indicate the toxicity of these soils to soil biota despite meeting legislated Australian safe levels and guidelines for disposal or use in residential areas.