Environmental forensics of compost-assisted bioremediation in complex co-contaminated soils

Legacy industrial activities have generated soils co-contaminated with petroleum hydrocarbons, metal(loid)s, and regulated organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). In these complex systems, conventional remediation approaches and concentration-based monitoring often fail to fully capture contaminant transformation processes and residual chemical complexity. Here, pilot-scale field bioremediation was combined with an environmental forensics framework to evaluate contaminant degradation, microbial response, and metal(loid) mobility under realistic field conditions. Six biostimulation strategies were assessed, including plant-derived compost, sludge-based compost, mineral fertilisers, and natural attenuation controls. After 150 days, hydrocarbon removal remained below 30% in control treatments, whereas nutrient-based treatments achieved intermediate efficiencies and compost-amended systems showed the highest performance, reaching up to 58% with plant-derived compost. Compost application was also associated with reductions in TCLP-extractable Zn, Pb, and Cd fractions, consistent with decreased metal mobility. Molecular fingerprinting revealed selective degradation of labile hydrocarbon fractions, while biomarkers, asphaltenes, and heavy waxes persisted after treatment. In parallel, non-target screening identified persistent polar contaminants, including organophosphorus flame retardants and phthalates, which were largely unaffected by remediation. Overall, the results demonstrate that compost-assisted bioremediation can enhance hydrocarbon degradation and reduce metal mobility in complex co-contaminated soils, while integrative environmental forensic approaches provide critical insight into residual contaminant complexity not captured by conventional concentration-based metrics.