The Thematic Strategy for Soil Protection specifies contamination as one of the eight main threats to soil, with 3.5 million sites potentially contaminated in Europe, and 0.5 million currently needing remediation. About 13% of those sites are contaminated with PACs in complex mixtures such as coal tar or creosote. Human exposure to PACs in soil or sediment is well known, but risk management at PAC-contaminated sites has barely changed since the 1970s, and is exclusively based on concentration levels of 16 regulated PAHs. NETPAC highlights the need for a more accurate risk assessment during bioremediation of PAH-contaminated sites. Previous reports revealed that, despite of effective PAH removal, bioremediation might have limited effect on or even increase the genotoxicity of the contaminated soil. Those works pointed to the formation of oxy-PAHs as a potential factor of risk. NETPAC has provided the first direct evidence of microbial activity as contributing to that increased genotoxicity (a bacterial metabolite of pyrene accumulated in bioremediated soil). This metabolite, not previously identified, suggests that previous knowledge on metabolic pathways for PAH biodegradation, gathered from the study of pure cultures, might not be sufficient to understand the reactions actually occurring in situ. The Stable Isotope Assisted Metabolomics method developed within NETPAC might be a decisive step forward towards the understanding of those reactions. Nontarget analysis of PAH-contaminated samples also revealed an unexpected diversity of N-PACs. The limited and isomer-selective biodegradability of higher molecular weight N-PACs, including well-known carcinogens, suggested their potential contribution to risk. Results from NETPAC should raise awareness on policy makers and stakeholders about these classes of co-occurring contaminants overlooked during risk management of PAH-contaminated soils, and contribute to broaden measures of risk beyond the current list of 16 regulated compounds.
NETPAC has identified microbial key players involved in the degradation of model PACs of concern in contaminated soils (HMW-PAHs, oxy-PAHs and N-PACs), and assessed their occurrence during active bioremediation processes. Of special relevance and novelty is the work on oxy-PAHs and N-PACs. The identification of the microbial communities and metabolic pathways involved in the in situ cycling, and eventual formation (for oxy-PAHs), of these three classes of contaminants opens new lines of evidence on how to modulate these processes in the environment. NETPAC provides new tools to achieve a more effective and predictable remediation, with PAC removal below current reference levels, and minimizing the potential risks associated with this biotechnology.
