Enhancing bioremediation of diesel/biodiesel blend (B20) impacted sites using in situ bioreactors: A nature-based solution for sustainable groundwater management

The rise of biodiesel content in diesel/biodiesel blends as a viable alternative to traditional diesel fuel presents several advantages, particularly in terms of reduced emissions of pollutants like carbon monoxide (CO), hydrocarbons (HC), and smoke. However, the increased production and utilization of biodiesel raise concerns about potential environmental impacts, such as groundwater pollution. This has led to a growing need for sustainable and effective bioremediation techniques to address these issues and ensure the safe closure of contaminated sites in accordance with environmental regulatory criteria. Two experimental areas contaminated with B20 (80% diesel and 20% biodiesel v/v) to evaluate ammonium acetate biostimulation (B20_BAA) and natural source zone depletion (B20_NSZD) as remediation strategies were monitored for 10 and 12 years, respectively. Although the benzene half-life was 1.49 and 4.08 years, respectively, hydrocarbon concentrations remained above the maximum contaminant level (MCL) allowed in Brazil for groundwater, requiring additional technologies for site cleanup. Then, two pilot-scale airlift bioreactors were employed as nature-based solutions (NbS) to reduce the concentration of persistent contaminants for site closure. Indeed, concentrations of benzene and 2-methylnaphthalene decreased significantly after the bioreactors began operation, reaching values below their respective MCL. 16S rRNA gene sequencing showed a beneficial response of microbial communities composed of Massilia, Burkholderia-Caballeronia-Paraburkholderia, Mycobacterium and Bacillus genus involved in hydrocarbons aerobic biodegradation. Moreover, predicted functional genes analysis demonstrated that the relative abundances of key aerobic degradation pathways for benzene and 2-methylnaphthalene increased, supporting the hypothesis that the NbS stimulated the hydrocarbons biodegradation. These findings demonstrated that combining different nature-based solutions (NbS) can effectively remediate petroleum hydrocarbons in contaminated groundwater through geochemical characteristics and exploration of indigenous microorganisms. To the best of the authors’ knowledge, this is the first study to employ bioreactors to treat B20-contaminated groundwater at a field scale.