Biodegradation of petroleum hydrocarbons in contaminated clayey soils from a sub-arctic site: the role of aggregate size and microstructure.

This study investigates the extent of biodegradation of non-volatile petroleum hydrocarbons (C16-C34) and the associated microbial activity in predominant aggregate sizes during a pilot-scale biopile experiment conducted at 15 °C, with a clayey soil, from a crude oil-impacted site in northern Canada. The in situ aggregate microstructure was characterized by N2 adsorption and X-ray CT scanning. The soils in the nutrient (N)-amended and unamended biopile tanks were comprised of macroaggregates (>2 mm) and mesoaggregates (0.25-2 mm). Nutrient addition significantly enhanced petroleum hydrocarbon biodegradation in macroaggregates, but not in mesoaggregates. At the end of 65-d biopile experiment, 42% of the C16-C34 hydrocarbons were degraded in the nutrient-amended macroaggregates, compared to 13% in the mesoaggregates. Higher microbial activity in the macroaggregates of the nutrient amended biopile was inferred from a larger increase in extractable protein concentrations, compared to the other aggregates. Terminal Restriction Fragment Length Polymorphism (T-RFLP) of 16S rRNA genes showed that there was no selection of bacterial populations in any of the aggregates during biopile treatment, suggesting that the enhanced biodegradation in nutrient-amended macroaggregates was likely due to metabolic stimulation. X-ray micro CT scanning revealed that the number of pores wider than 4 μm, which would be easily accessible by bacteria, were an order of magnitude higher in macroaggregates. Also, N2 adsorption analyses showed that pore surface areas and pore volumes per unit weight were four to five-times larger, compared to the mesoaggregates. Thus the higher porosity microstructure in macroaggregates allowed greater hydrocarbon degradation upon biostimulation by nutrient addition and aeration.