Low temperature soil petroleum hydrocarbon degradation at various oxygen levels

A laboratory incubation study was conducted on a petroleum-contaminated soil from Macquarie Island in sub-Antarctic Australia to develop a target O₂ level for bioventing. The soil was amended with NH₄NO₃ (175 mg N kg^? 1 soil) and ¹⁴C-hexadecane (250 mg kg^? 1 soil) and placed in sealed respirometry chambers. The headspaces in the chambers were adjusted to 0, 1, 2.6, 5.2, 10.5, and 20.9% O₂. Each chamber was connected to an NaOH CO₂ trap and to an O₂ feed line (except the 0% O₂ chambers were connected to an N₂ feed line). Chambers were supplied with O₂ in response to pressure drop resulting from CO₂ trapping. Soils were incubated at 6 °C for 12 weeks. O₂ consumption and petroleum degradation were maximized in chambers with 10.4% O₂. There was a slight decline in both O₂ consumption and petroleum degradation at 20.9% O₂. As O₂ concentrations declined below 10.4% O₂ both O₂ consumption and petroleum degradation declined markedly. ¹⁴C collected in the CO₂ traps did not follow this pattern, but was greater in the 1% O₂ chambers than in 2.6 or 5.2% O₂ chambers. Nitrogen remaining at the conclusion of the study indicated that nitrate was completely consumed in the 0, 1, 2.6, and 5.2% O₂ chambers. nC₁₇:pristane and nC₁₈:phytane ratios in the soil at the conclusion of the incubation were significantly lower in the 10.4% O₂ chambers than in those with 20.9% O₂, and more petroleum hydrocarbons were consumed in the 10.4% chambers. Preferential degradation of pristane and phytane in the presence of limited O₂ may be the result of denitrification, evidenced by lower residual nitrate levels in the 10.4 than the 20.9% O₂ environment. Ten percent O₂ is suggested as a target for O₂ enhanced bioremediation.