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.