Volatilisation and biodegradation during air sparging of dissolved BTEX-contaminated groundwater

The relative contributions of volatilisation and biodegradation are quantified for a field trial of air sparging for the remediation of groundwater contaminated with dissolved petroleum hydrocarbons. Groundwater in the unconfined sand aquifer at Kwinana in Western Australia was grossly contaminated by benzene, toluene, ethylbenzene, xylene (BTEX) and other dissolved organics from spills of gasoline. Multi-depth sampling bores, in situ oxygen sensors and neutron access tubes were used to determine changes in groundwater chemistry, oxygen utilisation and the fate of injected air in the aquifer. Oxygen utilisation was used to infer rates of biodegradation. A vadose zone soil vapour extraction system was used to quantify the volatile organic compounds (VOCs) that partitioned from the aqueous phase into the gas phase. Volatilisation was found to be the dominant mechanism for the removal of dissolved VOCs. This was indicated by the close correspondence between calculated masses and the timing of losses. The rate of removal was very rapid, with most organics removed within 3 days of the start of sparging. The rate of loss was also observed to follow the Henryʹs Law constant for the particular compounds. Estimating biodegradation of dissolved petroleum hydrocarbons was complicated by other sinks for dissolved O2, the presence of residual entrapped air in the aquifer and bulk movement of groundwater. However, biodegradation rates were at least an order of magnitude less than volatilisation rates over the period of greatest losses. It was also notable that dissolved VOCs were reduced over a larger volume of the aquifer than directly contacted by injected air. This may have been due to groundwater movement enhanced by stopping and starting sparging during the trial.