VOLATILIZATION AND BIODEGRADATION DURING AIR SPARGING OF DISSOLVED BTEX-CONTAMINATED GROUNDWATER

The relative contributions of volatilisation and biodegradation are qu antified for a field trial of air sparging for the remediation of grou ndwater contaminated with dissolved petroleum hydrocarbons. Groundwate r in the unconfined sand aquifer at Kwinana in Western Australia was g rossly contaminated by benzene, toluene, ethylbenzene, xylene (BTEX) a nd other dissolved organics from spills of gasoline. Multi-depth sampl ing bores, in situ oxygen sensors and neutron access tubes were used t o determine changes in groundwater chemistry, oxygen utilisation and t he fate of injected air in the aquifer. Oxygen utilisation was used to infer rates of biodegradation. A vadose zone soil vapour extraction s ystem 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 VO Cs. This was indicated by the close correspondence between calculated masses and the timing of losses. The rate of removal was very rapid, w ith most organics removed within 3 days of the start of sparging. The rate of loss was also observed to follow the Henry's Law constant for the particular compounds. Estimating biodegradation of dissolved petro leum hydrocarbons was complicated by other sinks for dissolved O-2, th e presence of residual entrapped air in the aquifer and bulk movement of groundwater. However, biodegradation rates were at least an order o f magnitude less than volatilisation rates over the period of greatest losses. It was also notable that dissolved VOCs were reduced over a l arger volume of the aquifer than directly contacted by injected air. T his may have been due to groundwater movement enhanced by stopping and starting sparging during the trial. (C) 1998 Elsevier Science B.V. Al l rights reserved.