Physical versus Biological Hydrocarbon Removal during Air Sparging and Soil Vapor Extraction

Although physical removal of contaminants may account for the majority of mass of petroleum hydrocarbons (PH) removed using air sparging and soil vapor extraction (AS/SVE), biological processes contribute to mass removal both during system operation and during system shutdown. Biodegradation rates are difficult to measure during active AS/SVE due to the forced entry of air. This study examined the relative rates of physical and biological removal of PH during AS/SVE. Before system startup, soil gas CO 2 and O 2 were measured to estimate biodegradation rates based stoichiometrically on the mass of hexane. Biodegrada tion rates calculated from average CO 2 production (0.08% d -1 ) were 1.3 times less than those based on average O 2 utilization (0.26% d -1 ). A simulation model (Stella II) incorporating air injection and microbial respiration measured during AS/SVE shutdown predicted initial O 2 concentrations during AS/SVE well but overestimated steady-state O 2 by up to 2.5% potentially because respiration may be underestimated if more O 2 depletion occurred during AS/SVE than was measured when the system was off. Although biodegradation accounted for 77 times less contaminant mass removal than physical removal, biological contaminant removal may be underestimated using standard respiration tests.