Quantification of aerobic biodegradation and volatilization rates of gasoline hydrocarbons near the water table under natural attenuation conditions

Aerobic biodegradation and volatilization near the water table constitute a coupled pathway that contributes significantly to the natural attenuation of hydrocarbons at gasoline spill sites. Rates of hydrocarbon biodegradation and volatilization were quantified by analyzing vapor transport in the unsaturated zone at a gasoline spill site in Beaufort, South Carolina. Aerobic biodgradation rates decreased with distance above the water table, ranging from 0.20 to 1.5 g m⁻³ d⁻¹ for toluene, from 0.24 to 0.38 g m⁻³ d⁻¹for xylene, from 0.09 to 0.24 g m⁻³ d⁻¹ for cyclohexene, from 0.05 to 0.22 g m⁻³ d⁻¹ for ethylbenzene, and from 0.02 to 0.08 g m⁻³ d⁻¹ for benzene. Rates were highest in the capillary zone, where 68% of the total hydrocarbon mass that volatilized from the water table was estimated to have been biodegraded. Hydrocarbons were nearly completely degraded within 1m above the water table. This large loss underscores the importance of aerobic biodradation in limiting the transport of hydrocarbon vapors in the unsaturated zone and implies that vapor‐plume migration to basements and other points of contact may only be significant if a source of free product is present. Furthermore, because transport of the hydrocarbon in the unsaturated zone can be limited relative to that of oxygen and carbon dioxide, soil‐gas surveys conducted at hydrocarbon‐spill sites would benefit by the inclusion of oxygen‐ and carbon‐dioxide‐gas concentration measurements. Aerobic degradation kinetics in the unsaturated zone were approximately first‐order. First‐order rate constants near the water table were highest for cyclohexene (0.21–0.65 d⁻¹) and nearly equivalent for ethylbenzene (0.11–0.31 d⁻¹), xylenes (0.10–0.31 d⁻¹), toluene (0.09–0.30 d⁻¹), and benzene (0.07–0.31 d⁻¹). Hydrocarbon mass loss rates at the water table resulting from the coupled aerobic biodgradation and volatilization process were determined by extrapolating gas transport rates through the capillary zone. Mass loss rates from groundwater were highest for toluene (0.20–0.84 g m⁻² d⁻¹), followed by xylenes (0.12–0.69 g m⁻² d⁻¹), cyclohexene (0.05–0.15 g m⁻² d⁻¹), ethylbenzene (0.02–0.12 g m⁻² d⁻¹), and benzene (0.01–0.04 g m⁻² d⁻¹). These rates exceed predicted rates of solubilization to groundwater, demonstrating the effectiveness of aerobic biodgradation and volatilization as a combined natural attenuation pathway.