Extent of Benzene Biodegradation in Saturated Soil Column During Air Sparging

Abstract

AbstractIn situ air sparging has emerged as a popular remedial alternative to traditional pump‐and‐treat for the remediation of volatile organic contaminants in saturated soils and ground water. During air sparging, a gas, usually air, is injected into the subsurface below the lowest known point of contamination. The air induces partitioning of dissolved‐phase and free‐phase contamination into the vapor phase. Additionally, the injected air increases subsurface dissolved oxygen concentrations, potentially stimulating aerobic biodegradation. To take advantage of the ability of subsurface microbial populations to degrade organic compounds, air sparging may be implemented in a manner different from a traditional air sparging application; a technique often referred to as biosparging. This paper presents the results of a laboratory study performed to demonstrate the potential contribution of aerobic biodegradation to the attenuation of VOCs during the use of air sparging. This study included a series of controlled laboratory column tests using a soil profile subjected to biomass infiltration and a range of dissolved‐phase benzene concentrations. The test results demonstrated that, although the partitioning of NAPL‐ and aqueous‐phase benzene into the vapor phase through volatilization was the dominant removal mechanism, biodegradation could account for a substantial portion of the benzene removal during the application of air sparging. In order for considerable aerobic biodegradation to occur, however, sufficient amounts of dissolved oxygen must exist within the subsurface to serve as an electron acceptor. The presence of a soil matrix reduced the rates of biodegradation within the column tests when compared to batch reactor tests due to the discontinuous contact of the biomass, aqueous‐phase benzene, nutrients, and dissolved oxygen. Additionally, the rates of biodegradation that were observed during testing were independent of the initial aqueous‐phase benzene concentrations and the biomass concentrations selected for this study. Finally, the dissolved‐oxygen concentration was observed to be the rate‐limiting factor for biodegradation.