Remediation of Chlorinated Solvent Plumes Using In-Situ Air Sparging—A 2-D Laboratory Study

Jeffrey A Adams,Lue Tekola,Krishna R Reddy

doi:10.3390/ijerph8062226

Abstract

In-situ air sparging has evolved as an innovative technique for soil and groundwater remediation impacted with volatile organic compounds (VOCs), including chlorinated solvents. These may exist as non-aqueous phase liquid (NAPL) or dissolved in groundwater. This study assessed: (1) how air injection rate affects the mass removal of dissolved phase contamination, (2) the effect of induced groundwater flow on mass removal and air distribution during air injection, and (3) the effect of initial contaminant concentration on mass removal. Dissolved-phase chlorinated solvents can be effectively removed through the use of air sparging; however, rapid initial rates of contaminant removal are followed by a protracted period of lower removal rates, or a tailing effect. As the air flow rate increases, the rate of contaminant removal also increases, especially during the initial stages of air injection. Increased air injection rates will increase the density of air channel formation, resulting in a larger interfacial mass transfer area through which the dissolved contaminant can partition into the vapor phase. In cases of groundwater flow, increased rates of air injection lessened observed downward contaminant migration effect. The air channel network and increased air saturation reduced relative hydraulic conductivity, resulting in reduced groundwater flow and subsequent downgradient contaminant migration. Finally, when a higher initial TCE concentration was present, a slightly higher mass removal rate was observed due to higher volatilization-induced concentration gradients and subsequent diffusive flux. Once concentrations are reduced, a similar tailing effect occurs.

Highlights

Air sparging has emerged as an innovative technique for soil and groundwater remediation [1,2,3]
Higher initial TCE concentrations were observed in the bottom of the soil profile as compared to those measured at the top of the soil profile [12]
The tests were performed under both static groundwater and induced groundwater conditions, using a variety of air injection rates and initial TCE concentrations

Summary

Introduction

Air sparging has emerged as an innovative technique for soil and groundwater remediation [1,2,3]. The well sparge point is located below the lowest known point of contamination. The injected air rises towards the ground surface, and through variety of mass transfer, transport, and transformation mechanisms, the contamination present within the subsurface partitions into the vapor phase or is degraded. As the contaminant-laden air continues to rise toward the subsurface it encounters the vadose zone of soil, where it is often captured using a soil vapor extraction (SVE) system. By applying a vacuum to the subsurface, the SVE system is able to control the movement of vapors; encouraging movement toward the extraction wells while preventing unwanted migration into soil areas previously unaffected by contamination. The contaminated air may be treated using conventional methods such as carbon filters or combustion

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Conclusion