Petroleum mass removal from low permeability sediment using air sparging/soil vapor extraction: impact of continuous or pulsed operation

https://doi.org/10.1016/S0169-7722(99)00071-6Get rights and content

Abstract

Air sparging and soil vapor extraction (AS/SVE) are innovative remediation techniques that utilize volatilization and microbial degradation to remediate petroleum spills from soils and groundwater. This in situ study investigated the use of AS/SVE to remediate a gasoline spill from a leaking underground storage tank (UST) in the low permeability, clayey soil of the Appalachian Piedmont. The objectives of this study were to evaluate AS/SVE in low permeability soils by quantifying petroleum mass removal rates, monitoring vadose zone contaminant levels, and comparing the mass extraction rates of continuous AS/SVE to 8 and 24 h pulsed operation. The objectives were met by collecting AS/SVE exhaust gas samples and vadose zone air from multi-depth soil vapor probes. Samples were analyzed for O2, CO2, BTEX (benzene, toluene, ethylbenzene, xylene), and total combustible hydrocarbon (TCH) concentrations using portable hand meters and gas chromatography. Continuous AS/SVE was effective in removing 608 kg of petroleum hydrocarbons from low permeability soil in 44 days (14.3 kg day−1). Mass removal rates ranged from 2.6 times higher to 5.1 times lower than other AS/SVE studies performed in sandy sediments. BTEX levels in the vadose zone were reduced from about 5 ppm to 1 ppm. Ten pulsed AS/SVE tests removed 78 kg in 23 days and the mean mass removal rate (17.6 kg day−1) was significantly higher than the last 15 days of continuous extraction. Pulsed operation may be preferable to continuous operation because of increased mass removal and decreased energy consumption.

Introduction

Air sparging (AS) and soil vapor extraction (SVE) are two growing remediation technologies for removal of volatile organic compounds from sediments and groundwater. AS involves injecting atmospheric air into the aquifer to induce mass transfer of volatile organic chemicals to the vapor phase and mass transfer of oxygen to the aqueous phase. The injected air forms channels through the contamination plume as it flows upward through the saturated zone and into the vadose zone. The injected air volatilizes the contaminants in the flow channels and transports them to the vadose zone where they are either biodegraded or removed by SVE. SVE strips volatile organic chemicals bound to soil particles and induces atmospheric–soil vapor exchange in the unsaturated zone. AS/SVE is considered to be best suited for remediating volatile contaminants from homogeneous sandy soils where air permeability is high and thus a large area of the contaminant plume will be affected by the injected air.

The main factors controlling the performance of SVE are the chemical composition of the contaminants, vapor flow rates through the vadose zone, and the distribution of air flow channels through the contaminant zone (Bedient and Johnson, 1992). The effectiveness and efficiency of AS depend on the airflow pathways that directly contact the contaminated area and the indirect remediation from contaminant diffusion and other aqueous phase transport processes (Dahmani et al., 1994). The injected air does not penetrate the entire contaminant plume. Therefore, contaminant diffusion into the flow channels is usually the rate limiting factor after several days of operation.

Silt and clay sediments are not considered appropriate for AS/SVE. The low permeability characteristics of clayey soil inhibit air flow through the subsurface thus lowering contaminant removal efficiencies by reducing mass exchange rates of volatile contaminants to the vapor phase. Air flow patterns are affected by hydraulic conductivity, soil permeability and soil structure with less permeable sediments causing the formation of distinct air flow channels up to the unsaturated zone and producing poor air distribution (Johnson et al., 1993). Silty and clayey sediments generally require higher air injection pressures to achieve air flow through the saturated zone. Excessive pressure can result in destruction of the soil formation and promote soil fracturing which reduces AS/SVE effectiveness (Marley et al., 1995). Loden (1992) suggested a hydraulic conductivity of 10−3 cm s−1 for effective use of AS/SVE. Thus, the term “low permeability” used in this study and other AS/SVE literature generally refers to sediments with hydraulic conductivities less than 10−3 cm s−1.

Numerous studies have suggested that AS/SVE efficiency may be reduced in low permeability sediments. Laboratory (Ji et al., 1993) and numerical simulation studies (McCray and Falta, 1996; McCray and Falta, 1997) have shown that subsurface heterogeneity may impact air flow through saturated media. Both laboratory and simulation studies demonstrated that low permeability lenses may cause air to migrate laterally from the sparge point leaving contaminants within and above the lens untouched by air flow. Remediation within these zones is limited by the rate of diffusion or convection to the air channels.

Strategies for operating SVE and AS systems include continuous operation or intermittent operation termed “pulsing”. Pulsing can be on the order of minutes or hours. Pulsed AS/SVE has been suggested to enhance mixing in the subsurface thereby providing increased oxygenation and volatilization of dissolved-phase and NAPL contaminants (Johnson et al., 1993). AS is known to cause groundwater mounding during the first minutes, hours, or even several days of operation depending on sediment grain size due to the displacement of water by the injected air. Conversely, there is a collapse of the air channels when AS stops which causes a temporary depression in the aquifer. Pulsed AS utilizes this mounding and depression of an aquifer to induce bulk groundwater mixing which redistributes dissolved-phase contaminants relative to the air channels and may yield a larger radius of influence and reduce diffusion limitations (Boersma et al., 1995). Continuous AS may induce groundwater mixing by the frictional drag from flowing air, physical displacement of groundwater forming channels, capillary interaction of air and water, thermal convection, migration of fine materials resulting in redirection of airflow, and evaporative loss of water in the air stream and the resulting groundwater inflow to maintain water balance (Payne et al., 1995). The magnitude of groundwater mixing resulting from these processes is suggested to be greater during pulsed than continuous operation (Payne et al., 1995).

Several studies have measured enhanced contaminant removal efficiencies of pulsed system operation compared to continuous operation in sandy sediments. Clayton et al. (1995) and Payne et al. (1995) suggested that pulsed AS operation may induce groundwater mixing and produce greater mass removal rates in sandy aquifers, but studies have not investigated continuous and pulsed AS/SVE in low permeability sediments where groundwater mixing may be hindered.

The effectiveness of AS/SVE operation is not well documented in the Southeastern U.S. Appalachian Piedmont where low permeability, saprolitic soil may affect remediation efforts. The objectives of this in situ study were to evaluate physical removal rates of petroleum hydrocarbons during AS/SVE operation in the low permeability residuum and overburden of the Appalachian Piedmont and compare continuous and pulsed operation of the AS/SVE system.

Section snippets

Site description

The field site is located in Columbia, SC at an abandoned gas station where leaking USTs released an undetermined amount of gasoline into the soil and groundwater. From December 12, 1989 to April 16, 1992, Phase I, II, and III assessments were performed by consulting companies. Monitoring wells MW1-MW12 were installed and soil borings and groundwater samples were collected for contamination analysis. From June, 1992 to January, 1994 an expanded site assessment was implemented and involved

Physical contaminant removal

Physical removal of the petroleum hydrocarbons was estimated by collecting stack gas samples from the AS/SVE system. The exhaust manifold was equipped with a sampling port where exhaust gas samples were dispensed into 5-l Tedlar bags. Samples were analyzed for O2, CO2, total combustible hydrocarbon (TCH), and BTEX concentration using portable meters and gas chromatography within 24 h of collection. Exhaust gas samples were analyzed using a CO2 meter (GasTech RA411A; range: 0–4.975%) and a

Continuous operation

The AS/SVE system removed an estimated 608 kg of hydrocarbon contamination from the subsurface during 44 days of continuous (24 h day−1) operation (Fig. 3). The mean hydrocarbon removal rate (Table 2) was 10 times less than the initial 7-day study at this site of 144 kg day−1 (Aelion et al., 1995) potentially due to reduced levels of NAPL near the SVE wells, or rapid NAPL volatilization around the SVE wells followed by a significant decrease in SVE off-gas concentrations (McCray and Falta, 1997

Discussion

AS/SVE was effective in removing petroleum hydrocarbons in low permeability soil with a mean mass removal rate comparable to other AS/SVE studies (Table 2). AS/SVE became contaminant diffusion limited after approximately 1–2 days of operation as high initial values of hydrocarbon, TCH, and BTEX removal quickly decreased to a steady-state level. AS/SVE mass removal corresponded to TCH and BTEX concentration in the exhaust stack. Thus, mass removal was contaminant concentration dependent as

Summary

Our results demonstrated that AS/SVE was effective in removing petroleum hydrocarbons from low permeability sediments at hydraulic conductivities lower than those previously recommended. Pulsed operation of AS/SVE was more effective than continuous AS/SVE in this short-term study and suggested that pulsed operation may optimize mass removal while reducing the electrical cost of operation. Although the AS/SVE removal rates estimated in this study were comparable to other AS/SVE studies, the

Acknowledgements

This research was funded by the National Science Foundation (93-50314; Environmental Engineering), South Carolina Hazardous Waste Management Research Fund, and South Carolina Department of Environmental Health and Control (DHEC). Special thanks are extended to Pete Stone, Robert Faller, Todd Adams, Greg Withycombe, and Christine Bucklin of DHEC, and Mark Widdowson, Nikki Shaw, and Richard Ray for their support and contribution to this study.

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