HAZARDOUS WASTE & HAZARDOUS MATERIALSVolume 11, Number 3, 1994Mary Ann Liebert, Inc., Publishers

OpinionSoil Vapor Extraction and Air Sparging:

Are We There Yet?

GEORGE E. HOAG

Department of Civil Engineering, andEnvironmental Research Institute

University of ConnecticutStorrs, CT 06268

Recently, in-situ subsurface remediation processes have gained considerable attention amongpractitioners and researchers concerned with site remediation involving volatile and semi-volatile organiccompounds (VOCs and SVOCs, respectively). Of the in-situ processes presently available, soil vaporextraction has potentially the most widespread application for the remediation ofVOCs and semivolatilecompounds in soils. Many complimentary technologies, using SVE as the primary treatment technique,hold additional promise for remediation ofproblematic sites. These include applications with particularlychallenging geological formations or those exhibiting chemical compounds in the subsurface which maynot be fully removed by SVE alone. The complimentary technologies include air sparging (AS), bio-venting and subsurface heating. In most cases, the addition of oxygen to the subsurface is a consequenceofemploying SVE and AS. These processes thus have the potential for aerobic bioremediation. In othercases, the vapor capture aspects of SVE, makes it an essential component of subsurface heating and airsparging processes. AS technology is potentially applicable to a large number of VOC and SVOCcontaminated sites. Further, it is a technology with the potential to be far more economical andsubstantially more effective and efficient than groundwater pump and treat systems for source controlpurposes.

Soil Vapor Extraction

Considerable background information and data presently exist for SVE technologies. Thisinformation includes practical operating experience and data, a strong research understanding of thefundamentals including existing computer models, and established engineering design criteria.However, SVE technology is frequently applied at remediation sites with little or no regard to the rolethat several fundamental factors have on achieving the desired remedial goals. These factors includeknowledge of the source area NAPL (Non-Aqueous Phase Liquid) distribution, and anisotropic andheterogeneous subsurface permeability distribution at the soil/atmosphere interface and at depth. Further,perched water zones and contaminant/soil/air interactions can also exert significantly affect successfulapplication ofthe technology. Lack of information regarding some of the above site-specific criteria can

lead to many systems being improperly and inadequately designed. The shortfalls include diffusionlimitations, short-circuiting of air flow and contaminated soil zones which are not appropriately treatedby SVE.

SVE is being increasingly deployed as a equipment-based rather than a design-based technology.During the past few years I have instructed a number of short courses on SVE and AS. Increasingly I

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hear anecdotal information regarding the deployment of SVE by engineers and scientists without anydesign or pilot testing. Often when SVE and AS pilot tests are conducted, the duration is less than one

day. The purpose is usually to identify anomalies, rather than to generate data for subsequent analysisor design and performance evaluation (e.g., meeting cleanup criteria). Based on my personal experience,these sites are not only limited to small service stations. When asked why systems are installed withoutpilot testing or design, a frequent response is that the solution is based on what "the client is willing topay" or that the system is a "typical" installation. I frequently refer to this type of technologydeployment as "design while you dig".

The reality is that SVE is a forgiving technology. Fortunately a poorly designed system can stillextract large quantities ofVOCs from the subsurface, even though larger quantities of contaminants mayremain behind. Emphasis here is given to the amount of contaminant removed, rather than residualcontamination. Time estimates to reach some remedial goals and cleanup criteria may never be met, buteven inadequately designed SVE systems may continue to show positive levels of contaminant removal.When listing the advantages and disadvantages of SVE, ease of deployment is increasingly becoming a

disadvantage.A general problem at many sites is that heterogeneities with respect to contaminant distribution,

permeability and air flow boundary conditions limit the ability of remedial goals to be met. Reasonableremedial goals can often only be met through use of comprehensive site assessments which may include:identification of contaminant source location(s), bench scale treatability tests, pilot testing, computermodeling, detailed engineering design and on-going system optimization. At specific sites, such as highlyanisotropic and heterogeneous landfills, SVE may not be feasible. Some firms will still claim that SVEis always feasible. Thus, once a system is installed, design and operational modifications are thought tobe capable of correction of any initial deficiencies. Unfortunately, experience tells us that in some, andperhaps many instances, conditions for effective SVE application will not exist.

Air SpargingState-of-the art and state-of-the practice are both clearly more developed for in-situ SVE than

for AS. While SVE and AS are frequently associated with each other, AS is a far more complex process.This follows in part because ofassociated multiphase flow and the fact that AS appears to be much moresensitive to minor changes in soil properties than is SVE. Few published articles exist on AS.Unfortunately, the literature appears to be restricted to only pilot-scale studies. Because AS is a

relatively new technology, time has yet to teach us much about the ultimate performance of thispromising process alternative. This situation, however, creates a major dilemma for both industry andgovernment who are currently in the process of evaluating and selecting technologies to achieve desiredremedial goals now. The fundamental cause of this dilemma is our poor understanding of the fluidmechanics involved when air is injected under pressure into water and/or NAPL saturated soils. Wesimply do not know the spatial distribution of air filled porosity and gas phase velocity distributions withinthe air filled porosity as well as the aqueous phase velocity distributions in water filled porosity that occuras a result of air sparging. How then can we ever begin to understand the corresponding mass transferlimitations that control the rate of contaminant removal? Determination of the spacing and depth of airinjection wells is an associated practical problem of inordinate difficulty. In cases where subsurfacereleases of gasoline have occurred, it is often difficult to determine whether volatilization or

biodégradation is the principal cause of contaminant removal. In summary, we know very little abouthow AS works. Our ignorance includes when it works and more importantly when it doesn't work.Several associated and perplexing questions then follow. Why is it that AS is now being evaluated atVOC sites ranging in scale from neighborhood gasoline service stations to the largest Superfund sites?Several technical factors are responsible for this:

1. Air sparging is an in-situ source control process viewed as one of the very few alternativesto groundwater pump and treatment technologies.

2. A water treatment system including provisions for discharge of the treated water, as required

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with groundwater pump and treat systems, can be avoided if not eliminated with VStechnology.

3. Air sparging takes advantage of more than one treatment mechanism (i.e. aerobicbiodégradation, volatilization from NAPL and stripping from water).

4. Many groundwater pump and treat systems require long-term operation and may not reachtarget cleanup criteria.

5. Air sparging is associated with SVE which is recognized as a cost effective source controlremedy.

Additionally, several market forces contribute to the current demand for air sparging.1. Capital and operating costs associated with air sparging are far less than those for

groundwater pump and treat systems.2. Time to achieve desired remediation is possibly up to an order ofmagnitude shorter with AS

than with groundwater pump and treat systems. The liability of site owners may then belessened.

3. Air sparging installation is less complex than groundwater pump and treat systems.

The above list presumably constitutes some compelling motivation for AS because air spargingis being evaluated and installed at a rapidly growing number of sites across the U.S. Unfortunately, onlyhighly limited and preliminary fundamental research has been conducted on AS. Recently, a few RFP'shave made it to public announcement, but only a handful. It seems inconceivable that regulatoryagencies, federal departments and large corporations with both substantial research budgets and VOCand SVOC contaminated sites have not been more aggressive at conducting basic and applied researchon air sparging. Site demonstrations and treatability studies (mostly at the pilot scale) are common andrapidly proliferating. In my opinion, however, this is an extreme case of "cart before the horse" or hasthe in-situ remediation field resorted to wishful thinking?

Although there is little fundamental knowledge as a result of insufficient scientific research on AS,it is evident based upon field demonstrations conducted to date, that this technology holds great promisefor reasons outlined above. Soil vapor extraction and AS may well provide protection of the publichealth and the environment at potentially far lower costs than many other alternatives. Fundamentalresearch programs take time to develop and researchers require time and funding to conduct the workand disseminate the results. If substantial fundamental and applied research is not conducted on airsparging in the short term, the scientific community may well be asking the same questions, if not more,for decades to come.

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