Copyright 2007 Air & Waste Management Association14 em october 2007 awma.org

Andrew S. Drucker, P.E., is an environmental engineer with the Weapons Acquisition Support and Environmental

Quality Technology Transfer Branch at the Naval Facilities Engineering Service Center (NFESC) in Port Hueneme, CA.

E-mail: andrew.drucker@navy.mil. Disclaimer: The views presented in this article are

those of the author and do not necessarily represent the views of the U.S. Navy or NFESC.

Air sparging is an effective groundwater remediation technology commonly deployed at sites con-taminated with volatile organic compounds (VOCs) or halogenated volatile organic compounds (HVOCs) as a result of leaking underground storage tanks, pipelines, or other accidental releases. The technology relies on two basic mechanisms—volatilization and biodegradation—working alone or in tandem. Volatilization occurs as pressurized air is injected into a groundwater zone where it proceeds to come in contact with the VOCs. The air strips the contaminants from the liquid, converting them to a vapor, and transports them via air channels to the unsaturated zone. While in the unsaturated zone, the stripped contaminants are bio- degraded or removed via soil vapor extraction (SVE).

Air DistributionThe application of air sparging to remove VOCs from groundwater has been proven successful to varying degrees. In many cases, air sparging is the most cost-effective remedia-tion alternative. As with any in situ technology, however, a well-engineered design followed by proper installation and operation are essential to achieve success. There are several design criteria, most of which are site-specific, that need to be properly addressed to ensure the successful application of an air sparge system. The quality of air distribution is paramount to an air sparging site’s successful application. The extent of air distribution within the saturated region is manipulated by

adjusting the design criteria. It is highly desirable to promote the greatest level of air distribution within the contaminated area, both in terms of air channel density and the reduction or elimination of dead airflow zones. In other words, the greater the extent of air distribution within the remedial zone, the greater the opportunity for air to come in contact with the target contaminants, resulting in their removal. This is true whether air sparging is being used to facilitate volatilization, biodegradation, or a combination of the two.

The primary components of a conventional air sparg-ing system include an air sparge/injection well, an air compressor or blower, monitoring points/wells, and an SVE system (optional). Most of these components are adjusted for size, quantity, and location, with the primary objective to achieve a well-developed distribution of air-flow. For instance, an air compressor or blower is typically sized to deliver air at a flow rate set between 5 and 20 standard cubic feet per minute (scfm). However, greater air distribution is typically realized when setting the airflow rate at the high end of the scale, at or near 20 cfm.

Air distribution may also be improved by reducing the spacing between the individual sparge wells. This lowers the risk of airflow dead zones and contributes to flow overlap from adjacent wells, thereby increasing channel density at the overlap regions. Nested or staggered wells are another design consideration that may improve air distribution by taking advantage of the less permeable subsurface layers within the saturated zone. This is accom-plished by setting individual well depths to provide airflow

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U.S. Naval Facilities Test Adjustable Depth Air Sparging for Groundwater Remediationby Andrew Drucker

Copyright 2007 Air & Waste Management Associationawma.org october 2007 em 15

Figure 2. Schematic of an ADAS well’s flow-through packer while in operation.

above and below these less permeable layers. However, in most instances, well spacing of less than 15 feet and use of nested or staggered wells are cost-prohibitive options that can quickly drive up capital expense. These costs are greatly magnified as the square footage and depth of the treatment zone increase in size and distance, respectively. Generally speaking, significant increases in costs are real-ized at sites that are greater than 5000 square feet and deeper than 40 feet below ground surface.1

Even if one approaches the sparge well design by using rules of thumb, such as placing the sparge wells 15 feet apart and setting a discrete injection depth of between 10 and 15 feet below the water table, an operator runs the risk of not being able to deliver air to where it is needed most due to subsurface heterogeneities. This risk increases as the subsurface’s semi-impermeable and impermeable layers and lenses reach a higher level of complexity throughout the remedial zone. A comprehensive site characterization, along with air sparge pilot testing, will help the operator identify the appropriate location and depth to sparge.

ADjustAble Depth Air spArgingComprehensive site characterization and testing may not necessarily identify the point of injection that will result in the optimal rate of contaminant removal—what is often referred to as the sweet spot—for each and every air sparge well installed at a site. The question then arises: How does one remove the guesswork with respect to finding the “sweet spot” for each air sparge well installed at a site? One answer

Figure 1. Schematic of an ADAS well installed at a site with impermeable subsurface layers.

is to install a set of adjustable depth air sparge (ADAS) wells. ADAS wells offer improved performance capabilities, in many cases better than staggered groupings or nested sparge wells, without the added expense of more conventional well system designs.

ADAS wells allow an operator to manually adjust the depths of air injection by inserting a flow-through packer into a single elongated screened well. Figure 1 illustrates how the ADAS system is intended to be used in saturated subsurface zones, containing semi-impermeable and impermeable layers that alter the upward mobility of injected air. For illustration purposes only, the packer in Figure 1 is shaded blue to rep-resent the packer position above the impermeable layer at injection depth “1” and shaded black to represent the packer position below the same impermeable layer at injection depth “2.” The wandering black lines represent air channel flow that corresponds to the black color of the packer at injection depth 2. Likewise, the blue lines represent air channel flow that corresponds to the blue color of the packer at injection depth 1. By injecting at depth 2, the impermeable layers cause the air to travel at a wider distribution, bypassing a potentially highly contaminated region above the imperme-able layer. It is therefore advantageous to also inject air above the impermeable layer at depth 1 to prevent airflow from bypassing the contamination at shallower depths. Subsurface heterogeneities vary by site and may be more complex than is shown in Figure 1. The ADAS system would be of even more value at sites with injection depth flexibility.

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The inflatable flow-through packer is designed to seal off the interior of the well so that injection air flowing out from the bottom of the packer does not travel back up into the interior of the well (see Figures 2 and 3). The same pressur-ized air that is injected (or sparged) into the groundwater is also used to inflate the flow-through packer. A pressure regulator mounted to the bottom of the flow-through packer provides backpressure to inflate the packer. Once airflow

stops, the packer deflates, allowing the operator to readjust the packer to inject air at a new depth. The packer system consists of 1-, 2.5-, and 5-foot sections, allowing the operator to adjust packer length to meet site-specific requirements. Simple to use, the ADAS system allows the operator to manu-ally raise and lower the packer to any depth within the well without the need for specialized equipment beyond what is employed at a typical air sparge site.

The well design of the ADAS system is similar to a fully screened well used for groundwater monitoring pur-poses. The full screen length allows for injection adjustability throughout the portion of the well submerged within the groundwater zone. To prevent the possible short circuiting of airflow along a well’s exte-rior packing material (typically associated with the well being installed by drill rig), direct push well installation methods are used that eliminate the need for exterior packing material through which short circuiting occurs.

The ADAS system is capable of locating the sweet spot(s) within a single well. It allows

Figure 5. Air bubbling observed at nearby monitoring wells, as air injection depth below the water table was varied.

Figure 4. ADAS well installed at the demonstration site.Figure 3. Flow-through packer prior to being lowered into anFlow-through packer prior to being lowered into an ADAS well.

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the operator to perform a pilot test at each well without the added cost of mobilizing a drill rig more than once (i.e., once for pilot testing and once for complete well installation) and without the need to perform a costly comprehensive site characterization. By being able to adjust the air injection depth, ADAS users are able to characterize impermeable layers within the subsurface.

DEMONSTRATIONA demonstration of the ADAS system was recently performed in an area of active groundwater remediation at a Superfund site in New Jersey.

Site DescriptionAt the time of demonstration, remediation systems were operating at the site to remove VOCs in groundwater. Installation of the ADAS system (see Figure 4) was completed alongside existing conventional air sparge wells, thereby allowing a performance comparison between ADAS wells and conventional discrete depth sparge wells.

ObjectivesTesting was designed to evaluate the ADAS system’s per-formance in two areas: subsurface and overall operability. Subsurface performance was evaluated by changes in con-taminant removal rates, air distribution, and water table levels. Because mass removal (stripping) is a key parameter for evaluating ADAS, testing was performed using air sparg-ing and SVE. Operability, adjustability, and overall ease of use were evaluated under field conditions, and modifications were made as needed to improve the system.

ResultsThe influence data presented in Figure 5 show evidence of lateral migration and potential air pocket formation. Subsurface air pressures were measured at several well loca-tions within 20 feet of the air sparging test well during the 5-foot depth test. The highest pressures during sparging were measured at observation wells 8.6 feet and 17.6 feet from the air sparging test well and within the main airflow pathway.

Figure 6. FID readings taken at the previously measured depth. Each bar represents a separate period of operation within a single day.

Figure 7. FID readings taken at 4- and 5-foot depths from the water table within a period of one day.

Air bubbling was rapid at these two wells, to the point where foam was forming in the well.

Pressures at these wells were approximately 25–30% of the air sparging well head pressure, indicating potential air pocket formation in this zone. Additional pressure measure-ments made at a third observation well during the 6-foot depth test were approximately 30–35% of the air sparging well head pressure. The data indicate some impediment to

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vertical airflow (e.g., silty layers) approximately 3- to 4-feet below the water table in the test area.

Flame ionization detector (FID) and photo ionization detector (PID) readings were made from vapor samples at the SVE well to measure VOC concentrations recovered during ADAS testing. The FID readings were higher than the PID readings, but changes in both readings were similar on a percentage basis. Some increase in FID/PID readings was observed during all tests. The rates of increase were generally slow, indicating sparging airflow paths to the unsaturated zone were indirect and/or not in close proximity to the SVE well. The greatest increases in FID readings were observed during the 5-foot depth test (76 parts per minute [ppm], or 68% increase) (see Figures 6 and 7).

Smaller increases were observed during the 6-foot depth test (38 ppm, or 36% increase), 4-foot depth test (23 ppm, or 27% increase), and open well test (27 ppm, or 40% increase). Normalizing for test duration, the estimated VOC mass stripped from the saturated zone during the 5- and 6-foot depth tests accounts for approximately 60% of the total VOC mass stripped from the saturated zone by air sparging during testing at six depth intervals.

Elevated levels of oxygen and carbon dioxide concen-trations were measured, shortly after shutting down the oxygen biosparging system. Oxygen and carbon dioxide concentrations decreased during testing as these gases were

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flushed from the unsaturated zone. Oxygen concentrations rose one day after the biosparging system was again set into operation.

SUMMARYThe vertical adjustments to air sparging depth clearly influ-enced the achievable VOC mass removal rate, as indicated by the increases in FID readings at the SVE well compared to sparge depth. VOC stripping is a key design criterion for air sparging systems. However, the depth of sparging that maximizes VOC stripping is sometimes not determined dur-ing pilot testing or full-scale operations when using set-depth wells. Certain unknowns, such as the vertical distribution of VOCs and fine grained subsurface layers at each sparging well location, can limit the effectiveness of some wells in a full-scale system designed using set-depth wells, resulting in the need for additional sparging wells or other modifications later in the project. Use of ADAS wells during pilot testing could significantly improve the understanding of subsurface air distribution and VOC mass removal, thereby improving full-scale operations. em

REFERENCES1. Final Air Sparging Guidance Document. Prepared by Battelle for the Naval

Facilities Engineering Command, Naval Facilities Engineering Service Center, Washington, DC, 2001.

2. Final Pilot Testing Report Adjustable Depth Air Sparging System. Prepared by Xpert Design and Diagnostics, Allentown, PA, 2006.

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