Site Assessment and Cleanup Part II

The second of a two-part series, this article deals with actions that may be required when an UST site has been contaminated as a result of spills, leaks and overfills.

Highlights
This is the second part of a two-part series dealing with the actions that may be required when an UST site has been contaminated as a result of leaks, spills and overfills. Such actions, which include an assessment and cleanup of the soil and ground water contamination, require the use of specialists in hydrogeology and other sciences. These articles are not intended as guidance for such experts. Nor do they attempt to describe what must be done at a specific site or occurrence. Rather, their purpose is to provide system owners and operators with a basic understanding of:

• Site assessment and cleanup requirements in federal regulations
• The purpose and nature of site assessments
• The various types of cleanup actions that may be taken

Part I (July 1998 issue) covered the federal site assessment and cleanup requirements and the basics of the site assessment process. Part II covers the contamination cleanup process.

This article is drawn from the Technology of Underground Liquid Storage Systems, Independent Study Course (ISC), a 20-lesson course. The course material has been developed for the University of Wisconsin-Madison’s live training courses, and is based on nationally recognized codes and standards.

Basis for cleanup actions
Contamination of soil and ground water must be cleaned up in accordance with a corrective action plan developed from a site assessment and approved by the agency implementing the EPA requirements. The owner or operator may begin cleanup actions before plan approval, provided that he notifies the implementing agency in advance and incorporates these early actions into the formal plan, along with any conditions imposed by the implementing agency.

No prescribed cleanup standards exist that can be applied to all cleanup actions, which must be tailored to the conditions found at each site. Also, soil cleanup actions are quite different from ground water cleanup.

Cleaning up the soil
Soil cleanup can be either on-site or off. Normally, on-site methods are preferred. Digging up the dirt and hauling it to another location for cleanup can be very expensive. In addition, other soil or backfill may have to be hauled in to replace what has been removed. On-site methods, however, may not always be acceptable, depending on such factors as the scope of contamination and state regulations.

Another factor that controls the choice of cleanup methods is the location of the property. If it is in a rural area with no one living nearby, cleanup techniques may be acceptable that could not be considered in a crowded urban location.

The principal options for cleaning up or disposing of contaminated soil are as follows:

On-site
• Soil venting
• Soil shredding
• Bioremediation
• In situ vitrification
• Soil washing

Off-site
• Land farming
• Asphalt batching
• Low-temperature thermal treatment

Soil venting may be used if the soil is fairly loose and the pores of the soil above the ground water level tend to be filled with air. As a result of the contamination, this air will tend to be saturated with gasoline vapors. One way to remove it is to ventilate the soil to make it possible for vapors trapped in the loose soil to escape. The venting technique takes two forms, passive and active.

Passive soil venting systems simply consist of vents inserted into the ground. Usually, these vents are slotted pipes that extend from the depth of the contamination to the surface. Gravel is placed around the pipes to enhance movement of escaping vapors. If the soil is porous enough, and a sufficient number of slotted pipes are sunk into the ground, the contamination—particularly if it consists of volatile gasoline vapors—will eventually find its way out of the soil and into the pipes. There the vapors will rise to the surface and dissipate in the air.

Often, however, it becomes necessary to assist the venting action by introducing an active system—a method of accelerating the movement of trapped vapors out of the soil. Typically, an active venting system consists of an energy source (a pump, for instance) that creates a vacuum and pulls the vapors through the soil and into the slotted pipes (see Figure 1). Or, instead of a pump that creates a vacuum, the energy source may be an industrial blower that forces air into the soil and blows the contaminants out through the slotted pipes.

If an active venting system is used (either a vacuum or blower type), it may be necessary to clean up the vented air before it is released into the atmosphere. That is, air being forced through the ventilation pipes may be so saturated with vapors that it would create an air pollution problem if allowed to rise into the atmosphere. Depending on the amount of contamination, these vapors can be routed through a carbon adsorption system, or piped to a burner for incineration.

When should active (soil vacuuming or blowing) techniques be used for venting contaminants from the soil? Here are the major considerations that influence the decision:

• The contaminant’s volatility.
Soil venting may work very well for gasoline-saturated soil, but not work at all for heavy bunker fuel. Gasoline tends to vaporize at ambient temperatures, while bunker fuel may require substantial heating to volatize it.

• The depth from the ground surface to the water table.
If soil venting is to be used, there should be at least a 10-foot depth between the surface and the ground water table. Otherwise, the action of the venting may be to force the contaminants into the ground water.

• Soil permeability.
Air flow will not be adequate in tightly packed impervious soils, such as heavy clays.

• Time.
Projects that require quick remediation would best be remediated by alternative methods, such as off-site disposal.

• Contamination scope.
If the soil contamination has not been widespread—less than 500 cubic yards, for instance—it may be less costly to simply dig up the dirt and dispose of it at an off-site location.

• Other hydrogeologic factors.
In deciding whether to use soil venting techniques, experts also evaluate soil stratification, the quantity and concentration of the contaminant and other factors.

Soil vacuuming is especially useful when the contamination location is beneath a building. If needed, the technique can also be used when the contamination has spread across boundary lines to an adjoining property. When soil venting is being done, the air emerging from the slotted pipes is periodically analyzed to determine whether it is necessary to cleanup the air (through use of a carbon adsorption system, for instance) before releasing it to the atmosphere. The sampling also reveals whether additional slotted pipes may be necessary, or whether the power of the blower or vacuum system should be increased or decreased.

Soil shredding works this way: Workers dig up the contaminated soil and pile it up alongside the excavation. A truck-mounted shredding machine is moved to the location. The machine’s conveyer carries the soil onto a shredding belt, where a continuous raking action occurs. Closely-spaced rows of steel cleats on the belt churn, toss and aerate the dirt as it moves along. Oversized clods are forced back into the conveyer system for further processing. The device automatically ejects sticks, stones, metal, glass and other non-shreddable material.

As the shredded soil comes off the end of the belt, it is piled up in several separated piles. From time to time the piles are subjected to a gas analysis test. Frequently, the test will reveal that a particular pile of soil must be given another pass through the shredding machine. Sometimes, the dirt must go through the system half a dozen times or more before it has been sufficiently cleaned up.

Soil shredders are large machines, measuring up to 25 feet long and 12 feet wide. The shredding belt itself is usually only a yard or so wide. A front-end loader, capable of handling three cubic yards of dirt, is normally used for loading soil onto the conveyer. (See Figure 2).

Bioremediation is one of the newer concepts for cleaning up both contaminated soil and ground water. The concept, which is schematically illustrated in Figure 3, operates on the principle that certain tiny “bugs” (micro-organisms), which exist normally in nature, thrive on the consumption of hydrocarbons. The idea is that if you inject the right type of these “bugs” into a contaminated area and provide certain nutrients to keep them alive until their cleanup job is finished, they will destroy the hydrocarbon compounds in the soil and water.

This approach offers a number of advantages. First, it may be less costly than most other remediation techniques. Second, it allows on-site cleanup with minimal disturbance of soil. Third, it permits simultaneous treatment of contamination caused by different product types. But bioremediation also has its disadvantages, including the following:

• Long start-up times. It may take as long as eight weeks, after the “bugs” are introduced into the soil, before they begin to do their work.

• Possible production of unfavorable gases. w Need to continually provide nutrients to keep the micro-organisms alive and working.

• Need to remove the micro-organisms if they have gotten into the ground water, after the work is finished (if human or animal consumption of the water is anticipated).

Quite clearly, the decision on what type of micro-organisms to use for a particular cleanup job should be left to a microbiologist. Certain organisms are said to be anaerobic, i.e., they grow without free molecular oxygen. Others are aerobic (grow with oxygen), and some are said to be facultative in that they grow either with or without free molecular oxygen.

A variety of micro-organisms—bacteria, yeasts and molds; cyanbacteria; and green algae—have been used to degrade various organic compounds resulting from petroleum product spills. Theoretically, every compound that humans can create in a refinery or chemical plant can be metabolized by some micro-organism. Unfortunately, in the process micro-organisms sometimes create other undesirable products. Anaerobic organisms, for instance, require certain inorganic compounds, such as sulfate, nitrate or carbon dioxide. However, when they consume carbon compounds, they tend to produce methane gas.

Some types of petroleum products are more difficult for micro-organisms to degrade than others. Aromatic additives in gasoline that contain more than 22 carbon atoms, for example, can be attacked only by certain types of organisms.

When a remediator makes an evaluation as to whether micro-organisms can be used to clean up a site contaminated with petroleum products, he or she will usually:

• Extract soil samples and run tests to determine just what type of organic contamination is present.

• Run tests to determine how concentrated the contamination might be. If the concentration is too high, it may injure the micro-organisms selected to degrade it.

• Based on these tests, select a particular type of “bug” for use in bioremediation.

• Set up a pilot operation to monitor the effectiveness of the bioremediation as it progresses.

For bioremediation to work, a fairly heavy population of microbes must be injected into the soil. Moreover, the microbes must be kept healthy while they are doing their work. This requires attention to such factors as temperature, nutrients, pH factor, soil moisture and oxygen content.

Microbial populations do best when the temperature ranges from 70ºF to 100ºF. In cold climates, it sometimes becomes necessary to pump in heated water and recirculate it through the soil.

To thrive, microbes need nutrients. The two elements that are generally the most important to them are nitrogen and phosphorous. Both are usually present in the soil at a contamination site, but it may be necessary to inject additional amounts. It may also be necessary to inject additional water into the soil, since microbes need moisture to perform their metabolic processes. If the microbes need additional oxygen, air can be introduced into the soil and ground water through use of air spargers positioned in wells at the site.

Does bioremediation work? Although the process is relatively new, studies show that it can be effective. In one case, for example, 900 gallons of leaded gasoline had been spilled into silty soil. Within a period of 18 months, bioremediation removed 99 percent of the contamination that was in the soil and ground water. Analysis showed that the masses of “bugs,” set to work at the site, had consumed 650 gallons of leaded gasoline.

Bioremediation is usually less expensive than other forms of contamination cleanup. Studies at some sites show, for instance, that the cost of cleanup via bioremediation was about half of what the cost would have been if active soil venting (vacuum or blower) methods had been used.

In situ vitrification is a Latin term meaning, approximately, to turn something into glass in the place where it is located. In effect, it describes a process in which enormous heat, supplied by electric current, is introduced into the soil. The heat is so intense that it melts the contaminated soil and destroys the contamination. When the heat source is removed and the area is allowed to cool, what remains is a block of glass-like material. That’s how the process gets its name. You heat up the soil to such a high temperature that, in effect, you melt it. When it cools, it fuses into a glass-like solid—just as sand melted in the glass-making process cools into a solid.

In general terms, this is how in situ vitrification works: First, four electrodes are installed into the ground, one at each corner of the contaminated area. Between the electrodes, a conductive mixture of flaked graphite and glass is placed, which acts as a starter path (see Figure 4).

When the electrodes are in place and connected by the graphite paths, the operator turns on the electrical power source to which the electrodes are connected. So much power is applied that the soil between the electrodes heats up to 3600ºF. (To understand how hot this is, you need to know that at temperatures of between 2000ºF and 2500ºF, soils normally melt and fuse.) When the power is turned on, the intense heat melts rocks and sediment.

This super heat also destroys organics, through an action referred to as pyrolosis. Pyrolosis is a chemical engineering term used to describe situations in which organic compounds are subjected to very high temperatures. This pyrolitic action, of course, is the purpose of the in situ vitrification process. The temperature in the contaminated soil is raised so high that it destroys the hydrocarbons present.

Vapors may be released as the soil “cooks,” thus it may be necessary to place a hood over the area to capture the vapors. Filtering or adsorption equipment is installed under the hood to filter contaminants out of the released vapors before they escape into the atmosphere.

The vitrification process can be used to decontaminate about five tons of soil an hour, in single melts up to 650 tons. The technique can be used to a depth of about 25 feet.

The principal advantages of in situ vitrification are as follows:

• The contamination in the soil can be completely destroyed.

• A variety of contaminants, whether from gasoline, diesel or other chemicals, can be “zapped” simultaneously.

• Whereas bioremediation may require months, in situ vitrification can be accomplished in hours.

• The need to excavate and transport contaminated soil to a disposal site is eliminated.

• Because the process produces a solid block of glass-like material, the process can also be used to impede movement of contaminated ground water through an area.

The method does have some disadvantages also. One is that if the contamination is too deep (say, more than 25 feet), vitrification loses its effectiveness. Another is that the end result of the process is a large, solid lump of glass-like material that was once soil. This may not be a desirable result at some locations.

The cost of a vitrification project depends upon the local power rates and the amount of water present in the soil. If the soil is very wet, longer operational times are required because the water must be vaporized before the contaminants and soil begin to melt. Normally, operators can treat from four to five tons of soil an hour. A ton of soil contains from 20 to 29 cubic feet.

Soil washing is also called soil flushing or solution mining. This method involves soaking the soil with a fluid (mainly water, with some chemicals added), recapturing the fluid and removing the contaminants picked up as the fluid moves through the soil. Figure 5 depicts a typical arrangement for a soil washing operation.

The washing fluid is sprayed onto the surface, above the contaminated soil, soaking its way through the soil until it reaches the water table below. From there, the fluid continues on to a recovery well, positioned downstream from the contaminated area. The fluid, which now contains contaminants, is then pumped to the surface and directed into a storage pond. The contaminants are removed from the fluid before the fluid is recycled through the system.

Soil washing works well at some sites but not at others. Variables that affect whether this method can serve as a practical treatment include:

• Whether the contaminants will readily become soluble in the fluid/chemical mixture used in the washing process.

• The degree of contamination. If there are heavy concentrations, they may not readily yield to soil washing.

• Soil texture.

• The cost of the chemicals that must be added to the water. Chemicals are selected largely based on the composition of the contaminants. For the phenols present in some gasolines, the recommended treatment includes sodium hydroxide.

The soil washing method has some obvious disadvantages. Chief among them is the fact that, even though the chemicals used in the washing process may get rid of the contaminants, they may also alter the chemistry of the soil. The permeability and biology of the soil could also be affected. In some situations, the solutions used for soil washing may, themselves, be considered pollutants.

Another disadvantage is that if the fluid solution moves along preferential flow paths in the soil, it may totally miss pockets of contamination or actually contribute to the further spreading of contamination.

In the mid-1980s, a soil washing experiment was conducted at a U.S. Air Force fire-training pit in Wisconsin. The washing process continued for a period of a week. Later, technicians made intensive studies of soil and water samples extracted at the site. The conclusion was that the soil washing technique had not been effective at this location, probably because of the high soil absorption values at the site.

Advantages of the technique include its relatively low cost and the ease with which it can be set up.

Land farming is one of the most common methods for treating petroleum-contaminated soils. The concept is fairly simple. You dig up the contaminated soil and haul it away to an open field. There, the soil is unloaded and spread thinly over the surface. From time to time, a piece of farming equipment (e.g., a harrow) is pulled across the field to turn the contaminated soil. In this process, the contaminants are either bioremediated or released into the atmosphere.

In some land farming cleanup operations, the contaminated soil is simply spread thinly in an open field. No subsequent plowing is done. Instead, there is reliance on the natural forces of sun, rain and wind to achieve volitization and release of the contaminants from the soil.

The land farming method can be used only at locations not close to human habitation. If people are living nearby, they may take offense to releasing contaminants into the atmosphere. Other factors affecting the acceptability of this method include soil characteristics, climate and proximity to water supplies.

Figure 6:
Modified Asphalt Plant

Asphalt batching is just what it sounds like. An asphalt road is an aggregate (light gravel, for instance) held tightly together by a binding agent derived from petroleum.

This fact caused someone to make a mental connection between petroleum-contaminated soils and asphalt batching plants. Why not, this person thought, take the oil-soaked soil to an asphalt plant and run it through the heat process used in the manufacture of asphalt aggregates? The volatile light ends would be cooked off. What remained would be a mixture of dirt and a petroleum derivative that could be mixed with other asphaltic materials and used for building roads and parking lots.

The result of this idea is that, in several states, the use of asphalt batching plants to process petroleum-contaminated soil has been approved.

The process begins with excavating the contaminated soil and transporting it to the nearest asphalt batching plant. At the plant, the soil is placed on a conveyer, which moves it into a tank-like dryer. The dryer is heated to a temperature of 500ºF to 800ºF. The soil aggregate remains in the dryer for about five minutes, during which many of the contaminants are either destroyed (by the heat) or vaporized. The aggregate then moves into another chamber where it is mixed with other materials for subsequent use as asphalt.

Some advantages of using asphalt batching plants for disposal of contaminated soil are that:

• The contaminated soil requires no other treatment either before or after it is brought to the asphalt plant and put through the batching process.

• Most asphalt plants are equipped with devices to remove contaminants from the vapors that are “boiled” off the oil-laden soils, and thus prevent contaminated vapors from being released to the atmosphere.

• Batching plants are usually closer to contaminated sites than approved landfills, which lowers transportation distances.

A clear disadvantage is that soil contaminated with a highly volatile product like gasoline does not normally lend itself to the asphalt batching process. After the light ends vaporize in the “cooking” chamber, there is not much left in the form of petroleum product that can be used in the manufacture of asphalt. Moreover, to meet environmental requirements, most asphalt plants must be extensively modified before they can receive contaminated soils (see Figure 6).

One option is to use the asphalt plant to treat the contaminated soil and, rather than use the cooked material as part of an asphalt aggregate, simply return it to the excavation site.

Figure 7:
Full Scale Thermal Treatment

Low-temperature thermal treatment gets rid of soil contaminants by “cooking” them. The soil is loaded into an oven-like chamber and subjected to temperatures in the 500-800ºF range. This either destroys the contaminants or causes them to vaporize and separate from the soil.

Low-temperature thermal treatment is one of the most promising methods for dealing with petroleum-contaminated soils. Soils subjected to the process can usually be returned to the excavation from which they were removed.

One such process uses thermal treatment units that are mounted on flatbed trailers. These units can be driven directly to the site where the contamination has occurred. The contaminated soil is excavated, loaded directly into the thermal treatment unit, cleaned up and then returned to the excavation.

Full scale thermal treatment units include a solids handling system, a hot oil system, a gas handling system and a water system. All these systems are mounted on the vehicle that brings them to the contamination site (see Figure 7).

In a typical configuration of a low-temperature thermal treatment system, workers begin the process by feeding excavated soil onto a scalping screen. The screen removes material larger than two inches in diameter. A drag conveyer from the hopper feeds the remaining material into the thermal processor. This processor is an oven-like chamber. It is actually an indirectly heated auger-type heat exchanger, operating in excess of 600ºF. The thermal processor mixes, agitates and heats the soil, driving off moisture and contaminants.

A hot oil (650ºF) system circulates oil through a hollow shaft in the heat exchanger. This system includes pumps, piping, valves and a hot oil tank. The system provides indirect heating to the contaminated soil that is being processed in the unit.

A mixture of air and exhaust gases from the hot oil heater sweeps the vaporized contaminants from the thermal processor. The sweep gas moves the volatile gases through a filter fabric, or baghouse, used for the control of particulate emissions. A condenser in the system liquifies the volatiles, and then an oil/water separator in the unit separates the condensate water and volatiles.

The water that emerges from the separator retains only a low concentration of soluble organics. Workers treat the water in a two-stage carbon adsorption system. This treated water is then sprayed on the cleaned up soils to cool them, and to suppress dust formation.

Soils contaminated with certain fuels (diesel, gasoline and jet fuel) can be treated with high-temperature incineration in the afterburner of a thermal treatment unit. Incineration of the soils in this afterburner destroys the contaminants. Exhaust gases from the hot oil system control the oxygen content in the afterburner, keeping it under lower explosive limits.

Some mobile (truck-mounted) thermal treatment units operate at higher temperatures and are designed, principally, for cleanup jobs at service stations, C-stores and similar sites where a high-volatile product has been spilled. Generally, these units employ a rotary kiln that bakes the contamination out of the soil.

Cleaning up ground water
Cleaning up the soil often represents only half of the cleanup job at a contaminated site. If contaminants have reached the water table in the area, or if contaminants are being carried through the soil by relatively free-flowing water, the correction process must also include cleanup of the water.

The planning of ground water cleanup is usually assigned to specialists who have been trained as hydrogeologists. Hydrogeologists are familiar with the forces that control water movement through various soil types. This movement is affected by a variety of factors, including:

• The attraction of soil surfaces, in various soil types, for water.

• The attraction of solutions, that may be present in the soil or in the contaminants, for water.

• Energy attributed to gravity’s downward pull on water.

• Air pressures exerted on water in the ground.

• Energy generated by hydrostatic pressure—the so-called pressure head.

In addition to these factors, hydrogeologists must take into account the differences in water movement through saturated and unsaturated soil zones.

When all of the various factors are calculated for a particular location, the hydrogeologist can reasonably estimate the ground water’s velocity moving through the area. Ground water can move several feet per hour through some soils. In other soils, it may move less than a foot per year. The potential velocity range typically falls somewhere between five feet per hour and five feet per day. Movement through gravel is very fast. Movement through heavy clay is very slow. Between these two extremes are silt, sand, peat, glacial till and other soil types.

Calculating ground water movement is further complicated by the infiltration process, the water’s penetration of the soil when it rains. The infiltration rate changes over time. When the backfill around a tank system is first installed, for instance, the infiltration rate may be high; with the passage of time, alterations in the backfill soil change the rate.

The factors discussed thus far are not all that must be considered in ground water movement calculations. Other factors include the presence of what soil scientists refer to as macropores, micropores and the capillary fringe.

It is not the purpose of this article to explain the details of these various factors that control the movement of ground water. Rather, the purpose is simply to introduce you to some of the considerations that enter into a determination of the scope and direction of ground water contamination at a site where there has been a leaking underground petroleum storage system.

Once the scope of ground water contamination has been established, the problem remains of what to do about it— what steps to take to remove the contamination.

Pump-and-Treat is the most straightforward ground water remediation technique. That is to say, you drill a well to the ground water level, at a point where the contaminated product is collecting in the water, and you pump out this water. When the water is brought to the surface, it is treated in some manner to remove the contaminant. (See Figure 8.)

Although the pump-and-treat technology is relatively simple, it is not always effective. It works best when the contaminated ground water is in a porous formation, and when the soil around the water is not strongly attractive to the contaminant in the water. This method may have the undesirable side effect of contributing to the further spreading of contamination.

Tailing is a process in which, as water moves through an aquifer, a slow decrease in the contamination occurs. An aquifer is a geological formation that contains water, especially water extracted through wells for human consumption.

Because of variances in the porosity and the absorptive qualities of different aquifers, contaminants will be more quickly dissipated in some aquifer formations than in others.

If the contaminant in the water is, say, gasoline, it won’t dissolve in the water. But as the gasoline moves through the aquifer, some of it will cling to the geological formation through which it is passing. Although this tailing action gets the contaminants out of the water, the contaminants remain in the formation. Chemicals such as benzene, toluene and xylene—all of which are present in most gasolines—are only slightly soluble in water. All, however, have a tendency to cling to geological formations in aquifers, and are especially difficult to dislodge and remove. These chemicals will eventually move from the geological structure into the ground water, at a predetermined rate. Accelerated ground water pumping will not usually cause them to move any faster.

How difficult is it to remove these chemicals? Consider a theoretical example. In one year, workers using pump-and-treat techniques could cleanup a 10-acre saltwater plume in a 55-foot-thick aquifer, with a water storage capacity of 30 percent. The workers would need to pump at a rate of 100 gallons a minute.

But if the same site were contaminated with gasoline in only 10 percent of the aquifer voids, it would require 1,500 years of pumping to remediate 80 percent of the toluene. Even if the pumping rate were increased 10-fold, the cleanup effort would require 150 years.

Among other things, this example underlines the importance of installing storage tank systems that won’t leak, particularly in areas where the systems are located above aquifers that provide the sole drinking-water source for the community.

Ground water treatment options
The three most widely used ground water treatment techniques are:

• Activated carbon adsorption

• Air and steam stripping

• Bioremediation

Activated carbon adsorption is a method in which water with hydrocarbon contaminants is passed through a bed of carbon, the contaminants in the water will adsorb onto carbon granule surfaces. Even such chemicals as benzene, toluene and xylene can be stripped from the contaminated water when they move through a bed of carbon.

Although the carbon adsorption method is used after contaminated water has been pumped out of the ground, some jurisdictions may allow for the cleaned water to be returned directly into the soil. A closed loop is created that reduces disposal costs, making carbon adsorption cost-effective as an in situ remediation technique.

Air and steam stripping is a method in which separation of volatile organic compounds (VOC) from ground water and soil-gas can be accelerated through the use of a stripping tower. Contaminated water is pumped from the ground, heated, and passed upward through plastic or ceramic packing material as air or steam is passed in the opposite direction. Contaminant-bearing liquid coats the many surfaces of the packing material. Vapors released from this liquid are picked up, in turn, by the air or steam moving in the opposite direction.

The volatile organic compounds thus collected are, in turn, removed from the saturated air or steam when it passes through a series of condensers and (in some systems) activated carbon filters. The stripped air and water is then recycled into the ground, completing the closed loop.

The contaminant-removal rate is based on the capacity of the stripping system employed, type and concentration of contaminants, ratio of air and water pumped into the tower and operating temperature (using steam instead of air speeds up the process). Variations in the design of an air/steam stripping system include the diameter and number of towers, extraction well depth, pump and fan speeds, type and density of packing materials, temperatures and vacuums, size of gravity separators and on-site storage.

Air stripping offers an effective and economical method of treating both ground water and soil-gas on-site. Capital, operating and maintenance costs are low; and the system does not require a full-time operator. The air/steam stripping technique is practical only when removing volatile organic compounds, and its efficiency is affected by changes in temperatures. Operating costs include normal servicing of the pumps, blowers and other mechanical equipment; removal of scale that builds up inside the tower; and reactivation of carbon.

Bioremediation is the third method of treating contaminated ground water and involves the use of micro-organisms. The essentials of this method were outlined above in the material on cleaning up contaminated soils.

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