Introduction

Prevention of ground-water contamination is far more logical, simple, and cost-effective than attempting to correct a problem-a problem that may have been in existence for years. A great deal of time, effort, and money are presently being expended to develop remedial measures to counteract the effects of contaminated aquifers and public water supplies. These include traditional as well as innovative construction techniques, water management, and research initiatives.

Several options or combinations of options are available to restore a contaminated aquifer: (1) provide inground treatment/containment, (2) provide aboveground treatment, (3) remove or isolate the source of contamination, (4) abandon the source of supply, or (5) ignore the problem. Generally, several techniques are coupled in order to achieve the desired results.

Restoration of contaminated aquifers to former background or near background conditions ortocontain contaminated ground water in certain locations is generally accomplished through one of two overall approaches. One approach involves natural or induced in situ treatment, while the other approach uses engineered systems to contain the contaminated ground water. In the latter case pumping wells or engineered structures are installed in order to develop hydraulic gradients that cause the contaminated water to remain in a specified, general location from which it may be removed for later treatment.

Regardless of the restoration approach, any source or sources that continue to contaminate the ground water should be removed, isolated, or treated. Treatment or removal of an existing contamination source eventually may result in restoration of ground-water quality through natural processes. In other situations, contaminated ground water is removed from the aquifer by pumping or is allowed to discharge to a stream in which the flow is sufficient to dilute the contaminant to nondeteclable concentrations. Natural replacement of the ground water is relied upon to eventually restore the quality of the water in the aquifer. Typically the natural restoration processes require many years or perhaps even decades for completion. As a result, ground-water restoration commonly requires a combination of approaches that involve ground-water removal and treatment or, if necessary, induced in situ treatment coupledwith source control (removal, isolation, treatment). Site-specific conditions, properly defined and understood, provide the ground-waterinvestigatorwith the basic information needed for the determination of a viable approach and for selecting and designing a cost-effective restoration scheme.

This chapter provides an overview of aquifer restoration technologies utilizing techniques derived from interrelated disciplines of geology, hydrology, geochemistry, engineering, construction, biology, and agronomy. The major emphasis of the chapter is on ground-water pumping systems and in situ biological treatment for organic contaminants, which are found at almost all hazardous waste sites. Many of the technologies have been developed by demonstration and research in conjunction with remedial activities in the Superfundprogram. Detailed information on selected techniques can be obtained from the references.

Contaminant Mobility

The design of a ground-water restoration program is complicated by the fact that all contaminants do not behave in the same manner. Although discussed previously, it is important to briefly redescribe the significance of contaminant mobility in developing and designing a ground-water restoration program.

The movement of most ground-water contaminants is controlled by gravity, the permeability and wetness of the geological materials, and the miscible character of the contaminants in ground water. When a material, particularly a hydrocarbon, is released to the soil, attraction arid gravity actively draw it into the soil. As the main body of material moves downward into the more moist regions of the soil, capillary forces become less important as the contaminants move through the more favorable channels by displacing air.

When the contaminants reach the water table, those less dense than water tend to spread laterally along the air-water interface or capillary fringe, while the heavier ones continue to move downward inthe saturated zone. In both cases, the contaminants tend to migrate in the direction of ground-waterflow. In unusual circumstances very dense contaminants may be more affected by gravity than by advective flow and move in directions other than that of the ground water.

The amount of a contaminant that reaches the water table depends on the quantity involved, the characteristics of the contaminant, the chemical and biological properties of the unsaturated zone, precipitation (ground-water recharge), and the physical and chemical characteristics of the earth materials. In general, the more permeable the earth material, the greater the quantity of contaminant that is likely to reach the ground water. The entire amount of a contaminant may be temporarily immobilized in the unsaturated zone so that it only migrates downward after rainfall events, becoming a continual or long-term contamination source. Material so immobilized in the unsaturated zone may remain there unless physically, chemically, or biologically removed.

A hydrocarbon liquid phase, for example, generally is considered to be immiscible with both water and air. Residual hydrocarbons can occupy from 15 to 40 percent of the available pore space. However, it is important to realize that various components of the hydrocarbon may slowly volatilize into the vapor phase and then dissolve into the liquid phase. A halo of dissolved components of the hydrocarbon precedes the immiscible phase, some of which becomes trapped in the pore spaces and is left behind as isolated masses. Even when the so-called residual phase is entirely immobile, ground water coming into contact with the trapped material leaches soluble components and continues to contaminate ground water.

Interaction of the contaminant and the aquifer materials is another consideration inthe evaluationof contaminant mobility. Some contaminants tend to partition between the liquid, solid, and vapor phases in amounts dictated by the characteristics of each contaminant, the nature of the aquifer material, particularly the amount of organic carbon, and other geochemical parameters. For many contaminants, these associations are not fixed but can be completely reversible. In addition, these compounds may move freely from one phase to another, depending upon their concentration in each phase. The processes of ion-exchange and sorption, chemical precipitation, and biotransformation all result in retardation or transformation of the contaminants. Ground water can become contaminated as freshwater moves through or past the aquifer material where contaminants are attached, or as infiltrating water moves through the unsaturated zone, which contains contaminants in the vapor phase. The subsurface transport of hydrophobic compounds is an active field of research.

Highly soluble contaminants, such as salts, some metal species, and nitrates, have little affinity for sorption to the solid phase. For aquifer restoration purposes, these contaminants can be considered to move essentially in the same direction and velocity as the ground water and are ideal candidates for pump-and-treat technology.

Site Characterization

In most restoration schemes, all too often the physical features of the subsurface are largely ignored and little understood, and most of the effort is involved with the design and construction of engineering structures. The important pointto consider, however, isthatthe physical features of the subsurface, that is, the distribution of permeability and porosity, and the resulting hydrogeologic characteristics control the movement and storage of fluids in the subsurface.

Ground-water restoration activities require dedication of sufficient resources to collect and understand site conditions. An adequate amount of field data must be collected to provide a detailed understanding of the geology, hydrology, and geochemistry of the site, as well as the types of contaminants to be removed, their concentrations, and distribution. The literature should be reviewed to determine, to the fullest extent possible, the contaminants characteristics of sorption, volatilization, partitioning, and ability to be degraded. Finally, laboratory investigations, including treatability studies, development of sorption isotherms, and column and microcosm examinations to determine contaminant transport and transformation parameters, assist in developing a full understanding of the site conditions, and potential alternatives forground-water remediation.

Many ground-water texts and reports, particularly the older ones, show ground-water flow nets to be homogeneous in both the horizontal and vertical dimensions-at least on a regional scale. In reality such depictions are rare and the actual water movement is much more complicated. Flow lines drawn on a watertable map, for example, imply that the fluids are moving directly downgradient when, in fact, the flow actually follows curvilinear paths (see Chapter 4). All too often significant amounts of the flow may be through limited parts of the aquifer, both horizontally and vertically. This could result from the spatial variability of permeability for water, or it could result from density or other considerations forcontaminants. In otherwords, neither the bulk of the water flow nor the distribution of the contaminants can be assumed as homogeneous.

Figure 7-1 is the map of a contaminated waste disposal site that shows the location of a number of monitoring wells and the altitude of the water surface in them. Notice that there is as much as 100 feet of difference in head in wells that are relatively close. The reason for this difference is well depth, with the deeper wells having the greatest depth to water. Figure 7-2 is a water-level map of the same area; contours were based on shallow wells of nearly the same depth and screen length. Flow lines depict the general direction of groundwater movement. Figure 7-3 is a hydrologic cross section, that is, a vertical flow net, constructed along the line A-A'. Notice in this example that in the upper 50 feet or so the ground water is flowing across different geologic units with little loss in head. This indicates that secondary permeability (fractures), rather than the Water Table and Flow Lines primary permeability of the various geologic units, is the major control on ground-water flow. In the lower part of the cross section the water-level contours or equipotential lines are closely spaced and roughly parallel land surface. This reflects the depth at which the fractures tend to disappear. The hydrologic cross section shows that fluid movement, both contaminants and groundwater, is largely limited to the upper 50 feet of the strata.

Figure 7-1 Map of a contaminated area showing
location of monitoring wells and elevation of water levels

Figure 7-2 Map showing configuration of the water table and flow lines

Figure 7-3 Hydrologic Cross Section Showing Equipotential and
Flow Lines. Numbers represent total head

Obviously, in either pump-and-treat or in situ restoration systems, or in ground-water monitoring, the location, depth, and length of the screens of monitoring or extraction wells are of paramount importance. If the wells are improperly located, monitoring results would not adequately represent the aquifer being studied, and its restoration would be more costly and less effective than necessary. Therefore, in planning and carrying out ground-water restoration activities, it is essential to dedicate adequate resources to the collection of background information. In designing remediation activities, it is more important to describe the most permeable zones so that it can be determined where the water can go, under a remediation system, rather than its natural state.

Source Control

The objective of source- control strategies is to reduce or eliminate the volume of waste, thereby removing or minimizing ongoing contamination of the ground-water environment. Source-control techniquesinclude removal of the source(s), surface-water controls, ground-water barriers, interceptors, and hydrodynamic controls.

Source Removal

Soil and water at a hazardous waste site may be removed for treatment or relocation to a site that is more acceptable from an engineering or environmental viewpoint. While the removal and treatment or reburial of contaminated materials at a more controlled site may appearto solve acontamination problem, various factors need to be evaluated before excavation commences. These factors include:

(1). Problems associated with the excavation of bulky, partially decomposed or hazardous waste.

(2) Distance to an acceptable treatment/reburial site.

(3)  Road conditions between sites.

(4) Accessibility of both sites.

(5) Political, social, and economic factors associated with locating a new site.

(6). Disposition of contaminated ground water.

(7). Control of nuisances and vectors during excavation.

(8). Reclamation of excavated site.

(9). Costs.

These considerations suggest that excavation and relocation may be a viable alternative only where costs are not significant compared to the importance of the resource being protected. In some cases, removal and reburial in an approved facility transfers a problem from one location to another, and possibly creates additional problems.

Surface Runoff Controls

Surface runoff control measures are used to minimize the infiltration and percolation of overland flow or precipitation at a waste site. It is the infiltration of these waters that serve as the moving or driving force that leaches contaminants from the surface or unsaturated zone to the water table. According to an EPA estimate (Schuller and others, 1983), a disposal site consisting of 17 acres with 10 inches per year of infiltration could produce 4.6 million gallons of leachate each year for 50 to 100 years. This estimate, of course, is site-specific. Reduction of infiltration through a contaminated site can be accomplished by contouring the site, providing a cap or barrier to infiltration, and revegetating the site.

Several standard engineering techniques can be used to change the topographic configuration of the land surface in order to control the movement of overland flow. Some of the more common techniques are dikes and berms, ditches, diversion waterways, terraces, benches, chutes, downpipes, levees, sedimentation basins, and surface grading.

A mounded and maintained cover or cap of low permeability material greatly reduces or even prevents water from entering the source, thus reducing leachate generation. Covers also can control vapors or gases produced in a landfill. They may be constructed of native soils, clays, synthetic membranes, soil cement, bituminous concrete, or asphalt, a combination of these materials.

Revegetation can be a cost-effective method of stabilizing the surface of a waste site, especially when preceded by capping and contouring. Vegetation reduces raindrop impact and the velocity of overland flow, and strengthens the soil mass, thereby reducing erosion by wind and water. It also improves the site aesthetically.

Schuller and others (1983) described the effect of regrading, installation of a PVC topseal, and revegetation of a landfill in Windham, Connecticut. As Figures 7-4 and 7-5 illustrate, field data clearly indicate that the cover reduced infiltration and leachate generation, which caused a reduction in the size and concentration of the leachate plume.

Ground-Water Barriers

Subsurface barriers are designed to prevent or control ground-waterflow into, through, orfrom a certain location. Barriers keep fresh ground water from coming into contact with a contaminated aquifer zone or ground water from existing areas of contamination from moving into areas of clean ground-water. Usually it is necessary to incorporate other technoligies, such as pump-and-treat systems, with ground-water barriers.

Figure 7-4 Distribution of specific conductance, May 19, 1981

Figure 7-5 Distribution of Specific Conductance, November 12, 1981

The types of barriers commonly used include:

1. Slurry trench walls
2. Grout curtains
3. Vibrating beam walls
4. Bottom sealing
5. Block displacement

Slurry trench walls are placed either upgradient from a waste site to prevent flow of ground water into the site, downgradient to prevent off site flow of contaminated water, or around a source to contain the contaminated ground water. A slurry wall may extend through the water-bearing zone of concern, or it may extend only several feet below the water table to act as a barrier to floating contaminants. In theformercase,thefoundation should lie on, or preferably in, an underlying unit of low permeability so that contaminants do not flow underthe wall. A slurry wall is constructed by excavating a trench at the proper location and to the desired depth, while keeping the trench filled with a clay slurry composed of a 5 to 7 percent by weight suspension of bentonite in water. The slurry maintains the vertical stability of the trench walls and forms a low permeability filter cake on the walls of the trench. As the slurry trench is excavated, it is simultaneously backfilled with a material that forms the final wall. The three major types of slurry backfill mixtures are soil bentonite, cement bentonite, and concrete. Slurry walls, under proper conditions, can be constructed to depths of 100 feet or so.

Slurry trench walls are reported to have a long service life and short construction time, cause minimal environmental impact during construction, and be a cost-effective method for enclosing large areas under certain conditions (Nielsen, 1983). A concern regarding the use of a scurry wall where contaminated materials are in direct contact with the wall is the long-term integrity of the wall (Wagner and others, 1986). In such cases, the condition of the wall needs to be verified over time by ground-water monitoring.

Two separate slurry walls were constructed along parts of the margin of the Rocky Mountain Arsenal near Denver in order to contain plumes that originate on the plant property (Shukle, 1982, Pendrell and Zeftinger, 1983, and Hager and others, 1983). Along the north boundary, where surficial, unconsolidated sand and gravel occur with a thickness that averages about 30 feet, the slurrywall, about 2 feet thick, is 6,800 feet long. On the upgradient side are a series of 35 12-inchdiameter discharging wells on 200 foot centers that pump contaminated ground water into a treatment facility. After flowing through a carbon filtration system the water is reinjected into 50 6-inch diameter recharge wells on 100 foot centers on the opposite side of the barrier.

Along the northwest boundary of the Arsenal is another bentonite slurry barrier, 1,425 feet long, that extends southwestward from a bedrock high. The wall, excavated into the sand and gravel with the bentonite slurry trench method, is 30 inches wide and extends 3 feet into the underlying bedrock. The barrier contains about 7,000 cubic yards of backfill that were obtained from a borrow pit and blended with the bentonite prior to emplacement. The barrier was constructed where the saturated thickness of the permeable material is less than 10 feet. Paralleling the downgradient side of the barrier is a series of 21 recharge wells, stretching nearly 2,100 feet along the Arsenal boundary. Directly behind (upgradient) the barrier and extending into the thicker part of the surficial aquifer are 15 discharge wells. The contaminated ground water is pumped to a treatment plant and then reinjected into the recharge wells, thus forming a hydraulic barrier. Farther southeast along the boundary is another hydraulic barrier system, about 1,500 feet long, that consists of two parallel rows of discharge wellswith 15wells per row and, downgradient, a row of 14 recharge wells. The contaminated water, originating from a spill, is pumped, treated, and then reinjected. This system and the one along the north boundary was put into operation in late 1981 and the system along the northwest boundary began operation in 1984.

Grouting is the process of pressure -injecting stabilizing materials into the subsurface to fill and, thereby, seal voids, cracks, fissures, or other openings. Grout curtains are underground physical barriers formed by injecting grout through tubes. The amount of grout needed is a function of the available void space, the density of the grout, and the pressures used in setting the grout. Two or more rows of grout are normally required to provide a good seal. The grout used may be either particulate (i.e., Portland cement) orchemical (i.e., sodium silicate) depending on the soil type and the contaminant present. Grouting creates a fairly effective barrier to groundwater movement, although the degree of completeness of the grout curtain is difficult to ascertain (Nielsen, 1983). Incomplete penetration of the grout into the voids of the earth material permits leakage through the curtain.

A variation of the grout curtain is the vibrating beam technique for placing thin (approximately 4 inches) curtains or walls. Although this type of barrier is sometimes called a slurry wall, it is more closely related to a grout curtain since the slurry is injected through a pipe in a manner similar to grouting. A suspended I-beam connected to a vibrating driver-extractor is vibrated through the ground to the desired depth. As the beam is raised at a controlled rate, slurry is injected through a set of nozzles at the base of the beam, filling the void left by the beam's withdrawal. The vibrating beamtechnique is most efficient in loose, unconsolidated deposits, such as sand and gravel.

Another method that uses grouting is bottom sealing, where grout is injected through drill holes to form a horizontal or curved barrier below the site to prevent downward migration of contaminants.

Block displacement is a relatively new plume management method, in which a slurry is injected so that it forms a subsurface barrier around and below a specific mass or "block" of material. Continued pressure injection of the slurry produces an uplift force on the bottom of the block, resulting in a vertical displacement proportional to the slurry volume pumped. Bru nsing and Cleary (1983) described an example of slurry-induced block displacement. Demonstrated in Whitehouse, Florida, a slurry wall was constructed around a small area, 60 feet in diameter, to a depth of 23 feet in unconsolidated material. Injection wells were then used to force a soil bentonite slurry outward along the bottom of the cell. Subsequent test holes indicated that the new floor of the cell contained 5 to 12 inches of slurry.

Sheet pile cutoff walls have been used for many years for excavation bracing and dewatering. Where conditions are favorable, depths of 100 feet or more can be achieved. Sheet piling cutoff walls can be made of wood, reinforced concrete, or steel, with steel being the most effective material for constructing a ground-water barrier. The construction of a sheet pile cutoff wall involves driving interlocking sheet piles down through unconsolidated materials to a unit of low permeability. Individual sheet piles are connected along the edges with various types of interlocking joints. Unfortunately, sheet piling is seldom water-tight and individual plates can move laterally several to several tens of feet while being driven. Acidic or alkaline solutions, as well as some organic compounds, can reduce the expected life of the system.

Membrane and synthetic sheet curtains can be used in applications similar to grout curtains and sheet piling. With this method, the membrane is placed in a trench surrounding or upgradient of the plume, thereby enclosing the contaminated source or diverting groundwater flow around it. Placing a membrane liner in a slurry trench application also has been tried on a limited basis. Attaching the membrane to an underlying confining layer and forming perfect seals between the sheets is difficult but necessary in order for membranes and other synthetic sheet curtains to be effective. Arlotta and others (1983) described a system that consists of a trench lined with 100 mil high density polyethylene and backfilled with sand. It was installed by the slurry trench construction method in New Brunswick, New Jersey, in the fall of 1982.

Hydrodynamic Controls

Hydrodynamic controls are used to isolate a plume of contamination from the normal ground-waterflow regime to prevent the plume from moving into a well field, another aquifer, or to surface water. Controlling the movement of ground water by means of recharge and discharge wells has been practiced for several years. The major disadvantages include the commonly long pumping periods, well construction and maintenance costs, and the fact that the subsurface geology dictates system design.

The extent of the cone of depression around a pumping well can be controlled by the discharge rate and thus the cone, which is a change in the hydraulic gradient, can be used to control ground-water flow directions and velocity. Managefüefit of the cone or cones permits the operator to capture contaminants, which can then be diverted to a treatment plant. Well placement is particularly important since proper spacing and pumping rates are required to capture the contaminants. Moreover, well placement should be optimized so that as little uncontaminated water as possible is produced in order to reduce treatment costs.

Recharge wells are used to develop a hydraulic barrier (an inverted cone of depression) or pressure ridge. In this way, recharge wells can be used to force the contaminant plume to move in preferred directions, such as toward a drain or discharging well.

The design of well systems is, in large part, based on trial and error methods coupled with experience. Herein also lies one of the more useful exercises of computer simulations, because with this approach one can quickly and easily evaluate different well location and pumping schedules, and estimate costs.

Gradient-control techniques are used at a great number of sites undergoing restoration and nearly always play some role in containment methods, as is the case at the Rocky Mountain Arsenal.

A well point system, which is a common technique used for dewatering at construction sites, consists of several closely spaced shallow wells connected to a main header pipe. The header pipe is connected to a suction lift pump. Well point systems are used only for shallow aquifers and are designed so that the drawdown produced by the system completely intercepts the plume of contamination.

Deep wells are similar to well point systems except they are generally deeper and normally are pumped individually. This system commonly is used in places where the ground-water surface is too deep for the use of a suction lift system.

Athorough knowledge of the hydrogeological conditions of a site is required for the development of a hydrodynamic control system. The effect of the injection wells on the drawdown and the radius of influence of the pumping wells must be analyzed. Of particular importance are the potential well yield or injection rate, and the effect of hydrologic boundaries. Monitoring of the system is essential.

Ground-Water Collection and Treatment

The cleanup of a contaminated ground-water site involves the collection and treatment of the contaminated water. Some of the techniques used for source control often are used as part of a ground-water cleanup program, including pumping well systems, interceptor systems, and some of the techniques used for source control. In addition, in situ treatment, enhanced desorption, encapsulation, and biodegradation may be part of a cleanup plan.

Pumping Systems

A ground-water pumping scheme combined with a treatment procedure, also called a pump-and-treat system, is usually designed for a specific ground-water contamination problem. The use of pump-and-treat systems is probably more widespread and successful than all other restoration techniques combined. Large expenditures are made each year to prepare for and operate pump-and-treat remediation of ground-water contamination (Keely, 1989). The hydrogeology of the site, the source of the contaminant, and the characteristics of the contaminant must be understood if an efficient and cost-effective program is to be conducted.

The operation of a well field to remove ground water causes the formation of stagnation zones downgradient from the extraction wells, which must be considered in the system design. For example, rf remedial action wells are located within the bounds of a contaminant plume, the portion of the plume lying within the stagnation zones will not be effectively remediated because the contaminants are removed only from the zone of advective ground-water flow. In this case, the only remediation in the stagnation zone will result from the process of chemical diffusion and degradation, which may be very slow. Proper location of wells based on pumping rates and drawdown tends to mitigate this effect.

The tailing effect also can affect the removal and renovation of ground water containing a low solubility contaminant. Tailing is the slow, nearly asymptotic decrease in contaminant concentration in ground water moving through contaminated geologic material. The contaminants migrate into the finer pore structures of the earth materials and are slowly exchanged with the bulk water present in larger pores and this results in "tailing."

Many human-made and natural organic compounds found in groundwater tend to adsorb to the organic and mineral components of the aquifer material. When water is removed by pumping, contaminants can remain on the aqu'rfer material, the amount depending on the geologic materials and characteristics of the contaminants. Once sorbed to the geologic material, contaminants may desorb slowly into the ground water, thus requiring extended periods of pumping and treating to attain desired levels of restoration.

The removal of awater-insoluble liquid, such as gasoline, can be difficult since the product may become trapped in the pores of earth materials and is not easily removed by pumping. Pumping ground water to remove the components of a residual phase initially may reduce the concentration, but this reduction may only be the result of dilution or lowering of the water table below the level of contamination. A contaminant will not be removed faster than it is released into the ground water, so if the pumping stops for a period of time, water-soluble residual phase components again will dissolve into the ground water bringing the concentrations back to the previous level.

An innovation in pump-and-treat technology is pulsed pumping. Thistechnique involves aftemating the periods of pumping, allowing contaminants time to come to equilibrium with the ground water in each cycle. Equilibrium is achieved by diffusion from stagnant zones or zones of lower permeability, and by partitioning of sorbed contaminants or those associated with residual contaminant phases. Alternating pumping among wells also can establish active flow paths in the stagnant zones.

Another innovation is the use of pump-and-treat systems in conjunction with other remediation technologies. Examples are the use of extraction wells with barrier walls to limit plume expansionwhile reducing the amount of clean water pumped, and the use of surface ponds or flooding to flush contaminants from the unsaturated zone prior to collection by a pumping system.

Interceptor Systems

Interceptor systems may be an alternate to pumping systems. The subsurface drains used in interceptor systems essentially function as an infinite line of extraction wells, and can perform many of the same functions. Subsurface drains create a continuous zone of influence in which ground water flows towards the drain. Subsurface drains are installed perpendicular to the direction of ground-water flow and collect ground water from an upgradient source for treatment. Interceptor systems prevent leachate or contaminated ground water from moving downgradient toward wells or surface water.

Two types of interceptor systems used for source control are the passive system, which relies on gravity flow, and the active system, which uses pumps. An interceptor system consists of a trench excavated to a specified depth below the water table in which a perforated collection pipe is installed in the bottom. Active interceptor systems have vertical removal wells spaced along the interceptor trench or a horizontal removal pipe in the bottom of the trench. Active systems are usually backfilled with a coarse sand or gravel to maintain the stability of the wall. These interceptor systems can be used as preventive measures, such as leachate collection systems, as abatement measures, such as interceptor drains, or in product recovery from ground water, such as the removal of gasoline or oil. Interceptor drains generally are used to either lower the watertable beneath a contamination source orto collect contaminated ground water from an upgradient source. Interceptor systems are relatively inexpensive to install and operate, but they are not well suited for soils with a low permeability.

In stratified soils with variable hydraulic conductivities, the drain is normally installed on a layer with a low hydraulic conductivity to minimize leachate leakage under the drain. An impermeable liner placed in the bottom of a trench also can be used to control underflow. The design, spacing, and location of drains for various soil and ground water conditions are described further in Wagner and others (1986).

A combined interceptor and ground-water dam installation was described by Giddings (1982). In this case, a landfill that began as a burning dump, was found to be discharging leachate both to the surface and to the ground water, much of which eventually flowed into an adjacent river. A leachate interceptor trench was constructed on the downgradient side of the disposal area, as shown in Figure 7-6. In the trench on the upgradient side was placed a perforated pipe in a gravel envelope that was covered with permeable material. The remainder of the trench on the downgradient side was then backfilled with fine-grained materials as shown in Figure 7-7. Leachatefromthe landfillflows into the filled trench, seeps into the perforated pipe, and then is collected for treatment. In this case, the main purpose of the ground-water dam was to prohibit water originating in the adjacent river from flowing into the trench, which would have substantially increased the volume of wastewater.

Figure 7-6 Site Layout

Figure 7-7 Site cross section

Ground-Water Treatment after Removal

Of course the technology of pumping and treating of ground water implies that a cadre of engineering processes are available for treating the extracted water at the surface. A detailed discussion of these is beyond the scope of this document. They will only be mentioned to give the reader a familiarity with the processes so that detailed searches can be made elsewhere.

Treatment technologies for pumped or intercepted ground water can be grouped into three broad areas: physical, chemical, and biological. Physical treatment methods include adsorption, density separation, filtration, reverse osmosis, air and steam stripping, and incineration. Precipitation, oxidation/reduction, ion exchange, and neutralization are commonly used chemical treatment methods. Biological treatment methods include activated sludge, aerated surface impoundments, anaerobic digestion, trickling filters, and rotating biological discs.

In Situ Treatment

In situ treatment is an alternative to the removal and subsequent treatment of contaminated ground water. This method requires minimal surface facilities and reduces exposure to the contaminant. The success of various treatment methods is highly dependent on physical factors including aquifer permeability, the characteristics of the contaminants involved, and the geochemistry of the aquifer material.

In situ treatment technology has not yet been developed to the extent of other currently available technologies for restoring contaminated aquifers. However, some in situ treatment technologies have demonstrated success in actual site remediations (Wagner and others, 1986). Laboratory and pilot-scale testing generally must be performed to evaluate the applicability of a particular technology to a specific site.

In situ treatment may be grouped into two broad categories: physical/chemical and biological. Brief descriptions follow of the available technologies that have potential for success at hazardous waste sites.

In Situ Physical/Chemical Treatment

Organic and inorganic contaminants may be treated chemically to cause immobilization, mobilization for extraction, ordetoxffication. The applicationofoxidation and reduction reactions to in situ treatment is largely conceptual, but potentially may be used to accomplish immobilization by precipitation, mobilization by solubilizing metals ororganics, ordetoxification of metals and organics (Wagner and others, 1986). The chemicals used in these processes, however, have the potential to degrade compounds other than those targeted and to form degradation products that may be more toxic than the original ones.

Precipitation, chelation, and polymerization are three methods usedto immobilize acontaminant. Precipitation using caustic solutions is effective in immobilizing dissolved metals in ground water. Chelation also may be effective in immobilizing metals, although considerable research is needed (Wagner and others, 1986). Polymerization is effective in immobilizing organic monomers. However, the chemicals added to the contaminants in the ground water may react to form toxic by-products. Solidification methods used for treatment of soils also can immobilize contaminants. Mobilization of contaminants is accomplished by soil flushing orvacuumextraction.Neutralization, hydrolysis, and permeable treatment bed technologies may be used for detoxification. Precipitation and polymerization will lower the hydraulic conductivities near the injection wells making closely spaced wells necessary for effective treatment.

One interesting example of polymerization, reported by Williams (1982), involved a 4,200 gallon leak of acrylate monomer from a corroded pipeline at a small plant in Ohio. The contaminant migrated through a layer of fill, consisting largelyof cinders, and then downward through a storm sewer trench into a thin sand and gravel aquifer. A test boring and soil sampling program delineated the plume and indicated that the contaminant was slowly beginning to undergo polymerization and, therefore, immobilization. To increase the rate of reaction, 2-inchdiameter perforated PVC pipe was buried, about 2 feet below land surface, in four narrow trenches that trended across the plume. A riser and manifold headerconnected each pipe to solution tanks containing a catalyst in one and an activator in the other. Both solutions contained a wetting agent. A total of 8,000 gallons of solution were injected during the two treatment operations and 1,000 gallons had been injected previously during the investigative phase. On the basis of pre- and posttreatment soil borings, it was estimated that 85 to 90 percentofthe liquid monomercontaminant was solidified, and in some places it exceeded 99 percent polymerization. It was assumed that the remaining material would polymerize naturally.

In situ physical/chemical treatment processes generally entail the installation of a series of injection wells at the head of or within the plume of contaminated ground water. An alternative technique that has been used in shallow aquiders, is the installation of in situ permeable treatment beds. Trenches are filled with a reactive permeable medium and contaminated ground water entering the trench reacts with the medium to produce a nonhazardous soluble product or a solid precipitate. Among the material commonly used in permeable bed trenches are limestone to neutralize acidic ground water and remove nonpolar contaminants, such as carbon tetrachloride, poychlorinatee biphenyls, and benzene, and zeolites and other ion exchange resins for removing solubilized heavy metals.

Permeable treatment beds are applicable only in relatively shallow aquifers because the trench must be constructed down to a layer of low permeability. They also are often effective for only a short time because they lose their reactive capacity or become plugged with solids. An over design o the system or replacemente of the reactive medium can lengthen the time during which permeable treatment is effective.

Mobilization for Extraction

Pump-and-treat remediation techniques often are inefficient when a preponderance of the contaminants are sorbed to the solid to the solid phase of the aquifer. The same can be said for in situ treatment if the reactive chemicals are unable to come into contact with the contaminants. In these cases, the enhanced desorption or mobilization of contaminants would be of considerable interest in aquifer restoration activities.

Soil flushing is the process of flooding a contaminated area with water or a solvent to mobilize the contaminant, followed by the collection of the elutriate. The process is based on the solvent solubilizing or chemically reacting with the contaminants and mobilizing them into te solvent phase. Water is used if the contamiant is readily soluble. Acid solutions tend to flush metals and basis organics.

The mobilization of contaminants by injecting surfactants into the aquifer matrix is possible. Techniques used for the secondary recovery of oil are being used experimentally, with moderate success. Both surfactant and alkaline floods have been attemped. Most oil-field surfactants are expensive, while alkaline floods produce lye, therefore, this approach promises little benefit to aquifer restoration.

In the recovery of  hydrocarbons, there are three possible physical-chemical methods. At shallow depths, thermal or steam flooding may be helpful while on a larger scale, alcohol flooding may at some future date prove to be helpful. Alcohol is easily produced and dissolves the hydrocarbon, but tentative research results indicate that the required alcohol-water ratio must be so high as to make the technique questionable.

Another emerging technology, which is increasingly being used, is alternately called in situ vacuum extraction or in situ volatilization. It is used to extract volatile organic contaminants from the unsaturated zone where contaminants exist as a result of underlying contaminated ground water, or free product riding on top of the ground water, or from leaks or spills. The technology has enjoyed considerable success in this and other industralized countries.

The plumbing associated with this type of remediation is obviously dictated by site conditions, including the thickness of the unsaturated sone, the volatility of the contaminants involved as well as their source and extent, and the porosity and permeability of the unsaturated zone (Pacific Environmental Services, 1989).

Generally these vapor extraction projects consists of a series of slotted PVC wells configured to span the area of contamination. Air inlet wells located both inside and outside of the plume increase the introduction of air from the atmosphere (fi. 7-8).

Like pump-and-treat remediation techniques, vacuum extraction projects usually require some type of surface treatment facility to deal with the collected vapors. When surface treatmente is required, activated carbon columns are widely in operation, however the use of biologically active columns is being studied, which will allow the intruduction of oxygen or other gases needed for biodegradation.

Vacuum extraction is best suited for area of high, relatively homogeneous, permeability. These should be no undergroung structures, and great care must be given to the explosive nature of the extracted vapors. The unit cost, which appears to be vey promising, varies widely according to the size of the area under remediation and the specific site characteristics.

Radio frequency heating has been under development since de mid-1970s and the concept is being applied to in situ decontamination of uncontrolled hazardous waste landfills and sites (Rich and Cherry, 1987). In this process, the ground is heated with radio frequency waves that vaporize he hazardous contaminants. The vapors emanating from the soil are then treated.

Detoxification

Neutralization of ground water may be accomplished by injecting dilute acids or bases into the aquifer through

Figure 7-8. Schematic of a Vaccum Extraction System

injection wells to adjust the pH to the desired level. Tolman and others (1978) recommended that neutralization only be applied to ground water at industrial waste disposal sites since municipal landfills, which constantly generate anaerobic decomposition products, would require neutralization over a long period of time.

Hydrolysis may be used for detoxification, however, the intermediate products formed during hydrolysis of a particular compound must be known since they may be more toxicthanthetargetedcompound. Esters, amides, carbamates, phosphoric and phosphonic acid esters, and pesticides are potentially degradable by hydrolysis (Wagner and others, 1986).

Biodegradation

There are two basic approaches to in situ biodegradation. The first relies on the natural biological activity in the subsurface. The second approach, called enhanced biorestoration, involves the stimulation of the existing microorganisms by adding nutrients.

Natural Subsurface Biological Activity

Biological treatment in the subsurface involves the use of microorganisms to break down hazardous organic compounds into nonhazardous materials. The site hydrology, environmental conditions, and the biodegradability of the contaminants are factors that determine the potential effectiveness of in situ biological treatment. Most compounds are more rapidly degraded aerobically, however some compounds will only degrade under anaerobic conditions. Biodegradation in ground water and solids can be a slow process and may take several years for completion depending on the compounds present. In situ biodegradation, however, is a desirable method of treatment because the contaminants are destroyed, thus, removal of ground water for external treatment and residual handling possibly can be avoided.

In situ biorestoration of the subsurface is a relatively new technology that has recently gained considerable attention. Scarcely more than a decade ago, conventional wisdom assumed that the subsurface below the root zone of plants was, for all practical purposes, sterile. Research during the last decade has indicated that the deeper subsurface is not sterile, but in fact, harbors significant populations of microorganisms. Bacterial densities of around a million organisms per gram of dry soil have been found in several uncontaminated aquifers. Water-table aquifers examined so far exhibit considerable variation in the rate of biodegradation of specific contaminants and rates can vary two or three orders of magnitude from one aquifer to another or over a vertical separation of only a few feet in the same aquifer. Although extremely variable, the rates of biodegradation are fast enough to protect ground-water quality in many aquifers.

Although not clearly defined, several environmental factors are known to influence the capacity of indigenous microbial populations to degrade factors include dissolved oxygen, pH, temperature, oxidation-reduction potential, availability of mineral nutrients, salinity, soil moisture, the concentration of specific contaminants, and the nutritional quality of dissolved organic carbon in ground water.

Natural biorestoration does occur in the subsurface environment. Contaminants in solution in ground water, as well as vapors in the unsaturated zone, can be completely degraded ortransformed to new compounds. Undoubtedly, thousands of contamination events are remediated naturally before the contamination reaches a point of detection. On the other hand, methods are needed to determine when natural biorestoration is occurring, the stage the restoration is in, whether enhancement of the process is possible or desirable, and what will happen if natural processes are allowed to run their course.

For information on in situ biorestoration of specific compounds and conditions see Bower and McCarty (1983), Jhaveri and Mazzacca (1983), Lee and Ward (1984), Parsons and others (1985), Parsons and others (1984), Sulflita and Gibson (1985), Sulflita and Miller (1985), Wilson (1985), Wilson and Rees (1985), Wood and others (1985), and Young (1984).

Enhanced Biorestoration

In the subsurface environment, populations of organisms capable of degrading contaminants increase until limited by metabolic requirements, such as mineral nutrients or oxygen. Once this point is reached, the rate of biodegradation or transformation of organic compounds is controlled by the transport mechanisms that supply the limiting nutrients.

The majority of microbes in the subsurface are firmly attached to soil particles. As a result, nutrients must be brought to the active sites by advection and diffusion of water in the saturated zone, or by soil gas, in the unsaturated zone. In the simplest and perhaps most common case, the compounds to be degraded for microbial energy and cell synthesis are transported in the aqueous phase by infiltrating water or by advective flow through the ground water. In the unsaturated zone, volatile organic compounds can move readily as vapors in the soil gas where oxygen is present. Below the water table, aerobic metabolism is limited by the low solubility of oxygen in water. Factors that control the rate of biological activity are the stoichiometry of the metabolic process, the concentration of the required nutrients in the mobile phases, the flow of the mobile phases, the opportunity for colonization in the subsurface by metabolically capable organisms, and the toxicity of the waste.

Much of the development work in the area of groundwater and soil remediation by biodegradation has been performed using petroleum products. The number of gasoline stations, underground tanks, and gasoline pipelines throughout the country and the potential for ground-water contamination have prompted considerable laboratory and field studies on in situ biodegradation of hydrocarbons.

Many of the enhanced biorestoration techniques now in use are variations on those developed by Raymond and his coworkers (Raymond, 1974; Raymond and others, 1986). This process reduces hydrocarbon contaminants in aquifers by enhancing the indigenous hydrocarbon-utilizing microflora. Nutrients and oxygen are introduced through injection wells and circulated through the contaminated zone by pumping one or more producing wells. The increased supply of nutrients and oxygen stimulates biodegradation of the hydrocarbons.

Raymond's process has been used with reasonable success to restore aquifers contaminated with gasoline. The overall removal of total hydrocarbons using this technology usually ranges from 70 to 80 percent. Some of the sites treated by this technique have been restored to the point where no dissolved gasoline was present in the ground water, and state regulatory standards were satisfied. State agencies charged with restoring other sites, however, have required that the operation continue until no trace of liquid gasoline could be detected. Most of the sites restored in this manner have had appropriate monitoring programs installed following remediation.

Usually the first step in the process is to use physical methods to recover as much of the gasoline as possible and then a detailed investigation of the hydrogeology is undertaken to determine the extentof the contamination. Laboratory studies are conducted to determine if the native microbes can degrade the contaminants and to determine the combination of minerals required to promote maximum cell growth at the ambient groundwater temperature and under aerobic conditions.

Considerable variations in nutrient requirements among aquifers have been noted. One aqu'rfer required only the addition of nitrogen and phosphorus, while anotherwas best stimulated by the addition of ammonium sulfate, mono- and disodium phosphate, magnesium sulfate, sodium carbonate, calcium chloride, and manganese and ferrous sulfate. Itwasfoundthat achemical analysis of the ground water was not helpful in estimating the nutrient requirements of the system.

Field investigations and laboratory studies guide the design and installation of a system of wells for injecting the nutrients and oxygen, and for the control of ground water flow. Controlling the ground-water flow is critical to moving oxygen and nutrients to the contaminated zone and optimizing the degradation process.

The technique developed by Raymond does not provide for treatment above the water table. Soils contaminated by leaking underground storage tanks may be physically removed during the process of removing the tank, however, this may not be practical with deep water tables or large areas of contamination. An alternative to soil removal is the construction of one or more infiltration galleries, which are used to recirculate the treated water back through the contaminated unsaturated zone. Oxygen may be added to the infiltrated water during an in-line stripping process forvolatile organic contaminants or through aeration devices placed in the infiltration galleries.

The rate of biorestoration of hydrocarbons, eitherabove or below the water table, is effectively the rate of supply of oxygen. Table 7-1 compares the number of times the water in the aquifer, orthe air above it, must be replaced to restore subsurface materials of various textures. The calculations assume typical values for the volume occupied by air, water and hydrocarbons (De Pastrovich and others,1979, Clapp and Horberger, 1978). The calculations further assume that the oxygen content of the water is 10 mg/L, that of the air is 200 mg/L and that the hydrocarbons are completely metabolized to carbon dioxide. These values are provided only to exemplify the processes involved and would differ at an actual site. The oxygen concentration in the water can be increased by using oxygen rather than air, which also would reduce the volumes of recirculated water required.

Hydrogen peroxide is an alternative source of oxygen in biorestoration and Raymond and others (1986) have patented a process of treatment with hydrogen peroxide. Iron or an organic catalyst may be used to decompose the hydrogen peroxide to oxygen. The rate at which hydrogen peroxide decomposes to oxygen must be controlled to limit the formation of bubbles that could lead to gas blockage and the loss of permeability. Hydrogen peroxide may mobilize metals, such as lead and antimony, and, if the water is hard, magnesium and calcium phosphates can precipitate and plug the injection well or infiltration gallery. To determine the microorganism's hydrogen peroxide tolerance level laboratory studies are performed.

Treatment Trains

In most contaminated hydrogeologic systems, the remediation process may be so complex, in terms of contaminant behavior and site characteristics, that no single system or unit is capable of meeting all requirements. Consequently, several unit operations may be combined in series or in parallel to effectively restore ground-water quality to the required level. Barriers and hydrodynamic controls may serve as temporary plume control measures, however, hydrodynamic processes are integral parts of any withdrawal and treatment or in situ treatment process.

Most remediation projects typically are started by removing the source. The next step may be the installation of pumping systems to remove free product floating on the water surface or the removal of soluble contaminants for treatment at the surface. Barriers also might be constructed to slow an advancing plume or to reduce the amount of water requiring treatment.

Table 7-1 Estimated volumes of water or air required to completely
renovate subsurface material that contained hydrocarbons at residual saturation

Enhanced biorestoration techniques maybe feasible in some of the more diluted areas of the plume. In some circumstances, a site may reach final restoration goals using natural chemical and biological processes. An adequate monitoring program would be required to establish data on the progress of the restoration program.

Steps in treatment of contaminated ground water include the removal, collection, and delivery of the contaminated water to the treatment units, and in the case of in situ processes, delivery of the treatment materials to the contaminated areas in the aqu'rfer. Athorough knowledge and understanding of the hydrogeologic and geochemical characteristics of the site are required to design a system that will optimize the remediation techniques selected, maximize the predictability of restoration effectiveness, and allowforthe development of a cost-effective and lasting remediation program.

The principal criteria for selecting remediation procedures are the water-quality level to which to restore an aquifer, and the most economical technology available to reach that level. Institutional limitations, however, sometimes override these criteria in determining if, when, and how remediation will be selected and carried out.

Response to a ground-water contamination problem is likely to require compliance with several local, state, and federal pollution control laws and regulations. If the response involves handling hazardous wastes, discharging substances into the airor surface waters, or injecting wastes underground, federal and state pollution control laws will apply. These laws do not exempt the activities of federal, state, or local officials or other parties attempting to remediate contamination problems. They apply to both generators and responding parties, and it is not unusual for these pollution control laws to conflict. A hazardous waste remediation project must meet RCRA permit requirements governing the transport and disposal of hazardous wastes, which can influence the selection of the remediation plan and the scheduling of cleanup activities.

In situ remediation procedures may be subject to permitting or other requirements under federal or state underground injection control programs. Withdrawal and treatment approaches may be subject to regulation under federal or state air pollution control programs or to pretreatment requirements if contaminated ground water is to be discharged to a surface water or to a municipal plan involving pumping from an aquifer may be subject to state ground-water regulations on well construction and well spacing, and may need to consider various competing legal rights to extract ground water.

Other factors influencing selection and design of a ground-water remediation program include the availability of alternative sources of water supply, political and judicial constraints, and the availability of funds. Where alternate water supplies are plentiful and economical, there may not be a demand for total remediation; adequate remediation to protect human health and the environment may be sufficient. In the final analysis, responsible agencies can pursue remediation measures to the extent that resources are made available.

References

Arlotta, S.V., G.W. Druback, and N. Cavalli, 1983, The Envirowall vertical cutoff barrier: Proc. 3rd Nat. Symp. on Aquifer Restoration and Ground-Water Monitoring, Nat. Water Well Assoc.

Bower, E.J. and P.L. McCarty, 1983, Transformation of halogenated organic compounds under denitrification conditions: Applied and Environmental Microbioloby, v. 45, no. 4.

Brunsing, T.P. and J. Cleary, 1983, Isolation of contaminated ground water by slurry induced ground displacement: Proc. 3rd Nat. Symp. on Aquifer Restoration and Ground-Water Monitoring, Nat. Water Well Assoc.

Clapp, R.B. and G.M. Homberger, 1978, Empirical equations for some soil hydraulic properties: Water Resources Research, vol.14.

Giddings, T., 1982, The utilization of a ground-water dam for leachate contaminant at a landfill site: Proc. 2nd Nat. Symp. on Aquifer Restoration and Ground-Water Monitoring, National Water Weil Assoc..

Hager, D.G., C.E. Smith, C.G. Loren, and D.W. Thompson, 1983, Ground-water decontamination at Rocky Mountain Arsenal: Proc. 3rd Nat. Symp. on Aquifer Restoration and Ground-Water Monitoring, Nat. Water Well Assn.

Jhaveri, V. and A.J. Mazzacca, 1983, Bio-reclamation of ground and groundwater, a case history: Proc. 4th Nat. Conf. on Management of Uncontrolled Hazardous Waste Sites, Washington, D.C.

Keely, J.F. 1989. Performance Evaluation of Pumpand-Treat Remediations. Superfund Issue Paper. EPA 540/8-89/005.

Knox, R.C., L.W. Canter, D.F. Kincannon, E.L. Stover, and C.H. Ward. 1984. State-of-the Art of Aquifer Restoration. EPA 600/2-84/182.

Lee, M.D., J.M. Thomas, R.C. Borden, P.B. Bedient, C.H. Ward, and J.T. Wilson, 1988, Biorestoration of aquifers contaminated with organic compounds: CRC Critical Reviews in Environmental Control, vol.18, no. 1.

Lee, M.D. and C.H. Ward, 1984, Reclamation of contaminated aquifers: Proc. of the 1984 Hazardous Spills Conference, Nashville, TN.

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Pacific Environmental Services, 1989, Soil vapor extraction VOC control technology assessment: EPA450/4-89-017.

Parsons, F., G.B. Lage, and R. Rice, 1985, Biotransformation of chlorinated organic solvents in static microcosms: Environmental Science and Technology, vol. 4.

Parsons, F., P.R. Wood, and J. DeMarco, 1984, Transformation of tetrachloroethene and trichloroethene in microcosms and ground water: Jour. Amer. Water Works Assn., vol. 76.

Pendrell, D.J., and J.M. Zeltinger,1983, Contaminated ground-water containmenVtreatment system at the northwest boundary, Rocky Mountain Arsenal, Colorado: Proc.3rd Nat. Symp. on Aquifer Restoration and GroundWater Monitoring, Nat. Water Well Assn.

Raymond, R.L., R.A. Brown, R.D. Norris, and E.T. O'Neill, 1986, Stimulation of bio-oxidation processes in subterranean formations: U.S. Patent Office, 4,588,506. Patented May 13, 1986.

Raymond, R.L., 1974. Reclamation of hydrocarbon contaminated ground waters: U.S. Patent Office, 3,846,290. Patented November 5, 1974.

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Schuller, R.M., A.L. Dunn, and W.W. Beck, 1983, The impact of top-sealing at the Windham Connecticut landfill: Proc. 9th Ann. Research Sympos. on Land Disposal of Hazardous Waste, EPA-600/9-83-018.

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Sulflita, J.M. and G. D. Miller,1985, Microbial metabolism of chlorophenolic compounds in ground water aquifers: Environmental Toxicology and Chemistry, vol. 4.

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Wilson, B.H. and J.F. Rees, 1985, Biotransformation of gasoline hydrocarbons in methanogenic aquifer material: Proc. of NWWA/API Conf. on Petroleum Hydrocarbons and Organic Chemicals in Ground Water, Houston, TX.

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Young, L.Y., 1984, Anaerobic degradation of aromatic compounds: Microbial Degradation of Aromatic

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