2
Complexity of the Contaminated Subsurface

No environmental problem can be solved rationally before it has been adequately defined. This chapter presents an overview of the characteristics of contaminated ground water that must be understood in order to assess prospects for remediation. The complexities of the subsurface environment and of contaminant distribution, which are documented in this chapter, significantly complicate the cleanup task (see Box 2-1). Although this chapter focuses on factors that complicate ground water cleanup, the arguments should not be interpreted to mean that restoration of ground water is impossible on theoretical grounds. Ground water contamination by hazardous substances is by no means intractable, although at many sites it is likely to prove extraordinarily difficult, time consuming, and costly to reverse.

THE SUBSURFACE ENVIRONMENT

When contaminants enter the subsurface, they become subject to a variety of physical, chemical, and biological processes that operate beneath the ground. The design of an effective ground water cleanup system requires an understanding of these processes because they control the fate of the contaminants and the ease with which they can be extracted.

Physical Characteristics

Ground water is stored in underground formations called aquifers. There are two broad categories of aquifers: consolidated (see Figure 2-1)



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Alternatives for Ground Water Cleanup 2 Complexity of the Contaminated Subsurface No environmental problem can be solved rationally before it has been adequately defined. This chapter presents an overview of the characteristics of contaminated ground water that must be understood in order to assess prospects for remediation. The complexities of the subsurface environment and of contaminant distribution, which are documented in this chapter, significantly complicate the cleanup task (see Box 2-1). Although this chapter focuses on factors that complicate ground water cleanup, the arguments should not be interpreted to mean that restoration of ground water is impossible on theoretical grounds. Ground water contamination by hazardous substances is by no means intractable, although at many sites it is likely to prove extraordinarily difficult, time consuming, and costly to reverse. THE SUBSURFACE ENVIRONMENT When contaminants enter the subsurface, they become subject to a variety of physical, chemical, and biological processes that operate beneath the ground. The design of an effective ground water cleanup system requires an understanding of these processes because they control the fate of the contaminants and the ease with which they can be extracted. Physical Characteristics Ground water is stored in underground formations called aquifers. There are two broad categories of aquifers: consolidated (see Figure 2-1)

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Alternatives for Ground Water Cleanup BOX 2-1 COMPLEXITY OF THE CONTAMINATED SUBSURFACE: A HYPOTHETICAL EXAMPLE Every conceivable subsurface remediation approach is subject to constraints posed by fundamental principles such as conservation of mass, conservation of energy, the theoretical limitation on energy efficiency embodied in the Second Law of Thermodynamics, and the thermodynamic relationships governing chemical and physical equilibria. Nothing in these principles precludes cleanup of contaminated sites to any desired level. However, consideration of these laws makes it evident that cleanup using presently known technology will often be extremely difficult to achieve in a reasonable period of time. To illustrate the situation, consider a barrel of trichloroethylene (TCE) that has leaked below ground. Because the TCE disperses, it would not generally be possible to pump the chemical out in pure form; rather, the TCE must dissolve in the ground water and be removed by pumping out the water. Based on the solubility of TCE in water, more than one thousand barrels of water would have to be removed for each barrel of TCE spilled, if the water removed was saturated with TCE. In practice, however, the pumped water may contain only one-tenth or one-hundredth or one-thousandth of the amount of TCE that would be present if the water were completely saturated with TCE, because most of the water will never have been in dose enough contact to the TCE long enough to allow complete saturation. Then, for each barrel of TCE it will be necessary to pump out 10,000 or 100,000 or 1 million barrels of water. Wells placed near the TCE would speed up the process, but it is seldom possible to precisely locate the concentrated pockets of chemical contamination. This example illustrates common elements of ground water contamination problems that greatly interfere with cleanup efforts; contaminants may be difficult to extract because they do not dissolve fully; the subsurface environment is neither simple in structure nor easy to characterize, and determining the precise location of the contamination is seldom easy. and unconsolidated (see Figure 2-2). Unconsolidated aquifers consist of uncemented granular materials such as sand and gravel; they store water in the interstitial pore space among the grains. Consolidated aquifers consist of more or less solid rock; they store water primarily in solution channels, fractures, and joints (although in material such as sandstone, some water may also be stored in interstitial pore spaces). Layers of such formations comprise what is called the saturated ground water zone. Here, water completely fills the pore openings. Overlying the saturated zone is a zone in which the pore spaces contain both air and water and thus are not saturated with water. This zone is known as the unsaturated, or vadose, zone.

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Alternatives for Ground Water Cleanup FIGURE 2-1 Simplified schematic of ground water flow in an unconsolidated aquifer. The flow lines indicate travel times to various parts of the subsurface, with longer travel times indicated by flow lines reaching deeper into the subsurface. SOURCE: Heath (1983). FIGURE 2-2 Simplified schematic of ground water flow in a consolidated aquifer. As the flow lines indicate, the direction of ground water flow in such aquifers depends on the locations of the fractures and thus is often tortuous and difficult to predict. SOURCE: From Heath (1980), as reprinted in LeGrand (1988). Hydraulic Properties of Aquifers Table 2-1 summarizes key hydraulic properties of aquifers and their importance to remediation efforts. (See the glossary for definitions of hydraulic properties and other technical terms used in this chapter.) As

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Alternatives for Ground Water Cleanup TABLE 2-1 Hydraulic Properties of Aquifers Important for Ground Water Cleanup Property Description Importance for Ground Water Cleanup Porosity Volume of pore space relative to the total volume Pores store water and contaminants Effective porosity Interconnected pore space that can transmit fluid Water and contaminants flow through interconnected pores Ground water velocity Rate of fluid movement Influences the direction and velocity of dissolved contaminant movement Hydraulic gradient Elevation and pressure differences that cause fluids to flow Influences the direction of contaminant movement Hydraulic conductivity Ease with which water can move through a formation Influences the rate at which fluid can be pumped for treatment Transmissivity Product of formation thickness and hydraulic conductivity Influences the rate at which fluid can be pumped for treatment Storage coefficient Volume released by pressure changes per unit area during pumping in a confined aquifer Influences the quantity of fluid that can be obtained by pumping Specific yield Fraction of total pore volume released as water by gravity drainage during pumping of an unconfined aquifer Influences the quantity of fluid that can be obtained by pumping Specific retention Fraction of total aquifer volume retained as water above the water table after pumping an unconfined formation Influences the quantity of contaminant that remains in the subsurface after pumping an illustration of why hydraulic properties are important in ground water remediation, consider the effect of variations in hydraulic conductivity. The hydraulic conductivity controls the amount of water that can be supplied to a well and therefore reflects the ease with which dissolved contaminants can be removed from the aquifer. The hydraulic conductivity of a sandy aquifer is approximately two orders of magnitude greater than the hydraulic conductivity of a silty sand aquifer (average hydraulic conductivities for these types of materials are 30 meters per day and 0.3 meters per day, respectively). Accordingly, water containing contaminants can be extracted from a sandy aquifer at a rate about 100 times greater than it can be extracted from a silty sand aquifer. Therefore, if the zone of contamination is the same in both aquifers, it will be

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Alternatives for Ground Water Cleanup possible to clean up the sandy aquifer much faster than the silty sand aquifer. However, the ease of cleanup of the clean sand aquifer relative to the silty sand aquifer may be offset by the fact that the zone of contamination will be larger in the clean sand aquifer because it is more permeable. This example illustrates that understanding the aquifer's key hydraulic properties and how they are likely to influence the remediation is essential to predicting the performance of the cleanup system. Determining the influence of hydraulic properties on remediation is far more difficult in fractured, consolidated rocks than in unconsolidated rocks because the hydraulic properties vary widely with location and therefore depend on the size of the sample of the aquifer being investigated. For example, a small volume of rock obtained from between fractures can have an exceedingly low porosity; on the other hand, a small part of aquifer material primarily from a fracture can have a porosity approaching 100 percent. Therefore, the true values for a reasonably sized portion of the aquifer are between these two extremes. An average value of the hydraulic parameters for such an aquifer is of little use in providing hydrogeologic information required for cleaning up aquifers. Predicting contaminant movement in fractured rock is extremely complex because contaminants will move along the line of least resistance, which is in the fracture and often in a direction that cannot be determined by conventional methods for hydrogeologic investigations. Because of the tendency of contaminants to move through the fractures to locations that are difficult to determine and to access, remediation of fractured rock aquifers poses an extreme technical challenge. Ground Water Flow The major influences on ground water flow are precipitation, which recharges aquifers, and gravity, which causes ground water to flow and eventually discharge to springs, rivers, and oceans. As an example, Figure 2-1 shows recharge and discharge in unconsolidated sediments in an idealized and simplified cross section. Water moves from a recharge area at high elevation to a discharge area at low elevation. Water may also move vertically through a series of less permeable layers, known as confining beds, that may separate aquifers at various depths. Figure 2-2 illustrates the complexities of ground water flow paths for fractured, consolidated rocks. Flow paths in fractured rock are often difficult to ascertain because the fractures are not uniformly distributed and may not be interconnected. The fundamental law describing ground water flow is known as Darcy's Law, which can be expressed as follows:

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Alternatives for Ground Water Cleanup In this equation, the term q is the volumetric flow rate per unit cross-sectional area of aquifer perpendicular to the direction of flow. The term K is the hydraulic conductivity (see Table 2-1), which is a measure of the ease with which water moves through the aquifer material and which decreases with pore size (just as the flow of water through a pipe decreases with pipe diameter). The term dh/dl is the hydraulic gradient (see Table 2-1), which quantifies the pressure and gravity forces that drive flow and which is influenced by aquifer recharge, elevation, and pumping. Darcy's Law states that the rate of ground water flow is determined by the magnitude of the hydraulic gradient and the magnitude of the hydraulic conductivity of the aquifer material. The Effect of Pumping on Flow Pumping ground water, as is done with pump-and-treat systems, causes complex perturbations in flow, even at sites with relatively simple hydrogeology. To illustrate this effect, consider a hypothetical site at which water is injected into the aquifer at one well and pumped out of the aquifer at the same rate from a second well. Placement of the wells is critical in determining the flow of water from the recharge well to the discharge well. In the top diagram in Figure 2-3, the wells are placed on a line perpendicular to the direction of ground water flow, and only a small amount of the recharged water (indicated by the stippled area) is removed by the pumping well. In the bottom diagram, the wells are aligned parallel to the direction of ground water flow, and nearly all of the recharged water is pumped out, with almost none moving downgradient. At an actual field site, ground water flow is far more complex than in this simple illustration because the hydraulic properties that describe an aquifer are not uniform. Nevertheless, this example demonstrates the necessity of understanding the flow system before installing a well to remove contaminants or to control the direction of movement of a contaminant plume. Geochemical Characteristics Once contaminants enter the subsurface, they become subject to control not only by the aquifer's physical properties but also by a variety of possible geochemical reactions. These reactions may cause the contaminants to change form, sorb to aquifer solids, or form complexes with other chemical species. In addition, geochemical characteristics of the site can influence the operation of aquifer cleanup systems. For example,

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Alternatives for Ground Water Cleanup FIGURE 2-3 Diagrams, called flow nets, that illustrate the effect of well placement on the direction and quantity of ground water flow in a homogeneous aquifer with recharge (R) and discharge (D) wells that are at the same depth and are pumping the same amount of water. The solid lines are flow lines, which indicate the pattern of ground water movement. A flow net, by definition, has an equal quantity of water flowing between each pair of flow lines. The dashed lines are lines of equal hydraulic head. Only the water in the stippled areas moves from the recharge well to the discharge well. In the top diagram, the wells are placed in a line perpendicular to the direction of ground water flow, and little recharge water reaches the discharge well. In the bottom diagram, the line from the recharge well to the discharge well is parallel to the direction of ground water flow, and nearly all of the water from the recharge well reaches the discharge well. SOURCE: Da Costa and Bennett, 1960.

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Alternatives for Ground Water Cleanup Core barrel used to obtain a sample from the subsurface. Courtesy of Rice University, Department of Environmental Science and Engineering. geochemical characteristics determine whether metals, such as iron, will precipitate when the contaminated water is extracted for treatment. Metal precipitates can clog extraction wells and require installation of special treatment systems, significantly increasing cleanup costs. The chemical composition of ground water reflects both the mineralogy of the aquifer and the flow path of the water from the point of recharge to the point of discharge. The movement of water and its interaction with soil and rock can be described by the concept of a hydrogeochemical cycle, shown in Figure 2-4. As the figure indicates, the chemistry of the infiltrating water is largely controlled by temperature, precipitation, soil mineralogy, and anthropogenic inputs, including airborne contaminants and chemicals in runoff water. Factors that affect the chemical composition of the water once it infiltrates the subsurface include the mineralogy of the aquifer matrix, the residence time that the water is in contact with the soil and rock, and mixing of ground water with other sources of water or subsurface contaminants. Contaminants and products from microbial activity in the subsurface can also cause important changes in the water chemistry. As shown in Figure 2-4, as water moves through the unsaturated zone the concentrations of solutes increase as soil gases and minerals (which may contain metals such as iron and manganese) dissolve. Ground water has a longer residence time

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Alternatives for Ground Water Cleanup FIGURE 2-4 Schematic showing the hydrogeochemical cycle of water moving through the subsurface. Water begins its path as rainfall, which may contain dissolved matter such as pollutants. Some of this rainfall infiltrates the soil, where the composition of the water changes as soil gases dissolve and as the water reacts chemically with soil components (through ion exchange, redox, sorption, and dissolution reactions). From the soil, the water migrates downward to the water table. Once in the aquifer, the water flows toward discharge points such as springs, lakes, rivers, and the ocean, continuing to change chemically along the way. It then evaporates from these surface bodies and once again begins its path as rainfall. SOURCE: Modified from Back et al., 1993.

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Alternatives for Ground Water Cleanup in the deep subsurface than in shallow systems and continues to change in chemical composition as it moves to discharge points in springs, lakes, rivers, estuaries, and oceans. In general, shallow aquifers (2 to 50 meters below land surface) are more vulnerable to ground water quality problems than deep aquifers because most contaminants enter the subsurface via use and disposal near the land surface or leakage from buried storage containers. Table 2-2 summarizes the many types of geochemical processes that can affect contaminant fate and transport in the subsurface. Many of these reactions occur together or sequentially in contaminated environments (Cherry et al., 1984). For example, the oxidation of organic contaminants to carbon dioxide can be coupled with the reduction of (insoluble) ferric iron oxides to aqueous (soluble) ferrous iron. When sulfide is present, the ferrous iron can precipitate as monosulfide or pyrite. These processes occur within the constraints of thermodynamics and in many cases are mediated by microorganisms. Biological Characteristics Microorganisms of many kinds, primarily bacteria but also protozoa and fungi, inhabit subsurface environments (Ghiorse and Wilson, 1988). Microorganisms are important in ground water systems because they consume organic matter, including contaminants, and because they alter the chemical state of the aquifer. Subsurface microorganisms are also extremely important in the development of new technologies that use biological processes to treat contaminated ground water in place rather than having to extract it (see Chapter 4). The activity of microorganisms in uncontaminated aquifers is often limited by the availability of metabolizable organic carbon, which the organisms require for growth and reproduction. The concentration of natural organic carbon is low in ground water (usually less than 2 mg/liter). Therefore, the presence of degradable organic contaminants in the subsurface generally stimulates microbial growth, although some organic compounds and trace metals inhibit microbial activity. Two broad classes of bacteria play important roles in the subsurface and in the development of new ground water cleanup technologies: aerobic and anaerobic. Aerobic organisms require oxygen to degrade organic compounds. They transfer electrons from the organic material to oxygen, which is termed the ''electron acceptor.'' The organic material is oxidized and the oxygen is reduced. This process generates energy for the organisms and transforms the organic material to carbon dioxide and new cell mass. Anaerobic organisms use substances other than oxygen as electron

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Alternatives for Ground Water Cleanup TABLE 2-2 Geochemical Processes in Subsurface Environments Process Definition Significance in Aquifers Effect on Contaminants Dissolution-precipitation Reactions that dissolve or precipitate solids such as natural minerals Primary control on the chemical composition of ground water Can increase or decrease the concentrations of dissolved constituents, including some types of contaminants Oxidation-reduction Reactions that add electrons to (reduce) or remove electrons from(oxidize) chemicals, altering their chemical form Determines the speciation of metals with more than one oxidation state and the possible chemical and biological degradation pathways of organic matter Can alter contaminant concentration either by direct chemical reactions or by increasing microbial degradation of the contaminant; can dissolve or precipitate metals Sorption-desorption Reactions that transfer a substance from the fluid phase (solvent) to the solid phase (sorbent), or vice versa Affects dissolved concentrations by the attachment and release of constituents on surfaces of aquifer sediment Sorption can slow the movement of contaminants Ion exchange Exchange of ions in clays for ions in solution, with charge balance maintained Reduces the concentration of one ion and increases the concentration of another Can remove contaminant ions from solution, particularly when clay is present, and thus slow their removal by pumping Complexation Interactions between chemicals in solution that generate combined chemical species, such as ion pairs, complex ions, or chelates Affects the availability of substances in ground water to participate in reactions Can alter the concentrations, reactivities, and mobilities of contaminants (especially metals)

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Alternatives for Ground Water Cleanup ple, Figure 2-10 illustrates the complexity of a bromide plume produced by a tracer test conducted in a relatively uniform sand and gravel aquifer in Cape Cod, Massachusetts. The complexity of the plume, as determined from 650 multilevel samplers, reflects the sensitivity of transport to spatial variations in hydraulic properties even in simple subsurface environments. The presence of low-permeability zones or lenses can cause rate-limited mass transfer behavior similar to that observed in aggregated soils. Over time, solutes will diffuse into such strata. These zones thus function alternately as sinks and then sources of solute mass for the bulk ground water (Valocchi, 1988; Gilham et al., 1984). The large amounts of time required for compounds to diffuse out of these low-permeability strata can add considerably to the time and volume of water required to flush out contaminants (Wilson, 1992). The presence of low-permeability lenses will also influence NAPL migration. For materials with sufficiently small pores, NAPL phases cannot displace the water in the pores and therefore cannot penetrate the pores. These strata therefore act as capillary barriers to NAPL flow, inducing lateral spreading, irregular migration pathways, and "pooling" of the NAPL (see Figures 2-5 and 2-6). Such pools may be extremely difficult to locate. In addition, ground water makes minimal contact with these NAPL pools, thus causing flushing to be an ineffective method for recovering the organic liquid. Heterogeneities can also create preferential pathways for NAPL migration, greatly enhancing travel distance and limiting lateral spreading. NAPLs have been observed to preferentially flow along fine-scale surfaces between subsurface layers (Kueper et al., 1993). In addition, small-scale permeability variations can trigger the onset of viscous fingering in an inherently unstable flow regime. Subsurface macropores, created by plant roots or chemical weathering, and formation joints and fractures can serve as preferential pathways. In addition to enhancing the contaminant transport rate, such pathways may permit the NAPL to penetrate otherwise impermeable strata and facilitate the transport of contaminants between formations. Preferential flow paths tend to create highly variable distributions of NAPL in the subsurface. Under such conditions, locating contaminants is a formidable task, making contaminant removal through excavation infeasible and posing great difficulties for contaminant containment. Chemical Heterogeneity Variable chemical composition within a formation may also affect contaminant transport over a range of scales. At the pore scale, a vari-

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Alternatives for Ground Water Cleanup FIGURE 2-10 The complexity of a plume created by a bromide ion solution in a sand and gravel aquifer in Cape Cod, Massachusetts. SOURCE: LeBlanc et al., 1991.

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Alternatives for Ground Water Cleanup able mineral composition will create a complex environment in which many sorption mechanisms may coexist (Weber et al., 1992) and behavior is not easily quantified. At a larger scale, spatial variability in organic carbon content or formation mineralogy will tend to produce zones of high contaminant retention. Some theoretical studies suggest that spatial variability of chemical factors controlling sorption can enhance contaminant dispersion (Garabedian, 1987; Valocchi, 1989). Relatively little is known, however, about the degree to which sorption characteristics vary and are correlated with variations in hydraulic conductivity (Sudicky and Huyakorn, 1991). Variable mineral composition and organic carbon content may also affect NAPL entrapment mechanisms, possibly creating zones of entrapped NAPL that are less accessible to flowing ground water. Source Variability In addition to physical and chemical variability, variability in the composition and distribution of the contaminant source also affects contaminant behavior in the subsurface. Contaminants rarely enter the subsurface individually. Thus, an understanding of the behavior of mixtures is critical to predicting contaminant migration and retention. Unfortunately, the composition of the contaminant source is usually imprecisely known. For NAPLs, properties such as density, viscosity, solubility, and interfacial tension depend on composition. For example, chlorinated solvents ordinarily enter the subsurface as DNAPLs, which tend to sink below the water table, but they may also enter as minor components of a hydrocarbon mixture constituting an LNAPL, which tends to spread laterally at the water table. As components of a NAPL mixture dissolve or volatilize over time, the NAPL properties change, a process termed "weathering." Very little information is available pertaining to the weathering of chemical mixtures of interest in aquifer remediation. The presence of multiple dissolved contaminants creates competition for sorption sites. In severely contaminated systems, the properties of dissolved contaminants will also be influenced by the presence of cosolvents. The effect of multiple solutes and cosolvents on sorption phenomena seriously complicates the task of characterizing contaminant distribution and transport (Weber et al., 1991; Nkedi-Kizza et al., 1985; Rao et al., 1990). The magnitude, temporal evolution, and spatial distribution of the source of contamination greatly influence the transport and retention of pollutants. These factors, however, are unknown at many sites. Temporal fluctuations in contaminant release will create zones of high concentration within a plume, although such zones tend to flatten over large

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Alternatives for Ground Water Cleanup travel distances due to natural dissipative mechanisms. The size of the spill may influence the amount of contamination that ultimately reaches the aquifer. For example, if a spill is small, it may never reach the saturated zone due to residual entrapment of the contaminant within the unsaturated zone. The spatial distribution of the original source of contamination may affect the extent of aquifer contamination. For example, DNAPLs will tend to remain localized within a small horizontal cross-sectional region because these chemicals have no tendency to spread laterally until they encounter a stratum of low permeability. In the majority of cases it is useful to separate the contamination conceptually into two parts: (1) the plume of dissolved contaminant and (2) contaminant source areas, which include the potentially substantial amounts of contaminant in precipitated or NAPL form. As Chapter 3 will explain, the plume can be contained and its size decreased by pumping. The sources of contamination are generally much more difficult to control and remove; they will often extend over a considerable volume, and no adequate methods are available for locating all of them. Removal of contaminant sources by pumping may require years or centuries, depending on factors such as the solubility of the contaminant in water, the size of the sources and their distribution, and the flow pattern of the aquifer. RESEARCH NEEDS FOR IMPROVING UNDERSTANDING OF THE CONTAMINATED SUBSURFACE This chapter has documented that the subsurface environment even in its uncontaminated condition—is a complex system that scientists do not fully understand. Advances in knowledge about subsurface processes are essential for improving ground water contamination assessments and cleanup technologies. These advances are needed in three broad areas: subsurface characteristics, contaminant transport and distribution, and reaction pathways and rates. The greatest progress will be made if site cleanups are accompanied by investigations aimed at identifying the critical conditions and processes controlling contaminant behavior, while gathering data helpful for optimizing performance of the cleanup system. Subsurface Characteristics This chapter has documented that hydrogeologic investigations of contaminated environments must provide both fine resolution and complete coverage of the contaminated area. Heterogeneities ranging from

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Alternatives for Ground Water Cleanup the microscale to the macroscale can have profound effects on transport of contaminants and performance of cleanup systems. Errors in predicting subsurface characteristics—and hence the performance of cleanup systems—frequently result from the inability to obtain representative samples at both sufficiently small and large scales. Improvements in subsurface sampling methods especially in noninvasive methods that do not jeopardize the integrity of the site with extensive drilling—would significantly advance the ability to design ground water cleanup systems and to predict the fate of contaminants. Research is needed to answer the following questions: How can sampling methods be improved to ensure that the samples are representative? What parameters—physical, chemical, and biological—must be included in the site characterization program for formulation of realistic cleanup objectives and optimization of the treatment system? How can the variability of aquifer properties be characterized over a sufficient range of scales—millimeters to kilometers—to understand the effect of variability on contaminant transport and cleanup system performance? What level of characterization is needed to support decisionmaking at the various stages of remedial investigation and design? Can the variability of aquifer properties be assessed adequately with statistical information developed from common geologic environments? How can the costs of sampling methods be reduced to allow more extensive sampling at sites? What new ideas and techniques can contribute to development of a reliable three-dimensional map of subsurface geology and ground water flow patterns at a site? Contaminant Distribution and Transport Considerable progress has been made in recent years in developing the ability to predict the distribution and transport of contaminants in the subsurface. For example, a greater understanding now exists pertaining to the coupling of permeability variations and dispersive contaminant transport. Similarly, the fundamental physical phenomena governing NAPL migration and entrapment have been identified in laboratory studies under simple, well-defined conditions. However, much still needs to be learned regarding contaminant behavior in complex natural systems. Most investigations of contaminant fate in the subsurface have been carried out under ideal conditions such as in homogeneous aquifers

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Alternatives for Ground Water Cleanup with single contaminants. As this chapter has emphasized, however, the subsurface is neither chemically, biologically, nor physically homogeneous. Unfortunately, relatively little is understood about the impact of heterogeneities on processes that control the fate and transport of contaminants, including sorption, abiotic and biotic reactions, and residual entrapment and dissolution. More information is needed about the behavior of contaminants at a fundamental level in nonideal systems to answer the following questions: How can the full range of contaminants and other organic compounds in the ground water—not just those targeted in the monitoring plan—be determined? What critical parameters and properties govern transport of contaminants in various types of complex subsurface systems? Trapped globules of NAPL in a laboratory column containing a porous medium (glass beads). Courtesy of Rice University, Department of Environmental Science and Engineering.

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Alternatives for Ground Water Cleanup How can the relationships between these critical parameters and properties and the transport of contaminants be quantified? How can these properties be measured, or how can they be inferred from more easily quantifiable media properties? How can detection methods for NAPLs or other concentrated sources of contamination be improved to provide more detailed characterization of the distribution of contaminant mass? How can data from sampling and pumping wells be used to locate contaminant sources more precisely? How can site-specific data best be incorporated into mathematical models for predicting contaminant transport? Reaction Pathways and Rates This chapter has documented that a variety of chemical and biological reactions influence the fate of contaminants in the subsurface. Time scales for ground water transport are generally large enough—on the order of many years—that even exceedingly slow reactions can register an impact. For this reason, classes of reactions of minimal importance in other environments can significantly affect the fate of ground water contaminants. Furthermore, the large fluid-solid interfacial area typical of aquifers increases the occurrence of reactions mediated by mineral constituents and microorganisms on the solid surfaces. The heterogeneity of subsurface conditions greatly complicates understanding of contaminant reactions. Because of the diversity of geochemical conditions, expressed as compositional gradients from the molecular scale to the macroscale, a bewildering variety of reaction behavior can occur. Furthermore, the study of subsurface reaction rates poses severe methodological problems, especially when working with natural aquifer solids of diverse composition. Special precautions are necessary to define and control the experimental system sufficiently so that reaction rates can be measured while at the same time preserving the connection to the real environment. It is not surprising that researchers have been reluctant to take up this challenge and that knowledge is often sparse regarding the reaction mechanisms and pathways that may determine the fate of ground water contaminants. The fundamental study of such reaction systems is relatively new, and if pursued may open new vistas for in situ ground water cleanup. Research is needed to address the following questions: What are the critical chemical and biological reactions affecting contaminants in various types of subsurface environments? How can rates of these reactions be quantified, and how do they

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Alternatives for Ground Water Cleanup depend on geochemical conditions and the presence of particular microorganisms? How can reaction rates measured in the laboratory be extrapolated to the field? Are naturally occurring microorganisms able to achieve desired biotransformations, or must specially cultured organisms be introduced? How can the subsurface microorganisms be assayed to enable prediction of biochemical transformations? To what extent do specific chemical or biochemical reactions depend on geochemical conditions such as pH, redox state, and mineralogy? To what extent do particular reactions modify geochemical conditions—pH, redox state, and surface composition—thus promoting or inhibiting other reactions? For localized contaminant sources, what chemical treatments could destroy or solubilize contaminants, particularly NAPLs? How can reaction rate information best be incorporated into mathematical models for predicting contaminant fate? CONCLUSIONS Based on consideration of the properties of the subsurface environment and contaminant behavior in the subsurface, the committee reached the following conclusions: Theoretically, it is possible to clean up contaminated ground water, subject to constraints imposed by the principles of mass and energy conservation, thermodynamics, and kinetics. However, practical limitations arising from the uneven distribution of contaminants, subsurface complexity, and the inherently slow rate of ground water movement severely restrict decontamination efforts. Subsurface environments have complexities and heterogeneities that make them inherently difficult to decontaminate. The complexity of the subsurface and the difficulty of characterizing it contribute in large measure to the problems experienced in ground water cleanup documented in the following chapters. While some generalizations are possible, each site has a character of its own and must be studied carefully to enable effective remediation. Contaminants found at hazardous waste sites are diverse in nature, manifesting a wide range of properties that may complicate remediation. Contaminants that are present as separate liquid phases, sorbed strongly to aquifer soil and rock, or precipitated as solids constitute a large reservoir that is difficult to remove. Immiscible liquid contami-

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Alternatives for Ground Water Cleanup nants (NAPLs) especially increase the complexity of remediation because they migrate into places inaccessible to extraction by hydraulic forces. Ground water contamination problems may become increasingly complex with the passage of time because of the potential for contaminants to migrate and accumulate in less accessible zones. Measures to remove contaminants from zones where the release occurred and to contain contaminants that cannot be removed should be taken as soon as possible after the contamination occurs. Transport and transformations in the subsurface occur relatively slowly, at time scales as long as years, decades, and centuries. All of those concerned with ground water remediation—scientists and technologists, as well as decisionmakers and the public—must recognize that ground water cleanup requires patience and perseverance to an extent considerably greater than for surface water cleanup. Expectations of quick and easy solutions are illusory. REFERENCES Adamson, A. W. 1982. Physical Chemistry of Surfaces, 4th ed. New York: John Wiley and Sons. Anderson, M. 1979. Modeling of ground water flow systems as they relate to the movement of contaminants. CRC Crit. Rev. Environ. Control 9:97-156. API (American Petroleum Institute). 1989. A Guide to the Assessment and Remediation of Underground Petroleum Releases, 2nd ed. Publication No. 1628. Washington, D.C.: API. Back, W., M. J. Baedecker, and W. W. Wood. 1993. Scales in chemical hydrogeology: a historical perspective. Pp. 111-129 in Regional Ground Water Quality, W. Alley, ed. New York: Van Nostrand Reinhold. Barker, J. F., and J. T. Wilson. 1992. Natural biological attenuation of aromatic hydrocarbons under anaerobic conditions. Pp. 57-58 in Proceedings of the Subsurface Restoration Conference, Dallas, Texas, June 21-24 . Houston: Rice University. Brusseau, M. L., and P. S. C. Rao. 1989. Sorption non-ideality during organic contaminant transport in porous media. CRC Crit. Rev. Environ. Control 19:22-99. Cherry, J. A., R. W. Gilham, and J. F. Barker. 1984. Contaminants in groundwater: chemical processes. Pp. 46-64 in Groundwater Contamination: Studies in Geophysics. Washington, D.C.:National Academy Press. Conrad, S. H., E. F. Hagan, and J. L. Wilson. 1987. Why are residual saturations of organic liquids different above and below the water table? In Proceedings—Petroleum Hydrocarbons and Organic Chemicals in Groundwater: Prevention, Detection and Restoration. Worthington, Ohio: National Water Well Association. Da Costa, J. A., and R. R. Bennett. 1960. The pattern of flow in the vicinity of a recharging and discharging pair of wells in an aquifer having areal parallel flow. Pp. 524-536 in General Assembly of Helsinki: Commission of Subterranean Waters Publication No. 52. Gentbrugge, Belgium: International Association of Scientific Hydrology. DOE (Department of Energy). 1992. Chemical Contaminants on DOE Lands and Selection of Contaminant Mixtures for Subsurface Science Research. Report No. DOE/ER-0547. Washington, D.C.: DOE.

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Alternatives for Ground Water Cleanup EPA (Environmental Protection Agency). 1982. Aquatic Fate Process Data for Organic Priority, Pollutants. Report No. 440/4-81-014. Washington, D.C.: EPA. Garabedian, S. P. 1987. Large-Scale Dispersive Transport in Aquifers: Field Experiments and Reactive Transport Theory. Ph.D. dissertation. Massachusetts Institute of Technology, Department of Civil Engineering, Cambridge, Mass. Ghiorse, W. C., and J. T. Wilson. 1988. Microbial ecology of the terrestrial subsurface. Adv. Appl. Microbiol. 33:107-172. Gilham, R. W., E. A. Sudicky, J. A. Cherry, and E. O. Frind. 1984. An advection-dispersion concept for solute transport in heterogeneous unconsolidated geological deposits. Water Resources Res. 20(3):369-372. Heath, R. C. 1980. Basic Elements of Ground-Water Hydrology with Reference to Conditions in North Carolina. U.S. Geological Survey Water Resources Investigations Open-File Report 80-44. Washington, D.C.: U.S. Government Printing Office. Heath, R. C. 1983. Basic ground-water hydrology. U.S. Geological Survey Water Supply Paper 2220. Washington, D.C.: U.S. Government Printing Office. Hem, J. D. 1985. Study and Interpretation of the Chemical Characteristics of Natural Water. U.S. Geological Survey Water Supply Paper 2254. Washington, D.C.: U.S. Government Printing Office. Kueper, B. H., D. Redman, R. C. Staff, S. Reitsma, and M. Mah. 1993. A field experiment to study the behavior of tetrachloroethylene below the water table: spatial distribution of residual and pooled NAPL. Ground Water 31(5):756-766. LeBlanc, D. R., S. P. Garabedian, K. M. Hess, L. W. Gelhar, R. D. Quadri, K. G. Stollenwerk, and W. W. Wood. 1991. Large-scale natural gradient tracer test in sand and gravel, Cape Cod, Massachusetts: 1. experimental design and observed tracer movement. Water Resources Res. 27(5):895-910. LeGrand, H. E. 1988. Region 21, Piedmont and Blue Ridge. Pp. 201-208 in Decade of North American Geology: GNA 0-2, Hydrogeology, W. Back, P. R. Seaber, and J. S. Rosenshein, eds. Boulder, Colo.: Geological Society of America. Lucius, J. E., G. K. Olhoeft, P. L. Hill, and S. K. Duke. 1992. Properties and Hazards of 108 Selected Substances. Open File Report 92-527. Golden, Colo.: U.S. Geological Survey. Mabey, W., and T. Mill. 1978. Critical review of hydrolysis of organic compounds in water under environmental conditions. J. Phys. Chem. Ref. Data 7:383-415. Mackay, D. M., D. L, Freyberg, P. V. Roberts, and J. A. Cherry. 1986. A natural gradient experiment on solute transport in sand aquifers 1: approach and overview of plume movement. Water Resources Res. 22:2017-2029. Mercer, J. W., and R. M. Cohen. 1990. A review of immiscible fluids in the subsurface: properties, models, characterization and remediation. J. Contain. Hydrol. 6:107-163. Montgomery, J. H., and L. M. Welkom. 1990. Groundwater Chemicals Desk Reference. Chelsea, Mich.: Lewis Publishers. Nkedi-Kizza, P., P. S. C. Rao, and A. G. Hornsby. 1985. Influence of organic cosolvents on sorption of hydrophobic organic chemicals by soils. Environ. Sci. Technol. 19:975-979. NRC (National Research Council). 1991. Environmental Epidemiology, Volume I: Public Health and Hazardous Wastes. Washington, D.C.:National Academy Press. Oostrom, M., J. S. Hayworth, J. H. Dane, and O. Given. 1992. Behavior of dense aqueous phase leachate plumes in homogeneous porous media. Water Resources Res. 28(8):2123-2134. Powers, S. E., C. O. Loureiro, L. M. Abriola, and W. J. Weber, Jr. 1991. Theoretical study of the significance of nonequilibrium dissolution of nonaqueous phase liquids in subsurface systems. Water Resources Res. 27(4):463-477. Powers, S. E., L. M. Abriola, and W. J. Weber, Jr. 1992. An experimental investigation of NAPL dissolution in saturated subsurface systems: steady-state mass transfer rates. Water Resources Res. 28(10):2691-2705.

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