2
Source Zones

Understanding the characteristics of subsurface source zones provides a foundation for addressing source characterization, technology options, and decision making. Following the definition given in Chapter 1, source zones are volumes that have been in contact with separate phase contaminants and that act as reservoirs that sustain a contaminant plume in groundwater, surface water, or air or that act as a source for direct exposure. Within these subsurface regions, non-aqueous, sorbed, and dissolved phase contaminants in hydraulically stagnant zones can provide persistent loading of contaminants to groundwater passing through them. First, the five hydrogeologic settings that typify most hazardous waste sites are described. The chapter then turns to an examination of contaminant releases and subsequent transport, storage, and fate, describing the many processes that act on contaminants in the subsurface and how this is manifested in the field-scale distribution of contaminants. The architecture of source zones is then considered for the five hydrogeologic settings. Although many of the processes that control contaminant fate and transport in the subsurface are the same for either chlorinated solvents or chemical explosives, these contaminants are discussed separately because their release mechanisms can be significantly different from one another.

HYDROGEOLOGIC SETTINGS

Subsurface settings are a product of a set of diverse geological processes that produce an abundance of variations. Common sedimentary systems include windblown (eolian) sands, beach sands, alluvial fans, river sequences, glacial outwash,



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Contaminants in the Subsurface: Source Zone Assessment and Remediation 2 Source Zones Understanding the characteristics of subsurface source zones provides a foundation for addressing source characterization, technology options, and decision making. Following the definition given in Chapter 1, source zones are volumes that have been in contact with separate phase contaminants and that act as reservoirs that sustain a contaminant plume in groundwater, surface water, or air or that act as a source for direct exposure. Within these subsurface regions, non-aqueous, sorbed, and dissolved phase contaminants in hydraulically stagnant zones can provide persistent loading of contaminants to groundwater passing through them. First, the five hydrogeologic settings that typify most hazardous waste sites are described. The chapter then turns to an examination of contaminant releases and subsequent transport, storage, and fate, describing the many processes that act on contaminants in the subsurface and how this is manifested in the field-scale distribution of contaminants. The architecture of source zones is then considered for the five hydrogeologic settings. Although many of the processes that control contaminant fate and transport in the subsurface are the same for either chlorinated solvents or chemical explosives, these contaminants are discussed separately because their release mechanisms can be significantly different from one another. HYDROGEOLOGIC SETTINGS Subsurface settings are a product of a set of diverse geological processes that produce an abundance of variations. Common sedimentary systems include windblown (eolian) sands, beach sands, alluvial fans, river sequences, glacial outwash,

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Contaminants in the Subsurface: Source Zone Assessment and Remediation deltaic sequences, and lake-deposited (lacustrine) clays. Common rock systems include limestone, dolomite, sandstone, shale, interbedded sandstone and shale, extrusive volcanic flow sequences, intrusive granitic bodies, and metamorphic systems of crystalline rock. To varying degrees these systems can be fractured, cemented, and/or opened by dissolution (karst). This diversity makes it challenging to develop general statements regarding the characteristics of source zones, the efficacy of remedial technologies, and what endpoints are attainable. For example, the flow of groundwater or remedial fluids (such as surfactants) is substantially different in beach sand than in karst systems, and the tools required to characterize alluvium are substantially different than tools used to characterize rock. Five general hydrogeologic settings that are broadly representative of the common conditions of concern are illustrated in Figure 2-1. The differentiating features between the five settings are the spatial variations in permeability and porosity (see Box 2-1, which describes the terminology relevant to the following discussion). These parameters control the mechanisms by which contaminants are stored and released from source zones under natural and engineered conditions. The scale (size) of the representative hydrogeologic settings is envisioned FIGURE 2-1 Five General Hydrogeologic Settings.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation BOX 2-1 Terminology for Hydrogeologic Settings The following terms help distinguish among the five hydrogeologic settings discussed in this chapter. Consolidated vs. Unconsolidated Media: Geologic media that are cohesive as a body, firm, or secure are described as consolidated (e.g., most rock formations). Geologic media that are not cohesive as a body, are loosely arranged, and that readily separate into granular components, are described as unconsolidated. Most alluvial deposits (e.g., beach sand) are unconsolidated. Both terms are geotechnical, in that different tools are used to probe consolidated vs. unconsolidated media. The term unconsolidated may not apply to all clays, although all clays are granular. Thus, unconsolidated is a more restrictive term than granular and is used sparingly in this report. Grain Size: From Press and Siever (1974), common labels describing the sizes of granular media are: Clay < 1/256 mm < Very Fine Sand < 1/16 mm < Fine Sand < 1/8 mm < Medium Sand < 1/2 mm < Coarse Sand < 1 mm < Very Coarse Sand < 2 mm < Granule < 4mm < Pebble < 8 mm < Cobble < 256 mm < Boulder. Grain size and the degree of mixing of different grain sizes (sorting) are primary factors that control the permeability of granular porous media. Permeability: Permeability (k) is a property of a porous medium that describes its capacity to transmit fluid. Permeability is independent of the fluid or fluids present in the porous medium and has the units length squared (e.g., m2). Permeability is used in this report as the primary metric for the capacity to transmit fluid because more than one fluid (e.g., air, water, and NAPL) can coexist in the pore space of the medium of interest. Low permeability media are considered herein to be < 10−14 m2. High permeability media are considered to be > 10−10 m2. Between 10−14 and 10–10 m2 is referred to as moderate permeability media. Secondary Permeability: Secondary permeability refers to the portion of the permeability of a porous medium that can be attributed to secondary (post-emplacement) features of the matrix. Examples of secondary features include fractures, animal burrows, root casts, and solution features. In some media, such as fractured clays or crystalline rock, the dominant factor controlling fluid transmission is commonly secondary permeability. Effective Porosity: Porosity is defined as the volume of void space in the medium divided by the total volume of the medium. In hydrogeology the more important term is the effective porosity of a porous medium, , which is a unitless parameter defined as the volume of the interconnected void space in the medium divided by the total volume of the medium. Throughout this report, when the term porosity is used, effective porosity is assumed.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation For fractured media, the components of porosity include the matrix porosity, , and the fracture porosity, : Hydraulic Conductivity: Within the field of groundwater hydrology, the term hydraulic conductivity (K) describes a porous medium’s capacity to transmit water. In contrast to permeability, conductivity is dependent on the properties of the porous medium and the fluid in the porous medium and has the units of length divided by time. Hydraulic conductivity is described as: where k is permeability, g is the gravitational constant, ρwater is the density of water, and µwater is the viscosity of water. K values are included at relevant points in this report for those more familiar with hydraulic conductivity. The relationship between permeability and hydraulic conductivity, and their values for common geologic media, are described in Figure 2-2. In all cases these values are based on the assumption that water is the fluid of interest and that water fully saturates the porous media. Low hydraulic conductivity is considered to be less than 10−7 m/sec, high is considered to be greater than 10−3 m/sec, and moderate would fall in between those two values. Heterogeneity: Heterogeneity is used to describe spatial variations in permeability. Heterogeneity can exist over a variety of scales and can be reflected in abrupt changes in permeability at discrete interfaces (caused, for example, by low-permeability inclusions) or by continuous variations in permeability over some length scale (caused, for example, by periodic gradations in grains size). Heterogeneity is of interest down to the scale of centimeters. In terms of the extent of heterogeneity, media with spatial variations in permeability of less than three orders of magnitude are referred to as mildly heterogeneous. This builds on (1) the classification of the Borden Aquifer (Canadian Forces Base Borden, Ontario) as “mildly heterogeneous” (Domenico and Schwartz, 1998) and (2) the observation of nearly three orders of magnitude variation in the permeability in the Borden Aquifer (Sudicky, 1986). Media with greater than three orders of magnitude spatial variation in permeability are described as having either moderate or high heterogeneity. Anisotropy refers to the condition in which the permeability of a geologic formation varies with the direction of measurement about a point. This commonly occurs in layered sedimentary deposits where vertical permeability is often less than 1/10th of the horizontal permeability. This anisotropy tends to foster lateral spreading and horizontal flow.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Transmissivity: Transmissivity describes the bulk capacity of a vertical interval of geologic media to transmit water. Transmissivity is the product of the average hydraulic conductivity of an interval and the thickness of the unit. The units of transmissivity are length squared divided by time. Layered: This term refers to horizontal beds of material with different permeability and porosity that are commonly encountered in natural geologic media. Individual layers typically reflect changes in the mode of deposition (e.g., flowing or stagnant conditions in water). The thickness and lateral extent of layers depends on the mode of deposition. FIGURE 2-2 Permeability and hydraulic conductivity for common geologic media. SOURCE: Adapted from Freeze and Cherry (1979).

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Contaminants in the Subsurface: Source Zone Assessment and Remediation in the range of a few meters, whereas the size of an entire source zone can be on the order of tens of meters. Source zones can occur within a single hydrogeologic setting (e.g., a sand dune deposit) or can include multiple hydrogeologic settings (e.g., alluvium overlying fractured crystalline rock). The latter case can be challenging in that the mechanism of contaminant storage and release can be substantially different in adjacent portions of a single source zone. The following section describes the hydraulic characteristics (primarily permeability and porosity) of the five general settings. The likely distribution of contaminants in each of these settings is developed in subsequent sections. Although they do not entirely capture the diversity of hydrogeologic systems, these five settings are useful for highlighting major differences in how source zones store and release contaminants. The taxonomy used in this chapter is purposefully general and could easily be expanded to more rigorously reflect the range of hydrogeologic conditions that exist. Type I – Granular Media with Mild Heterogeneity and Moderate to High Permeability Type I media include systems with porosities that are consistent with typical granular media (e.g., 5 percent to 40 percent), with permeabilities that are consistent with sand or gravel deposits (>10–14 m2 or hydraulic conductivity >10–7 m/s), and mild heterogeneity (less than three orders of magnitude). As conceptualized here, this material is about as uniform as it can be in nature and thus is relatively uncommon. Deposits of this nature are encountered in association with windblown sands and beach deposits. Examples include beach sands at the Canadian Forces Base Borden, Canada, and dune deposits at Great Sand Dunes National Park, Colorado (Figure 2-3). Due to its mild heterogeneity and moderate to high permeability, all portions of this media type can transmit groundwater. Type II – Granular Media with Low Heterogeneity and Low Permeability Type II settings have porosities that are consistent with typical granular media (e.g., 5 percent to 40 percent), low spatial variation in permeability (less than three orders of magnitude), low permeability consistent with silt or clay deposits (k < 10–14 m2), and low hydraulic conductivity (K < 10–7 m/s). An example is a clay deposit with no significant secondary permeability features (such as fractures, root holes, animal borrows, or slickenslides). These systems are somewhat uncommon (especially in the near-surface environment where releases typically occur), although some examples include TCE-contaminated clays at the Department of Energy’s Savannah River Site in South Carolina. More typically, low-permeability materials contain significant secondary permeability features and thus fit better into the Type V setting description (see below).

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Contaminants in the Subsurface: Source Zone Assessment and Remediation FIGURE 2-3 Example of Type I media from Great Sand Dunes National Monument. SOURCE: http://www.nps.gov/grsa Type III – Granular Media with Moderate to High Heterogeneity Type III encompasses systems with moderate to large variations in permeability (greater than three orders of magnitude) and porosities that are consistent with granular media (e.g., 5 percent to 40 percent). Given large spatial variations in permeability (at the scale of centimeters to meters), portions of the zone are comparatively transmissive while others contain mostly stagnant fluids. As an example, an interbedded sandstone and shale is shown in Figure 2-4. For the purpose of this report, the more transmissive zones in Type III media have a permeability greater than 10–14 m2 (K > 10–7 m/s). Near-surface deposits of this nature are common due to the abundance of alluvium with large spatial variations in permeability and are encountered in either rock or alluvium associated with deltaic, fluvial, alluvial fan, and glacial deposits. Examples include the Garber-Wellington Aquifer in central Oklahoma, the Chicot Aquifer in Texas and Louisiana, and varved sediments near Searchmont, Ontario (Figure 2-5). Type IV – Fractured Media with Low Matrix Porosity Fractured media with low matrix porosity are common in crystalline rock including granite, gneiss, and schist. Examples include bedrock in the Piedmont and Blue Ridge Mountain region of the southeastern United States and plutonic cores of mountain ranges in the western United States (see Figure 2-6 for an example). The primary transmissive feature in Type IV settings is secondary permeability caused by fractures, because little to no void space exists in the unfractured matrix. The permeability of the unfractured matrix is considered to

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Contaminants in the Subsurface: Source Zone Assessment and Remediation FIGURE 2-4 Interbedded sandstone and shale, shown as an example of Type III media. SOURCE: Reprinted, with permission, from http://geology.about.com. © 2004 About.com. be less than 10–17 m2 (K < 10–10 m/s). However, the bulk permeability of the media is dependent on the frequency, aperture size, and degree of interconnection of the fractures, such that the anticipated range of bulk permeability values is 10–15–10–11 m2 (K = 10–8–10–4 m/s). The porosity of both the matrix and the fractures is typically small—less than 1 percent. However, in regions where crystalline rock has been extensively weathered (e.g., at the top of bedrock), the bulk media can behave more like a porous medium than what would be expected from a fractured rock type setting. A primary feature that differentiates Type IV from Type I is that contaminants in Type IV will occur in a sparse network of rock fractures that may or may not be hydraulically interconnected. In general, sources zones in fractured media with low matrix porosity are less commonly encountered than sources zones in Type III and Type V settings. This reflects the fact that many surface releases never reach bedrock and, in the United States, crystalline bedrock occurs less frequently than sedimentary bedrock (Back et al., 1988). Type V – Fractured Media with High Matrix Porosity This setting includes systems where fractures (secondary permeability) are the primary transmissive feature and there is large void space in the matrix. The permeability of the unfractured matrix is considered to be less than 10–17 m2 (K <10–10 m/s). The anticipated range of bulk permeability values is 10–16–10–13 m2 (K = 10–9–10–6 m/s). The porosity of the fractures relative to the total unit

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Contaminants in the Subsurface: Source Zone Assessment and Remediation FIGURE 2-5 Interbedded sand and silt layers associated with annual depositional cycles from the Varved Sediments, near Searchmont, Ontario, shown as an example of Type III media. SOURCE: Reprinted, with permission, from http://geology.lssu.edu/NS102/images/varves.html. © 2004 Department of Geology and Physics, Lake Superior State University.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation FIGURE 2-6 Fractured crystalline rock shown as an example of Type IV media. Photo taken near Kitt Peak Observatory, Arizona. SOURCE: Reprinted, with permission, from http://geology.asu.edu/~reynolds/glg103/rock_textures_crystalline.htm. © 2004 Department of Geological Sciences, Arizona State University. volume is small (e.g., <1 percent). However, unlike Type IV, in Type V hydrogeologic settings the porosity of the unfractured matrix is anticipated to fall in the range of 1 percent to 40 percent. Fractured media with high matrix porosity are commonly encountered in sedimentary rock (e.g., limestone, dolomite, shale, and sandstone) and fractured clays. Examples include the Niagara Escarpment in the vicinity of the Great Lakes (see Figure 2-7) and fractured lake-deposited clay in Sarnia, Ontario, Canada. An important variant of the Type V setting is karst, which is common in carbonates (e.g., limestone or dolomite). In this scenario, transmissive zones include sinkholes, caves, and other solution openings that vary widely in aperture and have the potential to store and transport significant contaminant mass (see Figure 2-8). Permeability in karst terrains varies over tens of orders of magnitude from low permeabilities between fractures to open channel flow in channels and caves (Teutsch and Sauter, 1991; White, 1998, 2002). Karst is characterized by both rapid transport along sparse dissolution features and a high ratio of stagnant to transmissive zones. As such, it is one of the most challenging hydrogeologic settings to characterize and manage.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation FIGURE 2-7 Fractured limestone, Door County, Wisconsin, shown as an example of Type V media. SOURCE: Reprinted, with permission, from http://www.uwgb.edu/dutchs/GeoPhotoWis/WI-PZ-NE/BayshorePark/bayshcp3.jpg. © 2004 Natural and Applied Sciences, University of Wisconsin-Green Bay. Relating the Hydrogeologic Settings to Specific Sites The five hydrogeologic settings defined above represent distinct members in the continuum of settings observed at actual sites. Type I, with mild heterogeneity and moderate to high permeability, grades gradually into Type III as heterogeneity increases. With an increasing clay fraction, Type III grades to Type II. Natural systems range from clean sands to clayey sands to sandy clays to clays in a continuum. In a similar manner, the degree of importance of fractures may vary from insignificant in Type III to dominant in Type V. Because of these gradients, the presence of stagnant zones and the degree of diffusion and sorption vary continually. Source zones, especially those above a certain size, may also encompass more than one hydrogeologic setting. This commonly occurs in the instance of shallow alluvium over bedrock. For example, in the Piedmont of the southeastern United States, one can find fluvial deposits (Type III) and saprolite (Type V) overlying fractured crystalline rock (Type IV) (Figure 2-9). Selecting characterization tools and source management technologies is challenging under these conditions, because although contamination may exist throughout, the appropriate

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Contaminants in the Subsurface: Source Zone Assessment and Remediation BOX 2-2 DNT Production at Badger Army Ammunition Plant The Badger Army Ammunition Plant, built in 1942, was the principal DNT production facility in the United States until it changed to standby status in 1977. At this facility, waste pits were used to burn organic solvents, propellant wastes, and lumber. Three waste pits received up to 500 gallons (1893 L) per day of DNTs, solvents, and other constituents. Investigation of the waste pits showed concentrations of DNTs up to 28 percent. Interim remedial actions led to the excavation and incineration of the top 13–20 ft (4–6 m) of material from each waste pit. In addition, six soil vapor extraction wells installed at each waste pit removed 1,600 pounds (726 kg) of solvents. However, subsurface soils 15–25 ft (4.6–7.6 m) below the bottom of the waste pits still contained DNT well over 1 percent. Source treatment continues today at Badger with some success using in situ bioremediation along with in situ wetting to induce solid phase DNT mass transfer to soil pore water; however, delivery of nutrients and management of pH and nitrite are necessary to optimize field-scale biodegradation (Fortner et al., 2003). AAP, Radford AAP, Louisiana AAP, Longhorn AAP, Cornhusker AAP, and Iowa AAP. DNT was produced primarily at Badger AAP (see Box 2-2), while RDX was made at Holston AAP. Trinitrotoluene (TNT). TNT is the most prevalent explosive used in military ordnance. TNT production in the United State occurs solely at military arsenals and peaked at 65 tons per day during World War II (Kaye, 1980). In a refined form, TNT is one of the most stable explosives and can be stored for long periods of time. Commercial TNT production begins with a batch process through the sequential nitration of toluene. The first unit process produces mononitrotoluene (MNT or mono-oil) by the addition of nitric and sulfuric acids to toluene under heated conditions. The mono-oil is converted to dinitrotoluene (DNT or bi-oil) using nitric acid-fortified waste acid. In the final conversion to TNT, bi-oil was heated with oleum2-fortified sulfuric and nitric acids. In each step, the batch process produces a mixture of compounds. Thus, mono-oil and bi-oil are terms that include MNT and DNT, respectively, but also other manufacturing byproducts. Pre-product TNT is melted and washed with soda ash solution and then washed with sodium sulfite to separate 2,4,6-TNT from the other less desir- 2   Oleum is a heavy oily liquid mixture of sulfur trioxide in sulfuric acid.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation able isomers. Waste acids from each process are routed for use in the previous batch step. Wash water from the final purification steps is routed to a “redwater” (meaning explosives-contaminated water) treatment area by a network of flumes (Urbanski, 1967a). In 1968, continuous manufacturing of TNT began at Radford AAP, where the nitric acid and oleum were introduced countercurrent in a six-stage process (Kaye, 1980). Both batch and continuous TNT production use water-filled drown tanks at each production house to stop the process if an out-of-control reaction begins. Leaks in material transfers between production houses and from storage in holding tanks, other spills, and discharges of material to the drown tanks could be significant sources of explosives contamination in the subsurface. Historic literature uses the term “nitrobody” to represent the wide assortment of nitroaromatic molecules present in the production process prior to completion of the final product (TNT). The composition of the nitrobody in each stage of TNT production varies widely, as shown in Table 2-3 for the Radford AAP continuous production line. The nitrobody production materials discharged to the drown tanks contained mono-oil, bi-oil, and TNT in mixtures of nitric and sulfuric acids and residual toluene, often at elevated temperatures. Evaluations have shown that the drown-tank material from stage 1 of the Joliet AAP batch process and from stages 1 and 2 of the Radford AAP continuous process were liquids containing 75 percent to 85 percent MNT (mostly 2-MNT and 4-MNT) (Persurance, 1974) that could possibly behave like DNAPLs. However, the specific gravity of MNTs ranges from 1.155 to 1.160 g/cm3 depending on the isomer, which is much less dense than chlorinated solvents (see Table 2-1). The nitrobody material from later stages in both processes was found to be a solid at ambient temperature. Unfortunately, there is no information on the frequency with which drown tanks were used among the batch or continuous operations nor on the intervals at which TABLE 2-3 Composition of Nitrobody during Continuous TNT Production (percent).   Composition of Nitrobody, %   Process MNT DNT TNT Temp (°C) 1 77 18 4 50-55 2 0 71 29 70 3 0 30 69 80-85 4 0 10 90 90 5 0 2 98 95 6 0 0 100 100   SOURCE: Kaye (1980).

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Contaminants in the Subsurface: Source Zone Assessment and Remediation drown-tank materials were removed and destroyed. The physical–chemical properties of the TNT production-process discharge materials are further complicated by the high sulfuric acid content, which alters the density of the material and the solubility of the explosives compounds. The density of 100 percent sulfuric acid is 1.84 g/cm3 and 78 percent sulfuric acid has a density of 1.71 g/cm3. MNT isomers have a solubility of 34 percent in 90 percent sulfuric acid at 50°C; DNT isomer solubility is 20 percent in 90 percent sulfuric acid at 70°C; and TNT isomer solubility is 10 percent in 90 percent sulfuric acid at 80°C (Urbanski, 1967a). All these complexities make it very difficult to understand the miscibility of nitrobody production materials and to determine whether they consist of emulsions of separate nonaqueous phase liquids. Thus, the physical–chemical properties of explosives material that might be released at TNT production facilities vary depending on the process stage and on the mixed acid content of the material, the completeness of the reactions in that stage, and the extent of dilution in drown tanks or waste lagoons. In some situations, a separate phase NAPL containing mostly MNTs may be present, and in others, a dense miscible phase liquid (DMPL) containing very high concentrations of MNT, DNT, and TNT may be present. At most explosives sites, there is limited information on these factors, making it difficult to assess the distribution of explosives in various hydrogeologic settings. Dinitrotoluene (DNT). In general, the production of DNT mimics that of TNT, but the process stops after the second nitration. Thus, the factors that control the physical–chemical properties of any release material from DNT production are similar to those for TNT production. Nitration of MNT isomers produces various DNT isomers. For example, nitration of o-nitrotoluene produces 2,4-DNT and 2,6-DNT, while nitration of p-nitrotoluene produces only 2,4-DNT. Nitration of m-nitrotoluene produces 3,4-DNT, 2,3-DNT, and 3,6-DNT, all of which are undesirable in the production of 2,4,6-TNT. DNT production in the Army and the resulting subsurface contamination are discussed in Box 2-2. RDX (Royal Demolition eXplosive/Research Demolition eXplosive). Cyclotrimethylenetrinitramine, hexahydro-1,3,5-trinitro-sym-triazine, and Cyclonite are all synonyms for RDX, which is a white crystalline solid with a nitrogen content of 37.84 percent. RDX is usually used in mixtures with other explosives, oils, or waxes. It has a high degree of stability in storage and is considered one of the most brisant3 of the military high explosives. Pure RDX is used in press-loaded projectiles. Cast loading is accomplished by blending RDX with a relatively low melting point substance. RDX is also used as a base charge in detonators and in blasting caps. 3   Brisant is defined as of or relating to the power (the shattering effect) of an explosive.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Just prior to and during World War II, numerous methods to synthesize RDX emerged. The first method was through the direct nitration of hexamine with nitric acid. The yield on this process was low, and in 1941 Americans and Germans simultaneously developed a method where hexamine dinitrate is reacted with ammonium dinitrate in the presence of acetic anhydride (Urbanski, 1967b). This process is the principal one used in the United States today, and it contains a constant impurity of 8 percent to 12 percent HMX (see below). RDX was principally produced at the Holston Ordnance Works in Kingsport, Tennessee. Initial characterization efforts at this location have shown RDX in the groundwater, but only two sites located below production buildings have concentrations that imply a subsurface source zone (~ 2 percent to 4 percent of RDX’s aqueous solubility) (USACHPPM, 2003). RDX groundwater contamination is much more prevalent at other facilities where manufacturing process discharges occurred (as described below). HMX (High Melt eXplosive or Her Majesty’s eXplosive). Cyclotetra-methylenetrinitramine, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, and Octagen are all synonyms for HMX. HMX is used in military ordnance where the greatest explosive power per mass is needed. HMX is formed by the nitration of hexamine (or hexamethylenetetramine) in the presence of glacial acetic acid, acetic anhydride, ammonium nitrate, and nitric acid. The reaction produces a mixture of RDX and HMX, and the RDX is selectively destroyed by base hydrolysis. HMX is also a byproduct of production of RDX and has been produced at the Holston Ordnance Works. The groundwater investigations at Holston have found HMX in the same wells that RDX was found in, but at much lower concentrations and none over drinking water health advisory limits (USACHPPM, 2003). Manufacturing Process Discharges Manufacturing processes are defined here as post-production operations that operate with the solid phase explosive material. Examples include load, assemble, and pack facilities and demilitarization operations that remove the explosive fill from expired munitions. The most prevalent explosive fill material contains TNT and Comp B (60% RDX/40% TNT). Both operations typically use hot water or steam as a washdown or washout material. Spent cleaning solutions are typically filtered with coarse fabric to remove the suspended particulates and then are discharged via pipelines, open flumes, or ditches to infiltration or evaporation ponds. Continuous releases of aqueous solutions containing explosives as solutes can infiltrate into soils, causing large areas of contamination. The Umatilla Army Depot provides an example, where from about 1955 to 1965 a munitions washout facility was operated where hot water and steam were used to remove explosives from munitions bodies. An estimated 85 million gallons (322 million L) of wash water containing TNT and RDX was discharged to

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Contaminants in the Subsurface: Source Zone Assessment and Remediation two surface impoundments that covered about half an acre (0.2 hectares). Source control measures began by excavating the top 20 feet (6 m) of soil for treatment by composting. The remaining 30 feet (9 m) of soil above the water table is being treated with in situ flushing using groundwater pump and granular activated carbon treatment. Military Training and Testing Operations Military training and testing operations relevant to the release of explosive materials include live fire exercises that use military ordnance. Detonation of military ordnance typically consumes the majority of the explosive fill. Experimental characterization of detonations of 81- and 120-mm mortars and 105-mm artillery showed only trace residues of TNT, RDX, and HMX present on the ground surface (Jenkins et al., 2001). However, a low-order detonation can distribute solid phase energetic material onto and into near-surface soil (DeLaney et al., 2003). The low-order detonation process is very ill-defined, but it is generally characterized by initiation of detonation, which recedes before consumption of all of the explosive fill. Rupture of the case distributes large chunks to small particulates, and it can occur above the soil surface or in ground after impact. In addition, Explosive Ordnance Disposal methods (e.g., open detonation) to safely dispose of unexploded ordnance can also cause incomplete detonation of target objects (Lewis et al., 2003). These detonation methods have not been fully characterized for energetic material releases. Chemical Explosives Fate in the Subsurface The above sections have noted the various materials that might be released from military operations involving chemical explosives. For production and manufacturing process discharges, aqueous solutions are the most common type of waste. Under circumstances where the environmental temperatures are significantly lower than the discharge water temperatures, explosives may precipitate out of solution and create a separate solid phase material in the soil. For example, the aqueous solubility of TNT drops from 250 mg/L at 40°C to 110 mg/L at 20°C. Similarly, the solubility of RDX is 115 mg/L at 40°C and 45 mg/L at 20°C. Precipitation of explosive solids following production and manufacturing process discharges most likely occurs in the near soil surface (< 20 feet or 6 m), such that surface excavation and treatment (e.g., via incineration or composting) are effective remediation strategies. It is possible that leaks or spills from pipelines and flumes, and infiltration from unlined ditches, could lead to contaminant accumulation in unsaturated soil pores and fracture matrices and thus provide a long-term reservoir of contamination to groundwater. As mentioned previously, some highly concentrated wastes in production

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Contaminants in the Subsurface: Source Zone Assessment and Remediation process discharges might act like DNAPLs or DMPLs. However, even heated and concentrated production materials that might initially behave like a DNAPL are likely to undergo significant change once introduced into soil, where environmental conditions would tend to decrease both temperature and acidity, promoting the creation of a separate solid phase material. Of course, recharge of the subsurface during rain events can lead to dissolution of solid phase explosives and subsequent transfer of explosives mass to soil pore water and perhaps groundwater. Subsequent dilution of the more soluble, and/or degradation of the more labile, compounds could over long periods of time (decades) result in a mixture with a very different signature than the original release materials. Rainfall can dissolve the solid phase energetic material from the detonation of military ordnance and transfer the mass to soil pore water. The potential contaminant threat from detonation of military ordnance is determined by the depth to groundwater and the various contributions of recharge, dissolution, sorption, and degradation rates. The nature and the impact of this type of source are only beginning to be understood, and current programs are in progress to improve scientific understanding and develop mitigation approaches (SERDP, 2003). Management of these source areas is compounded by the presence of unexploded ordnance and continued operations. Once in the subsurface, biotic and abiotic redox reactions are the principal processes that degrade the chemical explosives discussed above, although there are significant differences within this group. TNT is rapidly transformed to mono-and then di-aminonitrotoluenes by naturally occurring microorganisms and soil minerals under both aerobic and anaerobic conditions (Ahmad and Hughes, 2000). TNT biodegradation appears to be mostly cometabolic. Reduction rates decrease with the successive reduction of each nitro group due to the destabilization of the aromatic ring and a decrease in the electrophilic nature of the remaining nitro groups. A portion of the aminonitrotoluenes can continue to participate in reactions with soil organic matter, becoming covalently bound in a multistep humification process (Thorne and Leggett, 1999). 2,4-DNT and the 2,6-DNT manufacturing impurity can be biodegraded under aerobic conditions where specific bacteria use these materials for carbon, nitrogen, and energy sources (Fortner et al., 2003). Environmental reactions with RDX are strongest under reducing conditions that sequentially reduce the nitro groups to mono-, di-, and trinitroso products of RDX, followed by ring cleavage to produce a variety of short-chain compounds (Hawari, 2000). Very little work has been done to understand the fate of HMX in the subsurface, although a similar sequential reduction of the nitro groups followed by ring cleavage, as with RDX, is hypothesized. Natural attenuation mechanisms favor the loss of TNT>DNT>RDX>HMX in the environment. TNT sorption and aerobic degradation provides for continuous elimination reactions. However, DNT appears to require nutrients and nitrite byproduct elimination to support significant biodegradation. RDX and HMX both sorb poorly to soils and require strongly reducing conditions for natural

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Contaminants in the Subsurface: Source Zone Assessment and Remediation elimination reactions. For this reason, and considering the toxicity of RDX, RDX has become one of the more challenging organic explosives for environmental remediation. Field-Scale Distribution of Explosives in Source Zones At this time, characterization of chemical explosive source zones is immature compared to the knowledge base for DNAPLs, primarily because when the major production operations ceased 20–30 years ago, the knowledge of past waste management practices and the physicochemical properties of the explosive production mixtures faded. Distinct source zones of explosives generally contain solid phase material, although for TNT production, the presence of mono-oil and bi-oil material as a significant source material has been speculated. Explosive debris distributed on or in surface soils by detonations is emerging as a potential source material at military training and testing ranges. How chemical explosives might be distributed in the five hydrogeologic settings described earlier is difficult to determine at this time. The presence of reprecipitated solid phase explosive compounds in granular media is suspected, based on phase partitioning laws and maximum aqueous phase limits. But the dynamics of explosive material reprecipitation, dissolution, and transport that would define source zone architecture are not well understood. In addition, no work has been performed to understand the miscible/immiscible flow characteristics of production process wastes in drown tanks or disposed of in surface impoundments. SUMMARY This chapter has outlined important physical and chemical features of contaminated sites that should be understood (or at a minimum discussed) prior to any site remediation. First and foremost, it is imperative to be able to categorize the hydrogeologic setting of a site, as this plays a significant role in determining the overall subsurface distribution of contamination. Furthermore, the existing hydrogeologic setting limits both the types of tools that can be used to characterize the source zone and the technologies that might achieve reductions in source mass. Given the combination of heterogeneity in hydrogeology and in physical-chemical properties, complex sites are the norm rather than the exception. Chlorinated solvents that exist as DNAPLs in the subsurface are the primary concern of the Army and many other potentially responsible parties, and thus constitute the major focus of this report. Among the many distinguishing features of DNAPL sites is the fact that the distribution of DNAPL in the subsurface is typically sparse and highly heterogeneous (depending on the site hydrogeology). Furthermore, depending on a site’s porosity, permeability, and sorption capacity, a substantial portion of the contaminant mass that might have been released to the

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Contaminants in the Subsurface: Source Zone Assessment and Remediation subsurface as a DNAPL may transition into stagnant zones as either sorbed or dissolved phase contamination. These sources have the potential to be a chronic supply of contamination to groundwater plumes. In comparison to DNAPLs, the state of the art for explosives source zone characterization is quite immature. Most explosives are released to the environment as aqueous mixtures, from which chemical explosives precipitate out. The source zone architecture created by production process discharges, manufacturing process discharges, and military training and testing operations requires scientific investigation before remediation technologies can be considered, designed, and deployed with confidence. In addition, an important constraint not found with DNAPLs is explosives safety. Management of detonation hazards, especially the drilling and handling of source material, will require additional resources and technologies. Even though five hydrogeologic settings are discussed in the chapter, there are many more than five typical contaminant distributions. A site’s contaminant distribution will be influenced by transformation and transport processes, by the nature of the contaminant release, and by the hydrogeologic setting (or combination of settings). Thus, this chapter should not be viewed as a cookbook for how to categorize sites and determine their contaminant distribution. Rather, source characterization is necessary, as discussed in the next chapter. REFERENCES Adamson, A. W., and A. P. Gast. 1997. Physical Chemistry of Surfaces, 6th ed. New York: Wiley. Ahmad, F, and J. B. Hughes. 2000. Anaerobic Transformation of TNT by Clostridium. In: Biodegradation of Nitroaromatic Compounds and Explosives. J. C. Spain, J. B. Hughes, and H. Knackmuss (eds.). Boca Raton, FL: Lewis Publishers. Anderson, W. G. 1987. Wettability literature survey—part 4: effects of wettability on capillary pressure. J. Pet. Technol. 39:1283–1300. Back, W., J. S. Rosenshein, and P. R. Seaber. 1988. Hydrogeology—The Geology of North America Volume O-2. Boulder, CO: Geological Society of America. Ball, W. P., C. Liu, G. Xia, and D. F. Young. 1997. A diffusion-based interpretation of tetrachloroethene and trichloroethene concentration profiles in a groundwater aquitard. Water Resources Research 33(12):2741–2758. Bradford, S. A., R. Vendlinski, and L. M. Abriola. 1999. The entrapment and long-term dissolution of tetrachloroethylene in fractional wettability porous media. Water Resources Research 35(10):2955–2964. Brewster, M. L., A. P. Annan, J. P. Grenhouse, B. H. Kueper, G. R. Olhoeft, J. D. Redman, and K. A. Sander. 1995. Observed migration of a controlled DNAPL release by geophysical methods. Groundwater 33(6):977–987. Cohen, R. M., J. W. Mercer, and J. Matthews. 1993. DNAPL Site Evaluation. C. K. Smoley (ed.). Boca Raton, FL: CRC Press. Conrad, S. H., E. F. Hagan, and J. L. Wilson. 1987. Why are residual saturation of organic liquids different above and below the water table? In: Proceedings—Petroleum Hydrocarbons and Organic Chemicals in Groundwater: Prevention, Detection and Restoration. Worthington, OH: National Water Well Association.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Cope, N., and J. B. Hughes. 2001. Biologically-enhanced removal of PCE from NAPL source zones. Environ. Sci. Tech. 35(10):2014–2021. De la Paz, T., and T. Zondlo. 2003. DNAPLs in Karst the Redstone Experience. Presentation to the Committee on Source Removal of Contaminants in the Subsurface. January 30, 2003, San Antonio, Texas. DeLaney, J. E., M. Hollander, H. Q. Dinh, W. Davis, J. C. Pennington, S. Taylor, and C. A. Hayes. 2003. Characterization of explosives residues from controlled detonations: low-order detonations. Chapter 5, Distribution and fate of energetics on DoD test and training ranges: Report 3, ERDC TR-03-2. Vicksburg, MS: U. S. Army Engineer Research and Development Center, Environmental Laboratory. Domenico, P. A., and F. W. Schwartz. 1998. Physical and Chemical Hydrogeology, 2nd ed. New York: John Wiley & Sons. Environmental Protection Agency (EPA). 1992. Evaluation of Ground-Water Extraction Remedies: Phase II, Volume I—Summary Report. Publication 9355.4-05. Washington, DC: EPA Office of Emergency and Remedial Response. Fortner, J. D, C. Zhang, J. C. Spain, and J. B. Hughes. 2003. Soil Column Evaluation of Factors Controlling Biodegradation of DNT in the Vadose Zone. Environ. Sci. Technol. 37(15):3382–3391. Freeze, R. A., and J. A. Cherry. 1979. Groundwater. New Jersey: Prentice-Hall. Hawari, J. 2000. Biodegradation of RDX and HMX: from basic research to field application. In: Biodegradation of Nitroaromatic Compounds and Explosives. J. C. Spain, J. B. Hughes, and H. Knackmuss (eds.). Boca Raton, FL: Lewis Publishers. Hiemenz, P. C., and R. Rajagopalan. 1997. Principles of Colloid and Surface Chemistry, 3rd ed. New York: Marcel Dekker. Huang, W. L., and W. J. Weber. 1998. A distributed reactivity model for sorption by soils and sediments. 11. Slow concentration dependent sorption rates. Environ. Sci Technol. 32(22):3549–3555. Jenkins, T. F., J. C. Pennington, T. A. Ranney, T. E. Berry, P. H. Miyares, M. E. Walsh, A. D. Hewitt, N. M. Perron, L. V. Parker, C. A. Hayes, and E. G. Wahlgren. 2001. Characterization of Explosives Contamination at Military Firing Ranges. ERDC TR-01-5. Vicksburg, MS: U. S. Army Engineer Research and Development Center. Kaye, S. M. 1980. Encyclopedia of Explosives and Related Items. PATR 2700, Volume 9. Dover, New Jersey: U.S. Army Armament Research and Development Command, Large Caliber, Weapon Systems Laboratory. Kueper, B. H., and D. B. McWhorter. 1991. The behavior of dense nonaqueous phase liquids in fractured clay and rock. Journal of Ground Water 29(5):716–728. Kueper, B. H., D. Redman, R. C. Starr, S. Reitsma, and M. Mah. 1993. A field experiment to study the behavior of tetrachloroethylene below the water table: spatial distribution of pooled DNAPL. Groundwater 31:756–766. Kueper, B. H., and E. O. Frind. 1991a. Two phase flow in heterogeneous porous media. 1. Model development. Water Resources Research 27(6):1049–1057. Kueper, B. H., and E. O. Frind. 1991b. Two phase flow in heterogeneous porous media. 2. Model application. Water Resources Research 27(6):1058–1070. Lemke, L. D., L. M. Abriola, and J. R. Lang. 2004. DNAPL source zone remediation: Influence of hydraulic property correlation on predicted source zone architecture, DNAPL recovery, and contaminant mass flux. Water Resources Research. In press. Lewis, J., S. Thiboutot, G. Ampleman, S. Brochu, P. Brousseau, J. C. Pennington, and T. A. Ranney. 2003. Open detonation of military munitions on snow: An investigation of energetic material residues produced. Chapter 4, Distribution and fate of energetics on DoD test and training ranges: Report 3, ERDC TR-03-2. Vicksburg, MS: U.S. Army Engineer Research and Development Center, Environmental Laboratory.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Liu, C., and W. P. Ball. 1998. Analytical modeling of diffusion-limited contamination and decontamination in a two-layer porous medium. Advances in Water Resources 24(4):297–313. Liu, C., and W. P. Ball. 2002. Back diffusion of chlorinated solvents from a natural aquitard to a remediated aquifer under well-controlled field conditions: predictions and measurements. Journal of Groundwater 40(2):175–184. Mackay, D. M., W. Y. Shiu, and K. C. Ma. 1993. Illustrated Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals, Vol. III. Chelsea, MI: Lewis Publishers. Mackay, D. M., R. D. Wilson, M. P. Brown, W. P. Ball, G. Xia, and D. P. Durfee. 2000. A controlled field evaluation of continuous versus pulsed pump-and-treat remediation of a VOC-contaminated aquifer: site characterization, experimental setup, and overview of results. Journal of Contaminant Hydrology 41:81–131. Meinardus, H. W., V. Dwarakanath, J. Ewing, G. J. Hirasaki, R. E. Jackson, M. Jin, J. S. Ginn, J. T. Londergan, C. A. Miller, and G. A. Pope. 2002. Performance assessment of NAPL remediation in heterogeneous alluvium. Journal of Contaminant Hydrology 54:173–193. Mercer, J. W., and R. M. Cohen. 1990. A review of immiscible fluids in the subsurface: properties, models, characterization and remediation. Journal of Contaminant Hydrology 6:107–163. Montgomery, J. H. 2000. Groundwater Chemicals, 3rd ed. Boca Raton, FL: Lewis Publishers and CRC Press. Morrow, N. R. 1976. Capillary-pressure correlations for uniformly wetted porous media. Journal of Canadian Petroleum Technology 15(4):49–69. O’Carroll, D. M., S. A. Bradford, and L. M. Abriola. 2004. Infiltration of PCE in a system containing spatial wettability variations. Journal of Contaminant Hydrology 73:39-69. Pankow, J. F., and J. A. Cherry (eds.). 1996. Dense Chlorinated Solvents and other DNAPLs in Groundwater. Portland, OR: Waterloo Press. Parker, B. L., D. B, McWhorter, and J. A. Cherry. 1997. Diffusive loss of non-aqueous phase organic solvents from idealized fracture networks in geologic media. Ground Water 35(6):1077–1088. Parker, B. L., J. A. Cherry, and R. W. Gillham. 1996. The effect of molecular diffusion on dnapl behavior in fractured porous media. Chapter 12 In: Dense Chlorinated Solvents and Other DNAPLs in Groundwater. J. F. Pankow and J. A. Cherry (eds.). Portland, OR: Waterloo Press. Parker, B. L., R. W. Gillham, and J. A. Cherry. 1994. Diffusive disappearance of immiscible–phase organic liquids in fractured geologic media. Journal of Groundwater 32(5):805–820. Persurance, R. 1974. Explosion Hazard Classification of Drowning Tank Material from TNT Manufacturing Process. Picatinny Arsenal Technical Report 4613. Dover, NJ. Poulsen, M. M., and B. H. Kueper. 1992. A field experiment to study the behavior of tetrachloroethylene in unsaturated porous media. Environ. Sci. Technol. 26(5):889–895. Powers, S. E., L. M. Abriola, and W. J. Weber, Jr. 1994. An experimental investigation of NAPL dissolution in saturated subsurface systems: transient mass transfer rates. Water Resources Research 30(2):321–332. Powers, S. E., W. H. Anckner, and T. F. Seacord. 1996. Wettability of NAPL-contaminated sands. Journal of Environmental Engineering 122:889–896. 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 Research 28(10):2691–2705. Press, F., and R. Siever. 1974. Earth. San Francisco, CA: W. H. Freeman and Company. Rathfelder, K. M., L. M. Abriola, M. A. Singletary, and K. D. Pennell. 2003. Influence of surfactant-facilitated interfacial tension reduction on organic liquid migration in porous media: observations and numerical simulation. J. Contaminant Hydrology 64(3-4):227–252. Robertson, B. K., and M. Alexander. 1996. Mitigating toxicity to permit bioremediation of constituents of nonaqueous-phase liquids. Environ. Sci. Tech. 30:2066–2070.

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Contaminants in the Subsurface: Source Zone Assessment and Remediation Rosenblatt, D. H., E. P. Burrows, W. K. Mitchell, and D. L. Parmer. 1991. Organic Explosives and Related Compounds. In The Handbook of Environmental Chemistry, Vol 3. G. O. Hutzinger (ed.). Berlin and Heidelberg: Springer-Verlag. Salathiel, R. A.. 1973. Oil recovery by surface film drainage in mixed-wettability rocks. J. Pet. Technol. 25(OCT):1216–1224. Sale, T., T. Illangasekare, F. Marinelli, B. Wilkins, D. Rodriguez and B. Twitchell. 2004. AFCEE Source Zone Initiative—Year One Progress Report. Colorado State University and Colorado School of Mines, Prepared for the Air Force Center for Environmental Excellence. Schwille, F. 1988. Dense Chlorinated Solvents in Porous and Fractured Media. Translated by J. F. Pankow. Chelsea, MI: Lewis Publishers. SERDP. 2003. Annual Report to Congress—Fiscal Year 2002, from the Strategic Environmental Research and Development Program. Arlington, VA: SERDP Program Office. Sudicky, E. A. 1986. A natural gradient experiment on solute transport in a sand aquifer: spatial variability of hydraulic conductivity and its role in the dispersion process. Water Resources Research 22(13):2069–2082. Sudicky, E. A., R. W. Gillham, and E. O. Frind. 1985. Experimental investigations of solute transport in stratified porous media: (1) the non reactive case. Water Resource Research 21(7):1035–1041. Sung, Y., K. M. Ritalahti, R. A. Sanford, J. W. Urbance, S. J. Flynn, J. M. Tiedje, and F. E. Loffler. 2003. Characterization of two tetrachloroethene (PCE)-reducing, acetate-oxidizing anaerobic bacteria, and their description as Desulfuromonas michiganensis sp. nov. Applied and Environmental Microbiology 69:2694–2974. Teutsch, G., and M. Sauter. 1991. Groundwater modeling in karst terranes: scale effects, data acquisition and field validation. Pp. 17–35 In: Proceedings of the Third Conference on Hydrogeology, Ecology, Monitoring, and Management of Ground Water in Karst Terranes, Nashville, TN. Thorne, P. G., and D. C. Leggett. 1999. Investigations of explosives and their conjugated transformation products in biotreatment matrices. Special Report 99-3. U.S. Army Corps of Engineers, Cold Regions Research and Engineering Laboratory. Hanover, NH: Army Corps of Engineers. Urbanski, T. 1967a. Chemistry and Technology of Explosives, Vol. 1. Oxford: Pergamon Press. Urbanski, T. 1967b. Chemistry and Technology of Explosives, Vol. 3. Oxford: Pergamon Press. USACHPPM. 2003. Interim Measures Report, Site-Wide Groundwater, Area B (Explosives Production Area), 28 May through 13 June 2003, Holston Army Ammunition Plant, Kingsport, Tennessee. Aberdeen Proving Ground, MD: U.S. Army Center for Health Promotion and Preventative Medicine, Ground Water and Solid Waste Program. Weber, W. J., W. L. Huang, and E. J. LeBoeuf. 1999. Geosorbent organic matter and its relationship to the binding and sequestration of organic contaminants. Colloids and Surfaces A-Physicochemical and Engineering Aspects 151 (1–2):167–17. White, W. B. 2002. Karst hydrology: recent developments and open questions. Engineering Geology 65(2–3):85–105 White, W. B. 1998. Groundwater flow in karstic aquifers. Pp. 18–36 In: The Handbook of Groundwater Engineering. J. W. Delleur (ed.). Boca Raton, FL: CRC Press. Yang, Y., and P. L. McCarty. 2000. Biologically enhanced dissolution of tetrachloroethene DNAPL. Environ. Sci. Tech. 34(14):2979–2984. Yang, Y., and P. L. McCarty. 2002. Comparison between donor substrates for biologically enhanced tetrachloroethene DNAPL dissolution. Environ. Sci. Tech. 36:3400–3404.