Contaminants in the Subsurface Source Zone Assessment and Remediation (2005) / Chapter Skim
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2 Source Zones
2 Source Zones
Pages 34-78
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From page 34... ...
Within these subsurface regions, nonaqueous, 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.
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From page 35... ...
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)
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From page 36... ...
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 trans mission is commonly secondary permeability.
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From page 37... ...
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.
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From page 38... ...
(m/sec) 10-7 1 10-8 10-1 Gravel 10-9 10-2 10-10 10-3 Basalt 10-11 10-4 Sand Limestone Clean Sand 10-12 10-5 Karst Silty rock ransmissiveT 10-13 10-6 Fractured 10-14 10-7 crystalline Dolomite 10-15 10-8 Limestone illT Sandstone 10-16 10-9 Glacial 10-17 10-10 clay 10-18 10-11 marine Unweathered Shale Rock 10-19 10-12 Unfractured Crystalline 10-20 10-13 FIGURE 2-2 Permeability and hydraulic conductivity for common geologic media.
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From page 39... ...
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)
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From page 40... ...
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)
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From page 41... ...
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.
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From page 42... ...
42 CONTAMINANTS IN THE SUBSURFACE 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.
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From page 43... ...
Fractured media with high matrix porosity are commonly encountered in sedimentary rock (e.g., limestone, dolomite, shale, and sandstone) and fractured clays.
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From page 44... ...
© 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.
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From page 45... ...
In the face of these complexities, the Army was nonetheless asked to estimate the percentage of its DNAPL sites that exist in the five hydrogeologic settings, for the purposes of providing context to the above discussion. Of the Army's 43 active and BRAC (base realignment and closure)
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From page 46... ...
© 2004 The Geology of Virginia, Department of Geology, College of William and Mary. DNAPLS How chlorinated solvents are distributed in the subsurface depends on the particular hydrogeologic setting, described above; on the chemical and physical properties of the solvents, the amount, mode, and timing of initial release of the solvents, and their fate and transport processes in the subsurface; and on human activities that may subsequently alter the source zone architecture (such as excavation)
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From page 47... ...
47 Law 4 atm 3 /mol) 3 9.1 3.4 1.1 3.4 2.0 0.46 3.4×10 2.7 0.34 Henry's Coefficient (10 m 15 15 15 30 ow viscosity.
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From page 48... ...
. Thus, it is important to make a distinction between a chemical component of a DNAPL, which may be denser or lighter than water in its pure liquid form, and the DNAPL itself, which is a separate phase organic liquid composed of various chemical species.
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From page 49... ...
for pure phase organic liquids. However, interfacial tension can be significantly affected by co-contaminants or additives in the DNAPL phase, including organic acids and bases and surfactants, and by dissolved pore water constituents, such as natural humic substances.
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From page 50... ...
or through the interaction of the released NAPLs with the solids. For example, contact with NAPL mixtures containing surface-active constituents can render a porous medium intermediate- to organicwet (e.g., Powers et al., 1996)
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From page 51... ...
Although capillary forces are well understood for simple pore geometry (e.g., cylindrical pores) , the complex pore structure of natural porous media makes precise predictions of interface positions difficult.
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From page 52... ...
Thus, DNAPL often follows a highly irregular path, resulting in a source zone that contains narrow vertical pathways connected to thin, laterally extensive horizontal lenses. This will be particularly true of a Type III geologic setting that contains persistent finer-textured horizontal layers of high permeability contrast or of Type IV or V media with extensive horizontal fractures.This could also be characteristic of a Type I setting, if fine-scale layering were present.
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From page 53... ...
, but the resident pore (wetting) fluid includes a surfactant that has lowered the interfacial tension between the PCE and aqueous phases (from 47.8 to 0.5 dynes/cm)
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From page 54... ...
54 CONTAMINANTS IN THE SUBSURFACE FIGURE 2-12 Effect of wettability on PCE (dark dye) migration in a sand box.
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From page 55... ...
. Local maximum residual saturations of DNAPLs measured in field-scale and laboratory experiments are typically in the range of 10 percent to 35 percent in saturated, unconsolidated media, with levels as high as 50 percent in materials of low permeability (Conrad et al., 1987; Schwille, 1988)
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From page 56... ...
Local DNAPL dissolution has been the subject of much investigation. Researchers have found that the rates of dissolution are controlled by a number of variables, including the solubility of the DNAPL constituents, the local groundwater velocity, the textural heterogeneity of the geologic media, the wettability characteristics of the solid, and the saturation of the DNAPL (e.g., Powers et al., 1992, 1994; Bradford et al., 1999)
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From page 57... ...
Sorbed contamination is the organic mass that is associated with the solid matrix material. For organic compounds, the sorption capacity is generally related to the fraction and character of the solid phase organic carbon, the surface area of the solids, and the compound's octanolwater partition coefficient (Kow)
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From page 58... ...
The DNAPL mass will be distributed within both residual ganglia and more saturated pools. During or immediately after a release, DNAPL will be the largest component of contaminant mass in the source zone.
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From page 59... ...
The tendency for the source to exist as DNAPL, sorbed, or dissolved phase mass is discussed below for each of the five hydrogeologic settings introduced earlier. Type I Settings In instances of granular media with mild heterogeneity and moderate to high permeability, all of the source zone can be viewed as transmissive.
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From page 60... ...
Type IV Settings Type IV settings involve fractured media with low matrix porosity, such that the primary space in which contaminants can be stored are fractures. Critical attributes of fracture networks are that they typically represent a small fraction of
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From page 61... ...
In (B) , the 2`and 4` fluxes are back diffusion into the transmissive zone post DNAPL dissolution.
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From page 62... ...
Because matrix porosity in this hydrogeologic setting is low, little if any contaminant is stored as sorbed or stagnant dissolved phase mass in the source zone. However, in instances where the fracture networks are poorly connected, a subset of the fractures may behave as stagnant zones, and DNAPLs in dead-end fractures may act as persistent sources of dissolved phase contaminants that are difficult to remediate.
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From page 63... ...
Upon reaching the upper fractured clay layer, the DNAPL continues to migrate downward through secondary permeability features. Large aqueous phase concentration gradients would likely drive contaminants into the clay matrix via diffusion (Parker et al., 1994, 1997)
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From page 64... ...
is common in many hydrogeologic settings. This phenomenon, which is shown schematically in Figure 2-17, may support the long-term contamination of groundwater.
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From page 65... ...
Therefore, not all groundwater contaminant plumes imply the presence of a source zone. CHEMICAL EXPLOSIVES The chemical explosives that have caused the greatest environmental impact, and which are of greatest concern to the Army, are the organic explosives 2,4,6trinitrotoluene (TNT)
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From page 66... ...
However, much less is known about how chemical explosive source material interacts with soil compared to chlorinated solvents. Hence, this section primarily describes the nature of the contaminant releases, highlighting several unique features that make sites contaminated with these compounds particularly challenging to remediate.
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From page 67... ...
military training and testing operations. More explosives problems are derived from historic production process discharges and from manufacturing processes (milling/machining or in demilitarization of ordnance)
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From page 68... ...
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)
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From page 69... ...
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)
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From page 70... ...
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.
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From page 71... ...
. Manufacturing Process Discharges Manufacturing processes are defined here as post-production operations that operate with the solid phase explosive material.
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From page 72... ...
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.
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From page 73... ...
Natural attenuation mechanisms favor the loss of TNT>DNT>RDX>HMX in the environment. TNT sorption and aerobic degradation provides for continuous elimination reactions.
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From page 74... ...
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.
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From page 75... ...
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.
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From page 76... ...
2004. DNAPL source zone remediation: Influence of hydraulic property correlation on predicted source zone architecture, DNAPL recovery, and contaminant mass flux.
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From page 77... ...
1997. Diffusive loss of non-aqueous phase organic solvents from idealized fracture networks in geologic media.
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From page 78... ...
1986. A natural gradient experiment on solute transport in a sand aquifer: spatial variability of hydraulic conductivity and its role in the dispersion process.
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