Groundwater and Soil Cleanup Improving Management of Persistent Contaminants (1999) / Chapter Skim
Currently Skimming:
4 DNAPLs: Technologies for Characterization, Remediation, and Containment
4 DNAPLs: Technologies for Characterization, Remediation, and Containment
Pages 129-201
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 129... ...
components, including polychlorinated biphenyls (PCBs) , also may be present, but chlorinated solvents are by the far most prevalent (see Table 1-4~.
|
From page 130... ...
The chlorinated solvents that comprise the most common DNAPL components are denser than water (Table 4-1) and tend to sink beneath the water table.
|
From page 131... ...
in Different Media Medium Required Entry Pressure (cm of TCE) Clean sand (K = 1 x 10-2 cm/see)
|
From page 132... ...
A plume of dissolved contaminants, known as the dissolved-phase plume, will form when groundwater contacts either residual saturation, DNAPL pools, or DNAPL lenses. The volume of soil that contains DNAPL at or above residual saturation is termed the source zone.
|
From page 133... ...
are primarily for treating source zones, whereas others are for plumes of dissolved contaminants. CHARACTERIZATION OF DNAPL CONTAMINATION The presence of DNAPL makes site characterization both more critical and more difficult due to the irregular distribution of DNAPLs.
|
From page 134... ...
134 o o u o cry ¢ 5o V)
|
From page 135... ...
135 | D , I , D ~ ~ I, ~= o c ~ ~ ~ ~° ~ E o u ~3 ~ ~ u E ~c ~° tic C ~° ~ E ¢ ~ U ~ U o .
|
From page 137... ...
Although hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs) , generally fluoresce, most chlorinated solvents do not.
|
From page 138... ...
At this time, there are no well-documented successes in locating DNAPL pools using seismic methods, although research is continuing.3 Ground Penetrating Radar (GPR) GPR has a proven capability to define shallow stratigraphy.
|
From page 139... ...
Tracer tests require a very large number of high-quality analyses; if DNAPL saturation is low, very precise data are needed over an extended time period. This technology has proven invaluable at several dozen sites for determining the quantity of DNAPL present and for evaluating the performance of remediation technologies in field trials through tests conducted before and after treatment.
|
From page 140... ...
As the air flows through the contaminated soils, the portion of volatile compounds present in the vapor phase flows toward the well and is removed through the well along with the extracted air. The volatile compounds associated with the soils (either adsorbed or dissolved in the soil moisture)
|
From page 141... ...
Bioventing is discussed in more detail in the bioremediation section of this chapter. Groundwater pumping may be used to lower the water table and thus increase the depth to which SVE may be applied.
|
From page 142... ...
Due to kinetic effects, actual vapor-phase concentrations may be lower than predicted from equilibrium calculations. The vapor pressure of a compound is a function of temperature: increasing the temperature increases vapor pressure, the partitioning of a compound into the vapor phase, and thus SVE efficiency.
|
From page 143... ...
As with vertical wells, various combinations of air extraction and air injection can be used. A recent innovation for lightly contaminated soil uses the daily changes in barometric pressure to induce air flow through wells.
|
From page 144... ...
For coarser-grained soils from which the water would drain fairly rapidly, large volumes of water must be recovered to lower the groundwater table sufficiently to have any benefit, and the recovered groundwater will require treatment in most cases. One other modification that has become widely used, although developed primarily for petroleum hydrocarbons, is high-vacuumenhanced vapor recovery.
|
From page 145... ...
Performance Generally, SVE has been most successful for treating volatile compounds in moderately to highly permeable soils. SVE systems have been applied at a large number of sites contaminated with chlorinated solvents.
|
From page 146... ...
Frequently, VOCs will accumulate preferentially on the surface of and within clay lenses and layers. Air flow will then be minimal in the most highly contaminated soils.
|
From page 147... ...
As the injected air moves through a formation and comes in contact with NAPLs, contaminated soil, or water containing dissolved-phase contamination, the volatile contaminants partition into the air. Partitioning from the dissolved phase is described by a compound's Henry's law constant; partitioning from DNAPLs is described by its vapor pressure.
|
From page 148... ...
Since DNAPLs at a contaminated site are found in discontinuous ganglia in the saturated zone and in low spots overlying less permeable zones in or at the bottom of aquifers, achieving a uniform air flow through the entire source zone may be difficult. Air moves through saturated media by a complex process.
|
From page 149... ...
In these cases, both volatilization and enhanced biodegradation are important processes. In addition, because fuel is less dense than water, fuel source zones and groundwater plumes tend to occur near the water table.
|
From page 150... ...
At the Savannah River Site, DOE conducted a field demonstration of air sparging on a site contaminated with chlorinated solvents that had leaked from an unlined sediment basin. During the demonstration, the air sparging process increased the recovery of VOCs from 49.4 kg (109 lb)
|
From page 151... ...
SOURCE: EPA, 1995b. the DNAPL source zone to remove DNAPL by dissolution and/or mobilization (see Figure 4-4~.
|
From page 152... ...
In one trial at Hill, researchers from EPA and the University of Florida tested a system that pumped approximately 10 pore volumes of a mixture of 70 percent ethanol and 12 percent pentanol in water through soil in a 3 x 5 m sheet-piling cell using a line drive array of injection and extraction wells. The contaminant treated was an LNAPL consisting of a complex mixture of weathered jet fuel and other components.
|
From page 153... ...
It is likely that these technologies would be more effective for chlorinated solvents and other easily dissolved DNAPLs such as those typically found at DOE sites. Limitations For any flooding technology to be effective, the entire contaminated volume of soil must be effectively flushed with treatment solutions; for solvent flooding, multiple pore volumes must be circulated.
|
From page 154... ...
Advantages The chemical principles of these systems are relatively simple when treating chlorinated solvents. Alcohols are effective solvents and are not sorbed significantly.
|
From page 155... ...
Increases in solubility of more than five orders of magnitude and solubilities of hundreds of thousands of milligrams per liter have been reported for common DNAPL components (Baran et al., 1994~. Early field trials used surfactants that produced modest increases in solubility (one or two orders of magnitude)
|
From page 156... ...
Application A typical system involves arrays of injection and extraction wells designed to sweep the DNAPL source zone. Hydraulic controls or containment walls contain the surfactant solution.
|
From page 157... ...
Approximately nine pore volumes of a mixture of 3 percent surfactant and 2.5 percent pentanol in water were pumped through soil within a 3 x 5 m sheet-piling cell using a line drive array of injection and extraction wells. The contaminant was an LNAPL consisting of a complex mixture of weathered jet fuel and other components.
|
From page 158... ...
· The University of Buffalo and DuPont tested surfactant flood of a chlorinated solvent DNAPL in 1991-1993. A total of 12.5 pore volumes of 1 percent surfactant in water were injected using six injection wells around the treatment zone and two extraction wells near the center.
|
From page 159... ...
The persistence of some NAPL in every test conducted suggests that heterogeneities will inevitably result in some contamination remaining after treatment, although the level may be minimal and may be suitable for treatment by natural attenuation. Advantages Surfactant-enhanced aquifer remediation systems can rapidly remove mass from DNAPL source zones and remove DNAPLs nearly completely from relatively homogeneous units of moderate to high permeability.
|
From page 160... ...
Application The reaction of potassium permanganate or hydrogen peroxide injected in source zones (with or without ferrous iron as a catalyst) with DNAPLs yields carbon dioxide and water, plus chloride and other by-products.
|
From page 161... ...
Performance A small test cell in the unconsolidated sands of Canadian Forces Base Borden was contaminated with TCE and PCE and was flushed with potassium permanganate at a concentration of 30 g/liter in a test conducted by the Solvents in Groundwater Program of the University of Waterloo. The system injected six pore volumes of permanganate, followed by clean water.
|
From page 162... ...
Advantages The initial results from Base Borden and Portsmouth suggest that potassium permanganate has considerable potential for effective destruction of PCE and ICE. Fenton's reagent has long been known to oxidize common chlorinated compounds if it can be delivered to the source zone before it degrades, and at the proper pH it is also an effective oxidizer.
|
From page 163... ...
(The use of electrical heating as a stand-alone treatment method is described in the next section.) Physical and Chemical Principles Steam injection promotes contaminant recovery through several mechanisms.
|
From page 164... ...
64 E ._ ._ o ~ .= at: o · ~ ~ I-: !
|
From page 165... ...
The water table was about 30-37 m (100-120 ft) below ground surface.
|
From page 166... ...
If the temperature increases to the boiling point of water, most common DNAPL components will partition strongly into the vapor phase and can be removed through vapor extraction. Heating the vapors also increases vapor flow, as vaporization of the pore water and contaminants increases vapor pressure, promoting vapor displacement.
|
From page 167... ...
· Six-phase soil heating. Six-phase soil heating is a variant of electrical resistance heating, differing in the way the alternating current is applied to the soil.
|
From page 168... ...
At sufficiently high temperature, volatile and semivolatile compounds will volatilize, and water will be driven off as steam. Most chlorinated solvent DNAPLs are volatile enough that heating groundwater to boiling temperatures should drive off the DNAPL as a vapor phase.
|
From page 169... ...
. The increased vapor pressures due to heating and the creation of a low-pressure zone in the overlying hood for the system cause some organic vapors to migrate to the hood, where they are captured and treated.
|
From page 170... ...
Volatilized compounds will move away from the melt because heating of the soil vapors increases the vapor pressure adjacent to the melt. The Geosafe, Inc.
|
From page 171... ...
REMEDIATION TECHNOLOGIES FOR PLUMES OF DISSOLVED DNAPL CONTAMINANTS Treatment of plumes of contaminants dissolved from DNAPLs generally poses less of a technical challenge than treatment of undissolved DNAPLs because of the increased mobility of dissolved-phase contaminants. The treatment methods described in the remainder of this chapter apply primarily to dissolved-phase organic contaminants.
|
From page 172... ...
The application of electrokinetics for treatment of organic contaminants is the objective of the LASAGNA process. Performance The only documented field trials of electrokinetic systems to treat DNAPL employed the LASAGNA process at DOE's Paducah Gaseous Diffusion Plant.
|
From page 173... ...
At many DNAPL-contaminated sites, the DNAPL is composed of one or more common chlorinated solvents, while the dissolved-phase plume emanating from the DNAPL source zone often contains additional compounds that are metabolites of the chlorinated solvents. Common examples include the metabolites of PCE and TCE: dichloroethenes and vinyl chloride.
|
From page 174... ...
~ ~ ~ ~ ~8 =4 _ ~ _ ~ ~ H ~ ~ ~ ~ ~ ~ ~ N ~ ~ .a ~ .~ Native Microorganisms Ground Water to Treatment (optional) Surface SoiVCap Unsaturated Zone Saturated Zone FIGURE 4-8 Process diagram for in situ bioremediation (batch or continuous addition)
|
From page 175... ...
Although aerobic degradation reactions (in which oxygen acts as the electron acceptor) are highly effective at remediation of hydrocarbons as well as some less chlorinated solvents and metabolites of chlorinated solvents, most DNAPL components resist aerobic degradation (NRC, 1993~.
|
From page 176... ...
and vinyl chloride can accumulate. Reductive dechlorination of these compounds requires stronger reducing conditions.
|
From page 177... ...
Application In practice, in situ bioremediation is implemented by introducing nutrients, typically nitrogen and/or phosphorus sources, air or other sources of oxygen (such as pure oxygen or hydrogen peroxide) , and easily degraded organic substrates that can serve as a source of energy for the indigenous microorganisms.
|
From page 178... ...
Several vendors have claimed success in inducing reductive dechlorination by adding molasses, but little peer-reviewed information is available on field trials and commercial implementations. A field demonstration using the combined technologies of aerobic in situ bioremediation and SVE was conducted at DOE's Savannah River Site (Brockman et al., 1995; Hazen et al., 1995; Federal Remediation Technologies Roundtable, 1997~.
|
From page 179... ...
One limitation is the fact that because biodegradation occurs only in the aqueous phase, it is not suitable for direct remediation of free-phase DNAPL sources. In addition, the dechlorination of highly chlorinated hydrocarbons produces metabolites that, if not themselves degraded, are more mobile and more toxic than the original compound.
|
From page 180... ...
Physical, Chemical, and Biological Principles Studies have confirmed that certain plant species can take up chlorinated solvents from groundwater in the root zone (Chappell, 1997; Schnoor, 1997~. Once the plant takes up the solvent, it may
|
From page 181... ...
Poplars can extend their roots to the water table, and research studies show that a grove of poplars can create a depression in the water table ranging from several inches to several feet (Chappell, 1997~. The rate at which trees pump water depends on the number of trees, tree age, time of day, season, amount of sunlight, climate, and geographic location.
|
From page 182... ...
Permeable Reactive Barriers Description Permeable reactive barriers for groundwater remediation consist of subsurface units constructed of permeable reactive media placed to intercept the contaminated groundwater. As groundwater flows through the reactive media, dissolved contaminants are either immobilized or transformed into a more environmentally acceptable form.
|
From page 183... ...
Physical and Chemical Principles Reactive barriers containing granular zero-valent iron are being used to degrade chlorinated hydrocarbons, the most common DNAPL components at DOE sites, by the process of reductive dechlorination (Gilham and O'Hannesin, 1994~. The metal serves as a source of electrons for the reduction step, which removes chlorine atoms from the hydrocarbons and releases chloride and ferrous iron into solution.
|
From page 184... ...
It has been commonly used in water and wastewater treatment and recently was applied successfully in a field demonstration of in situ remediation of DNAPL compounds (Siegrist et al., 1997~. Potassium permanganate, a purple-colored solid crystal at room temperature, readily dissolves in water.
|
From page 185... ...
are readily oxidized by potassium permanganate (Gates et al., 1995~. Potassium permanganate can be mixed with grout and injected or otherwise emplaced into the subsurface to form horizontal or vertical reactive barriers (Siegrist et al., 1997~.
|
From page 188... ...
Such technologies are not DNAPL remediation technologies but may be used to reduce the spread of contamination or to allow aggressive source zone remediation within the wall. These technologies are summarized in Chapter 3.
|
From page 189... ...
For chlorinated organic contaminants, natural attenuation evaluations generally focus on biodegradation since this is almost always the primary process responsible for reducing contaminant mass. Until relatively recently, scientists believed that chlorinated organic compounds were generally highly resistant to biodegradation in the environment, but in the past two decades a variety of biological processes have been discovered that can transform these compounds in nature (for review articles, see Semprini, 1997a,b)
|
From page 190... ...
. In addition, the Air Force has a detailed technical protocol for investigating natural attenuation of chlorinated solvents that is now widely used to guide studies of natural attenuation at non-Air Force sites (Wiedemeier et al., 1997)
|
From page 191... ...
In addition, the biological reactions responsible for attenuation of chlorinated solvents generally require the presence of other organic compounds to serve as electron donors or primary substrates; biodegradation will not occur in the absence of these other substances. Another limitation is that some transformation products that result during natural attenuation, such as vinyl chloride, are more harmful than the original contaminants and may accumulate at the site.
|
From page 192... ...
In some cases, this uneven treatment is acceptable if natural attenuation rates are sufficient to control contaminants in less permeable zones following treatment of the permeable zones. The slow movement of groundwater in less permeable soils also can result in reagents being spent before they penetrate into the formation.
|
From page 193... ...
Other technologies have demonstrated the ability to clean up contaminants that have dissolved from these source zones. Following are brief summaries of the demonstrated capabilities of the technologies reviewed in this chapter: · Soil vapor extraction is effective for mass removal of volatile compounds in homogeneous, permeable soils and, with the addition of thermal processes, can be extended to semivolatile compounds.
|
From page 194... ...
Degradation of DNAPL source zones may require an extended time. · In situ vitrification has demonstrated the ability to vitrify soil and produce temperatures that should lead to the destruction or mobilization of DNAPL compounds.
|
From page 195... ...
A successful demonstration of surfactant flooding of an alluvial aquifer contaminated with DNAPL. Submitted as a research communication to Environmental Science and Technology.
|
From page 196... ...
Pp. 81-86 in Natural Attenuation: Chlorinated and Recalcitrant Compounds, E
|
From page 197... ...
1998. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater.
|
From page 198... ...
1994. Enhanced reductive dechlorination of chlorinated ethenes.
|
From page 199... ...
1998. Sequential anaerobic/ aerobic biodegradation of chlorinated solvents: Pilot-scale field demonstration.
|
From page 200... ...
1996. Dense Chlorinated Solvents and other DNAPLs in Groundwater.
|
From page 201... ...
1997. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.