Hydraulic and earth engineering – Earth treatment or control – Chemical
Reexamination Certificate
1999-06-10
2001-07-17
Bagnell, David (Department: 3673)
Hydraulic and earth engineering
Earth treatment or control
Chemical
C405S128250, C405S128500, C210S747300, C166S245000, C588S261000
Reexamination Certificate
active
06261029
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to methods for remedying subsurface soil contamination. More particularly, the present invention relates to a method for remedying the contamination of a subsurface environment by dense non-aqueous phase liquids (DNAPLs).
The publications and other materials used herein to illuminate the background of the invention, and in particular cases, to provide additional details respecting the practice, are incorporated herein by reference, and for convenience, are referenced by author and date in the following text, and respectively grouped in the appended list of references.
Table of Abbreviations
CF
cosolvent flushing
DERD
density-enhanced remediation of DNAPLs
DNAPL
dense non-aqueous phase liquids
IBD
In situ biodegradation
IFT
interfacial tension
ISGS
in situ gas stripping
LNAPL
less dense non-aqueous phase liquids
NAPL
non-aqueous phase liquids
PAT
pump-and-treat
PCE
perchloroethylene
RW
reactive walls
IS
steam injection
SF
surfactant flushing
1,1,1-TCA
1,1,1-trichloroethane
1,1,2-TCA
1,1,2-trichloroethane
TCE
trichloroethylene
VE
vapor extraction
BACKGROUND ART
Contamination of subsurface environments by fluids that are immiscible with water has occurred routinely in the United States and industrialized countries around the world over the last 40 years. Mercer and Cohen (1990). Such fluids are often termed non-aqueous phase liquids (NAPLs) in general, or LNAPLs when less dense or DNAPLs when more dense than the groundwater present in the subsurface. Typical LNAPLs are petroleum products (e.g., gasoline, diesel fuel, jet fuels, heating oils), and typical DNAPLs are chlorinated solvents (e.g., trichloroethylene, tetrachloroethylene, dichloroethanes). Schwille (1988); Bartow and Davenport (1990).
Species present in such NAPL phases can solubilize to the aqueous phase, volatilize to the gas phase, or sorb to the solid phase present in the subsurface. Environmental concerns result when such species are linked to human or ecological health concerns and become present in sufficient quantities in a mobile aqueous or gas phase. Such health concerns are associated with common constituents of NAPLs found routinely in the environment, such as trichloroethylene and benzene.
Once released into the subsurface, NAPLs migrate and typically reach a stable, immobile state within a relatively short time scale (hours to days) after the source is removed. Immobile NAPLs can remain in the subsurface over time scales that can range from months to decades or longer under natural conditions, because they are comprised of species that are only sparingly soluble in water. Miller, Poirier-McNeill et al. (1990). As a result, NAPLs are considered a long-term source of groundwater contamination. Mayer and Miller (1996).
DNAPLs tend to be an even more significant problem than LNAPLs because of the following characteristics:
(1) they were routinely used in industrial practices, spilled, and intentionally disposed of in the subsurface in the United States, starting in the 1960's and continuing for two decades;
(2) they often migrate larger distances than LNAPLs;
(3) they often penetrate the water table;
(4) they can form pools contained by low-permeability materials;
(5) they are often comprised of species that tend to degrade slowly in many systems;
(6) they are extremely difficult to locate and remove; and
(7) they contain species that are typically regulated at low concentrations in drinking water (e.g., 5 &mgr;g/L).
Because of the above characteristics, remediation of subsurface contamination resulting from DNAPLs is a frequently encountered problem, which has proven to be extremely difficult. Methods that have been used to remove such contamination include: pump-and-treat, cosolvent flushing, surfactant flushing, steam injection, and in situ biodegradation. To varying extents, all of these strategies have been implemented at the laboratory and pilot or full field scale. Each of these methods results in the removal of some solute mass from a system contaminated with a DNAPL, but the rate at which the removal occurs and the expense involved with this standard set of methods leaves the problem of DNAPL remediation unsolved. The advantages and disadvantages of the common set of DNAPL removal and containment strategies are described in detail as follows.
Pump-and-Treat
Pump-and-treat (PAT) is perhaps the most common method of subsurface restoration. The method consists of installing a well in a region contaminated by a DNAPL and pumping it, which results in the induction of a flow of groundwater in all directions toward the extraction well for some local volume in the vicinity of the well called the capture zone. de Marsily (1986); Domenico and Schwartz (1990). As the groundwater passes through a capture zone that is contaminated with DNAPL, a portion of the DNAPL will dissolve into the groundwater and be transported with it. If a PAT scheme is continued for a sufficient period of time, all of the DNAPL present in the capture zone will be solubilized and exit the system through the pumping well. Mayer and Miller (1996). In theory, placement of a sufficient number of adequately designed pumping wells and appropriate operation for a sufficient period of time will result in the removal of all DNAPL contaminants from a contaminated system.
Once these contaminants are brought to the surface from the wells, treatment of the contaminated waste stream is accomplished and the water is either injected back into the subsurface or disposed of using other means. The actual treatment processes used depends upon the characteristics of the waste stream, treatment objectives, discharge stream quality constraints, and other factors.
The PAT method is appealing because of its simplicity. However, experience has shown that this approach is not very efficient, where efficiency is defined as the mass of DNAPL removal per volume of fluids removed. Mackay and Cherry (1989). This lack of efficiency results from the upper bound on the mass removal rate that results from the solubility of the DNAPL in the water phase, and because the water removed from the pumping wells may be far removed from the solubility limit of the DNAPL in the fluid phase. The solubility of the DNAPL forms the upper bound on the mass removal rate because capillary forces that trap a DNAPL are typically not overcome by the viscous forces induced by pumping; thus, mobilization of free-phase DNAPL typically does not occur in PAT operations. Further, if mobilization does occur, viscous forces must be greater than the sum of both capillary and gravity forces if free phase capture by a pumping well is to occur. Because efficiency is low for PAT methods and cleanup standards for DNAPLs are often stringent, PAT remediation is ineffective as a means of remediating DNAPL-contaminated groundwater over time scales of even years. For this reason, PAT methods have been classified in some instances as a contaminant containment strategy, rather than as a restoration method.
PAT methods are inefficient not only because they do not mobilize the DNAPL into a capture well, but also because DNAPL distribution patterns in typical heterogeneous subsurface systems results in the formation of regions of high saturations of DNAPLs called pools. These pools in turn block the pore space of porous media, deflecting groundwater flow around the DNAPL zone. Since contaminant removal occurs only through mass transfer from the DNAPL to the aqueous phase, the surface area that exists between the groundwater and DNAPL, which is relatively small, results in a significant mass transfer limitation for most systems, which is manifest as concentrations of dissolved contaminants that are far removed from equilibrium values.
Vapor Extraction
Vapor extraction (VE) is a frequently used method of remediation in which a well (or wells) are installed into the unsaturated zone and a vacuum is applied to the well(s). Walter (1994); U.S. Army Corps of Engineers (1995). Much like PAT, a pressure gradient is established in a local area around the well, whic
Bagnell David
Jenkins & Wilson, P.A.
Mayo Tara L.
The University of North Carolina at Chapel Hill
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