Contaminant source zones: remediation or perpetual stewardship?It has been some 20 years since I published my first paper on organic liquid contamination of the subsurface. That article was among the first to model the infiltration of organic solvents into aquifer systems. Before the mid-1980s, the importance of separate phase liquid pollutants was not appreciated, and most investigations into groundwater contamination had focused on solute solute /so·lute/ (sol´ut) the substance dissolved in solvent to form a solution.
n. (dissolved constituent) transport. Since that time, substantial resources have been dedicated to research on the behavior of what have become known as nonaqueous phase liquids (NAPLs). Within 5-10 years of those first articles, practitioners started to identify particular classes of NAPLs on the basis of their environmental persistence and ease of subsurface detection. The two most common classes are a) NAPLs composed of fuel hydrocarbons that are lighter (LNAPLs) than water and, thus, more easily detected, because they tend to remain within the unsaturated unsaturated /un·sat·u·rat·ed/ (un-sach´ur-at?ed)
1. not holding all of a solute which can be held in solution by the solvent.
2. denoting compounds in which two or more atoms are united by double or triple bonds. zone or capillary fringe The capillary fringe is the subsurface layer in which groundwater seeps up from a water table by capillary action to fill pores. Pores at the base of the capillary fringe are filled with water due to tension saturation. areas of an aquifer; and b) organic solvent or dense NAPLs (DNAPLs) that tend to migrate deep into formations, becoming entrapped in irregular finger-like structures or pooled on low permeability strata. Laboratory evidence, coupled with a series of careful field case studies, soon revealed that fuel hydrocarbon plumes emanating from LNAPL LNAPL Light Non-Aqueous Phase Liquid contamination sites tended to stabilize with time, due to a series of microbial microbial
pertaining to or emanating from a microbe.
the breakdown of organic material, especially feedstuffs, by microbial organisms. transformation processes (commonly termed "natural attenuation Loss of signal power in a transmission.
The reduction in level of a transmitted quantity as a function of a parameter, usually distance. It is applied mainly to acoustic or electromagnetic waves and is expressed as the ratio of power densities. "). As a consequence of these investigations, monitored natural attenuation has become an accepted environmental management strategy for plumes at many LNAPL sites [e.g., U.S. Environmental Protection Agency Environmental Protection Agency (EPA), independent agency of the U.S. government, with headquarters in Washington, D.C. It was established in 1970 to reduce and control air and water pollution, noise pollution, and radiation and to ensure the safe handling and (EPA EPA eicosapentaenoic acid.
n.pr See acid, eicosapentaenoic.
n. ) 1999; Wiedemeier et al. 1999].
For subsurface contaminant contaminant /con·tam·i·nant/ (kon-tam´in-int) something that causes contamination.
something that causes contamination. plumes that are attributable to organic solvent sources (of which estimates suggest there are as many as 25,000 in the United States United States, officially United States of America, republic (2005 est. pop. 295,734,000), 3,539,227 sq mi (9,166,598 sq km), North America. The United States is the world's third largest country in population and the fourth largest country in area. alone), however, characterization and environmental remedy prescription have proven more elusive, and clean-up investments have often failed to deliver their promised outcome [National Research Council (NRC NRC
1. National Research Council
2. Nuclear Regulatory Commission
Noun 1. NRC - an independent federal agency created in 1974 to license and regulate nuclear power plants ) 2005; Stroo et al. 2003; U.S. EPA 2003]. As I look back over the past two decades of research on DNAPLs, I am struck simultaneously by two realizations. First, we have certainly come a long way in improving our understanding of the migration, persistence, and recovery of DNAPLs in the subsurface environment. Second, we have failed to adequately incorporate this understanding into a long-term environmental management strategy. Below, I have attempted to summarize what I believe to be the most significant research findings and what I see as the most imposing challenges to implementing them. Although this editorial is devoted to the DNAPL DNAPL Dense, Non-Aqueous Phase Liquid problem, similar observations can also be made about any long-term contaminant source issue.
We now know that spatial variability Spatial variability is characterized by different values for an observed attribute or property that are measured at different geographic locations in an area. The geographic locations are recorded using GPS (global positioning systems) while the attribute's spatial variability is in DNAPL mass distribution within a source region is almost inevitable, and, consequently, that mass detection is extremely difficult and uncertain [e.g., Interstate Technology & Regulatory Council (ITRC ITRC Identity Theft Resource Center
ITRC Instructional Technology Resource Center
ITRC Interstate Technology and Regulatory Council
ITRC Interstate Technology and Regulatory Cooperation
ITRC Information Technology Research Centre (Canada) ) 2003]. Migration pathways of DNAPLs will depend almost totally on the organic release characteristics (location, volume, composition, and rate), which are often unknown, and on small-scale subsurface textural variations that cannot be described deterministically (e.g., Kueper et al. 1993). Neverthdess, we have refined our conceptual and mathematical models of DNAPL migration to the point that we are confident in our predictions of migration pathways and equilibrium mass distributions for a fully characterized release scenario (e.g., Rathfelder et al. 2001, 2003). Furthermore, given a specific DNAPL distribution, we are also confident in our ability to modal DNAPL dissolution, the process that will control source longevity and plume concentrations (e.g., Miller et al. 1998).
Given our current level of understanding, it has become clear that characterization of the source zone and the degree of uncertainty associated with that characterization are of critical importance in site assessment. Several DNAPL source zone characterization tools have been developed and demonstrated in the last few years (ITRC 2003). Although each of these has its potential uses, it is my personal perspective that a thorough delineation of mass distribution with these tools will not be feasible in the foreseeable future. However, reduction of uncertainty will likely be possible through the application of novel multistage mul·ti·stage
1. Functioning in more than one stage: a multistage design project.
2. Relating to or composed of two or more propulsion units. characterization tools and protocols that incorporate knowledge obtained from initial characterization efforts into follow-on measurements (NRC 2005).
Considerable effort has also been directed toward the development and demonstration of so-called innovative remedial technologies. Many of these technologies involve flushing of the formation with various chemical amendments to achieve mass recovery or in-place mass destruction (e.g., surfactant Surfactant Definition
Surfactant is a complex naturally occurring substance made of six lipids (fats) and four proteins that is produced in the lungs. It can also be manufactured synthetically. flushing, in situ In place. When something is "in situ," it is in its original location. chemical oxidation). Others are predicated on creating in situ phase changes to facilitate mass removal (e.g., air sparging The term sparging may mean:
supply of water available to animals for drinking supplied via nipples, in troughs, dams, ponds and larger natural water sources; an insufficient supply leads to dehydration; it can be the source of infection, e.g. leptospirosis, salmonellosis, or of poisoning, e.g. standards (NRC 2005). Because innovative technologies typically come with a large price tag and little guarantee of achieving regulatory end points, it is not surprising that there is a reluctance to implement these in the field. These issues, coupled with mass characterization difficulties, have led many to consider the DNAPL problem essentially intractable and to argue for containment as the best management strategy (Cherry et al. 1997; Freeze 2000).
Indeed, on an economic basis, simple cost/benefit analyses for remedial alternatives at DNAPL sites will typically lead to the selection of containment as a presumptive pre·sump·tive
1. Providing a reasonable basis for belief or acceptance.
2. Founded on probability or presumption.
pre·sump remedy. Few, however, seem to look beyond the 30-year present value cost horizon in such analyses. They also often fail to appreciate that the factors that limit the potential success of source remediation (insufficient characterization) may also limit the success of any proposed containment strategy. On the basis of simple mass partitioning calculations, one can derive estimates of typical DNAPL source zone longevities that span centuries under natural gradient conditions, or even longer time periods if hydraulic isolation is attempted. Such long time frames present an enormous challenge to site managers. Can we confidently guarantee adequate and consistent stewardship of physical or hydraulic containment strategies? Will the likelihood of containment failure and the costs of continued monitoring be factored into the cost/benefit analysis? Furthermore, will monitoring strategies be sufficient for timely detection of containment failure? A summary of 5-year reviews of existing physical and institutional controls under the U.S. EPA-administered National Contingency Plan suggests the answer to these questions is "no." Although some progress been made since a 1995 U.S. EPA assessment, 5-year reviews continue to be characterized as paperwork exercises that result in few executable recommendations (Nakamura and Church 2000).
Perhaps the most promising news on the horizon is that, since the late 1990s, researchers have been identifying and isolating organisms and microbial consortia that are capable of transforming chlorinated chlorinated /chlo·ri·nat·ed/ (klor´i-nat?ed) treated or charged with chlorine.
charged with chlorine.
some, e.g. solvents under a variety of subsurface conditions (Bradley 2003). Unlike the fuel-hydrocarbon scenario, however, the natural rates of these processes have typically proven too slow to handle contaminant loadings, and solvent plumes have been documented to persist and continue to expand for decades without apparent biotic biotic /bi·ot·ic/ (bi-ot´ik)
1. pertaining to life or living matter.
2. pertaining to the biota.
1. Relating to life or living organisms. attenuation. Fortunately, recent research suggests that coupling of innovative remedial technologies (partial mass removal) with biostimulation may lead to more effective remediation (Christ et al. 2005). For example, in a recent pilot-scale source zone remedial demonstration, our research group observed the stimulation of indigenous microbial populations after active flushing with a nonionic surfactant solution (Abriola et al. 2005; Ramsburg et al. 2004). This microbial activity resulted in the continued decline of source zone contaminant concentrations 450 days after active treatment. Although promising, much research is still needed to refine the design and explore the potential efficacy of such coupled treatment approaches.
The above observations lead to the conclusion that--from the standpoint of risk reduction---containment and perpetual stewardship as a DNAPL site management strategy is not easily justifiable. Even at sites where, at present, physical and chemical complexities permit no other viable management alternative, we routinely have failed to adequately estimate the uncertainty associated with the containment and monitoring plan. Uncertainty is difficult to quantify and thus often neglected. However, until we can evaluate the level of uncertainty associated with the observations and conceptual models upon which we base our site management decisions, assessing the cost and benefit of any characterization or remedial activity will be nearly impossible. Only after the development and employment of tools capable of quantifying uncertainty will we be able to assure the public that the actions taken are truly reducing risk. In tandem with the development of uncertainty tools, we must continue to pursue research into promising long-term source zone management strategies that couple aggressive remediation (mass removal or destruction) technologies with source zone biotic attenuation. Such a two-pronged approach promises substantial returns in the next 5 years.
The author declares she has no competing financial interests.
Abriola LM, Drummond CD, Hahn EJ, Kibbey TCG (Trusted Computing Group, Beaverton, OR, www.trustedcomputinggroup.org) The successor to the Trusted Computer Platform Alliance (TCPA), announced in 2003 by founding members AMD, HP, IBM, Intel and Microsoft. , kemke LD, Pennell KD, et al. 2005. Pilot-scale demonstration of surfactant-enhanced PCE PCE pseudocholinesterase; see cholinesterase.
Apo-Erythro (CA), Apo-Erythro-EC, Diomycin (CA), E-Base, E-Mycin, Erybid (CA), Erymax (UK), Ery-Tab, Erythromid (CA), PCE (CA), Rommix (UK), Tiloryth (UK)
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The use of biological agents, such as bacteria or plants, to remove or neutralize contaminants, as in polluted soil or water. J 7:81-109.
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University of California Press, also known as UC Press, is a publishing house associated with the University of California that engages in academic publishing. .
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v 1. made radioactive by the addition of small quantities of radioactive material.
2. made contaminated by adding infective or radiographic materials.
3. an infective surface or object. with DNAPLs. Washington, DC:Interstate Technology & Regulatory Council.
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Linda M. Abriola is dean of Engineering and professor of Civil and Environmental Engineering at Tufts University. Her primary research focus is the integration of mathematical modeling and laboratory experiments to investigate the transport, fate, and remediation of nonaqueous phase liquid organic contaminants. The author of more than 100 refereed publications, She recently served on the NRC's Committee on Source Removal of Contaminants in the Subsurface. She is a Fellow of the American Geophysical Union The American Geophysical Union (or AGU) is a nonprofit organization of geophysicists, consisting of over 50,000 members from over 140 countries. AGU's activities are focused on the organization and dissemination of scientific information in the interdisciplinary and and a member of the National Academy of Engineering.
Linda M. Abriola
Department of Civil and Environmental Engineering