In Situ Technologies for Reclamation of PCB-Contaminated Sediments: Current Challenges and Research Thrust Areas
Publication: Journal of Environmental Engineering
Volume 133, Issue 12
Polychlorinated biphenyls (PCBs) were manufactured for use mainly as insulators, coolants, and dielectric fluids in electrical equipment owing to their inert, stable, and flame-retardant nature. They were banned in 1979 because of their toxicity and persistence in the environment. Between 1929 and 1977, approximately 1.25 billion pounds of PCBs were manufactured in the United States. Because of past disposal practices and accidental releases that continue to the present day, 450 million pounds of PCBs have entered the environment over time (ATSDR 2000). Moreover, their hydrophobic nature has led to their occurrence predominantly in soils and sediments that act as sources for their long-term release to the environment. PCBs bioaccumulate and biomagnify in the ecosystem, starting from submarine sediments and benthic organisms to seaweeds and fish, progressively moving up the food chain to reach humans, posing serious health hazards.
The Yusho PCB poisoning incident occurred in 1968 in Japan and involved more than 1,860 individuals, who consumed rice bran oil contaminated with PCBs and its oxidation products. The victims suffered from chloracne, hyperpigmentation, dermal lesions, and ocular discharges—symptoms that slowly lapsed over many years. However, other symptoms relating to the enzymatic or endocrinal effects persisted after more than 30 years (Masuda 2001), with several patients complaining of chronic bronchitis, peripheral numbness, fatigue, and headaches decades after the incident (Aoki 2001). A similar exposure to PCB-contaminated oil led to the Yu-Cheng episode in Taiwan in 1979, resulting in more than 2,000 victims with related symptoms. Additionally, children with prenatal and lactational exposure showed lower birth weights and impaired intellectual and cognitive development (Aoki 2001).
More recently, the Belgian PCB incident occurred in 1999, when 2,500 farms were inadvertently supplied with animal feeds contaminated with a mixture of PCBs and dioxins (Bernard et al. 2002). Poultry was hardest hit: egg production and hatching decreased, and a chicken edema epidemic broke out (van Larebeke et al. 2001). As an aftermath of the contamination, poultry and derived products were ordered off the markets, and some 2 million chickens were destroyed (van Larebeke et al. 2001). Even though this episode did not cause or is unlikely to cause adverse health effects in the general population (Bernard et al. 1999), it resulted in a huge political, social, and economic crisis highlighting the general risk perception of PCBs.
Whereas those episodes relate to an acute high-level exposure to PCBs and their products, today, we experience low-level chronic exposure through seafood, dairy products, and other consumables. Prenatal exposure to PCBs has been linked to shorter gestation periods, lower birth weights, and impaired and delayed neuromuscular development. Chronic high-level exposure to PCBs (such as occupational exposure) has been associated with increased occurrence of cancers of the gastrointestinal and biliary tracts, liver, gallbladder, lung, and the brain (National Research Council 2001).
Because of hazards associated with exposure to PCBs, they are regulated in drinking water, food, the workplace environment, and the way that they are handled, stored, and disposed. The maximum allowable level of PCBs in drinking water is (EPA 816-F-03–016). The Code of Federal Regulations rules current tolerances for PCB residues on a fat basis (21 CFR 109.30) at in poultry, in milk and dairy products, in eggs, in fish and shellfish, in infant foods, and in animal feed components. Matrices greater than in PCBs have to be disposed of by landfilling or incineration (40 CFR 761.65). The Occupational Safety and Health Administration (OSHA) limits PCB exposure to for an workday (29 CFR 1910.1000).
Today, PCBs are found in numerous rivers, coastal waters, and in 432 of the 1,467 hazardous waste sites in the U.S. (ATSDR 2000). They tend to partition preferentially to the organic components of the environment, leading to their prevalence in fine-grained, organically rich sediments. In their sorbed state, they are tightly bound to the organic fraction of the sediments but may become more available over extended time (Harkness et al. 1993), thereby posing a lingering threat of exposure. In general, treatment of sediments is problematic because of their limited accessibility and high water content, difficulties in technology selection attributable to a multitude of ongoing natural and anthropogenic processes affecting contaminant fate and transport, threat of disruption of benthic ecosystems, limited tools for evaluation of site recovery, and unrealistic cleanup goals. Problems specific to PCB contamination may be categorized as follows:
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Scientific: Total PCB concentrations are commonly used for site risk assessment, which may be an inappropriate parameter because all congeners are not equally toxic and because risk depends on the toxicity of the active components, their interactions with other congeners (additive, synergistic, or antagonistic), and bioavailability. Toxicity studies, often done with individual congeners, are of limited use in such assessments, since PCBs exist as mixtures in sediments with highly varied composition that is constantly changing, because of numerous ongoing physicochemical and biotransformation processes.
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Technological: There is a dearth of technologies that effectively deal with strongly sorbed contaminants such as PCBs occurring in sediments that act as reservoirs for their prolonged gradual release, potentially causing contamination posttreatment “relapse.” Also, because of the nature of historical disposals, they exist with other toxic heavy metals, pesticides, and so on (Stone 1992), making risk assessment, estimation of contribution of PCBs to toxicity, and setting cleanup goals difficult. Additionally, more meaningful criteria for PCB levels—such as toxicity and bioavailability—lack standardized methods for rapid, accurate, and portable measurement.
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Regulatory: PCBs often exist in small concentrations with respect to remediation potential of current technologies, so realistic regulatory goals are often difficult to establish.
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Social: Because of the poisoning episodes in the past, as well as present-day exposure routes through contaminated food, the general population has an aggravated risk perception of PCB-contaminated sediments, even for those with low levels, which are especially hard to treat.
Dredging and Ex Situ Treatment
Currently, dredging followed by treatment and land filling are predominantly used, but they are disruptive and unsustainable. Dredging often has to be employed in shallow waters with limited equipment access and must be conducted slowly and deliberately with such controls as silt screens and water surface covers to limit resuspension and volatilization of sediments while aiming to remove limited amounts of sediments to conform to treatment and disposal constraints (Magar 2001). In spite of the control measures, sediment beds disturbed during dredging carry increased risk of exposure to operators and neighborhood communities (through volatilization from disturbed water columns and disposal facilities), almost certain ecological imbalances (through the destruction of benthic communities), residual risks (through incomplete removal and sediment fallback), and contaminant transport (through increased contaminant mobility and water column agitation because of dredging-related equipment activity).
Treatment usually involves dewatering and stabilization or incineration. Ex-situ solvent extraction or chemical treatment processes are too expensive and often only marginally beneficial to justify use. Treated sediments are disposed of in confined disposal facilities and landfills which are limited in number, and carry increased risks of exposure and accidents during handling and transportation. The decontamination chain of dredging, treatment, and disposal is expensive and often leaves behind residual contaminants in the dredged areas or taints a previously pristine zone through increased sediment transport.
In Situ Treatment
Capping
Capping involves isolating a contaminated sediment bed through deployment of a clean layer or “cap” commonly consisting of sand, gravel and pebbles. Such passive caps made of unreactive material mainly rely on containment, rather than treatment, of the sediments to limit exposure and hence risk to the surrounding biota (SERDP 2004). The cap cuts down on bioavailability of the contaminants by physically separating the sediments from the surrounding life forms, reduces exposure to the contaminant through pore-fluid fluxes from the sediment zone, and confines bioturbation to the top clean layer of the cap and hence limits the possibility of resuspension of the contaminated sediments.
Passive caps modified by additives that actively immobilize PCBs [activated carbon (Millward et al. 2005), coke (Zimmerman et al. 2004)] or destroy them constitute reactive caps. A carbon-based additive such as activated carbon, in addition to containing the sediments, may provide enhanced immobilization by strongly partitioning the PCBs onto its surface. Additionally, the carbon surface may act as a substrate on which microbiota grow, aiding in the natural attenuation of the toxicity through biodegradation. The high affinity of organics to such material may also cause a capillary effect, leading to possible extraction and sequestration of organics from deeper sediments. In general, reactive caps may be more suitable than passive caps for highly contaminated sites that carry a risk of imminent exposure.
Capping is an attractive risk-control alternative because of its relatively inexpensive, easy, and less invasive nature. It is especially suitable for sites with low to moderate natural hydrodynamics and navigational traffic, fine-grained cohesive sediments with net deposition (SERDP 2004). Despite numerous successful field demonstrations of capping, critical aspects require groundbreaking work:
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There is a need for standardized platforms for making feasibility decisions for cap deployment, determining optimal cap thickness, assessing erosion resistance of the cap, and shielding requirements to withstand storm events. Robust hydrological models that are based on the mechanistic understanding of the flow processes can be instrumental for such assessments. Developing such models and validating them with site-specific data gathered over multiple seasons of dry and wet weather conditions, along with characterization of sediment bed for shear and compressive strength, is warranted.
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In spite of intensive research on systems for PCB degradation, capping additives that can effectively sequester and/or degrade PCBs while having minimal impact on the surrounding ecosystem are limited, partially because most of those additives are being tested on a bench scale. Demonstrations at the pilot and field scale are necessary to understand their effectiveness in naturally contaminated environs and to ultimately develop materials for active deployment.
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Monitoring the water column above caps for contaminant and suspended solids concentrations and contaminant flux at the cap-water interface, as well as their comparison with background levels, are crucial in establishing sustained risk reduction by cap installation. Noninvasive techniques like sensors and sonar to determine the integrity and movement of the cap, especially after high-flow seasons, are critical for evaluating the longevity and functionality of deployed caps. Standardization of these techniques for effective monitoring and documentation of cap performance is warranted.
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Models integrating submarine hydrology, sediment strength attributes, and cap design parameters to simulate cap performance with respect to containment/resuspension, especially, under high-flow conditions, can be an effective way of instilling confidence in the public and regulatory agencies toward caps as an attractive technology for exposure and risk attenuation.
Bioremediation
Bioremediation can be employed for site reclamation by (1) enhancement of the indigenous microorganisms through addition of nutrients (nitrogen, phosphorus, ammonium chloride), supplementary carbon sources (sugars, organic acids, glutamate and so on), oxygen (peroxide), primers (polybrominated biphenyls), and by analog enrichment (adding biphenyl); (2) by augmentation of the indigenous population with exogenous cultured innoculums (established PCB degraders) (Allard and Neilson 1997); or (3) both. In the specific case of PCBs, lower congeners are more amenable to aerobic biodegradation, whereas higher homologues are susceptible to anaerobic processes (Magar 2003).
In situ bioremediation involves little increased risk of resuspension during deployment, no treatment residuals for disposal (is “green”), and by nature is a sustainable long-term approach to remediation. The high rates of PCB removal and complete mineralization achieved in the laboratory, the prevalence and diversity of PCB-metabolizing microorganisms in nature, and the principle of microbial infallibility (Singleton 1994) make bioremediation an attractive remedial alternative.
Despite its promise, bioremediation has several limitations, especially in field-scale applications. The rate of PCB removal may be orders of magnitude slower in nature than as established in the laboratory because of mass transfer limitations leading to reduced metabolic availability, shortage of one or more crucial nutrients, preferential metabolism of other easy-to-digest substrates, presence of microbial predators and toxins, and other environmental factors that can drastically constrain the microbial metabolism. Toxic intermediates produced during PCB metabolism may lead to extremely slow removal rates or a sudden end to the process (Abraham et al. 2002). Assimilation of oil fractions that harbor the PCBs preferentially over the PCBs themselves may cause their increased mobility and bioavailability. The following areas of research have been identified:
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Pure cultures grown under highly regulated conditions with single PCB congeners may not represent the population responsible for degradation or the natural environments in which remediation takes place. Development of effective methods for screening, identification, and extraction of naturally occurring PCB degraders and cultivating them in microcosms made from site sediments (Allard and Neilson 1997) could be useful for obtaining efficient consortia with predictable field performance.
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PCB uptake and removal pathways, including identification of intermediates and end products, are not well established. A better understanding of the complex, nonlinear PCB metabolism is required in natural sediments (or near natural conditions) consisting of intricate bilateral interactions of the microbial community to uncover mechanisms of PCB uptake and assimilation.
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Most microbes can metabolize only limited types of compounds because of the specificity of their enzymes. Genetic engineering can help expand their treatability spectrum (Singleton 1994) without a need to wait for microbial evolution to detoxify a variety of contaminants. Of course, this has to be accompanied by a thorough study of their fate and effects in the ecosystem before they are considered safe for deployment.
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Surfactants have been known to increase the availability of PCBs for microbial and abiotic degradation. Biosurfactants produced by microbes, such as those that metabolize petroleum oil, can be added to enhance PCB removal while avoiding chemically produced surfactants that could be pollutants themselves (Abraham et al. 2002). A better approach could be using mixed cultures consisting of microbes that can produce these surfactants in situ in conjunction with the PCB degraders.
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Mixed cultures may also consist of both the aerobic and anaerobic strains (Singleton 1994) to establish an in situ treatment train for higher removal efficiencies or using strains that feed off toxic intermediates and terminal metabolites produced by the PCB degraders.
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The extent of site reclamation is often evaluated on the basis of contaminant removal, although reduction of toxicity or bioavailability is often a more meaningful measurement. Their accurate quantification needs targeted research:
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Biosensors involving whole cells or microbial enzymes that may be deployed to detect and quantify PCBs in natural systems are very promising (Singleton 1994). Issues with contaminant specificity and interference from other organics need to be addressed before they can be used routinely for rapid assessment. Sensors, in conjunction with the Microtox assay, other mutagenic and physicochemical assays that may be more thorough, intense bioassays involving fish, higher plants and so on, may be standardized for toxicity assessments.
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Innovative methods to measure PCB biodegradation instead of using extractable concentration as the only parameter, including (1) monitoring the fate of chiral congeners and changes in their composition with respect to the original mixtures, since only biological processes can change their composition by selective enantiomeric degradation (Wong et al. 2001); (2) isotopic fractionation wherein enzymes preferentially act on an isotope resulting in reaction products richer in that isotope [ is preferred over (Abraham et al. 1998), ratios broaden significantly in aged environmental matrices over original compounds because of fractionation (Reddy et al. 2000)].
Natural Recovery
When applied as a remediation approach, natural recovery involves site reclamation by ongoing aquatic, sedimentary, physicochemical, and biological processes. Natural recovery can take place through two primary pathways: burial of contaminated bed by clean sediments (natural capping) and transformation via biodegradation, immobilization, dilution, or volatilization (National Research Council 2001). Long-term monitoring is an integral part of adopting natural recovery (SERDP 2004) and includes documentation of source control, statistically significant decline in contaminant concentration and bioavailability, biological recovery, and development of a reliable model to predict future sediment quality. In conjunction with monitoring, public advisories such as regulated fishing, navigation, recreation, and property development are routinely employed. Current important research needs are discussed in the following:
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Deployment decisions involve answering questions on expected reductions in exposure by burial and in toxicity by transformation processes. So, apart from developing models for site selection, a tremendous need exists for detailed models, as follows: (1) to predict recovery performance by incorporating seasonal flow variations, amount and nature of organic carbon in sediments, and the density, variety, and degradation potential of the microbial population, and so on; (2) to assess the cost of deployment by including direct costs of monitoring and implementation of regulated access and indirect costs incurred through restricting resource usage; and (3) for long-term risk assessment such as bioavailability and hence toxicity of contaminants in their respective partitioned phases, as well as increased exposure during high-flow conditions.
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Since burial tends to be the dominant attenuation pathway at most sites (contaminant transformation can take decades), development of standardized tools for measuring the stability and erosion resistance of sediment beds including in situ sensors, sediment bed profiling techniques, and measuring particulate content and fluxes are crucial from site selection to prolonged monitoring during recovery and should be targeted for research.
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In addition to routine sediment analysis for target contaminants and currently used bioassay tests that tend to be elaborate and expensive, a long-term monitoring intensive approach calls for development of passive samplers or endocrinal indicators and establishing sound correlations between responses in these surrogates and that in exposed organisms typical of the ecosystem.
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Evaluation of reductions in exposure and toxicity, along with measurement of contaminant fluxes, in situ redox potentials, and tracking burial rates of contaminated sediments by using novel in situ sensors should constitute standard protocols in the future to document the sustained occurrence and rates of natural recovery.
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The need for pilot and field-scale demonstrations cannot be emphasized enough, since they are critical for validating and fine-tuning models on recovery performance and risk assessment, as well as elucidating the intrinsic variability of natural recovery (Magar 2001), but perhaps they are most important for documenting the sustained recovery and achievement of benchmarks in cleanup goals.
Conclusion
Owing to their sustainable and largely nondisruptive nature, in situ reclamation technologies are the future of PCB-contaminated sediment sites. Currently, understanding of these technologies is limited, and successful demonstrations are too few to allow routine deployment with predictable remediation performance. Research thrust in the following aspects is warranted:
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Integrating processes related to site hydrodynamics, watershed layout, sediment stability and transport, and PCB partitioning, fate, and exposure to develop robust and portable models that can be used for technology screening and predicting recovery rates.
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Furthering the mechanistic understanding of remediation processes, especially for natural recovery and bioremediation. Technological evolution in such critical areas as engineered high-performance microbes or techniques for their identification and extraction, novel engineered geotextiles, or additives for caps, are required.
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Standardizing current tools and techniques for long-term monitoring critical to most in situ technologies and expanding them to include isotopic fractionation, biosensors for PCB concentrations, sensors for sediment stability and movement, bioassay surrogates, and so on, is needed.
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Establishing meaningful remediation criteria, such as reduced toxicity and bioavailability (versus total PCBs), and demonstrating PCB attenuation through these criteria are critical for assuaging the general no-action perception associated with some of these technologies.
Acknowledgments
This document has not been subjected to internal policy review of the U.S. Environmental Protection Agency. Therefore, the opinions presented herein do not necessarily reflect the views of the agency or its policy. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
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Published In
Journal of Environmental Engineering
Volume 133 • Issue 12 • December 2007
Pages: 1075 - 1078
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© 2007 ASCE.
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Published online: Dec 1, 2007
Published in print: Dec 2007
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