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4-3 Chapter 4: Monitored Natural Recovery Other advantages of MNR may include no construction or infrastructure is needed, and may, therefore, be much less disruptive of communities than active remedies such as dredging or in-situ capping. No property should be needed for materials handling, treatment, or disposal facilities, and no contaminated materials should be transported through communities.
Two key limitations of MNR may include it generally leaves contaminants in place and that it can be slow in reducing risks in comparison to active remedies. Any remedy that leaves untreated contaminants in place probably includes some risk of reexposure of the contaminants. When MNR is based primarily on natural burial, there is some risk of buried contaminants being reexposed or dispersed if the sediment bed is significantly disturbed by unexpectedly strong natural or man-made (anthropogenic) forces. The potential effects of reexposure may be greater if high concentrations of contaminants remain in the sediment, and likewise, lower if contaminant concentrations or risks are low.
There is also some risk of dissolved contaminants being transported to the surface water at levels that could cause unacceptable risk. The time frame for natural recovery may be slower than that predicted for dredging or in-situ capping. However, time frames for various alternatives may overlap when uncertainties are taken into account. In addition, realistic estimates of the longer design and implementation time for active remedies should be factored in to the comparison. Like any remedy that takes a period of time to reach remediation goals, remedies that include MNR frequently rely upon institutional controls, such as fish consumption advisories, to control human exposure during the recovery period. These controls may have limited effectiveness and usually have no ability to reduce ecological exposures.
Major areas of uncertainty frequently noted for MNR include the ability to 1) predict future sedimentation rates in dynamic environments and 2) predict rates of contaminant flux through stable sediment. It can be especially difficult to predict rates of natural recovery where contaminant levels and risks are already low because small additional factors become relatively more important. However, a higher level of uncertainty may be more acceptable in these situations as well.
4.3 NATURAL RECOVERY PROCESSES
The success of MNR as a risk reduction approach typically is dependent upon understanding the dynamics of the contaminated environment and the fate and mobility of the contaminant in that environment. The natural processes of interest for MNR may include a variety of processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, or
concentration of contaminants in the sediment bed. These natural processes may include the following:
• Physical processes: Sedimentation, advection, diffusion, dilution, dispersion, bioturbation, volatilization;
• Biological processes: Biodegradation, biotransformation, phytoremediation, biological stabilization; and • Chemical processes: Oxidation/reduction, sorption, or other processes resulting in stabilization or reduced bioavailability.
Highlight 4-3 illustrates some of the natural processes the project manager should consider when evaluating MNR. With few exceptions, these processes interact in aquatic systems, sometimes increasing 4-4 Chapter 4: Monitored Natural Recovery the risk-reduction effects of a process compared to what they might be for that process in isolation, and sometimes reducing those risk-reduction effects. For example, as recognized by the U.S. Environmental Protection Agency’s (EPA) Science Advisory Board (SAB) Environmental Engineering Committee, Monitored Natural Attenuation: USEPA Research Program - An EPA Science Advisory Board Review (U.S. EPA 2001j), sustained burial processes remove contaminants from the bioavailable zone, but can also impede certain degradation processes, such as aerobic biodegradation. Likewise, contaminant sorption to sediment particles may reduce both bioavailability and rates of contaminant transformation.
In addition, in the case of mixed contaminants, the same natural process may result in very different environmental fates. When dealing with mixed contaminants at a site, the project manager should not focus unduly on one contaminant without understanding the effects of natural processes on the other contaminants, including breakdown products. Understanding the interactions between effects and prioritizing the significance of these effects to the MNR remedy should be part of a natural process analysis.
Highlight 4-3: Sample Conceptual Model of Natural Processes Potentially Related to MNR
4.3.1 Physical Processes Generally, physical processes do not directly change the chemical nature of contaminants.
Instead, physical processes may bury, mix, dilute, or transfer contaminants to another medium. Physical processes of interest for MNR include sedimentation, erosion, diffusion, dilution, dispersion, bioturbation, advection, and volatilization (including temperature-induced desorption of semi-volatiles). All of these processes may reduce contaminant concentrations in surface sediment, and thus reduce risk associated with the sediment. Sedimentation normally reduces risk physically by containing contaminants in place.
Other physical processes, such as erosion, dispersion, dilution, bioturbation, advection, and volatilization may reduce contaminant concentrations in sediment as a result of transferring the contaminants to another medium or dispersing them over a wider area (e.g., via ground water or surface water). These processes may reduce, increase, or transfer the risk posed by the contaminants. As discussed previously in Section 4.1, project managers should carefully evaluate the potential for increased exposure and risk to receiving water bodies before selecting MNR where dispersion is one of the risk reduction mechanisms.
Physical processes in sediment can operate at vastly different rates. Some may occur faster than others, but may or may not have more impact on risk. In general, processes in which contaminants are transported by bulk movement of particles or pore water (e.g., erosion, dispersion, bioturbation, advection) occur at faster rates than processes in which contaminants are transported by diffusion or volatilization and, therefore, are frequently, but not always, more important when evaluating MNR.
Processes that result in particle movement are particularly important for hydrophobic or other contaminants that are strongly sorbed to sediment particles. Some physical processes are continuous, and others seasonal or episodic. Depending on the environment, any of these types of processes (i.e., continuous, seasonal, or episodic) may have the most impact on natural recovery of a site. For example, project managers should not assume that episodic flooding will have a positive or negative effect on risk over an entire site. Flooding is most likely to cause erosion in some areas, while causing significant deposition in others.
Transport and deposition of cleaner sediment in a watershed may lead to natural burial of contaminated sediment in a quiescent environment. Natural burial may reduce the availability of the contaminants to aquatic plants and animals and, therefore, may reduce toxicity and bioaccumulation. The overlaying cleaner sediment also serves to reduce the flux of contaminants into the surface water by creating a longer pathway that the desorbed contaminants must travel to reach the water column.
However, while bioturbation by burrowing organisms may promote mixing and dilution of contaminated sediment with the newly deposited cleaner sediment, for bioaccumulative contaminants it may also result in continued bioaccumulation into the food web until contaminant isolation occurs.
The long-term protectiveness provided by sedimentation depends upon the physical stability of the new sediment bed and the rates of movement of contaminants through the new sediment. Major events, such as severe floods or ice movements may scour the buried sediment, exposing contaminated sediment and releasing the contaminants into the water column. Ground water that flows through the sediment bed also may transport dissolved contaminants into the water column. Depending upon their extent, processes such as these may extend the natural recovery period or, in some cases, inhibit it altogether. Project managers should consider the potential influence of these processes on exposure rates and risk. A site-specific evaluation of both sediment and contaminant fate and transport are important to evaluating MNR as a remedy. There are a variety of empirical and modeling methods to assess rates of 4-6 Chapter 4: Monitored Natural Recovery various physical processes at specific sites. These are discussed in Chapter 2, Section 2.8, Sediment and Contaminant Fate and Transport, and Section 2.9, Modeling.
4.3.2 Biological and Chemical Processes Like most natural processes, biological processes also depend on site-specific conditions and are highly variable. During biodegradation, a chemical change is facilitated by microorganisms living in the sediment. One of the important limitations to the usefulness of biodegradation as a risk-reduction mechanism is that the greater the molecular weight of the organic contaminants, the greater partitioning to sorption sites on sediment particles (Mallhot and Peters 1988) and the lower the contaminant availability to microorganisms. Some degradation of high molecular weight organic compounds occurs naturally in soil and sediment with anaerobic and aerobic microorganisms (Brown et al. 1987, Abramowicz and Olsen 1995, Bedard and May 1996, Shuttleworth and Cerniglia 1995, Cerniglia 1992, Seech et al. 1993).
Degradation rates vary with depth in sediment partly due to the change from aerobic or anaerobic conditions. This changes frequently occur at depths of a few millimeters to a few centimeters where sediments have substantial organic content and conditions are quiescent, and may occur deeper in some circumstances. Longer residence times of contaminants in the sediment (aging) also usually result in increased sequestration (Luthy et al. 1997, Dec and Bollag 1997). These processes reduce the availability of the organic compounds to microorganisms and, therefore, reduce the extent and rates of biodegradation (Luthy et al. 1997, Tabak and Govind 1997). However, this can also reduce the availability of the contaminant to receptors living in the sediment and as well as at higher trophic levels.
Chemical processes in sediment are especially important for metals. Many environmental variables govern the chemical state of metals in sediment, which in turn affects their mobility, toxicity, and bioavailablity making natural recovery due to chemical processes difficult to predict. Much of the current understanding of the role of chemical processes in controlling risk is focused on the important geochemical changes resulting from changes in redox potential that can affect the bioavailability of metal and organic metal compounds. Formation of relatively insoluble metal sulfides under reducing conditions can often effectively control the risk posed by metal contaminants if reducing conditions are maintained.
Environmental variables include pore water pH and alkalinity, sediment grain size, oxidation-reduction (redox) conditions, and the amount of sulfides and organic carbon present in the sediments. Furthermore, many chemical processes in sedimentary environments are also affected by the biological community.
Biochemical Processes for Polycyclic Aromatic Hydrocarbons (PAHs)
The class of hydrocarbons known as polycyclic aromatic hydrocarbons (PAHs) is a common contaminant in sediment and biota at Superfund sites. Many organisms are capable of accumulating PAH contaminants in their tissue, but biomagnification does not generally occur in vertebrate species (Suedel et al. 1994). Fish do not generally accumulate higher tissue PAH concentrations than their prey due to their ability to metabolize and eliminate PAHs; however, the PAH metabolites may themselves cause chronic toxicity, such as reduced growth and reproduction as well as increased incidence of neoplasms in fish. The potential exists for bioaccumulation in some invertebrate species because of their lesser ability to metabolize and eliminate PAHs (Meador et al. 1995).
PAHs may be subject to physical, chemical and biological breakdown in the environment and where these processes are effective, may be especially amenable to natural recovery. The type of process that dominates may depend on time. For example, following a release of PAHs into the environment, 4-7 Chapter 4: Monitored Natural Recovery physical-chemical processes such as dispersion, volatilization, and photodegredation may dominate.
Where these processes are effective in attenuating the contaminants to less toxic levels, tolerant microbial species may cause further biodegradation. There is a wide variation in rates of biodegradation and toxicity reduction, depending on the levels of microbial activity and the physical and chemical conditions of the site (Swindoll et al. 2000). PAHs biodegrade more quickly through aerobic than anaerobic processes, although the degradation rate usually decreases as the number of aromatic rings increases (Shuttleworth and Cerniglia 1995, Cerniglia 1992, Seech et al. 1993). While biodegradation of PAHs may occur under anaerobic conditions, PAHs usually persist longer in anaerobic sediment compared to aerobic environments (U.S. EPA 1996d, Safe 1980).
Although low PAH degradation rates are often attributed to low bioavailability (see review by Reid et al. 2000), evidence reported by Schwartz and Scow (2001) demonstrates that it may be the lack of enzyme induction amongst the PAH-degrading bacteria that is responsible for low rates below a threshold PAH concentration. Other researchers have reported this phenomenon for PAHs (Ghiorse et al. 1995, Langworthy et al. 1998) and other aromatic organics (Zaidi et al. 1988, Roch and Alexander 1997). At elevated PAH concentrations in sediment, there is selective pressure for PAH-degrading bacteria, which can increase the capacity to attenuate PAHs naturally. However, there is uncertainty about whether and how fast this degradation may reach acceptable risk levels. Because of the variation among sites, sitespecific studies may be needed to resolve uncertainties concerning degredation rates and whether these rates will contribute to recovery within an acceptable time frame.
Biochemical Processes for Polychlorinated Biphenyls (PCBs)
Release of a PCB Aroclor (see PCB data information in Chapter 2, Section 2.1.2, Types of Data) into the environment may result in a change in its congener composition. This is a result of the combined weathering effects and such processes as differential volatilization, solubility, sorption, anaerobic dechlorination, and metabolism, and results in changes in the composition of the PCB mixture in sediment, water, and biota over time and between trophic levels (NRC 2001).