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• Elimination or treatment of contaminated waste water or ground water discharges (e.g., installing additional treatment systems prior to discharge);
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• Removal or containment of potentially mobile sediment hot spots.
EPA’s Contaminated Sediment Management Strategy (U.S. EPA 1998a) includes some discussion of EPA’s strategy for abating and controlling sources of sediment contamination. Source control activities may be implemented by state or local governments using combinations of voluntary and mandatory actions.
The identification of continuing sources and an evaluation of their potential to re-contaminate site sediment are often essential parts of site characterization and the development of an accurate conceptual site model, regardless of source areas within the site. When there are multiple sources, it is often important to prioritize sources to determine the relative significance of continuing sources versus on-site sediment in terms of site risks to determine where to focus resources. Where sources are a part of the site, project managers should develop a source control strategy or approach for the site as early as possible during site characterization. Where sources are outside the site, project managers should encourage the development of source control strategies by other authorities, and understand those strategies. Generally, a source control strategy should include plans for identifying, characterizing, prioritizing, and tracking source control actions, and for evaluating the effectiveness of those actions. It is also useful to establish milestones for source control that can be linked with sediment remedial design and cleanup actions. If sources can be substantially controlled, it is normally very important to reevaluate risk pathways to see if sediment actions are still needed. If sources cannot be substantially controlled, it is typically very important to include these ongoing sources in the evaluation of what sediment actions may or may not be appropriate and what RAOs are achievable for the site.
Generally, significant continuing upland sources (including ground water, NAPL, or upgradient water releases) should be controlled to the greatest extent possible before sediment cleanup. Once these sources are controlled, project managers should evaluate the effectiveness of the actions, and should refine and adjust levels of source control, as warranted. In most cases, before any sediment action is taken, project managers should consider the potential for recontamination and factor that potential into the remedy selection process. If a site includes a source that could result in significant recontamination, source control measures will be likely necessary as part of that response action. However, where sediment remediation is likely to yield significant benefits to human health and/or the environment after considering the risks caused by an unaddressed or ongoing source, it may be appropriate to conduct an action for sediment prior to completing all land-based source control actions.
2.7 PHASED APPROACHES, ADAPTIVE MANAGEMENT, AND EARLY ACTIONS
At some sediment sites, a phased approach to site characterization, remedy selection, or remedy implementation may be the best or only practical option. Phasing site characterization can be especially useful when risks are high, yet some important site-specific factors are unknown. Phasing in remedy 2-21 Chapter 2: Remedial Investigation Considerations selection and implementation may be especially useful at sites where contaminant fate and transport processes are not well understood or the remedy has significant implementation uncertainties. Phasing may also be useful where the effectiveness of source control is in doubt. By knowing the effectiveness of source control prior to implementing sediment cleanups, the risk of having to revisit recontaminated areas is greatly reduced. High remedy costs, the lack of available services and/or equipment, and uncertainties about the potential effectiveness or the risks of implementing the preferred sediment management approach, can also lead to a decision to phase the cleanup. At some sites, it may be advantageous to pilot less invasive or less costly remedial alternatives early enough in the process that performance could be tracked. If performance does not approach desired levels, then more invasive or more costly approaches could be pursued.
Phasing can also be used at large, multi-source, multi-PRP sites with primarily historic contamination where contaminated sediment is still near the sources. At these types of sites, working with a single responsible party to address sediment with higher contaminant concentrations near a specific source may be an effective risk reduction measure, while the more complex decision making for the rest of the site is ongoing.
Project managers are encouraged to use an adaptive management approach, especially at complex sediment sites to provide additional certainty of information to support decisions. In general, this means testing of hypotheses and conclusions and reevaluating site assumptions as new information is gathered.
This is an important component of updating the conceptual site model. For example, an adaptive management approach might include gathering and evaluating multiple data sets or pilot testing to determine the effectiveness of various remedial technologies at a site. The extent to which adaptation is cost-effective is, of course, a site-specific decision. Resources on adaptive management at sediment sites include the NRC’s report Environmental Cleanup at Navy Facilities (NRC 2003) and Connolly and Logan (2004).
Even before the sediment at a site is well characterized, if risk is obvious, it may be very important to begin to control significant ongoing land-based sources. It also may be appropriate to take other early or interim actions, followed by a period of monitoring, before deciding on a final remedy.
Highlight 2-7 provides examples of early actions taken to control sources, minimize human exposure, control sediment migration, or reduce risk from sediment hot spots at contaminated sediment sites. Early or interim actions are frequently used to prevent human exposure to contaminants or to control sources of sediment contamination. However, such actions for sediment are less frequent. Factors for determining which response components may be suitable for early or interim actions include the time frame needed to attain specific objectives, the relative urgency posed by potential or actual exposure, the degree to which an action may reduce site risks, and compatibility with likely long-term actions (U.S. EPA 1992b).
An early action taken under Superfund removal authority may be appropriate at a sediment site when, for example, it is necessary to respond quickly to a release or a threatened release of a hazardous substance that would present an immediate threat. At contaminated sediment sites, removal authority or state authorities have been used to implement many of the actions listed in Highlight 2-7. The NCP at 40 CFR §300.415 outlines criteria for using removal authority, as further explained in the EPA guidance and directives (U.S. EPA 1993a, U.S. EPA 1996d, U.S. EPA 2000d). Project managers may also consider separating the management of source areas from other, less concentrated areas by establishing separate operable units (OUs) for the site.
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2.8 SEDIMENT AND CONTAMINANT FATE AND TRANSPORTAn important part of the remedial investigation at many sediment sites is an assessment of the extent of sediment and contaminant transport and the effect of that transport on exposure and risk. This usually includes gaining an understanding of the processes and events in the past and predicting future transport and exposure.
Highlight 2-7: Potential Examples of Early Actions at Contaminated Sediment Sites
Actions to prevent releases of contaminants from sources:
• Excavation or containment of floodplain soils or other source materials in the floodplain • Engineering controls (e.g., sheet pilings, slurry walls, grout curtains, and extraction) to prevent highly contaminated ground water, NAPL, or leachate from reaching surface water and sediment • Engineering controls to prevent contaminated runoff from reaching surface water and sediment
Actions to minimize human exposure to contaminants (coordinated with other appropriate agencies):
• Access restrictions • Fish consumption advisories • Use restrictions and advisories for water bodies • Actions to protect downstream drinking water supplies
Actions to minimize further migration of contaminated sediment:
• Boating controls (e.g., vessel draft or wake restrictions to prevent propeller wash, anchoring restrictions) • Excavating, dredging, capping, or otherwise isolating contaminated sediment hot spots
Actions taken to reduce risk from highly contaminated sediment hot spots:
• Capping, excavation, or dredging of localized areas of contaminated sediment that pose a very high risk In most aquatic environments, surface sediment and any associated contaminants move over time.
The more important and more complex issue is whether movement of contaminated sediment (surface and subsurface), or of contaminants alone, is occurring or may occur at scales and rates that will significantly change their current contribution to human health and ecological risk. Addressing that issue requires an understanding of the role of natural processes that counteract sediment and contaminant movement and fate, such as natural sedimentation and armoring, and contaminant transformations to less toxic or less bioavailable compounds. For this reason, it is important for project managers to use technical experts to help in the analysis, especially where large amounts of resources are at stake.
Sediment movement also is a complex topic because it has both positive and negative effects on risk. For example, floods frequently transport both clean and contaminated sediment, which are subsequently deposited within the water body and on floodplains. This may spread contamination,
isolate (through burial) other existing contamination, and lower concentrations of contaminants (through dilution) within the immediate site boundaries.
Both natural and man-made (i.e., anthropogenic) forces may cause sediment and contaminants to move. Highlight 2-8 lists examples of each.
Highlight 2-8: Potential Causes of Sediment and/or Contaminant Movement
Natural causes of sediment movement include:
• Routine currents in rivers, streams, and harbors • Tides in marine waters and estuaries • Floods generated by rainfall or snow-melt induced runoff from land surfaces • Ice thaw and ice dam-induced scour • Seiches (oscillation of lake elevation caused by sustained winds), especially in the Great Lakes • Storm-generated waves and currents (e.g., hurricanes, Pacific cyclones, nor’easters) • Seismic-generated waves (e.g., tsunamis) • Earthquakes, landslides, and dam failures • Bioturbation from micro- and macrofauna
Anthropogenic causes of sediment movement include:
• Navigational dredging and channel maintenance • Placer mining as well as sand and gravel mining • Intentional removal or breaching of hydraulic structures such as dams, dikes, weirs, groins, and breakwaters • In-water construction • Boat propeller wash, ships’ wakes, ship grounding or anchor dragging
Causes of dissolved contaminant movement without sediment movement include:
• Flow of ground water through sediment • Molecular diffusion • Gas-assisted transport Many contaminated sediment sites are located in areas that are primarily depositional, or in areas where only a limited surface layer of sediment is routinely mobilized. In these fairly stable areas, other processes may contribute to sediment and contaminant movement and resulting exposure and risk. These include, for sediment, bioturbation, and for dissolved contaminants, ground water flow, molecular diffusion, and, potentially, gas-assisted transport. Like erosion and deposition, these processes continue 2-24 Chapter 2: Remedial Investigation Considerations to operate after remedies are in place, so an understanding of whether or not they are likely to be significant ongoing contaminant transport pathways at a particular site is especially important for evaluating in-situ capping and MNR alternatives.
Various empirical and modeling methods exist for evaluating sediment and contaminant movement and their consequences. The models normally rely upon site-specific empirical data for input parameters. Both empirical methods and models have limitations, so it is usually important to consider a variety of methods in evaluating a site and to compare the results. For large or complex sediment sites, project managers should approach an assessment of sediment and contaminant movement from the
• A site-specific assessment of empirical site characterization data (see Section 2.8.1);
• A site-specific assessment of the frequencies and intensities of expected routine and extreme events that mobilize sediment (see Section 2.8.2);
• A site-specific assessment of ongoing processes that mobilize contaminants in otherwise stable sediment, such as bioturbation, diffusion, and advection (see Section 2.8.3); and
As noted above, this assessment will frequently require the use of models. A wide variety of models is available, ranging from simple models with small numbers of input criteria to complex, multidimensional models that are data intensive. A discussion of model uses and selection is presented in Section 2.9.
Especially for larger sites, a “lines of evidence” approach should be used to evaluate the extent of sediment and contaminant movement and resultant exposure for various areas of the water body. Where multiple lines of evidence point to similar conclusions, project managers may have more confidence in their predictions. Where the lines of evidence do not concur, project managers should bring their technical experts together to determine the source of the discrepancies and understand their significance.
This approach is described in more detail in Chapter 4, Section 4.4, Evaluation of Natural Recovery.
2.8.1 Data Collection
An assessment of sediment and contaminant movement begins with the collection of a variety of empirical data (i.e., data derived from field or laboratory observation). Although literature values may be available for some parameters, project managers are encouraged to collect site-specific information for the most important processes at the site (as identified in the conceptual site model), especially where large resources are at stake in decision making.
The vertical and horizontal sediment and contaminant distributions present at a site are a result of all of the routine and extreme, natural and anthropogenic processes that contribute to the physical, chemical, and biological attributes of a water body. Site conditions at the time of investigation generally reflect a combination of influences. Project managers should not assume that current conditions represent 2-25 Chapter 2: Remedial Investigation Considerations stable conditions when, in fact, sediment may be actively responding to recent or current forces and events. Conversely, project managers should not assume a site or all areas of a site are unstable or contaminants are mobile at a scale or rate which significantly impacts risk. At many sites, the same areas of contamination persist over many years, despite some level of surface sediment and contaminant redistribution.
Processes that are important in terms of exposure and risk on a watershed scale may be less important in smaller, more isolated areas of a water body. Both scales of investigation may be needed.