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Existing commercial, municipal, or hazardous waste landfills are the most widely used option for disposal of dredged or excavated sediment and pretreatment/treatment residuals from environmental dredging and excavation. Landfills also are sometimes constructed onsite for a specific dredging or excavation project. Landfills can be categorized by the types of wastes they accept and the laws regulating their operation. Most solid waste landfills accept all types of waste (including hazardous substances) not regulated as Resource Conservation and Recovery Act (RCRA) hazardous waste or Toxic Substances Control Act (TSCA) toxic materials. Due to typical restrictions on liquids in landfills, most sediment should be dewatered and/or stabilized/solidified before disposal in a landfill. Temporary placement in a CDF or pretreatment using mechanical equipment may therefore be necessary (Palermo 1995).
6.8.2 Confined Disposal Facilities (CDFs)
CDFs are engineered structures enclosed by dikes and specifically designed to contain sediment.
CDFs have been widely used for navigational dredging projects and some combined navigational/environmental dredging projects but are less common for environmental dredging sites, due in part to siting considerations. However, they have been used to meet the needs of specific sites, as have other innovative in-water fill disposal options, for example, the filling of a previously used navigational waterway or slip to create new container terminal space (e.g., Hylebos Waterway cleanup and Sitcum 6-34 Chapter 6: Dredging and Excavation Waterway cleanup in Tacoma, Washington). In some cases, new nearshore habitat has also been created as mitigation for the fill.
Under normal operations of a CDF, water is discharged over a weir structure or allowed to migrate through the dike walls while solids are retained within the CDF. Typically effluent guidelines or discharge permits govern the monitoring requirements of the return water. Details regarding the use and engineering design of CDFs are available in the USACE Engineer Manual, Confined Disposal of Dredged Material (USACE 1987) and the USACE Testing Manual (USACE 2003).
A cross-sectional view of a typical nearshore CDF dike design is shown in Highlight 6-10. CDFs may be located either upland (above the water table), near-shore (partially in the water), or completely in the water (island CDFs). There are several documents available containing thorough descriptions, technical considerations, and costs associated with CDFs (U.S. EPA 1996e, U.S. EPA 1994d, U.S. EPA 1991c, and Averett et al. 1990). Additionally, USACE and EPA (2003) describes a history and evaluation of the design and performance of CDFs used for navigational dredging projects in the Great Lakes Basin, including a review and discussion of relevant contaminant loss and contaminant uptake studies.
Highlight 6-10: Cross Section of a Typical Confined Disposal Facility Dike with a Filter Layer
16' 1.5' 1.5' 1' 1'
Note: Adapted from US. EPA 1998d 6.8.3 Contained Aquatic Disposal (CAD) For purposes of this guidance, contained aquatic disposal is a type of subaqueous capping in which the dredged sediment is placed into a natural or excavated depression elsewhere in the water body.
A related form of disposal, known as level bottom capping, places the dredged sediment on a level bottom elsewhere in the water body, where it is capped. CAD has been used for navigational dredging projects (e.g., Boston Harbor, Providence River), but has been rarely considered for environmental dredging 6-35 Chapter 6: Dredging and Excavation projects. However, there may be instances when neither dredging with land disposal nor capping contaminated sediment in-situ is feasible, and it may be appropriate to evaluate CADs. The depression used in the case of a CAD should provide lateral containment of the contaminated material, and also should have the advantage of requiring less maintenance and being more resistant to erosion than levelbottom capping. The depression for the CAD cell may be excavated using conventional dredging equipment or natural or historically dredged depressions may be used. Uncontaminated material excavated from the depression may be subsequently used for the cap (U.S. EPA 1994d).
6.8.4 Losses from Disposal Facilities
Evaluation of a new on-site disposal facility for placement of contaminated sediment should include an assessment of contaminant migration pathways and should incorporate management controls in the facility design as needed. Landfill disposal options may have short-term releases, which include spillages during transport and volatilization to the atmosphere as the sediment is drying. As for any disposal option, longer-term releases depend in large part on the characteristics of the contaminants and the design and maintenance of the disposal facility.
For CDFs, contaminants may be lost via effluent during filling operations, surface runoff due to precipitation, seepage through the bottom and the dike wall, volatilization to the air, and uptake by plants and animals. The USACE has developed a suite of testing protocols for evaluating each of these pathways (U.S. EPA and USACE 1992), and these procedures are included in the ARCS program’s Estimating Contaminant Losses from Components of Remediation Alternatives for Contaminated Sediments (U.S. EPA 1996e). The USACE has also developed the Testing Manual (USACE 2003), which describes contaminant pathway testing. Depending on the likelihood of contaminants leaching from the confined sediment, a variety of dike and bottom linings and cap materials may be used to minimize contaminant loss (U.S. EPA 1991c, U.S. EPA 1994d, Palermo and Averett 2000). Depending on contaminant characteristics, CDFs for sediment remediation projects may need control measures such as bottom or sidewall liners or low permeability dike cores. Project managers should also be aware that permeability across these barriers can decline significantly with time due to the consolidation process and blockage of pore spaces with fine materials. Therefore, site-specific evaluation is important.
Contaminants may be released as a mud wave outside of the boundaries of the CAD, or to the water column or air during placement of the contaminated sediment. Seepage of pore water may also occur during the initial consolidation of the sediment following placement. Other releases common to insitu caps, such as through erosion of the cap or movement of contaminants through the cap (see Chapter 5, In-Situ Capping) may also occur. Whatever disposal options are evaluated, the rate and potential effects of contaminant losses during construction and in the long term should be considered.
Highlight 6-11 presents some general points to remember from this chapter.
6-36 Chapter 6: Dredging and Excavation Highlight 6-11: Some Key Points to Remember When Considering Dredging and Excavation • Source control should be generally implemented to prevent recontamination • A dredging or excavation alternative should include details concerning all phases of the project, including sediment removal, staging, dewatering, water treatment, sediment transport, and sediment treatment, reuse, or disposal • Transport and disposal options may be complex and controversial; options should be investigated early and discussed with stakeholders • In predicting risk reduction effects of dredging or excavation of deeply buried contaminants, exposure and risk are related to contaminants that are accessible to biota. Contaminants that are deeply buried have no significant migration pathway to the surface, and are unlikely to be exposed in the future may not need removal • Environmental dredging should take advantage of methods of operation, and in some cases specialized equipment, that minimize resuspension of sediment and transport of contaminants. The use of experienced operators and oversight personnel is very important to an effective cleanup • A site-specific assessment or pilot study of anticipated sediment resuspension, contaminant release and transport, and its potential ecological impacts should be conducted prior to full scale dredging • Realistic, site-specific predictions should be made of residual contamination based on pilot studies or data from comparable sites. Where residuals are a concern, thin layer placement/backfilling, MNR, or capping may also be needed • Excavation (conducted after water diversion) often leads to lower levels of residual contamination than dredging (conducted under standing water) • A dredging or excavation project should be monitored during implementation to assess resuspension and transport of contaminants, immediately after implementation to assess residuals, and after implementation to measure long-term recovery of biota and to test for recontamination 6-37 This page left intentionally blank.
Chapter 7: Remedy Selection Considerations
7.0 REMEDY SELECTION CONSIDERATIONSNo two sites are identical and therefore the risk-management strategy will vary from site to site... The strategy selected should be one that actually reduces overall risk, not merely transfers the risk to another site or another affected population. The decision process necessary to arrive at an optimal management strategy is complex and likely to involve numerous site-specific considerations...
Management decisions must be made, even when information is imperfect. There are uncertainties associated with every decision that need to be weighed, evaluated, and communicated to affected parties. Imperfect knowledge must not become an excuse for not making a decision.
In these two statements from the National Research Council’s (NRC’s) report A Risk Management Strategy for PCB-Contaminated Sediments (NRC 2001), the NRC identifies some of the key challenges faced by many project managers at the remedy selection stage. The program goal of the Superfund remedy selection process is to select remedies that are protective of human health and the environment, that maintain protection over time, and that minimize untreated waste [Title 40 Code of Federal Regulations (40 CFR) §300.430(a)(1)(i)]. Superfund remedies must also be cost-effective and use permanent solutions to the maximum extent practicable [Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) §121(b)]. The best route to meeting these and other requirements, as well as the best route to overall risk reduction, depends on a large number of site-specific considerations, some of which may be subject to significant uncertainty. Although final decision making in the face of imperfect knowledge may be necessary, it may be appropriate to postpone a final decision if there is significant doubt about the proposed action’s ability to reduce site risks substantially in light of the potential magnitude of costs associated with addressing certain sediment sites. Postponing a final decision may provide an opportunity to conduct additional investigation or pilot studies, and would not necessarily preclude carrying out appropriate interim response actions at the same time.
7.1 RISK MANAGEMENT DECISION MAKING
Consistent with the National Oil and Hazardous Substances Pollution Contingency Plan (NCP), each of the risk management principles in the U.S. Environmental Protection Agency’s (EPA’s) Principles for Managing Contaminated Sediment Risks at Hazardous Waste Sites (U.S. EPA 2002a; see Appendix A), is important to consider for achieving a successful sediment cleanup. Several of the principles apply more directly to the remedy selection stage, especially Principle 7, Select Site-Specific, Project-Specific, and Sediment-Specific Risk Management Approaches that will Achieve Risk-based Goals. Any decision regarding the specific choice of a remedy for a contaminated sediment site should be based on a careful consideration of the advantages and limitations of available approaches and a balancing of tradeoffs among alternatives.
A risk management process should be used to select a remedy designed to reduce the key human and ecological risks effectively. Another important risk management function generally is to compare and contrast the costs and benefits of various remedies. As noted in EPA’s Ecological Risk Assessment Guidance for Superfund: Process for Designing and Conducting Ecological Risk Assessment (U.S. EPA 1997d), risk assessments should provide a basis for comparing, ranking, and prioritizing risks. The
results can also be used in cost-effectiveness analyses that offer additional interpretation of the effects of alternative management options.
In addition, risk management goals should be developed that can be evaluated within a realistic time period, acknowledging that it may not be practical to achieve all goals in the short term. Risk management of contaminated sediment should comprehensively evaluate the broad range of risks posed by contaminated sediment and associated remedial actions, while recognizing that some risks may be reduced in a shorter time frame than others.
EPA’s Rules of Thumb for Superfund Remedy Selection (U.S. EPA 1997c, also referred to as the “Rule of Thumb Guidance”) is a helpful guidance for project managers to review when making riskmanagement decisions and selecting remedies at sediment sites. The Rules of Thumb Guidance describes key principles and expectations, interspersed with “best practices” based on program experience and policies. In addition, this guidance discusses how remedy selection may also be applicable to the Resource Conservation and Recovery Act (RCRA) Corrective Action Program. For more information on the two cleanup programs, the project manager should refer to Office of Solid Waste and Emergency Response (OSWER) Directive 9200.0-25, Coordination Between RCRA Corrective Action and Closure and CERCLA Site Activities (U.S. EPA 1996f).
Decisions regarding risk management and remedy selection should also consider pertinent recommendations from stakeholders, which frequently include the local community, local government, states, Indian tribes, and responsible parties. Remediation may significantly impact day-to-day activities of residents and recreation-seekers, and operations of commercial establishments near the water body for extended periods. Stakeholders should be involved when designing and scheduling remedial operations, not just during the remedy selection process. Documenting and communicating how and why remedy decisions are made are very important tasks at sediment sites. For guidance on documenting remedy decisions under CERCLA, project managers should refer to EPA’s A Guide to Preparing Superfund Proposed Plans, Records of Decision, and other Remedy Selection Documents, also referred to as the “ROD Guidance” (U.S. EPA 1999a).
7.2 NCP REMEDY SELECTION FRAMEWORK
In the NCP, EPA provides a series of expectations (see Highlight 7-1) to reflect the principal requirements under CERCLA §121 and to help focus the remedial investigation/feasibility study (RI/FS) on appropriate cleanup options. EPA developed nine criteria for evaluating remedial alternatives to ensure that all important considerations are factored into remedy selection decisions. Chapter 3, Section
3.2 outlines the NCP’s nine remedy selection criteria. These criteria are derived from the statutory requirements under CERCLA §121, as well as technical and policy considerations that have proven to be important for selecting among the remedial alternatives. In general, the nine criteria analysis comprises the following two steps: 1) an evaluation of all alternatives with respect to each criterion; and 2) a comparison among the alternatives to determine the relative performance of the alternatives and identify major trade-offs among them (i.e., relative advantages and limitations). Generally this comparison is made on a qualitative basis, although some have attempted a quantitative analysis (e.g., Linkov et al.
2004). Ultimately, the remedy selected must be protective of human health and the environment, attain (or waive) applicable or relevant and appropriate requirements (ARARs), be cost effective, use permanent solutions and alternative treatment technologies or resource recovery technologies to the maximum extent 7-2 Chapter 7: Remedy Selection Considerations practicable, and satisfy a preference for treatment or provide an explanation as to why this preference was not met.