Processes, Assessment and Remediation of Contaminated Sediments

The decades of the 1960s and 1970s reflected an awakening of an environmental consciousness in America and increasing efforts to reduce the uncontrolled or poorly controlled releases into the natural environment that had characterized past human activity. It is perhaps hard to understand today the common belief that the air, soil and water were effectively limitless and therefore appropriate for largely uncontrolled disposal of our wastes. It is also hard to imagine that during the 1940s through the 1960s there were acute air pollution episodes in various cities around the globe that led to the premature deaths of people at a rate that was easily observable. More than 4,000 excess deaths occurred during a “killer smog” episode in London in 1952 due to the combination of normal air emissions and adverse atmospheric conditions. These acute episodes helped galvanize public opinion, leading ultimately to regulations such as the Clean Air Act amendments of 1970. Following passage of these regulations, air quality improvements were rapid in many areas although we continue to work to manage the more difficult air pollution problems.

Contaminated sediments exhibit many features and challenges that differentiate its assessment and remediation from that of contaminated soil. Both soils and sediments tend to accumulate the hydrophobic organic and inorganic constituents that give rise to environmental contamination and risk. Sediments, however, are often found in dynamic environments that can lead to substantial contaminant migration. In general, contaminated sediment sites are a legacy of past contaminant discharge practices and the contaminants have accumulated in environments that are most conducive to such accumulation. Thus, a preponderance of contaminated sediment sites are in fine-grained, often organic-rich, sediments that are more likely to absorb hydrophobic contaminants and in environments where such sediments tend to accumulate, i.e., low energy depositional environments. Contaminated soils, however, often represent the source areas themselves and may exhibit a broader range of environmental and media properties. In addition, many of the processes that influence contaminant migration and fate in sediments (erosion, bioturbation, hyporheic exchange) are less pronounced or nonexistent at contaminated soil sites. Even when soils and sediments exhibit similar properties, there may be significant differences due to sediment characteristics. For example, the erosion characteristics of sediments are often controlled by the cohesive nature of fine-grained sediment depositions compared to the minimal cohesion of dry, wind-blown soils.

Moreover, sediments are often confined by one or more spatial dimensions, for example the containment of river sediments to the banks of a river and the adjacent floodplain, limiting the dilution often associated with migration. Thus, contaminated sediment sites may exhibit elevated concentrations and potential risks over large areas or distances compared to many contaminated soil sites. Contaminated sediment sites, by definition, are also associated with large amounts of water, which both complicates their assessment and management but also enhances the potential exposure and risk, for example to aquatic animals and organisms that depend upon them for food.

The goals of this chapter are to (1) describe the processes that govern the transport of sediment in surface waters, (2) provide guidance for use in assessing and/or quantify sediment transport, and (3) describe the procedure to use in modeling sediment transport. A basic knowledge of these topics is requisite to understanding many of the contaminant transport processes important in sediments due to the strong particle associations of most contaminants of concern. This chapter starts with brief overviews of sediment transport – sedimentation related problems and how sediment in surface waters responds to the forces that cause water movement. Basic sediment transport processes are also defined. Section 3.2 describes pertinent properties of sediments, and transport processes for cohesive sediments. Section 3.3 provides guidance to use to assess and/or quantify sediment transport. It is often necessary for remedial project managers to conduct a Sediment Transport Assessment in support of a remedial alternatives evaluation for contaminated sediment Superfund sites. The assessment involves using a systematic approach that (1) identifies the processes and mechanisms that might result in erosion, (2) determines the most appropriate methods to use to assess sediment resuspension and deposition, and (3) quantifies sediment resuspension and deposition rates under varying flow conditions. Section 3.4 provides an overview of the procedures following in performing a sediment transport modeling study. These procedures or steps include: (1) model selection and setup, (2) hydrodynamic modeling, (3) sediment transport modeling, (4) calibration and validation of the models, and (5) analyzing model results.

Natural, surficial, cohesive (clay-bearing), aquatic sediments are subject to a variety of phenomena in which physics, rather than say chemistry, plays an essential role; this includes, but is not limited to, bioturbation, self-weight compaction, and phase growth. Scientific monographs (e.g., Berner, 1971, 1980; Boudreau, 1997; DiToro, 2001; Burdige, 2006; Schultz and Zabel, 2006) that focus on early diagenesis, i.e., those changes occurring in the top 1–10 meters (m) of aqueous sediments, make only passing reference to the physics of early diagenetic phenomena. In contrast, civil engineers, soil physicists and geophysicists have afforded great attention to the physics/mechanics of compaction, particularly in soils, anthropogenic sediments and basin-scale studies (e.g., Yong and Warkentin, 1966; Giles, 1997; Wang, 2000; Craig, 2004; Mitchell and Soga, 2005; Das, 2008); yet, this knowledge has not been effectively transferred to obtain a better understanding of early diagenesis.

Over the past few decades, risk assessments have become an important component of remedial investigations (RI) and feasibility studies (FS) for contaminated sediment sites. In the United States, the National Contingency Plan (NCP) requires that risk assessments be conducted to address the threat posed by the release of contamination to the environment. Risk assessment is typically viewed as an important early step in the process of determining whether remediation of contaminated sediment is necessary. Risk assessments have, however, become increasingly process oriented, with more emphasis on how to do the risk assessment and less on how to ensure that the assessment is useful for decision-making. Although following a defined process that is supported by guidance is advantageous, a process-dominated approach that lacks consideration of other important factors for managing contaminated sediments has shortcomings. The risk assessment needs to include early and explicit consideration of potential risk management options.

Sediments are the ultimate sinks for most hydrophobic organic compounds (HOCs) and metals in aqueous systems. These contaminants can then pose a long-term risk to organisms that dwell or interact with the sediments or to higher organisms through the food chain. The starting point for the assessment of sediment toxicity or effects is bulk contaminant concentrations normalized by sediment mass (Chapman et al., 1999). The values are relatively easy to obtain and are useful as an initial screening tool to assess contamination. These values do not take into account important properties of the sediment, such as the concentration of sulfides, iron oxides, and organic contents, which greatly affect metals availability in sediments, or organic sequestering phases, which can reduce organic chemical availability. Hence, the toxic level of contaminants derived from bulk sediment loading has been proven to vary significantly among different sediments (Di Toro et al., 1990; Chapman et al., 1999).

Contaminated sediments present a serious and vexing problem. The legacy of poor environmental practices related to industrial, agricultural, and residential uses of chemicals, waste water treatment, storm water management, as well as numerous other activities affecting water quality are evident in the current challenges facing public and private organizations addressing the risks posed by contaminated sediments. Managing these risks involves detailed consideration of a complex set of processes (e.g., physical, chemical, biological, socioeconomic, etc.) operating over broad spatial and temporal scales. Large uncertainties related to these processes cloud projections about the future performance of remedies. The number of technologies that are currently available for application at contaminated sediment sites is limited to a small number of variations of dredging, capping, treatment (both in situ and ex situ) and monitored natural recovery (MNR). The economic and environmental costs of managing contaminated sediment risks are large (e.g., project costs for each of the Fox and Hudson River cleanup projects in the United States are pushing $1 billion). Sediment cleanup projects are further complicated by the diverse range of policies, perspectives, risk attitudes and personal values that pertain to risk management decisions. Government institutions, private organizations and local communities face a number of difficult problems connected to risk management for contaminated sediments. Our purpose here is to analyze the risk management problem posed by contaminated sediments, present a series of guidelines for advancing risk management practice, and describe the path toward more effective risk management solutions.

Monitored natural recovery (MNR) of contaminated sediments is a remedial approach that relies on natural physical, chemical, and biological processes to isolate, destroy, or otherwise reduce the bioavailability or toxicity of contaminants (USEPA, 2005a; NRC, 1997). Like other sediment remedies, MNR typically includes contaminant source control, site investigation, development of a conceptual site model (CSM), and long-term monitoring. Unlike other remedies, MNR does not include a construction phase; however, it is not a “no-action” approach. If monitoring indicates that recovery is not proceeding as predicted, site managers may implement enhanced MNR (EMNR, discussed later in this chapter), combine MNR with other remedies such as capping, removal, or institutional controls, consider alternate remedies, or adjust expectations of MNR recovery (Magar et al., 2009).

Challenges to sediment remediation include not only the sheer scope of contamination but also technical limitations and escalating costs associated with cleanup. The development of in situ sediment remediation technologies, mirroring the development of successful in situ groundwater remediation approaches, has recently been identified as a priority research need (SERDP and ESTCP, 2004) and could result in treatments that are more effective compared to traditional methods. Implementation of technically feasible and cost-efficient in situ remediation approaches, such as in situ biotransformation, provides numerous potential advantages which could contribute to successful contaminated sediment management. Most notably, in situ biotransformation can directly reduce contaminant concentrations and/or toxicity. When occurring naturally, in situ biotransformations could serve as a key component for management strategies based on monitored natural recovery (MNR) and could potentially be incorporated into capping and combined remedy designs. In other scenarios, engineering may be required to stimulate particular microbial populations and/or manipulate environmental conditions to optimize biotransformation (and biodegradation) activity.

Sediments accumulate hydrophobic organic compounds (HOCs) such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and dichlorodiphenyltrichloroethane (DDT). Sediments thus act as reservoirs, exposing HOCs to benthic biota, releasing HOCs into porewater, and contributing HOCs to the aquatic food web. It has been observed that certain sediment particle types, known as black carbon (BC), have stronger sorption capacity than inorganic particles with coatings or inclusions of natural organic matter (Ghosh et al., 2003). Char, charcoal, soot, and their derivatives are such types with strong sorption capacity. Once the HOCs are sorbed into the BCs, they become much less available than HOCs sorbed on other mineral-based particles (Ghosh et al., 2000, 2003). These findings motivated studies of a novel in situ sediment treatment strategy using carbonaceous strong sorbents such as activated carbon (AC) to sequester HOCs. Activated carbon has been selected for most studies due to its high affinity for HOCs. By incorporating AC into HOC-contaminated sediment, HOCs would be redistributed, sorbed onto AC particles, and become less available to porewater and biota.

The historical release of contaminants into the environment has generated a legacy of contaminated sites throughout the world. For years, the sediments in water bodies adjoining these pollution sources served as sinks for contaminants, particularly hydrophobic organic compounds (HOCs) and heavy metals. Many of these original sources have been eliminated, but the sediments that formerly served as a pollutant sink now serve as sources of contamination and residual environmental risk. Assessment and remediation of these contaminated sediment sites have been the subject of much scientific analysis, public debate and technological innovation (NRC, 2001).There are few economically viable options for management of contaminated sediments. Capping sediments with a layer of clean material is one of few alternatives with a proven record of success for sediment remediation. This chapter is intended to describe the tools and techniques that are applicable for 55 assessment, design, implementation and monitoring of capping as a remedy for contaminated sediment sites.

Removal of contaminated sediments from the water body with subsequent treatment and/or disposal of the contaminated dredged sediment is the most common approach for contaminated sediment remediation. Various excavation equipment types and approaches have been used, including both dredging (excavation underwater) and excavation of the sediments in the dry. Excavation can be used as the sole active remediation approach or can be used in combination with monitored natural recovery (MNR) and/or capping. This chapter focuses primarily on environmental dredging as a contaminated sediment remedy component. The various treatment and disposal options available for contaminated sediments are also described in this chapter with a description of how they relate to the environmental dredging process, but these options are not covered in detail.

Contaminated sediment remediation is a long-term, often decadal, process from initial characterization to achieving remedial action objectives (RAOs). Monitoring remedial effectiveness is critically important in contaminated sediment management. It seeks to answer the fundamental question of “Were we successful?” As a result, it is also a topic of great sensitivity. From a pragmatic point of view, there are many disincentives to conducting remedy effectiveness monitoring. What happens if the remedy is not “successful” and hundreds of millions of private and public dollars have been spent over many years of cleanup, after years of investigation and negotiation? Do we start over again? Determine it cannot be done? While this concern is very real, it does not outweigh the statutory requirements, cost accountability, human and ecological risk implications, and the standards of good governance and environmental stewardship that mandate remedy effectiveness be tracked and verified.