Contaminated Sediment Risk Assessment
Contaminated sediments in rivers and streams, lakes, coastal harbors, and estuaries have the potential to pose ecological and human health risks. The goals of risk assessment applied to contaminated sediments are to characterize the nature and magnitude of the current and potential threats to human health, wildlife and ecosystem functioning posed by contamination; identify the key factors contributing to the potential health and ecological risks; evaluate how implementation of one or more remedy actions will mitigate the risks in the short and long term; and evaluate the risks and impacts from sediment management, both during and after any dredging or other remedy construction activities.
- 1 Introduction
- 2 Fundamentals of Sediment Risk Assessment
- 3 Key Considerations
- 4 Role in Sediment Management
- 5 Summary
- 6 References
- 7 See Also
- Contaminated Sediments - Introduction
- In Situ Treatment of Contaminated Sediments with Activated Carbon
- Passive Sampling of Sediments
- Sediment Capping
- Contaminated Sediment Remediation Guidance for Hazardous Waste Sites
- Principles for Environmental Risk Assessment of the Sediment Compartment
- Assessing and managing contaminated sediments:
Improving the management of contaminated sediments is of growing concern globally. Sediment processes in both marine and freshwater environments are important to the function of aquatic ecosystems, and many organisms rely on certain sediment quality and quantity characteristics for their life cycle. Human health can also be affected by sediment conditions, either via direct contact, as a result of sediment impacts on water quality, or because of the strong influence sediments can have on the quality of fish and shellfish consumed by people.
Science-based methods for assessing sediment quality and use of risk-based decision-making in sediment management are important for identifying conditions suspected to adversely affect ecological and human services provided by sediments, and predicting the likely consequences of different sediment management actions.A common approach to achieving the explicit management goals inherent in different sediment assessment frameworks in North America and elsewhere is the use of the ecological risk assessment (ERA). An ERA “evaluates the likelihood and magnitude of adverse effects from exposure to a chemical for organisms, such as animals, plants, or microbes, in the environment”. An ERA provides information relevant to sediment management decision-making . It should be based on sound science and performed in a technically defensible manner that is cost-effective and aimed at protecting human health and the environment.
Sediment risk assessment is a specific application of ERA. It may include aspects of a human health risk assessment, as well, to examine the direct and indirect consequences of sediment conditions on human health. It is increasingly used by governmental agencies to support sediment management in freshwater, estuarine, and marine environments. Strategies for sediment management encompass a wide variety of actions, from removal, capping or treatment of contaminated sediment to the monitoring of natural processes, including sedimentation, binding, and bio- and photo-degradation that serve to reduce the potential threat to aquatic life over time. It is not uncommon to revisit a sediment risk assessment periodically to check how changed environmental conditions reflected in sediment and biotic sampling work has either reduced or exacerbated the threats identified in the initial assessment.
At present, several countries lack common recommendations specific to conducting risk assessment of contaminated sediments. In the European Union, sediment has played a secondary role in the Water Framework Directive (WFD), with most quality standards being focused on water with the option for the development of national standards for sediment and biota for bioaccumulative compounds. The Common Implementation Strategy (CIS) in 2010 provided guidance on the monitoring of contaminants in sediments and biota, but not on risk-based decision-making. Additional changes to the strategy were initiated in 2021 to incorporate guidance for management of contaminated sediment . Sediment risk assessment guidance from Norway, Canada, the Netherlands, and the US are most often referenced when assessing the risks from contaminated sediments. Some European countries, such as Norway, have focused their risk assessment guidance on the assessment of sediment conditions relative to general chemical thresholds, while in North America, risk assessment guidance focuses on site- or region-specific conditions.
There is general consensus from a regulatory perspective, globally, on the importance of sediment risk assessment. Technical guidance documents prepared by Canada , the European Union, and the United States Environmental Protection Agency (USEPA) advise a flexible, tiered approach for sediment risk assessment. Sediment quality guidelines in many countries reflect the scientific importance of including certain sediment-specific measurement and biotic assessment endpoints, as well as certain physical sediment processes and chemical transformation processes potentially affecting biotic responses to contaminant exposure in the sediment. New risk assessment methods continue to emerge in the scientific literature. These new methods, however, are likely to be considered supplemental to the more generalized framework shared globally.
Fundamentals of Sediment Risk Assessment
Whereas there is strong evidence of anthropogenic impacts on the benthic community at many sediment sites, the degree of toxicity (or even its presence or absence) cannot be predicted with absolute certainty using contaminant concentrations alone. A sediment ERA should include lines of evidence (LOEs) derived from several different investigations. One common approach to develop several of these LOEs in a decision framework is the triad approach. Triad-based assessment frameworks require evidence based on sediment chemistry, toxicity, and benthic community structure (possibly including evidence of bioaccumulation) to designate sediment as toxic and requiring management or control. In some decision frameworks, particularly those used to establish and rank risks in national or regional programs, all components of the triad are carried out simultaneously, with the various LOEs combined to support weight of evidence (WOE) decision making. In other frameworks, LOEs are tiered to minimize costs by collecting only the data required to make a decision and leaving some potential consequences and uncertainties unresolved.
Figure 1 provides an overview of a sediment risk assessment process. The first step, and a fundamental requirement, in sediment risk assessment involves scoping and planning prior to undertaking work. This is important for optimizing the available assessment resource and conducting an assessment at the appropriate level of detail that is transparent and free, to the extent possible, of any bias or preconceived beliefs concerning the outcome.
Screening-Level Risk Assessment (SLRA)
Technical guidance in many countries strongly encourages sediment risk assessment to begin with a Screening-Level Risk Assessment (SLRA). The SLRA is a simplified risk assessment conducted using limited data and often assuming certain, generally conservative and generic, sediment characteristics, sediment contaminant levels, and exposure parameters in the absence of sufficient readily available information.
The analysis is often semi-quantitative, and typically includes comparisons of various chemical and physical sediment conditions to threshold limits established in national or international regulations or by generally accepted scientific interpretations. US technical guidance encourages the comparison of contaminant measurements in water, sediment, or soil to National Oceanographic and Atmospheric Administration (NOAA) sediment screening quick reference tables, or SQuiRT cards, which list quality guidelines from a range of sources, based on differing narrative intent.
The screening level approach is intended to minimize the chances of concluding that there is no risk when, in fact, risk may exist. Thus, the results of an SLRA may indicate contaminants or sediments in certain locations in the original study area initially thought to be of concern are acceptable (i.e., contaminant levels are below threshold levels), or that contaminant levels are high enough to indicate immediate action without further assessment (e.g., contaminant levels are well above probable-effects guidelines). In other cases, or at other locations, SLRA may indicate the need for further examination. Further study may apply site-specific, rather than generic and conservative assumptions, to reduce uncertainty.
Detailed Risk Assessment
Detailed sediment risk assessment is conducted when SLRA results indicate one or more sediment contaminants exceed background conditions or regulatory threshold limits. For some contaminants, such as the dioxins and other persistent, bioaccumulative, and toxic substances (PBTs), technical guidance may mandate further examination, regardless of whether threshold levels are exceeded. Detailed sediment risk assessment typically follows a three-step framework similar to that described for ecological risk assessment - problem formulation, analysis, and risk characterization.
US sediment management guidance describes a detailed risk assessment process similar to that followed for US ecological risk assessment. The first step is problem formulation. It involves defining chemical and physical conditions, delineating the spatial footprint of the sediment area to be examined, and identifying the human uses and ecological features of the sediment. Historical data are included in this initial step to better understand the results of biota, sediment, and water sampling as well as laboratory toxicity testing results. The SLRA is often included as a part of problem formulation.
The second step is analysis, which includes both an exposure assessment and an effects assessment. The exposure assessment includes the identification of pathways by which human and aquatic organisms might directly or indirectly contact contaminants in the sediment. The exposure route (i.e., ingestion, dermal, or inhalation of particulates or gaseous emissions) and both the frequency and duration of contact (i.e., hourly, daily, or seasonally) are determined for each contaminant exposure pathway and human and ecological receptor. The environmental fate of the contaminant, factors affecting uptake, and the overall exposure dose are included in the calculation of the level of contaminant exposure. The effects assessment identifies the possible short-term (acute) and long-term (chronic) biological responses associated with different levels of exposure for each contaminant and human and ecological receptor.
The third step is risk-characterization. It involves calculating the risks for each human and ecological receptor posed by each sediment contaminant, as well as the cumulative risk associated with the combined exposure to all contaminants exerting similar biological effects. An uncertainty analysis is often included in this step of the risk assessment to convey where knowledge or data are lacking regarding the presence of the contaminant in the sediment, the biological response associated with exposure to the contaminant, or the behavior of the receptor with respect to contact with the sediment. A sensitivity analysis also may be conducted to convey the range of exposures (lowest, typical, and worst-case) and risks associated with changes in key assumptions and parameter values used in the exposure calculations and effects assessment.
Sediment management, particularly at the watershed or river basin scale, involves a wide variety of different environmental, governmental, and societal issues. Incorporating these different views, interests, and perspectives into a form that builds consensus for whatever actions and goals are in mind (e.g., commercial ports and shipping, navigation, flood protection, or habitat restoration) necessitates a formal stakeholder engagement process.
Results from a three-year (2008-2010) Sediment and Society research project funded by the Norwegian Research Council point to three important challenges that must be resolved for successful stakeholder engagement: (1) including people who have important management information and local knowledge, but not much influence in the decision-making process; (2) securing resources to ensure participation and (3) engaging and motivating stakeholders to participate early in the sediment remediation planning process.
Conceptual Site Model
The preparation of a conceptual site model (CSM) is a fundamental component of problem formulation and the first step in sediment risk assessment. The CSM is a narrative and/or illustrative representation of the physical, chemical and biological processes that control the transport, migration and actual or potential impacts of sediment contamination to human and/or ecological receptors. The CSM should include a “food web” because the aquatic food web is an important exposure pathway by which contaminants in the sediment reach humans and pelagic aquatic life.
The CSM provides an early opportunity for critical examination of the interactions between sediment and the water column and the influence of groundwater inputs, surface runoff, and hydrodynamics. For example, there are situations where impacts in the aquatic food web can be driven by ongoing inputs to the water column from upstream sources, but mistakenly connected to polluted sediments. Other considerations included in a CSM can be socio-economic and include linkages to the ecosystem services provided by sediments, or the social, economic and environmental impacts of sediment management alternatives. In such cases where the sediment risk assessment is intended to address the longer-term societal benefits of different management actions (including no action), the CSM could be viewed as part of a sustainable development strategy, or SustCSM. At a minimum, however, the purpose of the CSM is to illustrate the scope of the risk assessment and guide the quantification of exposure and risk.
An important consideration in exposure analysis is the determination of the bioavailable fraction of the contaminant in the sediment. There are two considerations. First, the adverse condition may be buried deep enough in sediments to be below the biologically available zone; typically, conditions in sediment below a depth of 5 cm will not contact burrowing benthic organisms. If there is no prospect for the adverse condition to come closer to the surface, then the risk assessment could conclude the risk of exposure is insignificant. The second consideration relates to chemistry and the factors involved in the binding to sediment particles or the chemical form of the substance in the sediment. However, these assumptions should be examined in the context of climate change, and the likelihood of more frequent and extreme events, putting burial at risk, higher temperatures and changing biogeochemical conditions, which may alter environmental fate of contaminants, compared to historical studies.
The above contaminant bioavailability considerations are important factors influencing assumptions in the risk assessment about contaminant exposure. There have been recent advances in the use of sorbent amendments applied to contaminated sediments that alter sediment geochemistry, increase contaminant binding, and reduce contaminant exposure risks to people and the environment. Passive sampling techniques have emerged to quantify chemical binding to sediment and determine the freely dissolved concentration that is bioavailable.
Assessment and Measurement Endpoints
Assessment and measurement endpoints used in sediment risk assessment are comparable to those described in USEPA ecological risk assessment guidance. A sediment risk assessment, and ecological risk assessments more broadly, must have clearly defined endpoints that are socially and biologically relevant, accessible to prediction and measurement, and susceptible to the hazard being assessed.
Assessment endpoints for humans include both carcinogenic and noncarcinogenic effects. Due to their assumed higher levels of exposure, human receptors in sediment risk assessment typically include recreational, commercial, and subsistence fishermen, i.e., people who might be at increased risk from eating fish or contacting the sediment or water on a regular basis such as indigenous peoples, immigrants from fishing cultures, and subsistence fishers who rely upon fish as a major source of protein. Special considerations are given to women of child-bearing age, pregnant women and young children. Assessment endpoints for ecological receptors focus on benthic organisms, resident fish, piscivorous and other predatory birds and marine mammals. Endpoints typically include mortality, reproductive success and population susceptibility to disease or similar adverse chronic conditions.
Measurement endpoints are related quantitatively to each assessment endpoint. Whenever practical, multiple measurement endpoints are chosen to provide additional lines of evidence for each assessment endpoint. For example, for humans, it might be possible to measure contaminant levels in both food items and human blood or tissue. For predatory fish, birds and mammals, it might be possible to measure contaminants in both prey and predator tissues. Measurement endpoints can be selected to assess non-chemical stressors as well, such as habitat alteration and water turbidity. Typically, measurement endpoints are compared to measurements at a reference site to ascertain the degree of departure from local natural or background conditions.
Sediment Toxicity Testing
Sediment bioassays are an integral part of effects characterization when assessing the risks posed by contaminated sediments and developing sediment quality guidelines. The selection of appropriate sediment bioassays is dependent on the questions being addressed, the physical and chemical characteristics of the sediment matrix, the nature of the contaminant(s) of concern, and preferences of the supervising regulatory authority for the test method and test organisms. Bioassay procedures have been standardized in several countries, and it is not unusual for different test methods to be required in different countries for the same sediment management purpose. Guidance documents in Australia, Canada, Europe and the US cover the wide range of sediment bioassay procedures most often used in risk assessment.
In general, sediment toxicity tests focus on either (acute) lethality in whole organisms (typically benthic infaunal species such as amphipods and polychaetes) following short-term or acute exposures (<14 days) or (chronic) sublethal responses (e.g., reduced growth or reproduction or both) following longer-term exposures. It is not unusual in sediment risk assessment to rely on more than one sediment bioassay. Both acute and chronic tests involving either solid-phase or pore-water sediment fractions can be useful to discern the contributions of different contaminants in whole sediment by examining the response of different endpoints in different test organisms. The application of more specialized techniques such as toxicity identification evaluations (TIEs) have also proved useful to help identify contaminants or contaminant classes most likely responsible for toxicity and to exclude potentially confounding factors such as ammonia.
As part of the overall analysis of risk from exposure to certain sediment conditions, it is generally understood there is a moderate degree of uncertainty associated with sampling and the environmental fate of contaminants; an order of magnitude of uncertainty associated with ecological exposure and dose-response; and greater than an order of magnitude of uncertainty associated with the quantification of potential human health effects. The sources of uncertainty and significance to sediment risk assessment can vary widely, thereby affecting confidence in the decisions made based on risk assessment.
Consequently, technical guidance in several countries encourages including a quantitative uncertainty analysis in sediment risk assessment. The aim of uncertainty analysis is to express either quantitatively or qualitatively the limitations inherent in predicting exposures and effects and, ultimately, the level of overall risk posed by sediment conditions. Sediment risk assessment increasingly relies on a weight-of-evidence process to improve the certainty of conclusions about whether or not impairment exists due to sediment contamination, and, if so, which stressors and biological species (or ecological responses) are of greatest concern. Recent advancements, including the use of Bayesian networks and geographic information systems, also help capture the range of variability in both measured and predicted exposures and responses. The level of sophistication applied to the uncertainty analysis is a subjective consideration and often decided by regulatory pressures, public perceptions, and the likely cost (not only economic, but also social and environmental) of mitigating or removing the contamination.
Role in Sediment Management
Whether or not remediation of contaminated sediments is warranted depends on the magnitude of direct or indirect health risks to humans, ecological threats to aquatic biota, and the extent of risk reduction that can be achieved by removal or containment of the contamination. As all sediment management also introduces risk pathways, such as sediment re-suspension leading to contaminant release, possible impacts due to land, water and energy usage, and risk to workers, remedial decision-making should also consider the risks posed by the remedial process. There are two types of remediation risks inherent in sediment remediation - engineering and biological. Sediment remedy implementation risks are predominantly short-term engineering issues associated with applying the remedy such that worker and community health and safety are protected, and equipment failures and accidents are minimized. Sediment residual risks are predominantly longer-term concerns associated with the consequences of residual chronic exposures and effects to humans, aquatic biota, and wildlife after the remedy has been implemented.
In addition to evaluating sediment conditions prior to remediation, sediment risk assessment can be useful to predict the extent to which engineering risks, contaminant exposure pathways, and different human and wildlife populations at risk might change with different remediation options. Decision tools such as multi-criteria decision analysis (MCDA), or sustainability assessment, for example, incorporate elements from sediment risk assessment to support remediation decision making. Sediment risk assessment also plays an important role in the implementation of monitored natural recovery (MNR) as a remediation strategy. Insofar as ecological recovery is affected by surface‐sediment‐contaminant concentrations, the primary recovery processes for MNR are natural sediment burial and transformation of the contaminant to less toxic forms by biological or chemical processes.
Since risk reduction is the long‐term goal of contaminated sediment management, predicting the rate at which contaminant exposures and risks are mitigated by sedimentation and degradation over time can be aided by including parameters in the risk assessment that calculate the rate of contaminant removal or decay in the sediment. Evaluating sediment management options in terms of risk reduction involves assessing risks for the plausible range of environmental conditions expected in the affected waterbody, which includes the current state of the site as well as the conditions that might occur during the remedy implementation and long after the work is complete and the ecosystem stabilizes.
Effective sediment risk assessment begins with an initial scoping and planning exercise. The work proceeds to a SLRA and, if warranted, detailed risk assessment using a process comparable to an ERA. The key elements of sediment risk assessment must include a well‐designed and site‐specific CSM; a transparent and well‐thought‐out biological and chemical data collection and analysis plan; carefully selected reference sites and decision criteria; and an explicit discussion of uncertainty. If the sediment risk assessment concludes that unacceptable risks exist, the plausible risk management strategies must be identified, evaluated, selected, implemented, and their success monitored relative to the outcomes predicted in the risk assessment.
Sediment risk assessments are designed to simulate and predict plausible interactions between contaminants or other stressors and both ecological and human receptors. The intent is to derive meaningful insights that provide conclusions that are both rational and protective, in that they err on the side of over-estimating the likely environmental risks. Although conservative assumptions should always be used early in the sediment risk assessment process, final decisions should be supported by refined, realistic estimates of risk provided by site‐specific data and sound analytical approaches. It is increasingly evident after nearly 50 years of application that sediment risk assessment is most useful when supported by a well‐designed, site‐specific, and tiered assessment process.
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