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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.

Related Article(s):

• Contaminated Sediments - Introduction
• In Situ Treatment of Contaminated Sediments with Activated Carbon
• Sediment Capping
• Passive Sampling of Sediments

Contributor(s):

• Richard J. Wenning
• Sabine E. Apitz

Key Resource(s):

• Contaminated Sediment Remediation Guidance for Hazardous Waste Sites

• Principles for Environmental Risk Assessment of the Sediment Compartment

• Assessing and managing contaminated sediments:

Part I, Developing an Effective Investigation and Risk Evaluation Strategy

Part II, Evaluating Risk and Monitoring Sediment Remedy Effectiveness

Introduction

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. 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 the management decision-making process. It should be performed in a scientifically based, defensible manner that is cost-effective and protective of human health and the environment. Therefore, 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.

Cap Design and Materials for Chemical Containment

An inert material such as sand can be effective as a capping material where contaminants are strongly associated with solids and where the operative site specific transport mechanisms do not lead to rapid contaminant migration through such a material. Additional contaminant containment can often be achieved through the placement of clean sediment, e.g. dredged material from a nearby location. Other materials as cap layers or amendments may be useful to address particularly mobile contaminants or when particular degradative mechanisms can be exploited. The Anacostia River was the site of a demonstration that first tested “active” or “amended” capping in the field. Amended caps are often the best option when groundwater upwelling or other advective processes promote significant mobility of contaminants and the addition of sorbents can slow that contaminant migration. Although a variety of materials have been proposed for sediment caps, a far smaller number of options have been successfully employed in the field.

Metals migration is very site dependent due to the potential for many metals to complex with other species in the interstitial water and the specific metal speciation present at a site. Often, the strongly reducing environment beneath a cap renders many common metals unavailable through the formation of metal sulfides. In such cases, a simple sand cap can be very effective. Amended caps to manage metal contaminated sediments may be advantageous when site specific conditions lead to elevated metals mobility, but should be supported with site specific testing.

For hydrophobic organic contaminants, cap amendments that directly control groundwater upwelling and also sorbents that can remove migrating contaminants from that groundwater have been successfully employed. Examples include clay materials such as AquaBlok^® for permeability control, sorbents such as activated carbon for truly dissolved contaminants, and organophilic clays for separate phase contaminants.

The placement of clean sediment as an in situ cap can be difficult when the material is fine grained or has a low density. Capping with a layer of coarse grained material such as clean sand mitigates this issue although clean sands have minimal sorption capacity. Because of this limitation, sand caps may not be sufficient for achieving remedial goals in sites where contamination levels are high or transport rates are fast due to pore water upwelling or tidal pumping effects. Conditions such as these may require the use of “active” amendments to reduce transport rates.

Capping with clean sand provides a physical barrier between the underlying contaminated material and the overlying water, stabilizes the underlying sediment to prevent re-suspension of contaminated particles, and can reduce chemical exposure under certain conditions. Sand primarily provides a passive barrier to the downward penetration of bioturbating organisms and the upward movement of sediment or contaminants. Although conventional sandy caps can often be an effective means of managing contaminated sediments, there are conditions when sand caps may not be capable of achieving design objectives. Some factors that reduce the effectiveness of sand caps include:

• erosion and loss of cap integrity
• high groundwater upwelling rates
• mobile (low sorption) contaminants of concern (COCs)
• high COC concentrations
• unusually toxic COCs
• the presence of tidal influences
• the presence of non-aqueous phase liquids (NAPLs)
• high rates of gas ebullition

Of these, the first three are common limitations to capping and often control the ability to effectively design and implement a cap as a sediment remedial strategy. In these cases, it may be possible to offset these issues by increasing the thickness of the cap. However, the required thickness can reach infeasible levels in shallow streams or navigable water bodies. In addition, increased construction costs associated with thick caps may become prohibitive. As a result of these issues, caps that use alternative materials (also known as active caps) to reduce the thickness or increase the protectiveness of a cap may be necessary. The materials in active caps are designed to interact with the COCs to enhance the containment properties of the cap.

Apatites are a class of naturally occurring minerals that have been investigated as a sorbent for metals in soils and sediments. Apatites consist of a matrix of calcium phosphate and various other common anions, including fluoride, chloride, hydroxide, and occasionally carbonate. Metals are sequestered either through direct ion exchange with the calcium atom or dissolution of hydroxyapatite followed by precipitation of lead apatite. Zeolites, which are microporous aluminosilicate minerals with a high cationic exchange capacity (CEC), have also been proposed to manage metal species.

It is possible to create a hydrophobic, sorbing layer for non-polar organics by exchanging a cationic surfactant onto the surface of clays such as zeolites and bentonites,. Organoclay is a modified bentonite containing such substitutions that has been evaluated for control of non-aqueous phase NAPLs and other organic contaminants. An organoclay cap has been implemented for sediment remediation at the McCormick and Baxter site in Portland, OR. A similar organic sorbing phase can be formed by treating zeolites with surfactants but this approach has not been reported for contaminated sediments.

Activated carbon is a strong sorbent of hydrophobic organic compounds and has been used as a treatment for sediments or as an active sorbent within a capping layer. Placement of activated carbon for sediment capping is difficult due to the near neutral buoyancy of the material but it has been applied in this manner in relatively low energy environments such as Onondaga Lake, Syracuse, NY. Alternatives in higher energy environments include placement of activated carbon in a mat such as the CETCO Reactive Core Mat (RCM)^® or Huesker Tektoseal^®, or as a composite material such as SediMite^® or AquaGate^®. In the case of the mats, powdered or granular activated carbon can be placed in a controlled layer while the density of the composite materials is such that they can be broadcast from the surface and allowed to settle to the bottom. In a sediment treatment application, the composite material would either be worked into the surface or allowed to intermix gradually by bioturbation and other processes. In a capping application, the mat or composite material would typically be combined or overlain with a sand or other capping layer to keep it in place and to provide a chemical isolation layer away from the sediment surface.

As an alternative to a sorptive capping amendment, low-permeability cap amendments have been proposed to enhance cap design life by decreasing pore water advection. Low permeability clays are an effective means to divert upwelling groundwater away from a contaminated sediment area but are difficult to place in the aqueous environment. Bentonite clays can be placed in mats similar to what is done to provide a low permeability liner in landfills. There are also commercial products that can place clays directly such as the composite material AquaBlok^®, a bentonite clay and polymer based mineral around an aggregate core.

Sediment caps become colonized by microorganisms from the sediments and surface water and potentially become a zone of pollutant biotransformation over time. Aerobic degradation occurs only near the solids-water interface in which benthic organisms are active and thus there might still be significant benthic organism exposure to contaminants. Biotransformation in the anaerobic zone of a cap, which typically extends well beyond the zone of benthic activity, could significantly reduce the risk of pollutant exposure but successful caps encouraging deep degradation processes have not been demonstrated beyond the laboratory. The addition of materials such as nutrients and oxygen releasing compounds for enhancing the attenuation of contaminants through biodegradation has also been assessed but not applied in the field. Short term improvements in biodegradation rates can be achieved through tailoring of conditions or addition of nutrients but long term efficacy has not been demonstrated.

Figure 2. A conceptualization of a cap with accompanying habitat layer

Cap Design and Materials for Habitat Restoration

In addition to providing chemical isolation and containment, a cap can also be used to provide improvements for organisms by enhancing the habitat characteristics of the bottom substrate. Often, contaminated sediment environments are degraded for a variety of reasons in addition to the toxic constituents. One way to overcome this is to provide both a habitat layer and chemical isolation or contaminant capping layer. Figure 2 illustrates just such a design providing a more appropriate habitat enhancing substrate, in this case by incorporation additional organic material, vegetation and debris, which is often used by fish species for protection, into the surface layer. In a high energy environment, it should be recognized that it may not be possible to keep a suitable habitat layer in place during high flow events. This would be true of suitable habitat that had developed naturally as well as a constructed habitat layer and it is presumed that if such a habitat is the normal condition of the waterbody that it will recover over time between such high flow events.

Summary

Clean substrate can be placed at the sediment-water interface for the purposes of reducing exposure to and risk from contaminants in the sediments. The cap can consist of simple materials such as sand designed to physically stabilize contaminated sediments and separate the benthic community from those contaminants or may include other materials designed to sequester contaminants even under adverse conditions including strong groundwater upwelling or highly mobile contaminants. The surface of a cap may be designed of coarse material such as gravel or cobble to be stable under high flow events or designed to be more appropriate habitat for benthic and aquatic organisms. As a result of its flexibility, simplicity and low cost relative to its effectiveness, capping is one of the most prevalent remedial technologies for sediments.