- Policy and Guidance
- Conceptual Site Models
- Fate and Transport of Contaminants
- Site Characterization
- Risk Assessment
- Additional Resources
For purposes of this webpage natural sediments are defined as the organic and inorganic materials found at the bottom of a water body. Sediments may include clay, silt, sand, gravel, decaying organic matter, and shells among other things, but exclude anthropogenic debris, such as vehicle tires.
Sediments can become contaminated in a number of ways. Urban runoff that discharges to surface waters often contains polycyclic aromatic hydrocarbons (PAHs), oil and grease, and heavy metals. Agricultural runoff may contain nutrients and pesticides. Industrial spills and releases, especially those that occurred before controls were in place, can put product into the water. Chemicals that are denser than water, such as polychlorinated biphenyls (PCBs) and some pesticides like DDT, will sink to the bottom of water bodies and directly contaminate sediments. Atmospheric deposition of substances such as mercury is another source of sediment contamination as is the discharge of contaminated groundwater through the sediments to the overlying surface water (USEPA 1999 and USEPA 2005).
The classes of contaminants that are most common in sediment contamination are pesticides, PCBs, PAHs, and to a lesser extent dissolved phase chlorinated hydrocarbons. With the right geochemical conditions heavy metals and metalloids can also occur in sediments or precipitate into them. The sediments of many marinas are contaminated with tributyltin, an organo tin compound that was used as a biocide in marine paints (USEPA 1999).
Sediment investigations are generally conducted in two parts. The first uses common sampling and analytical procedures to determine if the total concentrations of contaminants are high enough to warrant concern. The underlying assumption is that all the contaminant is bioavailable. If the data indicate there may be a problem, then the second part of the investigation is done. This part focuses on bioavailability and determining whether there is physical evidence of an impact such as less biodiversity in the impacted sediments and the presence of the chemicals in the tissue of flora and fauna (USEPA 2005).
In addition to evaluating contaminant concentrations, the site investigation needs to develop a very complete conceptual site model. Unlike conventional soil and groundwater investigations, where rapid change in the site conditions is not expected, sediment systems can be very dynamic and it is important for both the risk assessment and remedy selection to have a full understanding of potential changes in site dynamics. A fuller discussion on investigation techniques is found in the Site Characterization section (USEPA 2005).
The risk assessment estimates the potential impacts of the contaminated sediments on human and ecological receptors. Many common organic sediment contaminants are suspected carcinogens and some, such as PCBs and mercury, bioaccumulate in the food chain. Risk assessors have developed a triad, or weight-of-evidence approach, that integrates sediment chemistry, laboratory toxicity testing, and community structure indices to assess risk (Pinkney et al 2005). A more complete discussion of sediment risk assessment and related guidance documents can be found in the Risk Assessment section.
OSWER Directive 9285.6-08 is the main policy guidance for managing contaminated sediments in the Superfund and solid waste programs. Since Superfund responses are subject to other laws and regulations that may be applicable or relevant and appropriate, the directive discusses them and additional related guidance documents. More information can be found in the Policy and Guidance section of this webpage.
OSWER Directive 9285.6-08 emphasizes that remediation at sediment sites generally fail if the source of the contamination is not remedied first. It makes little sense to remove contaminated sediments if an active source will recontaminate the clean sediments. Even innovative technologies like reactive barriers have a finite treatment capability if the active source is not removed. Removal of active sources would include moving upland DNAPLs such as creosote and heavy metals deposition from mining site run off.
The most common sediment treatment technologies are monitored natural recovery, in situ capping, dredging, and excavation (USEPA 2005). Monitored natural recovery takes advantage of natural abiotic and biological degradation processes and includes natural burial of contaminated sediments with clean sediments. Lake Hartwell offers an example of the use of natural burial. The remediation section has a more in depth discussion with examples of these techniques as well as general guidance on sediment site remediation.
In situ capping involves the placement of clean material over the contaminated sediments. When contaminants are relatively immobile in sediment, a cap prevents flora and fauna from contacting them. A relatively impermeable cap can prevent groundwater from discharging through the contaminated sediment. The cap thereby diverts the groundwater away from the contaminated area. Reactive caps are those that use activated carbon mats or organoclays to sorb any contaminants that leave the contaminated sediment pore water (USEPA 2005).
Dredging uses either a hydraulic cutter head or a bucket to directly remove sediment to an onshore treatment and/or disposal area. Both of these techniques produce considerable water that requires treatment before it can be returned to the surface water (USEPA 2005).
Excavation is similar to bucket dredging except that the sediments are partially dewatered by either diverting the surface water from the natural channel or by constructing a coffer dam around them. The contaminated sediments are then removed using conventional construction equipment. This technique has an advantage over dredging in that it produces far less water for treatment (USEPA 2005).
These common sediment treatment technologies are advantageous in highly contaminated source zones where there is a need to quickly reduce the sources. However, in areas of low or moderate levels of contamination an alternative strategy may be more attractive depending on the sensitivity of the ecosystem, and the ultimate goal of the project. Recent research, including limited field studies, has shown the potential for applying amendments which can target certain or a variety of contaminants in effected sediments (Cho, Ghosh et. al. 2009). Strong sorbents, like activated carbon can be amended to sediments to reduce the bioavailability of hydrophobic organic contaminants (PCBs, PAHs, DDT, etc.). Similar sorbents exist for the remediation of various metal contaminants as well.
On this webpage, individual documents specific to a section topic are highlighted in each section with appropriate links. The Additional Resources section is devoted to webpages maintained by EPA and other agencies and groups. These webpages contain several downloadable sediment references.
Toxicological Exposure of Sediment-Bound Hydrophobic Organic Contaminants as a Function of the Quality of Sediment Organic Carbon and Microbial Degradation
Fredrickson, H.L., J.W. Talley, J.S. Furey, and S. Nicholl
USEPA. 1999. Introduction to Contaminated Sediments. EPA 823-F-99-006, Office of Science and Technology, 24 pp.
USEPA. 2004. The Incidence and Severity of Sediment Contamination in Surface Waters of the United States, Second Edition. EPA 823-R-04-007, Office of Science and Technology, 278 pp.
USEPA. 2005. Contaminated Sediment Remediation Guidance for Hazardous Waste Sites, EPA-540-R-05-012. Office of Superfund Remediation and Technology Innovation, 236 pp.
Pinkney, A.E., B.L. McGee, P.C. McGowan, D.J. Fisher, J. Ashley, and D. Velinsky. 2005. Using the Sediment Quality Triad Approach to Characterize Toxic Conditions in the Chesapeake Bay (2002): An Assessment of Tidal River Segments in the Bohemia, Elk, Northeast and Severn Rivers, CBFO-C05-01. USEPA, Chesapeake Bay Program Office, 234 pp.
Field Application of Activated Carbon Amendment for In-Situ Stabilization of Polychlorinated Biphenyls in Marine Sediment. Yeo-Myoung Cho, Upal Ghosh, Alan J. Kennedy, Adam Grossman, Gary Ray, Jeanne E. Tomaszewski, Dennis W. Smithenry, Todd S. Bridges, Richard G. Luthy. Environmental Science & Technology 2009 43 (10), 3815-3823