How Superfund Cleans Up Sediment Sites
EPA uses the Superfund cleanup process to discover, evaluate and remediate sites with unacceptable levels of sediment contamination.
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During a preliminary assessment/site investigation, EPA reviews historical information and visits the site to determine if it poses a threat to people and the environment.
EPA generally conducts a remedial investigation in conjunction with a risk assessment.
During the remedial investigation, EPA conducts sampling and analysis to determine the nature and extent of the contamination.
The risk assessment estimates the potential impacts of the contaminated sediments on humans and the environment to determine if contaminant concentrations are high enough to warrant concern. Many sediment contaminants are suspected carcinogens and some, such as polychlorinated biphenylsA group of toxic, persistent chemicals used in electrical transformers and capacitors for insulating purposes, and in gas pipeline systems as a lubricant. The sale and new use of PCBs were banned by law in 1979 although large reservoirs of PCBs remain in the environment. (PCBs) and mercuryHeavy metal that can accumulate in the environment and is highly toxic if breathed or swallowed., bioaccumulate in the food chain. Risk assessors have developed an approach to assess this risk.
- Learn more: EPA Risk Assessment
Next, EPA develops a feasibility study to present a range of options for remediating the contaminated sediments. EPA evaluates the options and selects the best one by using the nine remedy selection criteria in the National Oil and Hazardous Substances Pollution Contingency Plan:
- Protection of human health and the environment;
- Compliance with Applicable or Relevant and Appropriate Requirements;
- Long-term effectiveness and permanence;
- Toxicity, mobility or volume reduction through treatment;
- Short-term effectiveness;
- State agency acceptance; and
- Community acceptance.
The selected remedy is intended to be protective of human health and the environment, maintain protection over time, and minimize untreated waste.
Remediation is when EPA designs the technical specifications for cleanup remedies and technologies, and then constructs and implements those approaches.
Generally, for remediation at sediment sites to work properly, the source of the contamination needs to be remedied first. It makes little sense to remove contaminated sediments if an active source will recontaminate the area.
Once the contamination source is addressed, remediation of the sediment can begin. The most common sediment treatment technologies are monitored natural recovery, capping, and dredging and excavation.
Monitored natrual recovery takes advantage of natural processes, including natural burial of contaminated sediments with clean sediments.
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.
Dredging removes contaminated sediment from a water body without draining or diverting the water. Dredging also removes a certain amount of water with the sediment. The sediments are usually dewatered on land, and the water is usually treated before discharge back to the water body or public treatment works. The contaminated sediment is then disposed of in a landfill or a confined disposal facility. Highly contaminated sediment may be treated before disposal.
Excavation is similar to dredging except that the sediments are partially dewatered by either diverting the surface water from the natural channel or by constructing a dam around them. The contaminated sediments are then removed using conventional construction equipment. This technique produces less water for treatment.
After remedial work is complete, these sites are typically subject to long-term monitoring to support five-year reviews, which provide a means of evaluating the implementation and performance of a remedy and determining if it is or will be protective.