2.3 Superfund — A Deeper Dive | DART – Deloitte Accounting Research Tool

2.3.1 Site Assessment

For a hazardous waste site to be considered a Superfund site, a site assessment must be completed. Superfund site assessments evaluate potential or confirmed releases of hazardous substances that may pose a threat to human health or the environment.

The Superfund site assessment process begins with site discovery or notification of a release or potential release into the environment. The EPA may be notified of hazardous waste activity by citizens, states, tribes, or other environmental programs. After notification, nonfederal sites undergo prescreening for a determination of whether the Superfund site assessment process is appropriate. Sites that are identified as appropriate for the process are assigned a site discovery date and added to the EPA’s active CERCLA site inventory.

Once a site is added to the CERCLA site inventory, a site assessment for determining whether the site warrants short-term or long-term cleanup is performed by the EPA or under the environmental program of a state, a tribe, or another federal agency. In site assessments, data are collected so that hazardous waste sites can be identified, evaluated, and ranked on the basis of Hazard Ranking System (HRS) criteria. The HRS is a numerically based screening system that uses information from initial limited investigations to assess the relative potential threat that sites may pose to human health or the environment. Sites with an HRS score below 28.51 generally require no further Superfund remedial attention and receive a “no further remedial action planned” designation. Sites that require additional study are referred to appropriate cleanup programs, including (1) the Emergency Response and Removal Program, (2) the RCRA Corrective Action program, (3) state and tribal cleanup initiatives such as voluntary cleanup programs (VCPs), (4) the Superfund alternative approach, and (5) the NPL. Note that only sites listed on the NPL are eligible for Superfund Trust Fund–financed remedial actions. The flowchart below illustrates the site assessment process.

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For example, the RCRA Corrective Action Program, a VCP, or the Superfund alternative approach.

In 2017, the EPA added subsurface intrusion as a component of the HRS. This addition allows the EPA to consider and score the threat posed by subsurface intrusion in its HRS analysis. Subsurface intrusion is the migration of hazardous substances or pollutants and contaminants from the unsaturated zone and the surficial groundwater into overlying structures. Although subsurface intrusion can occur through multiple mechanisms, the most common form is vapor intrusion. In 2018, the EPA added two sites to the NPL on the basis of vapor intrusion alone. The listing of these sites may serve as precedent for other sites to be added solely for vapor intrusion. Although most sites that have vapor intrusion are also ranked under the HRS for other exposure pathways, manufacturers may want to consider their current and former facilities to determine whether vapor intrusion poses a significant risk and, if so, whether further investigation or remediation is warranted.

Companies directly affected by the 2018 NPL listing, together with industry groups, expressed concern about the EPA’s unprecedented decision to include a site on the NPL solely on the basis of a subsurface intrusion pathway. One company filed suit against the EPA in the hope of overturning that decision. In July 2020, however, the U.S. Court of Appeals for the D.C. Circuit denied the company’s petition for review. The federal appellate court’s ruling is likely to affect the evaluation and remediation of contamination beneath manufacturing, chemical, and other industrial facilities around the country. In addition, the court’s affirmation of the EPA’s first NPL listing of a Superfund site due exclusively to subsurface intrusion of hazardous substances will inform listing decisions at other sites with subsurface contamination. The precedent that the court’s ruling establishes may result in the inclusion of more sites on the NPL based solely on the existence of subsurface intrusion pathways.

2.3.2 Placement on the NPL

A site’s HRS score determines whether the site is placed on the NPL. Placement is primarily intended to guide the EPA in:

Determining which sites warrant further investigation to assess the nature and extent of the human health and environmental risks associated with a site.
Identifying any appropriate CERCLA-financed remedial actions.
Notifying the public about sites that the EPA believes warrant further investigation.
Serving notice to PRPs that the EPA may initiate CERCLA-financed remedial action.

Therefore, placement on the NPL does not in itself (1) reflect a judgment of the activities of site owners or operators, (2) require those persons to undertake any action, or (3) assign liability to any person.

Generally, after a site has been placed on the NPL, the EPA will send “notice of liability” letters to PRPs for contaminating the site (see Section 2.3.7).

2.3.3 Remedial Investigation and Feasibility Study

A remedial investigation and feasibility study is performed at all sites listed on the NPL. Under the EPA’s Enforcement First for Remedial Action at Superfund Sites policy, the EPA usually asks a PRP to conduct the remedial investigation and feasibility study before the agency uses Superfund money. However, if a PRP cannot be located or is delayed in conducting the remedial investigation and feasibility study, the EPA can use Superfund money to perform the remedial investigation and feasibility study. In some instances, the EPA may conduct its own remedial investigation and feasibility study even though a remedial investigation and feasibility study is being prepared by a PRP. This may occur, for example, if the EPA is attempting to accelerate the cleanup process by focusing on a smaller area within a larger Superfund site while the PRP is completing the remedial investigation and feasibility study for the larger site.

As part of the remedial investigation, the PRP, EPA, or both collect data to characterize site conditions, determine the nature of the waste, assess risk to both human health and the environment, and conduct treatability testing to evaluate the potential performance and cost of the treatment technologies being considered. The data are then evaluated in the feasibility study across a range of alternative remedial actions.

The feasibility study has two main components: (1) development and screening of remedial action alternatives and (2) comparison of each alternative that passes screening in a detailed analysis. A range of remedial action alternatives, including cost estimates for each, is developed during the feasibility study as data from the remedial investigation site characterization become available. Treatability studies help reduce uncertainties related to cost and performance of treatment alternatives.

2.3.4 Remediation Decisions

The EPA has developed nine criteria for evaluating remedial alternatives to ensure that all important factors are considered in the remedy selection. The criteria are derived from the statutory requirements of CERCLA Section 121 (codified in 42 U.S.C. Section 9621), as well as technical and policy considerations. The analysis of remediation alternatives based on the nine criteria comprises two steps: (1) an individual evaluation of each alternative with respect to each criterion and (2) a comparison of options to determine the relative performance of the alternatives and to identify relative advantages and disadvantages. The diagram below depicts the relationship between the nine criteria and the statutory requirements of CERCLA.

The EPA has also established a two-step remedy selection process, in which a preferred remedial action is presented to the public for comment in a proposed plan. This plan summarizes preliminary conclusions based on information available and considered during the feasibility study. After the receipt and evaluation of public comments on the proposed plan (which may include new information), the EPA will issue a record of decision (ROD) documenting the selected remedy.

The selection of the preferred remedial action (and, ultimately, the remedy specified in the ROD) is based on factors such as the following:

Environmental media affected (i.e., soil, sediment, groundwater, indoor air).
Chemical contaminants.
Remedial objectives.
Current use of the site.
Future use of the site.
Intended cleanup timing.
Cost.

The types of contaminants of concern (COCs) discovered at a site will depend on current and historical operations and releases, as well as such events at neighboring properties. During environmental remediation projects at industrial facilities such as manufacturing, aerospace, railroad, and oil and gas operations, the most commonly found COCs include the following:

Heavy metals — These troublesome metals include arsenic, chromium, lead, and mercury.
Volatile organic compounds (VOCs) — Chlorinated VOCs, including perchloroethylene (PCE) and trichloroethylene (TCE), are common contaminants. Gasoline-related chemicals such as benzene and methyl tert-butyl ether (MTBE) are also VOCs.
Semivolatile organic compounds (SVOCs) — Polyaromatic hydrocarbons (PAHs) are a subset of SVOCs found in petroleum.
Polychlorinated biphenyls (PCBs) — PCBs were historically used in electrical equipment and are considered persistent organic pollutants because of their longevity.
Non-aqueous phase liquids (NAPLs) — NAPLs are chemicals or mixtures of chemicals (e.g., dry cleaning fluids, fuel oil, and gasoline) that do not dissolve in water. Light non-aqueous phase liquids (LNAPLs) are lighter than water and will float atop the water table. Dense non-aqueous phase liquids (DNAPLs) are heavier than water and will sink in the water column.

As part of the Federal Remediation Technologies Roundtable (FRTR), the EPA helped develop the Technology Screening Matrix tool to screen for remediation technologies on the basis of some of the factors mentioned above. In addition, the matrix features cost and performance reports developed by members of the FRTR.

The primary driver of the selection of a remediation technique is the type of environmental medium that is affected. Consequently, the discussion of remediation techniques and technologies in Sections 2.3.4.1 through 2.3.4.3 is organized by the type of affected environmental medium. Further, most of these remedial techniques also require a term of operations, maintenance, and monitoring (OM&M). See Section 2.3.6.1 for further discussion of the related OM&M considerations.

2.3.4.1 Soil Remediation

The most important driver of the remedial technique for contaminated soil is the remedial objective in conjunction with the current and future land use of the site. This is because the soil remediation technologies allow responsible parties to (1) remove the contaminated soil, (2) treat or stabilize the contaminated soil in place, or (3) control exposure to the contaminated soil by implementing engineering and institutional controls.

2.3.4.1.1 Soil Removal

Physical removal of chemical contaminants is the most intuitive method for eliminating contaminated soil, and the appropriate technology for doing so is soil excavation. Because excavation physically removes COCs, it is effective for all COCs entrained in the soil matrix; this technique may also remove buried drums of chemicals or other debris that is potentially contaminated. Further, as long as all of the contaminated soil is excavated, excavation can be performed for all future land uses, including those that are most restrictive, such as residential or recreational. However, if a site is currently being used for active operations, soil excavation activities will disrupt operations because of the physical space and access that the excavation machinery requires.

Contaminated soil is excavated with standard construction equipment such as backhoes, and soil may be loaded into dump trucks. The equipment chosen depends on the area’s size, the depth of the contamination, and whether access is limited by the presence of buildings or other immovable structures. Although long-arm excavators can reach as deep as 100 feet below ground, excavations are generally limited to shallower depths for reasons such as cost, health, and safety. Soil excavation below the water table is possible, but it requires dewatering the excavation (i.e., walling off the contaminated area and pumping out the water). If the pumped water comes into contact with the contaminated soil, the water will most likely require treatment before disposal, which is an additional cost. Deep excavations also typically require shoring3 to (1) keep the excavation open for collection of postremediation confirmation samples and (2) ensure workers’ health and safety.

Once the contaminated soil has been excavated, responsible parties have the following options:

Transporting the soil to a licensed landfill for disposal — Costs of this alternative include equipment and labor for excavation, transportation and disposal fees, and, potentially, additional taxes. Since the excavated and disposed-of soil must be replaced on-site, backfill material and vegetation are additional cost considerations.
Treating the soil on- or off-site — Treated soil may be disposed of at a licensed landfill or returned to the site for use as backfill. To reduce disposal costs, which are higher when hazardous waste is transported and disposed of, a responsible party may elect to treat soil before disposing of it at a landfill. Costs of this alternative, which may be more cost-effective than the transportation and disposal of hazardous waste, include the equipment and labor required for excavation, as well as the treatment, transportation, disposal, backfill, and restoration (e.g., vegetation or pavement). See the next section for a discussion of the various soil treatment options.

Excavation is commonly used when in situ (i.e., “in place”) treatment methods are too slow or expensive. Because of its effectiveness in removing all contaminated soil, excavation can also be used when unrestricted land use (e.g., residential) is required by a regulator or desired by the property owner. Off-site disposal is often the fastest method for removing high levels of contamination that pose an immediate risk to human health or the environment. Excavation is also a cost-effective approach for removing small amounts of contaminated soil. Finally, there are no long-term OM&M activities related to excavation since it eliminates the contamination and also removes the source of the contamination from underlying groundwater.

2.3.4.1.2 Soil Treatment and Stabilization

When soil excavation is not feasible because of overlying buildings or is impracticable because of the depth or large area of contamination, in situ treatment and stabilization may be effective. In situ technologies involve the application of chemical, biological, or physical processes to soil to degrade, remove, or immobilize contaminants without removing the bulk soil. Compared with excavation and ex situ treatment (i.e., treatment after excavation), these technologies offer several benefits, such as addressing deep contamination and generally costing less.

In situ treatment technologies include the following:

Soil vapor extraction — A vacuum is applied to unsaturated zone soil to induce the controlled flow of air and remove VOCs and some SVOCs from the soil.
Air sparging — Air is injected through a contaminated saturated zone to remove VOCs and SVOCs by volatilization.
Bioremediation — Microorganisms are used to degrade organic contaminants in soil, groundwater, sludge, and solids. The microorganisms break down the contaminants by either using them as an energy source or cometabolizing them with an energy source.
In situ chemical reduction — A reductant or reductant-generating material is placed in the subsurface to convert toxic organic compounds into potentially nontoxic or less toxic compounds. The technology uses adsorption or precipitation to immobilize metals, and it degrades nonmetallic oxyanions.
In situ oxidation — This technology involves reduction/oxidation (“redox”) reactions that chemically convert hazardous compounds to nonhazardous or less toxic compounds that are more stable, less mobile, or inert.
In situ thermal treatment — Such treatment includes many methods and combinations of techniques for applying heat to polluted soil, groundwater, or both. The heat destroys or volatilizes organic chemicals, and the gases are extracted through collection wells for capture and cleanup in a treatment unit.
Solidification — This technology encapsulates waste to form a solid material, coats the waste with low-permeability materials to restrict contaminant migration, or both. Solidification can occur as a result of either mechanical processes or a chemical reaction between a waste and binding reagents, such as cement, kiln dust, or lime/fly ash.

The time required for cleanup depends on the technique selected, the nature and extent of contaminated soil, and the area of contamination. In addition, regulatory agencies typically require a postremediation monitoring period to verify the efficacy of the remedy. As a result, OM&M activities would be expected components of a soil treatment remediation system.

2.3.4.2 Groundwater Remediation

The most important factor determining the selection of a remedial technology for contaminated groundwater is the remedial objective. Objectives can be a combination of the following:

Preventing ingestion of groundwater whose contaminant levels exceed drinking water standards (either state-enumerated standards or EPA-regulated maximum contaminant levels).
Preventing contact with, or inhalation of, VOCs from contaminated groundwater.
Restoring groundwater aquifers to predisposal or prerelease conditions.
Preventing migration of contaminated groundwater off-site.

When assessing these potential remedial objectives, the responsible party must also consider the current and future use of the groundwater. If groundwater is currently being used for drinking water or will be used in such a way in the future, the groundwater remediation technology must be robust enough to restore the groundwater quality to drinking water standards. However, if the groundwater is not used for potable water purposes, and other sources of drinking water are provided to the surrounding community, some other remedial alternatives may be available. These factors are important because with the many groundwater remediation technologies available, responsible parties may elect to (1) remove and treat the contaminated groundwater, (2) treat the contaminated groundwater in place, (3) contain the contaminated groundwater, or (4) control exposure to contaminated soil and groundwater. As discussed below, groundwater can be treated ex situ or in situ.

2.3.4.2.1 Ex Situ Groundwater Treatment

Pump-and-treat systems are among the most common treatment technologies used to remove contaminated groundwater. Groundwater is pumped from extraction wells to an aboveground treatment system that removes the contaminants. Once the groundwater is extracted from the groundwater-bearing unit or aquifer, selection of a treatment technology or technologies will depend on the contaminants found in the water. A treatment system may consist of a single cleanup method; however, treatment often requires several methods if the groundwater contains multiple types of contaminants or high concentrations of a single contaminant.

Commonly used ex situ groundwater treatment methods include the following:

Air stripping — This method involves the mass transfer of VOCs from water to air. For groundwater remediation, the process is typically conducted in a packed tower or an aeration tank.
Liquid phase carbon adsorption — Groundwater is pumped through a series of vessels containing activated carbon to which dissolved contaminants are adsorbed. When the concentration of contaminants in the effluent from the bed exceeds a certain level, the carbon can be (1) regenerated in place, (2) removed and regenerated at an off-site facility, or (3) removed and disposed of.
Precipitation/flocculation and sedimentation — This process removes metals from groundwater, typically through the use of precipitation with hydroxides, carbonates, or sulfides. Generally, the precipitating agent is added to water in a rapid-mixing tank along with flocculating agents such as alum, lime, or various iron salts. The mixture then flows to a flocculation chamber that agglomerates particles, which are separated from the liquid phase in a sedimentation chamber. Filtration or other physical processes may follow.
Filtration — This method isolates solid particles by running a fluid stream through a porous medium. The chemicals are not destroyed; they are merely concentrated, making reclamation possible.
Ion exchange — Toxic ions are removed from the aqueous phase in an exchange with relatively innocuous ions held by the ion exchange material. Modern ion exchange resins consist of synthetic organic materials containing ionic functional groups to which exchangeable ions are attached. Other ion exchange materials include clays, zeolites, and peat derivatives. They can be tailored to show selectivity toward specific ions. All metallic elements that are present as soluble species, either anionic or cationic, can be removed by ion exchange.

Note that pump-and-treat systems may also be used to “contain” the contaminated groundwater (commonly referred to as the “plume”). By pumping contaminated water to the surface, a pump-and-treat system controls the movement of contaminated groundwater, preventing the continued expansion of the contaminated zone.

2.3.4.2.2 In Situ Groundwater Treatment

The main advantage of in situ treatment is that groundwater can be treated without being brought to the surface, thus resulting in significant cost savings. However, in situ processes generally require longer periods, and there is less certainty about the uniformity of treatment because of the variability in aquifer characteristics and difficulty in verifying the efficacy of the process. Depending on the site and contamination characteristics, in situ chemical treatment may include (1) injection of reactive chemicals into subsurface soils or aquifers or (2) installation of a permeable chemical treatment wall across the groundwater flow path. Other in situ technologies include thermal treatment, bioremediation, and phytoremediation.

In situ treatments use the physical properties of the contaminants or the contaminated medium to destroy the contamination. In situ groundwater treatment technologies include the following:

Air sparging — Air injected through a contaminated aquifer traverses horizontally and vertically in channels through the soil column, creating an underground stripper that removes contaminants by volatilization. This injected air helps flush the contaminants up into the unsaturated zone, where a vapor extraction system is usually implemented to remove the generated vapor phase contamination.
Chemical oxidation — Chemical oxidants are used to convert hazardous contaminants to nonhazardous or less toxic compounds that are more stable, less mobile, or inert. Peroxide, ozone, and permanganate are some of the most commonly used chemical oxidants. These oxidants have been capable of achieving high treatment efficiencies (e.g., greater than 90 percent) for unsaturated aliphatic compounds (e.g., TCE) and aromatic compounds (e.g., benzene), with very fast reaction rates (90 percent destruction in minutes).
Thermal treatment — Steam is forced into an aquifer through injection wells to vaporize VOCs and SVOCs. Vaporized components rise to the unsaturated zone, where they are removed by vacuum extraction and then treated. Hot water or steam injection is typically of short or medium duration, lasting a few weeks to several months.
Bioremediation — Indigenous or inoculated microorganisms (i.e., fungi, bacteria, and other microbes) degrade (metabolize) organic contaminants found in soil or groundwater. “Enhanced bioremediation” attempts to accelerate the natural biodegradation process by providing nutrients, electron acceptors, and competent degrading microorganisms that may otherwise limit the rapid conversion of contamination organics to innocuous end products.
Phytoremediation — Plants are used to remove, transfer, stabilize, and destroy organic and inorganic contamination in groundwater. Phytoremediation mechanisms include enhanced rhizosphere biodegradation, hydraulic control, phytodegradation, and phytovolatilization.

These in situ treatment technologies may be combined with pump-and-treat systems to enhance the mobility of contaminants, thus increasing the recovery of subsurface contamination.

A permeable chemical treatment wall, or permeable reactive barrier (PRB), is a physical wall created below ground that contains reactive materials within the wall filling. Groundwater can flow through a PRB, and the reactive chemicals that make up the wall trap harmful contaminants or reduce their harmfulness. The treated groundwater then flows out of the other side of the wall. It may take many years for PRBs to clean up contaminated groundwater. A PRB is generally used as part of a “treatment train” rather than as a stand-alone remedy. For example, a PRB may act as a polishing technology after active source removal such as physical removal, thermal treatment, soil vapor extraction, or bioremediation.

2.3.4.2.3 Monitored Natural Attenuation

Natural attenuation is an in situ method that relies on natural processes to decrease or “attenuate” concentrations of contaminants in groundwater. Responsible parties monitor the groundwater conditions to ensure that the natural processes are working; therefore, the remediation process is referred to as monitored natural attenuation (MNA). The degradation of chemicals can be effected by a range of physical and biological processes. For example, naturally occurring microorganisms can break down target contaminants, such as fuels and chlorinated solvents, into less toxic or nontoxic substances. Physical mechanisms, including sorption, dispersion, dilution, and volatilization, may also work to remediate the groundwater.

Since MNA is most effective at a site where the source of contamination has been removed, it is most often combined with other soil and groundwater remediation technologies. Further, MNA may take several years, or even decades, to clean up a site. The actual cleanup time will depend on the size of the plume, the contaminant levels, and the chemistry of the groundwater to support bioremediation processes. As with other treatment technologies, when a party estimates the duration of MNA for cost estimation purposes, it should reference objective evidence related to remaining concentrations and observable trends. The EPA and states have published robust guidance and documentation about the evidence that should be collected when the efficacy of MNA is evaluated.

2.3.4.2.4 Groundwater Containment

Proper design allows groundwater extraction within pump-and-treat systems to serve as hydraulic containment. The extraction wells capture contaminated water for treatment and disposal, thereby preventing further migration of contaminated water downgradient. Other remedial technologies rely on physical containment of the groundwater.

Subsurface barrier walls (often referred to as “slurry walls” or “cut-off walls”) aim to (1) prevent further migration of contaminant plumes, (2) divert contaminated groundwater from the drinking water intake, (3) divert uncontaminated groundwater flow, and (4) provide a barrier for the groundwater treatment system. Slurry walls consist of a mixture of soil, bentonite clay, and water that is poured into trenches as a “slurry.” Other construction technologies include subsurface sheet pile walls (made of materials such as steel, precast concrete, and aluminum) and jet grouting, which injects a grout mixture at high velocities directly into the pore spaces of the soil or rock. The grout replaces and mixes the soil, creating a homogenous mass.

Slurry walls can be constructed in various configurations to manipulate the flow of groundwater. A barrier wall can be keyed into a low-permeability layer, such as underlying bedrock or clay, or it can be hanging in such a way that the wall does not extend into a low-permeability material. Hanging walls are typically used for containing floating contaminants (e.g., LNAPLs) or deflecting the flow of groundwater. Similarly, the aerial geography of slurry walls can be (1) configured to fully encapsulate the source material on all sides or (2) aligned along the downgradient extent of the plume to prevent migration. Slurry walls are used in conjunction with groundwater pump-and-treat systems to collect contaminated groundwater, and a low-permeability cap on top of the slurry wall can be used to eliminate infiltration into the wall.

2.3.4.2.5 Controlling Exposure to Contaminated Soil and Groundwater

For contaminants to pose a risk to human health or the environment, there must be (1) a source of chemical release, (2) a human or ecological receptor that is potentially exposed to the released chemicals, and (3) an environmental exposure pathway connecting the source and the receptor(s). If any one of these elements is absent, the exposure pathways are incomplete, and there is no risk. One technique for remediating contaminated soil and groundwater is elimination of the exposure pathway through the use of engineering controls, institutional controls, or both.

Engineering controls include various engineered and constructed physical barriers (e.g., soil caps, subsurface venting systems, subsurface walls, mitigation barriers, and fences) that contain or prevent exposure to property contamination. In contrast, institutional controls are administrative or legal instruments (e.g., deed restrictions or notices, easements, covenants, and zoning) that impose restrictions on the use of contaminated property or resources. Although institutional controls are often found without engineering controls, institutional controls are usually an integral part of engineering control protectiveness. The most common institutional controls for environmental remediation projects (e.g., deed restrictions or notices, covenants) (1) provide information or notification about residual contamination that may remain on a property and (2) identify engineering controls such as soil caps, mitigation barriers, or fencing, which are intended to restrict access and exposure to contamination and eliminate further migration of contamination.

With respect to soil and groundwater remediation, engineering and institutional controls are more cost-effective than removal and treatment technologies. However, because a remedy’s effectiveness depends on whether engineering or institutional controls are in place and in good condition, these techniques involve long-term OM&M activities. For engineering controls, OM&M activities may include routine inspections of soil caps, fences, or slurry walls, as well as maintenance when damage or wear is identified. For institutional controls, OM&M activities may include routine inspections to verify that (1) the land at issue is being used commercially or industrially rather than residentially or agriculturally and (2) the groundwater is not being used for drinking.

2.3.4.3 Sediment Remediation

Contaminated sediment is soil, sand, organic matter, or other minerals accumulated on the bottom of a water body that contain contaminants at levels that may adversely affect human health or the environment. Contamination sources include (1) direct pipeline or outfall discharges to a water body from industrial facilities, (2) chemical spills that migrate to a water body, (3) surface runoff or erosion of soil from contaminated sources on land, and (4) up-welling of contaminated groundwater or NAPLs into a water body. Remediation of contaminated sediment tends to be costly and logistically complex for the following reasons:

Water bodies may be affected by several sources of historical contamination.
Contamination is often diffuse, and sites are often large.
The aquatic environment is dynamic, and it is difficult to understand the various effects on sediment movement.
Logistics associated with conducting physical remediation activities are frequently complicated.
Many sediment sites contain ecologically valuable resources or legislatively protected species or habitats.
Several riparian landowners may be affected and involved in the process.
Navigational abilities must be considered on larger waterways.

2.3.4.3.1 Dredging and Excavation

The two most common means of removing contaminated sediment from a water body are dredging (for submerged sediment) and excavation (for sediment from which water has been diverted or drained).

Such removal is effective for source control (i.e., removal of hot spots), but it could be less effective for overall risk reduction because of resuspension and residual contamination. Both methods typically require transporting the sediment to a location for treatment, disposal, or both. They also frequently include treatment of water from dewatered sediment before discharge to an appropriate receiving water body. Key components for an entity to evaluate in deciding whether to use dredging or excavation as a cleanup method include sediment removal, transport, staging, treatment (any necessary pretreatment or treatment of water and sediment), and disposal (liquids and solids).

Dredging and excavation are usually more complex and costly than other sediment remediation techniques because of (1) the removal activities themselves; (2) the need for transport, staging, treatment (if necessary), and disposal of the dredged sediment; and (3) the accommodation of equipment maneuverability and portability or site access.

2.3.4.3.2 In Situ Techniques

The most common in situ sediment remediation techniques include in situ treatment and capping.

In situ sediment treatment mixes an amendment into sediment (1) passively through natural biological processes, such as bioturbation, or (2) actively through mechanical means. Amendment materials are used to transform, degrade, stabilize, or solidify contaminated sediment and may include components that are biological (e.g., cultured microorganisms), chemical (e.g., zerovalent iron), or physical (e.g., clay and concrete). In situ treatment technologies can reduce risk in environmentally sensitive ecosystems such as wetlands and submerged aquatic vegetation habitats, where sediment removal or containment by capping might be harmful. Treatment works to reduce concentrations of freely dissolved chemicals that are exposed to organisms or that may be mobilized and transferred from sediment to the overlying water column.

Capping involves the placement of a subaqueous covering or cap of clean material over contaminated sediment to mitigate the risks posed by the sediments. Caps are generally constructed of granular material (e.g., clean sand or gravel). A more complex cap design can include (1) geotextiles to aid in layer separation or geotechnical stability, (2) amendments to enhance protectiveness, or (3) additional layers to protect and maintain the cap’s integrity or enhance its habitat characteristics. Depending on the contaminants and sediment environment, a cap is designed to reduce risk by (1) physically isolating the contaminated sediment, (2) stabilizing the contaminated sediment to reduce transport downgradient, and (3) chemically isolating the contaminated sediment to reduce dissolution into the water column. A cap can be used after partial removal of contaminated sediment or as a stand-alone technique.

2.3.4.3.3 Monitored Natural Recovery

The National Research Council defines monitored natural recovery (MNR) as a remediation practice that uses natural processes to protect the environment and receptors from unacceptable exposures to contaminants. These processes may include physical, biological, and chemical mechanisms that act together to reduce the risks posed by the contaminants. Enhanced MNR (EMNR) involves application of materials or amendments to enhance these natural recovery processes (e.g., the addition of a thin-layer cap or a carbon amendment). The caps enhance ongoing natural recovery processes while minimizing effects on the surrounding aquatic environment. MNR and EMNR can be used alone or in combination with active remediation technologies to meet remedial objectives.

MNR usually involves acquisition of information about ongoing physical, chemical, and biological processes over time to confirm that these risk-reduction processes are occurring. Consequently, MNR is similar to the MNA remedy used for groundwater; however, while degradation or transformation of contaminants is usually the major attenuating process for contaminated groundwater, these processes often work too slowly for sediment remediation to occur in a reasonable time frame. Therefore, physical removal (dredging or excavation) and physical isolation (capping) are the most frequently used processes for sediment remediation. Two key advantages of MNR are its relatively low implementation cost and its noninvasive nature. Two key limitations of MNR are that it generally leaves contaminants in place and may reduce risks more slowly than active remedies do.

2.3.5 Remedial Design/Remedial Action

During the remedial design stage, the technical specifications for the selected remedy are designed. Once the design has been finalized, actual construction and implementation of the remedial action are conducted. If PRPs have been identified, the remedial design and action are conducted and funded by the PRPs, with oversight by the EPA and other regulatory agencies if applicable.

2.3.6 Postconstruction Completion

Postconstruction activities ensure that Superfund response actions provide long-term protection of human health and the environment. Such activities include OM&M, long-term response actions (LTRAs), institutional controls, five-year reviews, and site deletion from the NPL.

2.3.6.1 Operations, Maintenance, and Monitoring

With the exception of removal activities, in which contaminated soil or sediment has been excavated and the site has been restored to prerelease conditions, all remedial technologies require a period of OM&M. For example, after a groundwater pump-and-treat system is constructed, the actual remediation process is the long-term operation of that system, along with contemporaneous monitoring of the groundwater quality to evaluate whether the system is remediating the groundwater. In addition, if a landfill or other source of soil contamination is capped, the cap must be inspected and repaired over time so that the remedy remains protective of human health and the environment. OM&M measures may also include maintaining institutional controls.

The purpose of OM&M is to ensure that the selected remedy is performing as intended. Adequate performance of OM&M activities over the lifetime of the remedy or project is critical to ensuring that the remedy continues to protect human health and the environment. The table below illustrates activities commonly performed as part of OM&M.

	Typical OM&M Activities
Inspection	Review sampling records for compliance with discharge permits and deviations Observe site conditions such as landscape, drainage, erosion, and integrity of structures and fences Inspect wells, piping, treatment facilities, and other mechanical and electrical systems and equipment
Sampling, monitoring, and analysis	Sample and monitor leachate, groundwater, and surface water Sample gas collection system and air Sample influent/effluent of treatment systems
Routine operations and maintenance	Operate treatment plants Perform site maintenance and repair, including maintenance of cap integrity, drainage systems, roads, and erosion control Maintain institutional controls, fencing, site access, and security measures Maintain treatment plant, wells, pumping systems, pollution control devices, and other operating mechanical and electrical equipment
Reporting	Provide routine reports Provide special reports

For PRP-led remedies, the PRP continues to operate and maintain the remedy during OM&M. However, the EPA has oversight to ensure that OM&M is being performed adequately. The EPA and the applicable state may require the PRP to submit periodic reports, maintain records, and host site visits from the EPA.

For Superfund-financed remedies, CERCLA Section 104 (codified in 42 U.S.C. Section 9604) requires states to pay for or ensure payment for all future maintenance. Although the states are responsible for OM&M, the EPA retains responsibility for determining when OM&M is complete and conducting five-year reviews. OM&M activities may continue for decades, and costs for OM&M are considered during the development of the feasibility study and should be included in the cost estimates for remedial alternatives.

Past EPA guidance recommended the general use of a 30-year period of analysis for estimating the present value costs of remedial alternatives during the development of the feasibility study. Current EPA guidance acknowledges that while this may be appropriate in some circumstances and is a commonly made simplifying assumption, the use of a 30-year period of analysis without site-specific considerations is not recommended. Site-specific justification should be provided for the period of analysis selected. As noted above in connection with groundwater MNA, the EPA and state regulatory agencies have published guidance identifying evidence to be used for evaluating the efficacy of remedial actions. Responsible parties may use data analytics and modeling to evaluate groundwater trends and estimate the time it will take for concentrations of COCs to meet remedial standards. With the appropriate amount of supporting data, responsible parties may also use their experience at a similarly situated site that has attained regulatory closure to estimate the OM&M duration. Most importantly, the underlying assumptions should be documented, with reference to authority when applicable. Otherwise, determination of OM&M duration may appear arbitrary. Further, responsible parties may sometimes be required to perform OM&M indefinitely for remedies that contain wastes on-site or include institutional controls.

For remedies involving soil, sediment, or groundwater restoration, OM&M may be terminated with regulatory agency approval if all work is completed, cleanup goals have been achieved, and additional monitoring or institutional controls are unnecessary. The estimated time for completing the work (and therefore the assumed duration of OM&M activities for estimating costs) is a significant judgment that should be substantiated with objectively verifiable data.

2.3.6.2 Long-Term Response Action

Section 435(f)(3) of the National Oil and Hazardous Substances Pollution Contingency Plan (NCP) states, in part:

For Fund-financed remedial actions involving treatment or other measures to restore ground- or surface-water quality to a level that assures protection of human health and the environment, the operation of such treatment or other measures for a period of up to 10 years after the remedy becomes operational and functional will be considered part of the remedial action.

The 10-year period from the “operational and functional” determination to the start of OM&M is defined as an LTRA. As noted in Section 435(f)(2) of the NCP, a remedy becomes operational and functional at the earlier of (1) “one year after construction is complete” or (2) “when the remedy is determined concurrently by the EPA and the state to be functioning properly and is performing as designed.” Section 435(f)(2) further states that the “EPA may grant extensions to the one-year period, as appropriate.” The most common LTRA remedies are (1) groundwater pumping and treatment and (2) MNA remedies with objectives of aquifer restoration.

2.3.6.3 Five-Year Reviews

A five-year review (FYR) is a statutory requirement that applies to all remedial actions selected under CERCLA Section 121. Under this mandate, the EPA is required to conduct a review every five years, or more frequently if necessary, of the remedies at Superfund sites where hazardous substances remain at levels that potentially pose an unacceptable risk. Removal actions conducted under CERCLA Section 104 and corrective actions conducted under RCRA are not subject to the FYR requirement; however, EPA regions may conduct FYRs for these or other remedies as policy or at its discretion. FYRs are performed throughout the life of a site until hazardous substances, pollutants, or contaminants no longer remain on site at levels that do not allow for unlimited use and unrestricted exposure.

2.3.6.4 Deletion From the National Priorities List

A site may be deleted from the NPL once all response actions are complete and all cleanup goals have been achieved. The EPA is responsible for processing deletions with concurrence from the state in which the Superfund site is located. Deleted sites may still require FYRs to assess protectiveness. If future site conditions are warranted, additional response actions can be taken through the Superfund Trust Fund or by PRPs. Relisting on the NPL is not necessary, but sites can be restored to the list if extensive response work is required. The EPA can also delete portions of sites that meet deletion criteria.

2.3.7 EPA “Notice of Liability” Letters to PRPs

This section provides further detail on the Superfund process and explains how an entity is identified as a PRP and put on notice.

The nature of PRPs and their potential liability is provided in ASC 410-30-05-15 and 05-16 as follows:

ASC 410-30

05-15 Superfund places liability on the following four distinct classes of responsible parties:

Current owners or operators of sites at which hazardous substances have been disposed of or abandoned
Previous owners or operators of sites at the time of disposal of hazardous substances
Parties that “arranged for disposal” of hazardous substances found at the sites
Parties that transported hazardous substances to a site, having selected the site for treatment or disposal.

05-16 This liability is imposed regardless of whether a party was negligent, whether the site was in compliance with environmental laws at the time of the disposal, or whether the party participated in or benefited from the deposit of the hazardous substance. Parties that disposed of hazardous substances many years ago — including the years preceding the enactment of the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 — at sites where there is, was, or may be a release into the environment, may be liable for remediation costs.

When a site has been proposed for inclusion on the NPL, the EPA typically determines which entities fall within the categories listed above before issuing a “notice of liability” letter. After identifying PRPs, the EPA uses “general notice” letters and “special notice” letters to communicate with them.

A general notice letter informs the recipient that it (1) has been identified as a PRP at a Superfund site and (2) may be liable for cleanup costs at the site. The letter explains the process for negotiating the cleanup with the EPA, includes information about the Superfund and the site itself, and may include a request for additional information. General notice letters are typically sent to PRPs early in the process, such as when a site has been proposed for inclusion on the NPL. Upon receiving a general notice letter from the EPA, a PRP should evaluate whether recognition of an environmental remediation liability is required under ASC 410-30. See Section 3.3 for further discussion of the recognition of environmental remediation liabilities.

The EPA issues a special notice letter when it is ready to negotiate with PRPs to clean up a site (i.e., at either the remedial investigation and feasibility study stage or the remedial design/remedial action stage). A special notice letter explains to PRPs why the EPA thinks that they are liable and informs them about the EPA’s plans for the site cleanup. The letter also invites parties to participate in negotiations with the EPA on performing future cleanup work and reimbursing the EPA for any site-related costs already incurred. The issuance of a special notice letter triggers a “negotiation moratorium,” meaning that the EPA agrees, for a certain period, not to unilaterally order the PRP to conduct the cleanup. Although the EPA generally issues special notice letters to PRPs, it may decide not to do so in the following circumstances:

Past experience with the PRPs indicate that a settlement is unlikely.
No PRPs have been identified.
PRPs lack the resources to do what is needed.

2.3.8 Liability Schemes Under CERCLA

The liability schemes of CERCLA differ from traditional common law and statutory liability schemes. Specifically, under CERCLA, the following three liability schemes may apply:

Strict liability — The government does not need to prove that the defendant was at fault. Rather, the government is required to prove only that the party falls within one of the four categories of PRPs, as described in the previous section.
Retroactive liability — Parties found responsible are liable even if their actions occurred before CERCLA was enacted.
Joint and several liability — Each PRP is potentially liable for the entire cost of cleanup, and it is the responsibility of the PRPs to allocate shares of liability among themselves.

ASC 410-30 includes the following guidance on liability under CERCLA:

ASC 410-30

Strict Liability

05-17 The courts have interpreted the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 to impose strict liability. Thus, a waste generator that disposed of its waste at approved facilities, in accordance with all then-current requirements, having exercised “due care,” would nevertheless be liable. Further, a waste generator that is responsible for a small percentage of the total amount of waste at a site may be held liable for the entire cost of remediating the site.

05-18 Also noteworthy is that wastes need not be hazardous wastes for there to be environmental remediation liability. If the waste generator “arranged for disposal” of wastes containing hazardous substances (at any concentration level and regardless of whether the substances were defined as, or known to be, hazardous at the time of disposal), and a “release” of hazardous substances has or could occur, the waste generator could be subject to environmental remediation liability.

05-19 Hazardous substance is a much broader term than hazardous waste. It includes any substance identified by the Environmental Protection Agency by regulation, pursuant to a number of federal statutes. Covered, for example, are substances considered to be toxic pollutants under the Clean Water Act or hazardous air pollutants under the Clean Air Act. The various lists of hazardous substances identified by the Environmental Protection Agency contain more than one thousand chemicals and chemical compounds.

05-20 The possibility of becoming subject to liability for environmental remediation costs associated with past waste disposal practices based on strict liability can affect transactions involving the acquisition or merger of an entity or the purchase of land.

Joint and Several Liability

05-21 Through Environmental Protection Agency initiated legal action, liability under the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 may be joint and several. If a potentially responsible party can prove, however, that the harm is divisible and there is a reasonable basis for apportionment of costs, the potentially responsible party may ultimately be responsible only for its portion of the costs.

05-22 In order to mitigate the potentially harsh effects of the strict, joint and several, and retroactive liability scheme, however, Superfund does permit responsible parties to sue other responsible parties to make them contribute to the cost of the remediation or to recover money spent on remediation.

2.3.9 Superfund Settlement Agreements

As discussed in Section 2.3.7, the EPA issues general notice letters and special notice letters to communicate with PRPs about Superfund liability. A general notice letter puts a PRP on notice that it may be liable for costs associated with the cleanup of a Superfund site. A special notice letter invites a PRP to enter into good-faith negotiations with the EPA. Typically, a PRP has 60 days to provide the EPA with a good-faith offer to do site work or pay for cleanup. If the PRP provides such an offer, the entity generally has an additional 60 days for negotiation. If the PRP does not submit a good-faith offer at the end of the 60 days, the EPA may start the cleanup work or issue a unilateral administrative order requiring the PRP to do the work.

PRPs can enter into various types of Superfund settlement agreements with the EPA. Such settlement agreements are summarized in the table below.

Settlement Agreement Type	Description	Typical Uses	Court Approval Required
Administrative order on consent (AOC)	A legal document that formalizes an agreement between the EPA and one or more PRPs to address some or all of the parties’ responsibilities at a site.	Removal activity (short-term cleanup). Investigation. Remedy design work. Cost recovery when payments are made as part of an agreement for work and for de minimis cash-out payments.	No
Administrative agreement	A legal document that formalizes an agreement between the EPA and one or more PRPs to reimburse the EPA for costs already incurred (cost recovery) or costs to be incurred (cash-out) at a Superfund site.	All types of payment agreements that do not include performance of work.	No
Judicial consent decree (CD)	A legal agreement entered into by the United States (through the EPA and the DOJ) and PRPs. A CD is the only settlement type that the EPA can use for the final cleanup phase (remedial action) at a Superfund site.	Final cleanup. Recovery of cleanup costs in cost recovery and cash-out settlements. Performance of (1) removal work or (2) a remedial investigation and feasibility study.	Yes
Work agreement	The EPA and a PRP negotiate an agreement (in the form of an AOC or CD) that outlines the work to be done. The term “work agreement” covers a variety of agreements under which the PRP (rather than the EPA) performs the work.	Site investigation (remedial investigation and feasibility study). Short-term cleanup (removal actions). Long-term cleanup (remedial design/remedial action).	No
Cost recovery agreement	An agreement between the EPA and a PRP that addresses only the reimbursement of EPA costs. It takes the form of an administrative agreement.	Cost recovery. AOCs for work (1) may include a provision that requires the PRP to reimburse the EPA for past work costs and (2) will include a provision that requires the PRP to pay the EPA’s future costs for overseeing the PRP’s work. Such provisions are considered “cost recovery” because the costs are billed to the PRP after they are incurred by the EPA.	No
“Cash-out” agreement	Sometimes it is more appropriate for PRPs not to be involved in performing work at a site. In such cases, the EPA may negotiate a “cash-out” agreement with a PRP, under which the PRP pays an appropriate amount of estimated site costs before the work is done. Agreements to cash out de minimis PRPs take the form of AOCs, and agreements to cash out peripheral and other parties that have the ability to pay take the form of administrative agreements.	The EPA uses the money to help pay for the cleanup.	Yes, if a judicial CD