Fact sheet: Monitored natural recovery (MNR) and enhanced natural recovery (ENR) – Sediments

From: Public Services and Procurement Canada

On this page

Description

Monitored natural recovery (MNR) is a remediation technology that makes use of natural processes to reduce risks to receptors at contaminated sediment sites to acceptable levels within an appropriate time frame. Natural recovery processes refer to a variety of physical, chemical, and biological mechanisms that occur naturally without human intervention. The four main modes of natural recovery are chemical transformation, physical isolation, reduction in bioavailability, and dispersion. MNR can be achieved by eliminating exposure pathways through one of these modes.

Enhanced natural recovery (ENR) is a form of MNR in which materials or amendments are added to augment natural recovery processes (e.g., addition of a thin-layer cap or a carbon amendment). Adequate control of sources of contamination is also essential to ensure the effectiveness of recovery processes.

Monitored natural recovery is one of the most frequently used approaches for the rehabilitation of contaminated sediments, and is often used in conjunction with other remedial approaches. MNR is considered as a passive in situ approach, a practice that relies on natural processes (unstimulated and non-accelerated) to reduce the toxicity of sediments. Monitoring is necessary to confirm that the processes are working as anticipated to reduce risks to acceptable levels within a reasonable time frame.

Not all natural processes result in reduced risk to human health and the environment; some processes may increase or move the risk to other sites or receptors. Therefore, before implementing an MNR project, it is essential to identify and evaluate the processes that contribute to risk reduction at a site. Processes that reduce toxicity via chemical transformation or reduced bioavailability through increased adsorption are generally preferable as basic mechanisms of natural recovery. However, highly stable contaminants in sediments are not easily converted or destroyed. In these cases, risk reduction through physical isolation via natural sedimentation or ENR is a common option for site recovery. Dispersion is a less desirable process for natural restoration; although it can reduce risk in the area of origin of the contaminated sediment, it usually increases contaminant exposure in downstream areas and may therefore result in an unacceptable risk.

MNR is not a “do nothing” approach equivalent to the status quo. Although it relies on natural processes that generally would incur no construction costs, characterization and monitoring of MNR sites do require significant long-term effort, and the associated costs can sometimes exceed the expense of in situ recovery or dredging. Implementation of MNR requires detailed site characterization, thorough risk analysis, and predictive modelling of the ecosystem recovery. It also requires planning and focused monitoring to demonstrate control of contaminant sources, identify site-specific natural processes, and establish remediation goals and timelines to confirm that natural processes are occurring as expected, and contributing to risk reduction over time.

Internet links:

ITRC (Interstate Technology & Regulatory Council) 2014, Contaminated Sediments Remediation, Remedy Selection for Contaminated Sediments.

http://www.itrcweb.org/contseds_remedy-selection/

ESTCP (Environmental Security Technology Certification Program) 2009, Technical Guide: Monitored Natural Recovery at Contaminated Sediment Sites.

https://www.serdp-estcp.org/Tools-and-Training/Environmental-Restoration/Contaminated-Sediments/(sort_by)/publish_date

RTDF (Remediation Technologies Innovation Forum) 2006, Weight-of-Evidence Approach for Evaluating Monitored Natural Recovery.

https://rtdf.clu-in.org/public/sediment/mnrpapers.htm

U.S. EPA (United States Environmental Protection Agency) Clu-In 2016, Contaminated Site Clean-Up Information: Sediment Remediation, Monitored Natural Recovery.

https://clu-in.org/contaminantfocus/default.focus/sec/Sediments/cat/Remediation/p/6

Implementation of the technology

MNR should only be selected as a remediation technique if it can be demonstrated that natural processes are occurring and will reduce risks to acceptable levels within a reasonable time frame, as determined through site evaluation, regulatory involvement, and stakeholder engagement. Before selecting MNR as an option for site management, the manager must be able to answer at least the following three questions:

  • Is there evidence that natural remediation processes are occurring in the ecosystem under consideration, such as sedimentation with clean material, weathering of contaminants to less-toxic forms, or reduction of contaminants in fish tissue?
  • Why is ecosystem recovery occurring (and what are the physical, chemical, and biological processes involved)?
  • How will the processes change over time, and what can we expect in the future?
  • The MNR process may include source control, development of a conceptual site model (CSM), long-term monitoring, and ENR.

Source Control

It is essential that the source of contamination is controlled and there is little or no additional release of contaminants into the environment before implementing an MNR project. All restoration approaches share the goal of source control, but it is especially important for natural restoration because of the slow pace of environmental recovery.

Development of a Conceptual Site Model (CSM)

A CSM is needed to portray how specific natural processes proceed and interact to reduce the risk associated with a contaminated site. The CSM forms the basis for evaluating natural processes during the development of the remedial action plan and during the phases of implementation. It illustrates the relationship between natural recovery processes and the reduction of risks to ecological and human health receptors. The CMS should include:

  • physical, chemical, and biological site characteristics
  • contaminant sources
  • fate and transport processes
  • exposure pathways and possible receptors
  • Developing the CSM increases the understanding of the exposure pathways that allow contaminants to reach human and ecological receptors. The model assists site managers in differentiating between important and inconsequential exposure routes. The CSM also highlights data gaps requiring further investigation.

Monitoring of Indicators

Site characterization and monitoring is critical for establishing the feasibility of MNR as a remedy, and these activities may be more intensive when compared to sites using other remedies. The case for MNR is developed based on rigorous analyses (e.g., literature review, laboratory and field studies, modelling hydrodynamic testing) to define the role of natural processes in risk reduction. Potential indicators that natural restoration is occurring at the study site include observations of long-term reductions in containment concentrations (1) in organisms at higher trophic levels and (2) in the water column. Using data collected from sediment cores to demonstrate a decrease over time in historical contaminant concentrations, toxicity, or mass would be another indicator.

Monitoring efforts should be targeted to confirm source control, identify natural processes acting upon contaminants (e.g., sedimentation), set expectations for recovery, and confirm that natural processes continue to act as predicted, reducing the risk over time.

Enhanced Natural Recovery

Should natural attenuation occur too slowly for MNR to be considered a feasible remediation technique, acceleration may be considered through Enhanced Natural Recovery (ENR). ENR involves laying down a thin layer (10 to 20 cm) of clean sediment on top of the contaminated sediment to reduce the exposure of benthic organisms to contaminated sediments and dilute contaminated concentrations through mixing with clean material.

The process of placing the thin layer of clean sediment is similar to placement of a sediment cap. In addition, amendments (e.g., nutrients, activated carbon) may be incorporated into the clean sediment to further expedite remediation efforts. More information on the placement of a cap and addition of amendments may be found in the fact sheet, Capping – Sediments.

Materials and Storage

No equipment or material inputs are required for successful performance of MNR, except in the use of ENR, for which clean sediment and/or amendments may be stockpiled, and specialized equipment may be required for transport and placement. Sediment being incorporated into a remediation project should be stored in covered piles. This will reduce dust and prevent rainfall from eroding the material and transporting it into the receiving environment and adjacent surface waters. Amendments may be incorporated into the ENR design to improve the remediation process, similar to amended capping (see Capping – Sediments). Common amendments include activated carbon, apatite, organophilic clay, and nutrients. When amendments are incorporated into the ENR design, treatment materials may also be stored on site.

Waste and Discharge

No significant waste discharge is created with MNR remediation, except in the use of ENR. The following waste and discharges may occur if ENR is used:

  • excess clean sediment and amendments may remain on the site. Excess materials may be reused or disposed of according to local waste disposal guidelines
  • soluble and/or buoyant sediments and amendments may enter the water column during placement, increasing the loading into the water column and potentially migrating downstream
  • rainfall runoff from the stockpiled material can enter the water body, causing sedimentation and negatively affecting biota. Stockpiled material should be covered, and potential runoff should be captured and treated prior to discharge

Recommended analyses for detailed characterization

Biological analysis

  • Total heterotrophic and specific bacterial counts (according to the contaminants of interest)
  • Biological Oxygen Demand
  • Characterization of the benthic community

Chemical analysis

  • pH
  • Oxidation reduction potential (Eh)
  • Organic matter content
  • Nutrient concentrations includes:
    • lammonia nitrogen
    • total Kjeldahl nitrogen
    • nitrates
    • nitrites
  • Chemical oxygen demand
  • Metals concentrations
  • Electron acceptor concentrations includes:
    • dissolved oxygen
    • nitrate
    • sulphate
    • ferrous and ferric iron
    • methane
    • dissolved manganese
    • carbonate
  • Dissolved salt concentration in water
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free
  • Salinity/conductivity
  • Chemical solubility, hydrophobicity, or volatility
  • Sulfides (AVS)
  • Radioisotopic analyses 
    • Determination of the age of deposited sediments.

Physical analysis

  • Dissolved oxygen concentration
  • Vadose zone oxygen, nitrogen dioxide, and methane concentrations
  • Temperature
  • Soil granulometry
  • Evaluation of biological conditions and ecological factors
  • Sediment particle size distribution
  • Sediment shear strength 
    • Allows prediction of sediment scour potential under a variety of conditions
  • Sediment bulk density
  • Sediment cohesiveness

Recommended trials for detailed characterization

Biological trials

  • Microcosm mineralization trial
  • Biodegradation trial

Other information recommended for detailed characterization

Phase II

  • Regional climatic conditions (precipitation, temperature, etc.)
  • Physical-chemical characterization of sediments and interstitial water
  • Bathymetry
  • Presence of potential environmental receptors
  • Characterization of the present and proposed surface water usage and the water body in general (including the required draft for vessels)

Phase III

  • Geotechnical characterization of sediment deposition
  • Characterization of hydrodynamic conditions 
    • Water depth and velocity under a range of conditions, used to determine the sediment bed stability.
  • Sediment core profiles 
    • Used to determine historical trends in contaminant concentrations and loadings, including previous or potential chemical weathering and biodegradation.
  • Testing of diffusion and advection processes 
    • Modelling created to determine the required overlay thickness to ensure physical and/or chemical isolation of the contaminants.

Notes:

The analyses, trials, and characterization recommendations are dependent on the type of recovery employed at the site: chemical transformation, physical isolation, reduction in bioavailability and mobility, and/or dispersion. Not all analyses listed will be necessary at every site. For more information on required analyses, trials, and characterization, see ESTCP (2009).

Applications

MNR applies under the following conditions:

  • contaminant concentrations in the biota and biologically active zones are naturally recovering
  • effective source control has been achieved
  • the human health and ecological risks are low to moderate, or contaminated sediment posing a high risk has been appropriately remediated or removed
  • the sediment bed is stable, and the rate of deposition is greater than the rate of erosion
  • planned future land use and estimated changes in water levels are compatible with natural recovery
  • operational physical, biological and chemical processes are sufficiently active to reduce the toxicity, mobility, and bioavailability of contaminants in an acceptable time frame
  • toxicological risk associated with exposure to contaminants is low and/or controlled by institutional controls (e.g., prohibition of fishing, restrictions to navigation)
  • the site contains unique sensitive habitats that could be irreversibly damaged by in situ remediation or sediment removal
  • natural processes are capable of eliminating risk pathways through degradation, transformation, or burial
  • MNR is most appropriate when:
  • contaminants are adsorbed into the solid phase or dissolved into interstitial water, readily biodegraded or transformed to less toxic forms, and do not easily biomagnify; and
  • contamination is low level and widespread, and active remediation methods are not feasible or cost-effective

Applications to sites in northern regions

MNR relies on naturally occurring processes of sedimentation and chemical and biological degradation. Sedimentation and chemical degradation occur at relatively similar rates in northern environments as compared to temperate environments; however, biological degradation has been demonstrated to occur at a much slower rate. MNR is implemented easily where site-specific conditions allow and is very applicable to remote and northern sites. MNR requires very little mobilization of equipment; however, it requires robust long-term monitoring. Site access and transportation for regular monitoring may pose the most significant logistical and fiscal constraint.

Institutional controls, such as navigational barriers and moratoriums on fishing, are required to limit risk associated with human exposure during the recovery period, and may be required for a longer period of time in comparison to other remedies. These controls may cause hardship in locations where the local communities rely on fishing to provide a significant portion of their nutritional necessities and exports. Communities in remote locations may be more negatively affected by such controls than people in temperate climates. A risk assessment is recommended and must account for northern lifestyles, cultures, and unique ecological systems.

As more active remediation technologies (i.e., dredging, excavation, capping, and confined aquatic disposal) are often cost-prohibitive in northern environments, MNR may be viewed by the community to be akin to inaction. Community concerns regarding the use of a passive remediation approach and the potential need for long-term fish-consumption advisories should be considered in remedy selection and risk assessment.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
Biological
Applies
Chemical
Applies
Control
Does not exist
Dissolved contamination
Applies
Free Phase
Does not exist
Physical
Applies
Residual contamination
Applies
Resorption
Applies
Thermal
Does not exist

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
With restrictions
Chlorobenzenes
Applies
Explosives
With restrictions
Metals
With restrictions
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
With restrictions
Pesticides
With restrictions
Petroleum hydrocarbons
Applies
Phenolic compounds
With restrictions
Policyclic aromatic hydrocarbons
With restrictions
Polychlorinated biphenyls
With restrictions

Notes:

MNR has been successfully conducted in a number of commercial contaminated sites remediation projects, and can be considered a commercialized technology. ENR is commercially available for certain amendments and in the testing phase for others. More information on possible amendments may be found in the fact sheet Capping – Sediments.

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Does not apply
1 to 3 years
Does not apply
3 to 5 years
Does not apply
More than 5 years
Applies

Notes:

Restrictions involve contaminants that require the presence of specific natural processes (e.g., sedimentation) or that may require the use of ENR.

The time required for risk reduction is generally long relative to other remedy options and is highly uncertain. The time required for MNR to remediate a contaminated site depends on the types and amounts of contaminants, the size and depth of the contaminated area, the type of sediment present, and the physical conditions at the site (Declercq et. al., 2012). It may take years to decades to clean up a site using MNR.

Long-term considerations (following remediation work)

If MNR achieves the remediation cleanup goals, there are few to no post-remediation long-term considerations, particularly when MNR occurs through chemical transformation and reduction in contaminant bioavailability.

Locations where sedimentation has buried contaminants under clean sediment are at risk of re-exposure. Institutional controls, such as restrictions on navigation, may be required to avoid disturbance of the overlain clean sediment. Additional monitoring following extreme weather events (e.g., floods, earthquakes) should also be considered in sedimentation locations.

Another risk in MNR remediation is that contaminant dispersion may redistribute contaminants downstream, resulting in increased risk off-site while giving the appearance of moving toward site-remediation objectives. Robust monitoring during remediation periods will identify this risk early, so measures may be taken to prevent further loss of contaminant off-site.

Secondary by-products and/or metabolites

Depending on site conditions and contaminant characteristics, intermediate degradation products may be formed or contaminants may be mobilized. The degradation of certain contaminants may generate metabolites, which can be more harmful than the original products. Chemicals that have toxic metabolites include metallic mercury, which can transform into methylmercury under microbial action; many polychlorinated biphenyls (PCBs), an insulator in electrical equipment; tetrachloroethene (PCE); and dichlorodiphenyltrichloroethane (DDT).

Limitations and Undesirable Effects of the Technology

MNR is not an appropriate approach when there are immediate moderate-to-severe risks to human health and ecological receptors and remedial objectives are not achievable within a reasonable amount of time

Contaminants are left in place, and therefore could be reintroduced back into the environment.MNR is not viable when sediment deposition rates are inadequate for timely burial, or when sediment erosion is likely. When natural recovery depends on natural sedimentation, it is possible for contaminants to be re-exposed after a significant modification of the sediment due to an unexpected severe event (e.g., flood, storm)

Institutional controls, such as restrictions on navigation or fish consumption, are required to control human exposure, and may be required for a longer period of time as compared to other remedies. Ecological exposure is unaffected by institutional controls, requiring physical barriers (e.g., a thin-layer sediment cap) to prevent exposure

Monitoring associated with MNR occurs over long periods, and the cost may be significant. Identifying when contaminant concentrations are trending toward remediation may take years to achieve, due to natural variability. Funding commitments should be guaranteed before moving forward with MNR

Care must be taken to address potential migration of contamination, particularly when natural recovery processes involve off-site dispersion

MNR often has poor public perception. Acceptance can be increased by communicating the pros and cons of available remedial options to the public.

ENR may have water-depth limitations similar to other in situ treatments (e.g., Capping – Sediments)

Complementary technologies that improve treatment effectiveness

Alternative technologies that have been shown to enhance the overall time and effectiveness of MNR include:

  • dredging, excavating, or capping areas of contamination posing a high human health and/or ecological risk
  • applying a thin (10-20 cm) sediment cap (i.e., ENR); and
  • implementing flow-control structures (e.g., stone wall) to accelerate sediment deposition

Required secondary treatments

No secondary treatment is required if target risk reduction levels are achieved.

Application examples

Documents containing examples of applied natural recovery can be found at the following addresses:

Performance

MNR performance and recovery time varies depending on sedimentation rates, the dispersion of contaminants within the sediment, the degree of bioturbation as well as contaminant sequestration and biodegradation processes. For example, MNR occurs more quickly in net dispositional environments, where contaminated sediments are buried by a layer of clean sediment. Performance will also be impacted by the use of complementary technologies and/or acceleration methods (e.g., introduction of a thin capping layer, presence of structures favouring sedimentation).

The implementation of MNR as a remedial process relies on extensive chemical and site-specific risk assessment and modelling to predict the anticipated time required for natural recovery without direct intervention. Continued monitoring during the baseline and recovery periods is critical for establishing feasibility and assessing the MNR model against monitoring results. Monitoring efforts should be targeted to confirm source control, identify natural processes acting upon contaminants, set expectations for recovery, and confirm that natural processes continue to reduce the risk over time as predicted. Site managers should be prepared to adjust and readjust performance forecasting, and to consider alternative technologies if measured results deviate significantly from original predictions. Detailed guidance on the implementation of MNR, establishing lines of evidence and carrying out predictive modelling, is provided in ESTCP (2009) and FCSAP (2015).

Measures to improve sustainability or promote ecological remediation

Sustainable remediation is the application of technologies and approaches to enhance a remediation project’s environmental, social, and economic benefits. MNR is inherently a more sustainable remediation technology than active treatment. It has minimal transportation and disposal requirements, and does not involve the use of heavy machinery that could disrupt the site. A key ongoing activity for MNR is monitoring of site conditions that are likely to affect MNR processes (i.e., chemical transformation, immobilization, sedimentation and stability, and dispersion). Ongoing monitoring is required to verify that recovery process rates can meet risk-based goals in the desired time frame and that off-site risks remain acceptable.

Potential impacts of the application of the technology on human health

Human health impacts

The environmental and human health effects arising during remediation through MNR are the result of exposure to contaminants already in the system, which are allowed to continue as the site moves toward remedial objectives. When human exposure levels are unacceptably high, MNR may be combined with risk management techniques (e.g., institutional controls preventing site access) or barrier technologies (e.g., ENR or capping) to reduce exposure.

Aquatic impacts

The placement of a thin-layer sediment cap may change the existing sediment concentration, which may lead to changes in the food supply and nutrients available to aquatic and benthic organisms. The added sediment may also change the existing habitat structure and protective cover. These changes may result in mortalities and difficulty re-establishing habitats and colonies within the site.

Negative effects associated with ENR may be mitigated through a thorough site characterization and relocation of threatened or endangered species and associated habitats. Using materials that are similar to native sediments will encourage reestablishment of the benthic communities and decrease the time required to restore biological activity at the site. In addition, including large debris, such as boulders, will provide cover for aquatic and benthic organisms and increase the likelihood of survival.

References

Author and update

Composed by : Bruno Vallée M.Sc, LVM Inc.

Latest update provided by : Bruno Vallée, M.SC., LVM inc. and Ashley Hosier, Ing., Royal military college

Updated Date : January 2, 2017

Version:
1.2.1