Guide to Monitored Natural Recovery at Federal Aquatic Contaminated Sites

List of figures

Figure 1: Overview of the 10 steps outlined in the Federal Contaminated Sites Action Plan (FCSAP) Decision-Making Framework (FCSAP, 2025) and the Framework for Addressing and Managing Aquatic Contaminated Sites (FCSAP 2021c).

Figure 2: Mathematical/Computer Models for MNR (adapted from Magar et al. 2009).

List of acronyms

BTEX

Benzene, toluene, ethylbenzene and xylenes

CCME

Canadian Council of Ministers of the Environment

CEQGs

Canadian Environmental Quality Guidelines

COA

Canada-Ontario Agreement

COPC

Contaminant of potential concern

CSM

Conceptual Site Model

CSMWG

Contaminated Sites Management Working Group

DMF

Decision-Making Framework

DOC

Dissolved Organic Carbon

DQRA

Detailed Quantitative Risk Assessment

ERA

Ecological Risk Assessment

EQG

Environmental Quality Guidelines

ESA

Environmental Site Assessment

FCSAP

Federal Contaminated Sites Action Plan

ISQG

Interim Sediment Quality Guidelines

LCA

Life Cycle Analysis

LOE

Lines of Evidence

LTM

Long-Term Monitoring

MNA

Monitored Natural Attenuation

MNR

Monitored Natural Recovery

MTBE

Methyl tert-butyl ether

NA

Natural Attenuation

NAPL

Non-aqueous phase liquid

NR

Natural Recovery

PAHs

Polycyclic aromatic hydrocarbons

PCBs

Polychlorinated biphenyls

PQRA

Preliminary Quantitative Risk Assessment

RAP

Remedial Action Plan

RMP

Risk Management Plan

ROA

Remedial Options Analysis

R/RM

Remediation/Risk Management

1 Introduction

1.1 Background

The Federal Contaminated Sites Action Plan (FCSAP) is a federal program established in 2005 with the goal of reducing environmental and human health risks from known federal contaminated sites in Canada and their associated federal financial liabilities. To achieve this objective, FCSAP provides guidance, tools and resources to federal departments, agencies and Consolidated Crown corporations (collectively referred to as “custodians”) to ensure that federal contaminated sites are managed in a scientifically sound and a nationally consistent manner. To manage contaminated sites, the FCSAP program adopted the Decision‐Making Framework (DMF), a 10-step roadmap that outlines the specific activities, requirements and key decisions to effectively address federal contaminated sites in Canada. This risk-based approach serves as a proactive management tool that ensures the necessary steps are taken to characterize, classify, prioritize, and remediate/risk-manage federal contaminated sites in a step-wise and iterative process. This systematic approach ensures that limited resources are efficiently and effectively allocated to those sites representing the highest potential risks to human and ecological receptors, as well as liability. The DMF along with other FCSAP-related resources can be found on the FCSAP website.

The objectives of the 10 step framework are to assess risks to human health, aquatic biota, wildlife, and the natural environment under the current and future intended land use scenarios, and to implement risk management solutions considered to be protective of those land use scenarios. This process involves conducting site assessments and classification, identifying contaminants of concern, identifying potential receptors, determining potential exposure pathways, and estimating the level of risk based on the exposure pathways, as well as developing and implementing a remediation / risk management strategy to reduce the risks.

Specific guidance regarding management of federal aquatic contaminated sites is provided in the Framework for Addressing and Managing Aquatic Contaminated Sites under the FCSAP (FCSAP 2021c), which is based on information provided in the Canada-Ontario Decision-Making Framework for assessment of Great Lakes Contaminated sediment (COA 2008), as well as the 10 steps outlined in the DMF. The DMF (FCSAP, 2025) further provides guidance on advice and expert support from relevant government departments. See Figure 1 for an overview of the 10 steps outlined in the DMF and the Framework for Addressing and Managing Aquatic Contaminated Sites under the FCSAP.

The size and scope of federal contaminated sites vary greatly and range from abandoned mines on Crown land in the North, airports, government laboratories, working harbours, ports, lighthouses, military bases to facilities on reserve lands under the Indian Act (1985). The assessment of aquatic sediments at these federal contaminated sites is generally evaluated using the Canadian Council of Ministers of the Environment (CCME) Canadian Sediment Quality Guidelines (CSQG) for the Protection of Aquatic Life (CCME 1999). The CSQG document provides guidance for deriving Interim Sediment Quality Guidelines (ISQG) and for developing site-specific sediment quality objectives.

Monitored natural recovery (MNR), a passive in situ remediation approach, is one potential technique available for the remediation of contaminated sediment sites. Information regarding the application of MNR in the regulatory environment in the United States has been reviewed by Magar et al. (2009). The guidance developed by Magar et al. (2009) has been adapted for use in the current document and can be applied on federal aquatic contaminated sites in a manner that is consistent with the DMF (FCSAP, 2025) and the Framework for Addressing and Managing Aquatic Contaminated Sites (FCSAP 2021c). The evaluation of MNR as an appropriate remediation approach, its development, implementation, and confirmation of success, fits into steps 7, 8, and 9 of these frameworks, as represented by the red boxes in Figure 1.

The objectives of Step 7 (“Develop Remediation/Risk Management (R/RM) Strategy”) are to:

1. Consider various guideline-based approaches, risk-based approaches, or a combination of approaches (hybrid approach) to establish R/RM objectives.
2. Establish, evaluate, and select corresponding R/RM objectives.
3. Consider the needs, knowledge, and experiences of nearby Indigenous groups through engagement activities.
4. Consider climate change impacts that may lead to changes in the affected media and future exposure scenarios and receptors when developing the R/RM strategy.
5. Consider and integrate feasible sustainability measures into the R/RM strategy, such as GHG reduction; and
6. Where applicable, consider and integrate Gender-based Analysis Plus (GBA+) measures into the R/RM strategy to ensure it doesn’t have negative affects

Information gathered from previous steps is used to develop a R/RM strategy for the contaminated aquatic sites, which often includes some form of active or passive remediation techniques. Before selecting potential R/RM strategies, there should be evidence that the selected remedial actions are not likely to cause more environmental damage than they remedy. Furthermore, several criteria should be considered when selecting potential remedial options, including protection of human and ecological receptors, long-term effectiveness, environmental sustainability, costs, community acceptance, and clear understanding of the goals for site management (FCSAP 2021c). Selection of MNR, or an integration of MNR with other potential R/RM strategies, should be considered at this step in the development of the appropriate overall site management strategy which may include a Remedial Action Plan (RAP) or Risk Management Plan (RMP). A contingency plan must be developed as well for use in the event that MNR does not meet the expected performance standard. Even where active remediation is initially selected, MNR may be employed in the later stages of an overall R/RM strategy.

Since MNR is a strategy that deals with the management of contaminant risks within the aquatic environment, site custodians considering MNR as a remedial option may wish to consult Fisheries and Oceans Canada (DFO) and Environment and Climate Change Canada (ECCC) FCSAP Expert Support for protection of ecological receptors. Health Canada (HC) expert support may also be consulted for protection of human health. This guide is not in itself any form of default regulatory approval for MNR as a remedial approach for contaminated sediments; rather, it is meant to provide information for those custodians considering MNR as an option in the management of contaminated sediments.

The objective of Step 8 (“Implement Remediation/Risk Management Strategy”) is to implement the remediation or risk management strategy, which includes the RAP/RMP, in order to reduce the site risk to an acceptable level. This step includes:

1. Meeting requirements under the Impact Assessment Act (2019);
2. Obtaining all permits and approvals required to undertake any work at the site;
3. Implementing the Sustainability Plan (SP) and the associated sustainable contracting clauses;
4. Selecting the contractor;
5. Conducting operations, maintenance and monitoring during implementation of the remediation during the RAP/RMP; and
6. Verifying the efficacy of the RAP/RMP.

Step 9 (“Confirmatory Sampling and Final Reporting”) involves confirming the achievement of the R/RM objectives/goals following the implementation of the R/RM strategy, including the RAP and RMP. Confirmatory sampling is used to demonstrate that the contamination has been removed or stabilized effectively and that R/RM objectives have been attained within the expected timeline. The site conditions as well as activities carried out during site decommissioning and clean-up, including drawings, records, and monitoring data will be documented in a report in accordance with the DMF (FCSAP, in press). This report may be  used to confirm that the site may be closed, or to inform future LTM and/or remedial activities.

In a sustainable development context where the overall environmental impact of remediation should be considered, MNR can be evaluated as an appropriate remediation approach in combination with, or as an alternative to other methods. Therefore, a good understanding of the MNR approach and the associated mechanisms is important. In addition, it is also important to incorporate climate change and its potential implications to the MNR when considering this R/RM approach for a federal contaminated site.

1.2 Objectives of the guidance document

This guidance document is meant to be an orientation guide on the use of MNR as a remediation approach for contaminated aquatic sediments. The goal of the document is to offer a summary of oversight measures and implementation practices for MNR, and to provide guidance on how they should be used.

Specific objectives of the guide include the following:

• Define the different concepts involved in MNR;
• Identify the requirements for MNR to be a feasible remediation option;
• Define and present the main components of the evaluation and use of MNR; and,
• Provide recommendations for determining how to select and conduct a successful MNR-based site R/RM approach and site closure.

Monitored Natural Attenuation (MNA) is a similar remediation tool that can be used for contaminated groundwater and soils. MNA is described in the Guide to Monitored Natural Attenuation in Soil and Groundwater for Federal Contaminated Sites (FCSAP 2021d).

This guide is not intended to provide detailed technical guidance on evaluating natural recovery processes in sediments. Several existing references should be used when evaluating whether natural recovery processes are occurring at a specific site. See Appendix A for a list of these references.

2 Natural recovery and monitored natural recovery

2.1 What is natural recovery?

Natural recovery (NR) is a natural process, or a combination of natural processes, that reduces the mass, toxicity, mobility, bioavailability, volume, and/or concentration of contaminants or their breakdown products in aquatic bedded sediment environments. The term “bedded sediment” is used to distinguish these environments from sediment particles suspended in the water column. NR processes include physical, chemical, and biological processes that act without human intervention under favourable conditions. These in situ processes include dispersion, dilution, burial by clean accumulating sediment, sorption, volatilization, radioactive decay, and chemical or biological stabilization, degradation, transformation, or destruction of contaminants (Magar et al. 2009).

2.2 Natural recovery processes

Natural recovery generally comprises physical, biological or chemical processes that reduce contaminant concentration, availability, or toxicity. These processes can be either destructive or non-destructive:

• Destructive: Transformation of the contaminants, resulting in a decrease of their total mass. The processes involved are usually biological and chemical ones, including aerobic and anaerobic biodegradation by endogenous micro-organisms (naturally present in the environment), radioactive decay, and abiotic degradation (photolysis, hydrolysis and chemical reduction).
• Non-destructive: Reduction of the concentration, state (dissolved or solid), and/or mobility of a contaminant, without reducing its total mass. These are usually thermodynamically reversible reactions (such as precipitation to a solid phase) or other physical processes acting on the transport and mobility of the contaminants, such as advection, dispersion, diffusion, sorption, dilution, and volatilization.

NR processes must be well-understood to fully comprehend and predict the fate of the various contaminants and to assess the exposure pathways for receptors. See below for more details on each NR process.

Advection

• Description: movement of dissolved contaminant (solute) by bulk porewater movement.
• Important factors: dependent on porewater properties, sediment porosity, and hydraulic gradient. Independent of contaminant properties.
• Effects: usually refers to unidirectional movement of contaminant with the prevailing porewater movement.

Dispersion

• Description: fluid mixing due to porewater movement heterogeneity.
• Important factors: dependent on porewater properties, sediment porosity. Independent of contaminant properties.
• Effects: causes longitudinal, transverse and vertical spreading of the plume. Reduces local contaminant concentration but elevates concentration in other areas.

Dissolution

• Description: partitioning of contaminant from solid phase of sediment to pore water phase.
• Important factors: dependent on porewater properties, sediment porosity. Independent of contaminant properties.
• Effects: may initially increase contaminant exposure at a site, but may also facilitate dispersion and ultimately reduce contaminant concentrations at a site.

Sorption sequestration immobilization

• Description: reaction between sediment matrix and solute whereby contaminants become adsorbed to organic carbon or clay minerals. Deposits of Fe/Mn hydrous oxides at the anoxic/oxic interface of groundwater discharge areas can also concentrate heavy metals and other contaminants due to co-precipitation and adsorption reactions.
• Important factors: dependent on sediment matrix properties (e.g. organic carbon and clay mineral content, bulk density, grain size, and porosity) and contaminant properties (e.g. solubility, hydrophobicity, octanol-water partitioning coefficient).
• Effects: tends to reduce apparent solute transport velocity and remove solutes from the porewater via sorption to the sediment matrix. Requires verification of permanence in support of MNR.

Recharge / Discharge (simple dilution or flushing)

• Description: reducing concentration of a contaminant due to groundwater infiltration to the contaminated area. Most likely to occur in near shore sediments.
• Important factors: dependent on groundwater-surface water interactions and climate.
• Effects: may cause dilution (i.e. if sediments contain contaminant and groundwater is not a source) or deposition of contaminant (i.e. if infiltrating groundwater is a source). Infiltrating groundwater may replenish electron acceptor concentrations, especially dissolved oxygen.

Volatilization

• Description: volatilization of contaminants into the vapour phase (e.g. volatilization of organoselenium, BTEX vapour losses).
• Important factors: dependent on chemicals' vapour pressure and Henry's Law constant.
• Effects: removes contaminants from interstitial pore water and transfers them to interstitial gas. Release of gases from submerged sediments may be a source of contaminants in surface water.

Biodegradation / biotransformation

• Description: microbially-mediated oxidation-reduction reactions that degrade or transform contaminants.
• Important factors: dependent on porewater geochemistry, microbial populations and contaminant properties. Biodegradation can occur under aerobic and/or anaerobic conditions.
• Effects: may completely degrade contaminants. Typically the most important process to truly reduce contaminant mass.

Resuspension

• Description: disturbance to sediment increasing dispersion to the overlying water column.
• Important factors: dependent on the extent of consolidation in sediment and potential for energetic disturbance to sediments.
• Effects: may initially increase contaminant exposure at a site, but may also facilitate dispersion and ultimately reduce contaminant concentration at a site.

Redox transformation

• Description: oxidation or reduction reactions reducing mobility of contaminants into porewaters.
• Important factors: dependent on oxygen penetration of sediments (depth of redox boundary).
• Effects: reduces mobility of contaminants, but may be reversed if redox conditions change.

Bioturbation

• Description: reworking of sediment microenvironments by benthic invertebrates, including burrowing and irrigation of constructed sediment tubes.
• Important factors: depemdent on hydrological exchange between surface and interstitial waters. Dependent on activity and density of benthic invertebrates contributing to physical sediment disturbance.
• Effects: can enhance microbial activity and biogeochemical processes by increasing surface area for oxygen and substrate exchange.

Phytoremeditation

• Description: introduction of plants to stabilize or metabolize contaminants in sediments. Primarily applicable to freshwater wetland and salt marsh environments.
• Important factors: dependent on physical environment factors governing plant growth, contaminant- and plant-specific assimilation efficiencies.
• Effects: reduces contaminants available to other ecological receptors.

Burial (sedimentation)

• Description: physical accumulation of clean sediment over a contaminated area. Includes consolidation and bed armouring.
• Important factors: dependent on sedimentation rate and sediment characteristics (e.g. organic carbon and clay mineral content, bulk density, grain size and porosity).
• Effects: reduces direct exposure vector at sediment surface and eliminates scour and resuspension. Verification of permanence required for MNR.

Mineralization

• Description: chemical transformation of an organic contaminant to its basic elements (e.g. CO₂, H₂O, Cl^-).
• Important factors: dependent on microbial community and factors affecting their activity (e.g. pH, temperature, nutrients, degradable carbon sources, redox, alkalinity and organic carbon content).
• Effects: can result in partial or complete degradation of contaminants.

Abiotic degradation

• Description: chemical transformations that degrade contaminants without microbial facilitation, such as hydrolysis.
• Important factors: dependent on contaminant properties and porewater geochemistry.
• Effects: can result in partial or complete degradation of contaminants.

Partioning from NAPL

• Description: partitioning from non-aqueous phase liquid (NAPL) into porewater. NAPL plumes, whether mobile or residual, tend to act as a continuing source of groundwater contamination.
• Important factors: dependent on sediment-water interface matrix, site-specific degradation potential of contaminants, and porewater exchange.
• Effects: continued primary source of dissolved contamination in sediment, with the potential for changing contaminant profile (e.g. weathering oil entrained in sediments)

2.3 What is monitored natural recovery?

Monitored natural recovery (MNR) is a remediation approach for aquatic contaminated sites that relies on un-enhanced natural recovery processes to protect human and environmental receptors from unacceptable exposure of contaminants (U.S. NRC 2000), while monitoring effectiveness over time. MNR may be utilized in situations where contaminants present a less than immediate or substantial risk to human or ecological receptors; and where it can be demonstrated that naturally-occurring processes will reduce contaminants to acceptable levels within a reasonable timeframe. In addition, MNR can also be utilized in situations where other active remedial activities (e.g. dredging) may cause more environmental damage and risks (U.S. EPA 1998). MNR is often used in combination with other engineered remediation strategies (e.g. removal, capping, in situ mixing) as part of an overall site management strategy. In this context, MNR can be an excellent remedial option in order to achieve most cost-effective, highest degree of sustainability, and optimal site closure.

MNR is not a “do nothing” or “walk away” approach. In fact, MNR may involve costs that can be less than, similar to, or greater than other R/RM strategies. MNR also includes costs to conduct performance monitoring, characterize natural recovery, and assess whether remediation goals are, or will be, met. However, it should be noted that these elements should be considered regardless of the remediation approach selected under the 10-step approach of the DMF (FCSAP, in press).

There is a clear distinction between MNR performance monitoring and LTM as described in Step 10 of the DMF (FCSAP, in press). Typically, sediments are monitored during LTM (Step 10) after remediation goals have been achieved, including when MNR is employed as a remediation strategy. MNR should not be considered as a form of LTM unless remedial targets have already been met and monitoring is deemed to be required (FCSAP 2013a). Then, LTM is used to verify sediment stability and confirm that the dynamic conditions of the sediments and contaminant concentrations continue to remain within levels that are considered acceptable, as supported by the ecological risk assessment (ERA) or human health risk assessment (HHRA). Additional LTM sampling may also be prescribed if during Step 10 there is an occurrence of an extreme weather event like storms, high winds, or ice scour where there is the potential for contaminants to be remobilized (Magar et al. 2009).

2.4 Advantages and disadvantages of MNR

The advantages of MNR include the use of natural processes to reduce risk, reduced environmental damage and footprint associated with active remedial techniques, and potentially lower costs (Magar et al. 2009). MNR is also likely to be the most sustainable alternative for sediment remediation because it involves minimal equipment and no hauling or treatment of contaminated materials (ITRC 2014). However, MNR may present challenges for regulatory, stakeholder, and Indigenous community acceptance due to the longer-term time horizon and the lack of active physical remedial works (i.e. leaving contaminants in place), which likely requires additional investment in engagement and risk communication with Indigenous groups and stakeholders. Below are  the advantages and disadvantages of MNR compared to other remediation approaches.

Advantages

• Minimizes disturbance of landscape and receptors in the area (important for wetlands or other sensitive environments), thereby avoiding environmental costs and providing a sustainable remedial alternative.
• Has the potential to lower monetary construction costs and maximizes environmental benefit.
• Reduces risk of exposure for humans, aquatic biota, wildlife, or the environment because contaminated media are left undisturbed.
• Utilizes inherent natural processes and reduces environmental footprint (e.g., energy consumption and emissions) of the overall remediation strategy. It is a more sustainable approach compared to other remediation approaches, such as select ex-situ techniques.
• Limits the impact on operations because no property is needed for materials handling, treatment, or disposal and contaminated materials do not need to be transported through communities.

Disadvantages

• Usually requires a longer time frame for remediation and monitoring. Some risks are associated with exposure of contaminants that are left in place.
• Requires site characterization and data collection that may be more demanding and complex than capping or dredging techniques.
• Requires more education and outreach to ensure public (e.g., lower fish consumption guidelines during the recovery period) and regulatory acceptance. Limited control of ecological receptor exposures during the recovery period.
• Cannot be used on high concentrations of contaminants that pose immediate risk.
• May allow for re-mobilization of contaminants not removed by NR processes due to changes in hydrological, and geochemical, and climate conditions. Results in uncertainty regarding sedimentation rates, homogeneity of sediments, and rates of natural recovery at low contaminant concentrations and future climate change scenarios.

2.5 MNR feasibility

MNR is generally considered most feasible for sites with low or diffuse contaminant concentrations, where human and ecological risks are low, and where dredging or capping might impart excessive environmental impacts (Perelo 2010; Menzie 2010). MNR should be selected only when it meets all relevant cleanup selection criteria and R/RM objectives for the site in question. MNR must adequately protect the health and welfare of humans, aquatic life, wildlife, and the environment. Further, the complete R/RM strategy may include institutional controls such as temporary fish consumption advisories. MNR should also meet site cleanup objectives within a time frame that has been accepted by regulatory bodies, Indigenous groups, and other stakeholders. In general, application of MNR is most appropriate for contaminants that do not bioaccumulate, where the sediment bed is stable with little to no resuspension, and where declining trends in contaminant concentrations have been demonstrated (Magar et al. 2009; FCSAP 2021c). On the other hand, MNR may not be feasible where long-term or permanent institutional controls are required. Unstable sediment beds with greater potential for disturbance or resuspension will also limit the applicability of MNR at a site (FCSAP 2021c). Finally, control of the source of contamination is especially important for MNR because gaps in understanding or managing the sources will limit the ability to demonstrate MNR processes and success (Magar et al. 2009). However, in some cases, applying MNR can be the most effective option for remediation both monetarily and environmentally (Magar and Wenning 2006) and the least invasive option, allowing the ecosystem to remain undisturbed and enabling continued use of the affected waterbody (Nadeau 2008).

The list below  contains factors/criteria that should be considered to qualitatively rate the feasibility of MNR and can be used as an additional tool to determine MNR feasibility. The criteria consider the feasibility of MNR on a continuum from highly supportive to less favourable, rather than a dichotomous “yes” or “no”.

Contaminant and media properties

Source of contamination

• High MNR feasibility: it is removed, controlled, or ceased.
• Medium MNR feasibility: it is being removed or under control.
• Low MNR feasibility: it is continuing or unknown.

Chemical composition of contamination

• High MNR feasibility: low acute toxicity, bioaccumulation and/or biomagnification potential.
• Low MNR feasibility: acutely toxic, high bioaccumulation and/or biomagnification potential.

Dominant attenuation processes

• High MNR feasibility: irreversible/destructive.
• Low MNR feasibility: reversible/non-destructive.

Extent and severity of the contamination

• High MNR feasibility: well defined.
• Low MNR feasibility: poorly defined.

Dimensions of contaminated areas

• High MNR feasibility: shrinking.
• Medium MNR feasibility: stabilized.
• Low MNR feasibility: growing.

Contaminant phase

• High MNR feasibility: thin.
• Low MNR feasibility: thick.

Geochemistry of sediment

• High MNR feasibility: well understood.
• Low MNR feasibility: poorly understood.

Porewater advection/diffusion

• High MNR feasibility: infiltrating flow is not a contaminant source (i.e. dispersion/flushing is facilitated).
• Low MNR feasibility: infiltrating flow represents a continuing source.

Physical environment and porewater exchange

• High MNR feasibility: low energy (favours deposition); high energy (favours dispersion).
• Low MNR feasibility: frequent scour events exposing or mobilizing contaminants.

Potential receptors

Ecological receptors

• High MNR feasibility: low potential for impact to primary productivity, benthic community, and fish and aquatic wildlife.
• Low MNR feasibility: high potential for impact to primary productivity, benthic community, and fish and aquatic wildlife.

Water intake near the site

• High MNR feasibility: none, or wells mainly used for industrial supply.
• Medium MNR feasibility: intake used for domestic gardens.
• Low MNR feasibility: intakes for drinking supply within 300 m of the site.

Presence of species at risk

• High MNR feasibility: no species at risk present.
• Medium MNR feasibility: species at risk present but unlikely to be substantially exposed to contaminated media.
• Low MNR feasibility: species at risk present in area, with high chance of contact with contaminated media.

Practical receptors

Level of confidence in study data

• High MNR feasibility: high (e.g., >2 years of available seasonal data).
• Low MNR feasibility: low (e.g., only one study, one year, or no seasonal data).

Objectives of custodian

• High MNR feasibility: long-term interest in the site.
• Medium MNR feasibility: medium-term interest in the site.
• Low MNR feasibility: short-term interest in the site.

Financial and institutional provisions for monitoring and implementation of a contingency plan

• High MNR feasibility: long-term, legally binding budget provisions secured.
• Medium MNR feasibility: long-term, non-legally binding budget provisions secured.
• Low MNR feasibility: no long-term budget provisions.

Access to off-site monitoring locations (e.g., upstream and sentinel monitoring wells)

• High MNR feasibility: long-term access secured.
• Medium MNR feasibility: long-term access possible.
• Low MNR feasibility: limited or no access possible.

In most cases where MNR is proposed, it is used as one component of the site management strategy, either in conjunction with active remediation (e.g. in situ capping or removal) or as a follow-up measure. For example, MNR may be used after dredging or capping highly contaminated areas to manage residual low level contamination in adjacent areas. MNR can also be effectively combined with capping and armouring in areas subject to erosion. The box below indicates other potential roadblocks to identifying MNR as a specific remediation approach.

The feasibility of MNR as a management option should be supported by detailed site-specific information that demonstrates the efficacy of this approach. Typically, multiple lines of evidence are required to demonstrate the feasibility and effectiveness of MNR, including documentation of source control, evidence of burial or reduced surface sediment concentrations, assessment of surface sediment mixing to determine depth of contamination and remedial targets, measurement of sediment stability, evidence of transformation and risk attenuation, modeling of long-term recovery in water, sediment, and biota, and consideration for future site use (Magar and Wenning 2006). Several assessment tools, guidance documents, and additional resources for selecting appropriate monitoring tools and approaches, including MNR, are available from the United States Naval Facilities Engineering Command website.

Progress should be carefully monitored using appropriate and defendable sampling strategies, and reporting should be performed based on a robust plan. As well, source control and performance monitoring are fundamental to the MNR approach. Section 4 describes these steps in greater detail.

3 Process for planning and implementing monitored natural recovery

The first stage of MNR planning, i.e., information gathering (as described in Section 4), consists of investigating the opportunity of utilizing MNR at a given site using a preliminary assessment of technical and practical constraints. The objective of this stage (i.e. Steps 3 and 5 of DMF) is to characterize the contaminated site and to develop a CSM which describes specific relationships among contaminant sources, exposure routes, interaction with potential human or ecological receptors, and mechanisms for release, transport, and NR.

The R/RM strategy development stage (i.e. Step 7 of the DMF), as described in Section 5, consists of investigating NR processes using lines of evidence and models that quantitatively assess contaminant fate and transport as well as rates of NR. The intent is to document and support the use of MNR as a remedial strategy based on reliable scientific data. Modelling should produce scientifically defensible outputs to support the use of NR processes as a remedial method. Next, R/RM goals and objectives are established. When establishing the R/RM goals and objectives, the potential impacts of climate change are considered, and feasible sustainability measures are integrated. A contingency plan that can be implemented in case MNR does not meet the expected R/RM goals and objectives is also developed at this stage.

The R/RM strategy implementation stage (i.e. Step 8 of the DMF), as described in Section 6, involves the execution of the RAP and/or RMP, as well as performance monitoring of the MNR to verify that R/RM goals and objectives are being, or will be, met. The performance monitoring program includes rules for decision-making and triggers for actions, detailed data analysis methodologies, and data quality objectives clearly established well in advance.

The confirmatory sampling and final reporting stage (i.e. Step 9 of the DMF), as described in Section 7, involves confirming the attainment of R/RM objectives after implementing the R/RM strategy including MNR. Confirmatory sampling is conducted to demonstrate that the contamination is removed or efficiently stabilized, and that the R/RM objectives were met. A site closure report, documenting risk reduction at an acceptable level and describing the use of sustainable approaches at the site, is prepared if no additional measure is required. An example to follow is presented in the FCSAP Site Closure Report Template document (FCSAP, 2022b).

Finally, the long-term monitoring (i.e. Step 10 of the DMF) stage is described in Section 8. Long-term monitoring is not necessarily part of MNR; however, where MNR is selected as part of the remedial approach, long-term monitoring design and implementation concepts (FCSAP; 2013a) can be consulted in the design of the R/RM strategy.

Collaborating with FCSAP expert support departments at each stage is recommended to verify all considerations were taken into account to determine whether MNR is an appropriate strategy for a given site.

4 Information gathered prior to MNR

Before considering any remediation action, including MNR, a robust conceptual site model (CSM) must be developed. The CSM describes the site-specific relationships among suspected contaminant sources, release and transport mechanisms, contaminated media, exposure routes, and potential for adversely affecting human and ecological receptors. It is a narrative and graphical representation of what is known or suspected about the contaminant sources, release mechanisms, migration, and fate. The CSM also includes hypotheses regarding the dominant processes that may promote natural recovery and how these processes will meet remediation objectives.

An initial CSM is developed with information gathered during Step 2 (Historical Review), including reports, imagery, regulatory agency records, and information available from adjacent sites (FCSAP 2021c; COA 2008). To refine the CSM, additional data is gathered over time and in three spatial dimensions to develop an understanding of the contaminant source and behaviour at the site. The following quantitative parameters are determined in steps 3 and 5 of the DMF (e.g. in Phase II and III Environmental Site Assessments; ESAs), contributing to further refinement of the CSM:

• Relevant contaminant(s), mass of the contaminant(s) sources, horizontal, and vertical distribution.
• Human or ecological receptors of potential concern that may be adversely affected by contaminants of potential concern (COPCs).
• Chemical properties of contaminant(s) sources:
  • Solubility;
  • Volatility; and,
  • Rates of biological and non-biological transformation.
• Contaminant phase distribution and partitioning between sediment and porewater, and potential for volatilization.
• Climatic characteristics under current conditions and future projections.
• Geochemical characteristics of the sediment:
  • Porosity, critical shear strength, permeability, density, grain size, extent of armouring, dissolved oxygen, organic material, and redox potential;
  • Presence of nutrients; and,
  • Microbiological analysis to determine the presence and viability of an appropriate microbial population.
• Hydrology:
  • Hydraulic conductivity and connectivity with groundwater;
  • Hydraulic gradient and energetics of overlying waterbody;
  • Groundwater flow direction and velocities;
  • Location and type of surface water; and,
  • Recharge and discharge zones.

The temporal and spatial variations of all of these factors, and how they interact with each other, create the processes that govern contaminant behaviour, fate, and transport dynamics. Only when these processes have been well understood can MNR be considered and appropriately applied at a site (Magar et al. 2009). The CSM must be refined enough at Step 7 (Develop Remediation/Risk Management Strategy) to provide an understanding of the dominant mechanisms of contaminant behaviour, including whether natural recovery processes are suitable as remediation options. Since MNR potentially requires more time as a remediation strategy, the influence of climate change and how it may affect recovery should also be incorporated into the CSM. The CSM serves as a foundation for the quantitative analyses that will be carried out during steps 8 and 9.

Several examples are available for CSMs (COA 2008; FCSAP 2012a), including CSMs constructed for biomagnification of methyl Hg from sediment through an aquatic food chain (FCSAP 2021c), MNR recovery of polychlorinated biphenyl (PCBs) contaminated sediments in Lake Hartwell, South Carolina (Magar et al. 2009), and hydrophobic organic contaminant dynamics (U.S. EPA 2014).

Specific assessment of MNR as a prospective remediation option can begin as early as Step 3 (Initial Testing Program/ESA Phase II) and would be further investigated at Step 5 (Detailed Testing Program/ESA Phase III). At steps 4 and 6, custodians have the option to assign one of five classes to aquatic sites (FCSAP 2021a):

• Class 1 (High priority): The available information indicates that action (e.g., further site characterization, risk management, remediation, etc.) is required to address existing concerns. Typically, Class 1 sites indicate high concern for several factors, and measured or observed impacts have been documented.

• Class 2 (Medium priority): The available information indicates that there is potential for adverse impacts, although the threat to human health and the environment is generally not imminent. Off-site contamination may not have been detected, however, the potential impacts for this contamination were rated high, and therefore some action is likely required.
• Class 3 (Low Priority): The available information indicates that this site is currently not a high concern. However, additional investigative work may be carried out to confirm the site classification, and some form of action may be required.
• Class N: The available information indicates there are probably no significant environmental impacts or human health threats. There is likely no need for action unless new information becomes available indicating greater concerns, in which case the site should be re-examined.
• Class INS: There is insufficient information to classify the site. In this instance, additional information is required to address data gaps.

There is potential for MNR to be applied at Class 1, 2, and 3 sites, and it is often applied in combination with other remediation strategies. Data from all these steps, as well as the Human Health and Ecological Risk Assessment (HHERA) from steps 5 and 7, can be combined and used to contribute to the assessment of MNR as a remediation option. The confirmation of MNR as an appropriate course of action and its implementation for a particular site would be determined during steps 7, 8, and 9 of the DMF.

5 Develop a remediation/risk management strategy

During Step 7 (Develop a Remediation/Risk Management Strategy), the viability of using remediation actions, including MNR, are considered based on the CSM; this is also known as a remedial options analysis (ROA) exercise. One of the primary goals of this step is to develop site-specific numeric remediation objectives by adapting generic guidelines that reflect site-specific conditions and/or are based on risk assessment (FCSAP 2021c). In most cases, a detailed quantitative risk assessment (DQRA) is conducted as part of Step 7 if it has not been done already at Step 5. The ROA begins once site testing programs (ESA Phase I, II, and III) have been carried out in accordance with Step 5, and once a comprehensive CSM has been developed. Both technical and practical constraints are used to establish whether MNR can be considered as a remediation option either alone or, more often, in conjunction with other actions in an overall remediation plan.

Several general requirements must be met for MNR to be specifically considered as part of an R/RM strategy, including:

1. Control of contamination source;
2. Achievement of remediation goals within an acceptable timeframe; and,
3. Protection of potential receptors.

i. Control of contamination source: Controlling, minimizing, or isolating the contaminant source so that recovery can be effective is one of the requirements to obtain FCSAP R/RM funding (FCSAP 2021b). If evidence for source control is lacking, removal or engineered isolation of the source may be sought using active remediation techniques (e.g., dredging or capping), either before MNR is initiated or in tandem. Lines of evidence (LOE) to describe source control can include literature and historical data, modeling to link sources with likely chemical transport mechanisms in sediments and towards receptors, and examination of site-specific data that may be available to support chemical transformation, reduction in bioavailability or mobility, physical isolation, or dispersion.

ii. Achievement of remediation goals: MNR should be capable of meeting the remediation objectives within a reasonable time period. While the acceptable limits of remediation timeframes depend on site conditions and the COPCs involved, it should be emphasized that recovery based on MNR can take 5-30 years, or longer for some recalcitrant (e.g., recovery resistant) contaminants (Magar et al. 2009). The ROA requires a preliminary estimate of the time for MNR to achieve remediation objectives (e.g., generic environmental quality guidelines [EQGs] or site-specific target levels). Empirical data from the site are used later to examine the estimated recovery trajectory. Additional discussion on remediation goals is provided in section 5.4.

Consultation with Expert Support and stakeholders should consider the funding restrictions of FCSAP regarding any long-term remediation approach. The custodian may be required to provide funding from their own operating budgets for long-term solutions to their contaminated sites beyond the FCSAP program time horizon.

iii. Protection of potential receptors: For MNR to be selected as a remediation option, an assessment of risk to existing and potential future receptors must be considered, including the following:

• Site-specific hydrodynamics and sediment geophysical properties;
• Contaminant fate and transport mechanisms;
• Potential exposure pathways for human and ecological receptors (e.g., species at risk, protected areas, sensitive habitats); and,
• Potential changes to site use in the future (e.g., dam removal, navigational dredging).

5.1 Contaminants conducive to MNR

Another major consideration is whether contaminants at the site are amenable to MNR. Some contaminants are better suited than others for a successful MNR approach. Whereas guidance for applying MNA to soil is primarily focused on organic contaminants, both organic and inorganic contaminants may be amenable to MNR in aquatic sediments. For example, microbial-mediated dechlorination of higher chlorinated PCBs can result in less toxic but more mobile lower chlorinated congeners (Abraham et al. 2002). Petroleum hydrocarbons and nitrotoluenes are potentially mineralized in the sediment environment (Haritash and Kaushik 2009; Walker et al. 2006). For inorganic contaminants (e.g., metals), transformation reactions are dominated by geochemical conditions that favour sorption and/or complexation (e.g., with dissolved organic carbon or as metal oxide or sulfide precipitates). Depending on the metal, this binding may be relatively stable and permanent (e.g., chromium), or could be subject to dissolution and remobilization if redox conditions change in the sediment (e.g., arsenic). Finally, metals that form organo-complexes can be transformed such that their bioavailability is increased (e.g., methylation of Hg under reducing conditions) or decreased (e.g., debutylation of tin under primarily aerobic conditions). As such, it is important to consider the transformative characteristics of the contaminant(s) of concern on site and its suitability to MNR strategy.

The dominant factors contributing to recovery depend on the nature of the contaminants and on the physical, biological, and chemical characteristics of the site. Where immobilization or dispersion are determined to be a dominant recovery process contributing to MNR, hydrodynamic conditions and sediment transport processes will be especially important to investigate. Furthermore, because of the potential for dispersion to incur exposure over a wide area, the dispersion process may require a more robust and comprehensive effort to analyze downstream and offsite risk. Alternatively, where chemical transformation or reduction of contaminant bioavailability are deemed to be primary MNR drivers, geochemistry, microbiology, and site-specific physicochemical conditions will dominate the CSM (Magar et al. 2009). The list below presents general guidance regarding the potential success of MNR implementation with different classes of compounds, as well as the dominant recovery process involved. Consideration of these recovery processes is required to further refine the CSM for a given site.

Hydrocarbons

BTEX

• Dominant recovery process: dispersion, dissolution, flushing, volatilization.
• Likelihood of success in aquatic environment: high.

Gasoline and fuel oil

• Dominant recovery process: biotransformation.
• Likelihood of success in an aquatic environment: low.

Non-volatile aliphatic compounds

• Dominant recovery process: biotransformation, immobilization.
• Likelihood of success in an aquatic environment: low.

PAHs

• Dominant recovery process: biotransformation, immbobilization.
• Likelihood for success in an aquatic environment: high for low molecular weight compounds; low for high molecular weight compounds.

Creosote

• Dominant recovery process: biotransformation, immobilization.
• Likelihood for success in an aquatic environment: low.

Oxygenated hydrocarbons

Alcohols, ketones and esters of low molecular weight

• Dominant recovery process: biotransformation.
• Likelihood of success in an aquatic environment: high.

MTBE

• Dominant recovery process: biotransformation.
• Likelihood of success in an aquatic environment: moderate.

Chlorinated aliphatics

Perchloroethylene (PCE), trichloroethylene (TCE), carbon tetrachloride

• Dominant recovery process: biotransformation.
• Likelihood of success in an aquatic environment: moderate.

Trichloroethane (TCA)

• Dominant recovery process: biotransformation, abiotic degradation.
• Likelihood of success in an aquatic environment: moderate to high.

Methylene chloride

• Dominant recovery process: biotransformation.
• Likelihood of success in an aquatic environment: high.

Vinyl chloride (VC)

• Dominant recovery process: biotransformation.
• Likelihood of success in an aquatic environment: moderate to high.

Dichloroethylene (DCE)

• Dominant recovery process: biotransformation.
• Likelihood of success in an aquatic environment: moderate.

Chlorinated aromatics

PCBs, Dioxins, Furans, multichlorinated benzenes

• Dominant recovery process: sorption to organic carbon, burial, slow transformations of higher chlorinated congeners.
• Likelihood of success in an aquatic environment: moderate (may require removal).

Pentachlorophenol (PCP)

• Dominant recovery process: sorption to organic carbon, burial.
• Likelihood of success in an aquatic environment:

Less chlorinated PCBs, dioxins

• Dominant recovery process:
• Likelihood of success in an aquatic environment: low.

Monochlorobenzene

• Dominant recovery process: biotransformation.
• Likelihood of success in an aquatic environment:

Nitroaromatics

TNT and RDX (explosives)

• Dominant recovery process: abiotic and biotic transformation, sorption to organics.
• Likelihood of success in aquatic environment: high.

Metals, metalloids and organometals

Divalent cations (e.g. Ni, Cu, Cd, Ag, Zn, Pb)

• Dominant recovery process: precipitation as hydroxides and sulfides, sorption to organic carbon, and burial.
• Likelihood of success in an aquatic environment: high.

Arsenic

• Dominant recovery process: complexation as iron oxides or sulfides.
• Likelihood of success in an aquatic environment: moderate - subject to anaerobic dissolution and remobilization.

Chromium

• Dominant recovery process: abiotic reduction from soluble CrIV to insoluble and less toxic CrIII.
• Likelihood of success in an aquatic environment: high.

Mercury

• Dominant recovery process: complexation with DOC, iron oxides, sulfides.
• Likelihood of success in an aquatic environment: low - potential for methylation under anaerobic conditions, or redox mediated mobilization.

Tin

• Dominant recovery process: microbial debutylation.
• Likelihood of success in an aquatic environment: high in organic rich sediments.

Radionuclides

• Dominant recovery process: sorption, co-precipitation, complexation with organic matter. Will not change external exposure for gamma emitters.
• Likelihood of success in an aquatic environment: dependent on half-life, redox sensitivity. May cause concentration of contaminant at redox boundaries.

Contaminated site managers can also consult the Government of Canada’s Guidance and Orientation for the Selection of Technologies (GOST) website for additional information. The GOST tool recommends remediation technologies for given contaminants. The Contaminant Fact Sheets list contaminant properties, sources, health and safety considerations, as well as analytical parameters and field trials for Phase II and III assessments to determine appropriate remediation technologies for a given site. The Monitored Natural Recovery and Enhanced Natural Recovery Fact Sheet contains information directly pertaining to the use of MNR as a remediation technique at contaminated sites.

5.2 Lines of evidence to evaluate MNR effectiveness

To further evaluate the potential effectiveness of MNR strategy, a robust CSM should be utilized to identify multiple LOEs that support the MNR approach. The LOEs are focused on four primary mechanisms responsible for natural recovery of sediments: chemical transformation, reduced bioavailability, physical isolation, and dispersion. Each mechanism may include multiple natural recovery processes identified in lists 1 and 4. A summary of the data type collected for each LOE is provided below.

Chemical transformation

• Changes in contaminant concentrations over time due to transformation:
  1. Trends of contaminant transformation focused on the dominant transformation processes
  2. Chemical indicators of degradation (e.g., weathering or degradative by-products)
  3. Analysis of factors that regulate transformation, including:
     • Solubility, partitioning, volatility
     • Redox potential
     • Microbial community
     • Availability of cofactors (e.g. organics, nutrients, electron donors/acceptors)
     • Geochemical and physicochemical factors

Reduced bioavailability

• Changes in concentrations of contaminant available for uptake by biota:
  1. Trends demonstrating reduced bioavailability and/or uptake
  2. Reduced concentrations in porewater
  3. Chemical solubility, hydrophobicity, or volatility
  4. Demonstration of contaminant weathering
  5. Evidence of sequestration or precipitation
  6. Demonstration of favourable geochemical or physicochemical conditions

Physical isolation

• Physical containment of contaminants in an area of sediment:
  1. Vertical sediment profiles demonstrating burial and reductions in surface sediment concentrations over time.
  2. Radiogeochemistry (e.g., lead-210 or cesium-137) to demonstrate sediment deposition rates.
  3. Evidence of burial may be supported by:
     • Hydrodynamic flow conditions
     • Geophysical conditions (e.g., bathymetry or sub bottom profiling)
     • Sediment armouring and scour potential under a range of flows
     • Bioturbation assessments

Dispersion

• Changes in localized containment concentrations via movement to adjacent areas
  1. Desorption or dissolution processes and kinetics.
  2. Analysis of contaminants in water column at the site, in downstream reaches and receptors.
  3. Evidence of dispersion may be supported by:
     • Hydrodynamic flow conditions
     • Sediment armouring and scour potential under a range of flows
     • Demonstration of contaminant or sediment transport

5.3 Modelling in support of evaluating MNR effectiveness

Modelling or simulation based on the CSM can be used to quantitatively understand current site conditions and predict future conditions based on data assembled for the site. The main objectives of modelling are to identify the best locations for monitoring, assess the extent and availability of contamination over time, the risk of exposure and impact on receptors, and the time required to achieve remediation objectives. Models are used to focus data gathering and analysis so that specific hypotheses can be tested.

Model selection and its level of complexity is guided by the CSM, which in turn is a product of the hydrological, biological, chemical, and physical complexity of the site. The initial model should only be as complex as it needs to be to answer existing management questions, although different environments (e.g., wetland, river, lake, estuary, marine environment) will require consideration and collection of different types of data. Initial models are typically based on statistical trend analysis of potential relationships that were identified in the early CSM (e.g., concentrations of contaminants in water and resident biota). As more information is gathered, the complexity of the model increases, as depicted in Figure 2.

The progressive complexity of models roughly encompasses four categories of models for MNR. In order of increasing complexity, these models include: hydrodynamic, sediment bed, sediment transport, and integrative. A brief description of each type of model is provided below. In addition, a list of the available models, their primary applications, advantages, and limitations is summarized in U.S. EPA (2014) and Appendix B.

1. Hydrodynamic Models
   These models incorporate information on the movement of water and predict how shear forces at the sediment water interface will affect distribution of contaminants.
2. Sediment Bed Models
   These models consider the physical, biological, and chemical factors affecting contaminant partitioning between sediment, porewater and overlying water column.
3. Sediment Transport Model
   These models involve longer term modeling of hydrodynamic forces and sediment bed shear strength.
4. Integrative Models
   These are merged hydrodynamic, sediment bed, and sediment transport models that also consider contaminant speciation and partition kinetics.

Regardless of the model selected, hypotheses, variables, and parameters used in the model must be documented, described, and justified based on site-specific field testing, field or laboratory testing, and verification or sound technical assumptions. The specific measurements required to calibrate a model are highly site-specific but may include measurements of sediment particle transport in the system, sediment bed shear strength, sources, sinks, transport of contaminants, and bottom water flow velocity (Magar et al. 2009). Additional guidance for physical, biological, and chemical tools used for monitoring and providing modelling data is included in the FCSAP LTM Guidance (2013a). Limitations of the selected model should also clearly be outlined. A sensitivity analysis should be carried out to determine which variables have the greatest influence. When data from literature are used, documentation and justification regarding applicability to the site and limitations must be included. When estimating the time required to achieve contaminant site-specific target levels, supporting calculations should be presented confirming that MNR is the most cost-effective and suitable remediation alternative.

5.4 Remediation goals

MNR is a remediation approach that generally requires longer periods of time to achieve remediation goals, often ranging from 5 to 30 years or more (Magar et al. 2009). Estimating contaminant recovery rates is therefore a critical function of the modelling approach and an important determinant of whether MNR is an acceptable remediation strategy. The estimated time required for remediation will depend on the specific R/RM objectives for the site. R/RM objectives should be determined with FCSAP Expert Support, federal and provincial regulators, Indigenous peoples, and relevant stakeholders, and may be based on reducing contaminant concentrations in sediments to compliance with existing environmental guidelines. However, as outlined in the COA (2008), remediation goals are not typically based only on concentrations of contaminants in sediment, but also site-specific goals defined through a risk assessment.

The Canadian Council of Ministers of the Environment (CCME) has established protocols for determining sediment quality guidelines using the available toxicological information for a given contaminant (CCME 1999). However, only Interim Sediment Quality Guidelines (ISQG) and Probable Effects Levels (PEL) for some contaminants have been developed to date. The PEL represents the lower limit of the range of chemical concentrations that are usually or always associated with adverse biological effects. If insufficient information exists to establish a full sediment quality guideline, a guideline value from another jurisdiction can be provisionally adopted. However, in most cases, the custodian may develop a risk-based standard instead of using CCME numerical sediment guideline values (COA 2008). Environmental factors that influence the toxicity of a specific contaminant can be considered in the derivation or refinement of a risk-based standard. For example, total organic carbon content (TOC), acid volatile sulphides (AVS), or hardness may influence the bioavailability of metals and affect the threshold concentrations of certain metals (U.S. EPA 2014).

To develop sediment remediation objectives for a given site, a HHRA or ERA may be required, depending on the presence of relevant receptors and exposure of the receptors to the contaminants at the site. Guidance for conducting ERAs is available in the main FCSAP ERA guidance document (FCSAP 2012a) and several specific technical guidance modules (FCSAP 2013b; 2013c; 2012b). The document Guidance for Developing a Contract Statement of Work (SOW) for Human Health Preliminary Quantitative Risk Assessment (PQRA) and Detailed Quantitative Risk Assessment (DQRA) is also available from Health Canada (2010).

5.5 Contingency plan

The contingency plan establishes a replacement remedial technology, an alternative remedial approach, or additional measures that can be implemented if MNR is ineffective or insufficient to achieve the remediation objectives (Magar et al. 2009). The contingency plan should be re-evaluated during performance monitoring to ensure they remain appropriate.

A contingency plan must be developed when MNR is adopted, given the uncertainties associated with this approach. Such a plan can include, for example, changes in the remediation approach (e.g. dredging or capping) or other remedial actions (e.g. Enhanced Natural Recovery); funding mechanisms may also be described. A contingency plan should be flexible, allowing for the incorporation of new information about site risks and remediation technologies.

When the RAP is developed in Step 7, it is recommended to include one or more criteria (“triggers”), as appropriate, that will signal unacceptable performance of MNR as the selected remediation option, and indicate when to implement contingency measures. Such criteria may include the following:

• Contaminant concentrations in sediment or porewater exhibit concentration increases indicative of a new or renewed release;
• Contaminants are identified at locations outside the original contaminated area, indicating renewed contaminant migration;
• Contaminant concentrations are not decreasing at a sufficiently rapid rate to meet the R/RM objectives; and,
• Changes in site use and/or hydrology adversely affect the appropriate use of MNR.

When establishing triggers or contingency measures, care is needed to ensure that sampling variability or seasonal fluctuations do not set off a trigger inappropriately. For example, an anomalous spike in porewater contaminant concentrations that sets off a trigger might not be a true indication of a change in trend. A flowchart may help guide the process of evaluating the efficacy of MNR and whether the contingency plan should be implemented.

5.6 Integration of MNR and sustainable remediation considerations

In some situations, uncertainty or unacceptable human or ecological health risks preclude the use of MNR as a feasible remediation strategy while in other situations, the impacts of more active remediation alternatives will likely cause more environmental harm than leaving the contaminants in place (U.S. EPA 1998). It is generally accepted that there are no zero-risk options for managing contaminated sediments. As such, both monetary and environmental cost-benefit analysis should be undertaken to assist in determining the optimum risk management strategy. Moreover, it is important that remedial actions not cause more environmental damage than they remedy (FCSAP 2021c). The singular focus on excessively conservative environmental health protection may lead to extensive physical remediation strategies with potential significant cost and negative environmental impacts (Sparrevik et al. 2011). Under this context, MNR as an integral part of a sustainable remediation strategy may serve to optimize the three pillars of sustainability - economic, social, and environmental. MNR is likely to be the most sustainable alternative for sediment remediation; since there is minimal equipment, contaminated material hauling and/or treatment (ITRC 2014).

A tool such as the Life Cycle Assessment (LCA) can be used to consider the benefits and costs of different remediation approaches or combined approaches more thoroughly, including the use of MNR. A LCA considers the environmental impacts of remediation for the entire lifespan of the project, including the immediate effect of the remedy, the use of resources and energy during implementation and the resulting use or exclusion from the area for recreation, fishing, and other activities. It also provides a framework where trade-offs between primary impacts (e.g., contamination risk in the environment) and increased local, regional and global impacts from potential remedial options (e.g., secondary impacts) can be assessed (Lemming et al. 2012). In a recent example at a contaminated sediment site in Norway, a LCA was applied to investigate the environmental footprint of different active and passive thin-layer capping alternatives as compared to natural recovery. The results showed that capping was preferable to natural recovery when analysis is limited to effects related to the site contamination. However, the incorporation of impacts related to the use of resources and energy during active remedial activity increases the environmental footprint by over 1 order of magnitude, making capping inferior to the natural recovery alternative (Sparrevik et al. 2011). This example illustrates that MNR approach can be an excellent component to a sustainable site remediation strategy when properly integrated and implemented – leading to overall positive environmental outcomes while successfully managing the contamination risk on site. Such life cycle assessments may become increasingly important as custodians take steps to mitigate climate change impacts (such as greenhouse gas emission reduction), or other environmental impacts of remediation.

5.7 MNR and climate change considerations

The effect of climate change can constitute a significant consideration in the development of R/RM objectives and the final selection of the R/RM approach that is most appropriate for a federal contaminated site. There are numerous areas where climate change considerations may play a significant role in how site management proceeds along the 10-step DMF. For example, in Step 7, custodians should consider the effect of climate change on their current site conditions, including contaminant media, pathways, and receptors, when considering either guideline or risk assessment approach. Short, medium, or long-term climate change implications on contaminant types, concentrations, and distribution or changes in the residency media should be incorporated into site assessment as early as Step 3 (FCSAP; 2022c). Additional guidance on how to adapt contaminated site management approaches to include climate change considerations can be found in the Integrating Climate Change Considerations into Federal Contaminated Sites Management guidance document (FCSAP 2022a).

The potential effect of climate change can play a significant role in the employment of MNR approach at a contaminated sediment site. Effective MNR also depends on other factors, such as physical sedimentation rate, sequestration, and immobilization of contaminated sediment material. Due to the often longer timeline associated with MNR, climate change may present a particular challenge for the success of MNR and should be given additional consideration. Future changes in the hydrological cycle in terms of rainfall, runoff, severe weather events and erosion will also affect transport, dilution, and fate processes. Important pathways that will become altered include volatilisation, adsorption, hydrolysis, biodegradation, photodegradation, photo-enhanced toxicity, uptake, and metabolism. Whilst rates of some of these processes are increased with increasing temperature, quantitative predictions and assessments of interactions are complex (Schiedek et al. 2007). These climate change associated factors in turn would potentially affect the MNR efficacy on a long-term basis, thus requiring additional consideration during Step 7.

6 Implementing a remediation/risk management strategy

If contaminants are amenable to MNR approach, there is acceptable source control and protection of receptors, and the ROA indicates that MNR may be appropriate for the site either alone or in tandem with other remediation actions, then the risk management strategy developed in Step 7 may be implemented in Step 8. Implementation of a risk management strategy includes the execution of a RAP/RMP and an Environmental Management Plan. This step also includes performance monitoring of MNR as designed in the context of adaptive management and the execution of a contingency plan if necessary. Additional guidance and resources for these requirements are provided in the Framework for Addressing and Managing Aquatic Contaminated Sites (FCSAP 2021c).

6.1 Performance monitoring

As a result of the potentially long remediation time frames that can be associated with the MNR approach, there is an elevated potential for contaminant migration and other uncertainties inherent to the MNR approach. Therefore, carefully designed monitoring is imperative for successful application of MNR. The design of the monitoring plan and performance sampling, as well as the development of a contingency plan, are key elements of the MNR process. Performance monitoring during Step 8 is not considered LTM; however, the basic principles in the development of a performance monitoring program can be derived from the LTM Guidance document (FCSAP 2013a).

Performance monitoring should include: 1) reference locations up-gradient from the contaminated site, 2) locations at the core of the contamination, 3) locations near the periphery, and 4) down-gradient from the contaminated area. By carefully selecting locations to monitor and by choosing the types of samples to include, an ideal performance monitoring plan would:

• Demonstrate that NR is either occurring or not, according to expectations;
• Detect, in a timely manner, changes in site physical and chemical conditions that might reduce the efficacy of MNR;
• Identify any toxic and/or mobile transformation products;
• Verify absence of unacceptable effects on receptors;
• Detect new releases of contaminants to the environment that could generate a risk or reduce the effectiveness of MNR;
• Verify attainment of remediation objectives;
• Verify that the area of contamination is not expanding;
• Demonstrate the effectiveness of receptor protection;
• Support a decision to terminate activities or implement the contingency plan, if necessary; and,
• Respond with suitable action should MNR prove inappropriate.

The goals and targets of the performance monitoring plan should be clear. Moreover, institutional and financial mechanisms for maintaining the monitoring program should be identified in the remedial plan or other site documents,as appropriate. Data collection must be conducted with a predetermined understanding of how the information will be used to assess remediation goals and MNR approach success. The main components of the monitoring program include the number and type of samples, monitoring locations, frequency of sampling, and the parameters monitored.

The performance monitoring framework should allow for a robust and defensible set of methodologies and parameters that progress toward remediation objectives. Performance monitoring programs should define the desired statistical power as well as the nature of the statistical tests to be performed. The desired power can then be used to determine the minimum number of samples required to achieve that level of confidence in the data (Magar et al. 2009).

For porewater and the sediment matrix, the number of samples and sampling locations should be sufficient to analyze the horizontal and vertical distribution of the contaminant and to predict contaminant movement and spatial distribution over time. Contaminants in sediment and porewater are most commonly monitored in the biologically active horizon of the sediment, which is typically 5 to 10 cm thick in freshwater systems but can be as thick as 1 m in estuaries and marine environments (U.S. EPA 2014). Sediment core sampling should be considered to determine stratigraphy at the monitoring locations. Monitoring of uncontaminated sediments and/or porewater surrounding the contaminated area must be conducted to ensure that a trend toward lower contaminant concentrations is not the result of horizontal or vertical migration from the primary contaminated area. The sampling strategy should include sediments in reference or background locations, the contaminated zone and at locations peripheral to the contaminated area; this will allow for a comprehensive assessment of the dispersion or migration patterns of the contaminant. Technical guidance for the planning and implementation of a field sampling program for sediment and porewater has been established (EC 2002a; 2002b).

Sediment samples can be obtained using a number of techniques (e.g., EC 2010, CCME 2016). EC (2010) describes criteria for selecting sediment sampling devices (e.g., grab or core samplers) based on desired sediment sample depth and volume, required stratigraphic resolution, and methods for sample handling and analysis. Similarly, guidance is available on porewater sampling using methods such as peepers (Teasdale et al. 1995), suction filtration, dialysis, resin and gel samplers, and centrifugation (U.S. EPA 2001). Please refer to the following section, 6.2 for more information on sampling parameters.

6.2 Sampling parameters

The parameters chosen for MNR confirmatory sampling are site- and contaminant-specific and depend on R/RM objectives. The parameters measured must be capable of describing the behaviour of the contaminants of interest, determining unacceptable exposure of human or environmental receptors, providing data that directly address hypotheses to allow the CSM to be refined, and verifying the remediation timeline as predicted. For example, appropriate parameters to analyze a site contaminated with arsenic are provided below.

Porewater

• Monitoring frequency: seasonal, based on results.
• Number and location of samples:
  • Statistically significant number of samples to delineate the entire zone of contamination and from reference locations.
  • Porewater obtained from appropriate strata (e.g., 1-2cm intervals from the sediment/water interface) to demonstrate redox boundary.
• Monitoring parameters:
  • Arsenite and arsenate
    • Dissolved Fe, Mn
    • Dissolved oxygen
    • Redox potential
    • Dissolved inorganic carbon
      • pH
      • SO₄^2-
      • NO₃^-
      • Fe2+
      • CH₄
      • CO₂
      • Mn²⁺
        • Groundwater inputs
        • Nutrient input (total and bioavailable P)
        • Total organic carbon
        • pH

Sediment matrix

• Monitoring frequency: seasonal analysis, adjusted based on results.
• Number and location of samples: statistically significant number of core samples covering the entire zone of contamination. Sediment obtained from appropriate strata (e.g., 1-2cm intervals from the sediment/water interface) to demonstrate the redox boundary.
• Monitoring parameters:
  • Arsenite and arsenate
    • Total Fe, Mn
    • Total sulfides and phosphorous
    • Total organic carbon
    • Grain size

Ecological receptors

• Monitoring frequency: sampling frequency dependent on risk analysis.
• Number and location of samples: statistically significant number of samples to assess contamination gradient.
• Monitoring parameters
  • Total arsenic (surface water)
  • Total Fe, Mn
  • Total sulfides and phosphorous
  • Total organic carbon
• Arsenic speciation (biota)
  • Abundance and diversity (benthic inverts)
  • Fish health measures 

6.3 Sampling timeline/frequency

Performance monitoring continues as long as contamination remains above acceptable levels, until the site specific R/RM objectives have been achieved. The sampling frequency will depend on a range of factors, including the type of contaminant and the level of risk represented by the concentrations remaining on site. The performance monitoring plan should allow for some flexibility in the sampling frequency: it can be reduced when the situation is stable or increased when conditions change (i.e., consider adaptive management principles). At every sampling event, the performance of MNR can be evaluated by comparing monitoring results to predictions made in the CSM and the R/RM strategy. If remediation is progressing as expected, MNR can continue. If at any time during monitoring activities the data shows that MNR is not achieving remediation objectives within the expected period, a contingency plan must be implemented.

For contaminated sediments, the first year of performance monitoring should include seasonal sampling to assess the performance of the existing MNR processes throughout the year. Sampling frequency may be reduced to yearly or biennially when MNR processes are most active once seasonal variation in NR is sufficiently understood. If objectives are not being met, additional remediation and consideration of a change in R/RM strategy may be warranted.

7 Confirmatory sampling

Once R/RM objectives have been achieved, additional confirmatory sampling (part of Step 9) should continue to a predetermined period in order to verify that concentrations are stable and remain below acceptable levels, and that the site presents no further environmental or human health risks. Confirmatory sampling within this period may not be more frequent than during performance monitoring and should be established based on performance monitoring results. The final sampling event (Step 9) in the MNR process would relate to the establishment of an Exit Criteria and associated Management Decision for site closure (FCSAP 2013a).

8 Long-term monitoring and site closure

Once confirmatory sampling has demonstrated that the level of contamination is stable at or below the R/RM objective, further LTM (Step 10) may also be implemented in the overall site management decision prior to site closure. Where MNR is selected as part of the remedial approach, LTM design and implementation concepts, as described in the FCSAP LTM Guidance document, can be consulted in the design of the MNR performance monitoring (FCSAP 2013a). As with other remediation strategies, LTM may be terminated for a site when it can be clearly demonstrated that R/RM objectives have been met and it is certain that contamination does not pose an unacceptable human or ecological risk in the foreseeable future and no further management action is necessary (FCSAP 2021c).

In instances where physical isolation and/or burial processes were relied upon to reduce contaminant levels, a site may be closed after the sediment is shown to be stable for extended periods, even after high-energy events (FCSAP 2013a). As with other R/RM actions, traditional “no further action” site closure may not always be attainable at all MNR sites (Magar et al. 2009). As such, contingency plans may need to be implemented and a complementary LTM strategy may need to be established. Guidance for site closure is currently provided in the Site Closure Report Guidance for Federal Contaminated Sites (FCSAP, 2022a).

9 Successful application of MNR

Application of MNR as a formal R/RM approach has been limited to date, but Magar et al. (2009) presents several examples where MNR has been employed in freshwater and marine environments in the United States. Since that review was published, other peer reviewed publications have examined the effectiveness of MNR in Canadian marine harbours. For example, the potential for MNR to be applied to a marine harbour environment was examined in Sydney Harbour, Nova Scotia. The harbour was contaminated with polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and metals from a coking and steel plant that discharged into the Sydney Tar Ponds and ultimately into the harbour (Walker et al. 2013). Following confirmation of source control and implementation of physical remediation measures, PAH and PCB concentrations were modelled (Smith et al. 2009) and the reduction of contaminant concentrations in surficial sediments was demonstrated to occur as a result of NR processes (Walker et al. 2013). Corresponding risk reduction was documented from declining concentrations of contaminants in resident biota (Walker et al. 2013b; 2013c).

Nadeau (2008) reviewed three cases studies based in the United States where MNR was employed, usually in conjunction with other remedial actions. MNR was applied to the remediation of mercury in Bellingham Bay in Washington, PCBs in Grasse River in New York, and PAHs in Commencement Bay, Washington.

At least two other prominent and ongoing examples may be cited to illustrate the effectiveness of MNR in Canadian harbour environments. These case studies, Saglek Bay, Labrador and Halifax Harbour, Nova Scotia, are presented below, along with additional resources for each site.

10 Summary of the MNR process

Below is an outline summarizing the integral elements of an effective MNR process.

Feasibility:

MNR may be used when:

• The source of contamination has been controlled;
• Site testing demonstrates that NR processes are occurring;
• The risks to receptors can be managed. If that is not possible, MNR might be combined with other techniques; and
• A long remediation timeframe is acceptable under the circumstances surrounding the contaminated site.

Remediation time:

• The necessary remediation time should be assessed before MNR is selected as the remediation approach. MNR should be capable of achieving remediation objectives within 20 years.

Site characterization:

• Robust characterization is required in any MNR assessment or demonstration.

Conceptual site model:

• A detailed site-specific conceptual model must be developed for the demonstration of MNR to show a clear understanding of the contamination, migration, exposure pathways, and receptors, and to identify risks. When modelling is performed, site-specific data should be used, particularly for key elements such as sedimentation or sorption rates. The impact of climate change on any of these factors should also be considered in the CSM. Modelling can be used to quantitatively refine the CSM, determine the trajectory of NR processes taking place, and more accurately predict the time of remediation.

Lines of evidence:

• Multiple lines of evidence are required to support the use of MNR for a given site. MNR is typically supported by four lines of evidence: chemical transformation, reduced mobility and/or bioavailability, physical isolation, and dispersion.

Monitoring plan:

• The monitoring plan is specific to the site and to the contaminants present. The monitoring framework should allow a statistical analysis of changing site conditions and progress toward remediation objectives. Monitoring should include reference locations up-gradient from the contaminated site, locations at the core of the contamination, and locations near the periphery and down-gradient from the contaminated area. The horizontal and vertical distribution of contaminant should be determined to predict its movement and the spatial distribution of contamination over time. Seasonal sampling should be required until an adequate understanding of seasonal variation is attained. Sampling frequency may be reduced depending on results. While LTM is not part of MNR, it is developed when MNR is selected as the remediation action for follow-up post-confirmatory sampling to demonstrate site stability.

11 Conclusion

Monitored natural recovery (MNR) can be an effective and sustainable approach for remediating contaminated sediments when the source of contamination is controlled, where NR processes are robust, and where potential risk to human and ecological receptors can be acceptably managed. MNR is most often combined with other remediation strategies and best applied to stable sediments that are not subject to resuspension. MNR is especially applicable to sensitive habitats where implementation of other remedial activities could present additional unacceptable environmental damage, determined by using tools such as Life Cycle Assessment. MNR is relevant to steps 7-9 of the 10-step  DMF (FCSAP, in press), after site characterization has been performed and a comprehensive CSM has been developed but can also be considered as a remedial option as early as Step 3 (Initial Testing Program/ESA Phase II). Costs associated with MNR can be either more or less than other remediation approaches such as dredging or capping; it nevertheless requires robust effort to characterize the site, planning and implementing performance and confirmatory monitoring. Typically, multiple LOEs are used to support MNR for a given site, including the four primary recovery mechanisms: chemical transformation, reduced mobility and/or bioavailability, physical isolation, and dispersion. During the development of MNR as a remediation strategy, contingency plans (to be applied if remediation goals are not being achieved) and long-term monitoring plans must be considered and adequately planned. Applicability of MNR as a remediation approach has been successfully demonstrated in several real-world examples and there is growing evidence that the approach can be employed as part of a successful remediation approach in aquatic environments.

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Appendix A – existing technical guides and protocols for MNR

Appendix B – selected computer models for application to MNR
(based on U.S. EPA 2014)