Guidance document on monitored natural attenuation for soil and groundwater remediation

Acronyms

BTEX

Benzene, toluene, ethylbenzene and xylenes

CCME

Canadian Council of Ministers of the Environment

CEQGs

Canadian Environmental Quality Guidelines

COPC

Contaminant of potential concern

CSM

Conceptual site model

DCE

Dichloroethylene

DFO

Department of Fisheries and Oceans (also known as Fisheries and Oceans Canada)

DMF

FCSAP decision-making framework

DNAPL

Dense non-aqueous phase liquid (or heavy immiscible liquid [HIL])

ECCC

Environment and Climate Change Canada

EMNA

Enhanced monitored natural attenuation

ESA

Environmental site assessment

FACS

Federal approach to contaminated sites

FCSAP

Federal Contaminated Sites Action Plan

FCSI

Federal Contaminated Sites Inventory

IC

Ion chromatography

ICP-ES

Inductively coupled plasma emission spectrometry

ITRC

Interstate Technology and Regulatory Council

LNAPL

Light non-aqueous phase liquid (or light immiscible liquid [LIL])

LTM

Long-term monitoring

MDNR

Missouri Department of Natural Resources

MNA

Monitored natural attenuation

MNR

Monitored natural recovery

MTBE

Methyl tert-butyl ether

NA

Natural attenuation

NAPL

Non-aqueous phase liquid

NRC

National Research Council

NSZD

Natural source zone depletion

PAHs

Polycyclic aromatic hydrocarbons

PCBs

Polychlorinated biphenyls

PHCs

Petroleum hydrocarbons

R/RM

Remediation/risk management

ROA

Remedial options analysis

SCT

Site Closure Tool

SSTL

Site-specific target level

TBA

Tert-butyl alcohol

TCE

Trichloroethylene

TOR

Time of remediation (t_remediation)

TRAV

Tool for Risk Assessment Validation

US EPA

United States Environmental Protection Agency

USGS

United States Geological Survey

VC

Vinyl chloride (or chloroethene)

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. The FCSAP Decision‐Making Framework (DMF) is a 10-step roadmap that outlines the specific activities, requirements and key decisions to effectively address federal contaminated sites in Canada. The DMF along with other FCSAP-related resources can be found on the FCSAP website.

This guidance document provides recommendations on how to apply monitored natural attenuation (MNA) as an in situ passive remediation approach at locations where soils and groundwater are contaminated with petroleum hydrocarbons, chlorinated organic compounds, metals, or radionuclides. The objective is to define different concepts involved in MNA, assess feasibility of MNA to be implemented at federal contaminated sites and define and present the main MNA implementation phases. In the end, the successful implementation of MNA will lead to site closure, which is the ultimate objective of FCSAP program. In a remediation or risk management strategy (R/RM) for contaminated sites, MNA, which may be used in combination with risk management/remediation measures, can be a sustainable solution to assist contaminated site managers with meeting targeted remedial objectives for a specific site within a reasonable time frame.

The implementation of MNA includes four phases: (1) MNA feasibility assessment, (2) effectiveness demonstration, (3) performance monitoring and (4) confirmatory sampling and site closure. This guidance document is intended to provide contaminated site managers with recommendations for each of these stages, which are part of Steps 7 (Develop R/RM Strategy), 8 (Implement R/RM Strategy) and 9 (Confirmatory Sampling and Final Reporting) of the FCSAP decision-making framework, illustrated by the red boxes on Figure 1.

Figure 1: The FCSAP Decision-Making Framework 10-step process

1.2 Overview of monitored natural attenuation in the context of the FCSAP Decision-Making Framework

The objective of the DMF Step 7 (Develop R/RM Strategy) is to establish R/RM goals and develop a site management strategy to mitigate potential exposure to contaminants. Information gathered from previous DMF steps (Steps 3 through 5) is evaluated against the proposed R/RM objectives to develop a R/RM strategy for the site. Step 7 enables checking the presence of critical elements required for the successful implementation of MNA as the chosen R/RM strategy for the site. The selection of MNA must be supported by lines of evidence to confirm its feasibility. The NA processes must be effective for the majority of contaminants present at the site and not only the predominant ones, as NA processes affecting one contaminant may have adverse effects on others.

The objective of the DMF Step 8 (Implement R/RM Strategy) is to implement the R/RM strategy developed in Step 7. In implementing the R/RM strategy, certain aspects of the process may have to be reassessed, depending on the site management strategy established in Step 7. The performance monitoring program, as described in Section 6 of this document, covers the necessary monitoring requirements to verify whether NA is taking place consistent with predictions made during MNA feasibility assessment and effectiveness demonstration stages (see sections 4 and 5). It should include decision rules or triggers for action, detailed data analysis methods and data quality objectives that have been clearly established in advance. This step includes the preparation of detailed tender documents, the development of health and safety plans and the selection of a contractor, if necessary.

The objective of the DMF Step 9 (Confirmatory Sampling and Final Reporting) is to verify that remedial objectives are met following the implementation of the R/RM strategy and performance monitoring (Step 8 of the DMF). In addition, the site history and final conditions should be documented in a report for future reference (site closure report). Integration with other contaminated site management requirements, such as long-term monitoring (Step 10 of the SMF), may be required when MNA is based on non-destructive processes (e.g., sorption, dispersion, dilution, and volatilization. Planning for these requirements should be considered at Step 7, i.e., during the development of the R/RM site strategy.

The environmental impact of a proposed remediation strategy, whether it consists of MNA or any other method, should be factored into the overall sustainability of the contaminated site management plan. Therefore, it is important to have a good understanding of the MNA approach and the conditions under which this approach could be considered for a site, as well as the mechanisms that may affect its success.

1.3 Objectives of the guidance document

The general objective of this FCSAP guidance document is to provide a summary of MNA mechanisms and the relevant implementation measures to be considered when selecting MNA as a remedial strategy for a contaminated site. The specific objectives are to:

• identify the different processes involved in MNA;
• identify the requirements to be met for MNA to be a feasible remedial option;
• document the potential for success, depending on the contaminant nature and the impacted site characteristics;
• define and present the main steps involved in the assessment and implementation of MNA;
• make recommendations on the development of a performance monitoring program for MNA as well as monitoring data analysis; and
• provide guidance on confirmatory sampling and site closure where MNA is implemented.

This guidance document is primarily concerned with sites where soil and groundwater are contaminated with petroleum hydrocarbons (PHCs), chlorinated organic compounds, metals, or radionuclides. Other contaminants may also be considered for MNA; however, enough lines of evidence must be available to support the use of MNA as an appropriate remedial method. For some contaminant classes, such as per- and polyfluoroalkyl substances (PFAS), the available literature is too limited to properly support models, and therefore implementation of MNA would require extensive experts involvement. The use of MNA to remediate sediments is not covered in this guidance, as this topic is discussed in the FCSAP Guide to Monitored Natural Recovery (MNR) in Aquatic Sediments for Federal Contaminated Sites (FCSAP, in development).

This document does not provide detailed technical guidance for the evaluation of natural attenuation (NA) processes, nor does it provide details related to the unique challenges associated with northern sites, particularly the time and costs involved. Several reference sources are available and should be used when identifying whether natural attenuation processes are occurring at a specific site. These sources can be found in Appendix A.

2 Natural attenuation (NA) and monitored natural attenuation (MNA)

2.1 What is natural attenuation?

Natural attenuation is a natural process, or a combination of natural processes, that results in the reduction in mass, toxicity, mobility, volume and/or concentration of contaminants and their breakdown products in the environment (e.g., in soil or groundwater). The United States Environmental Protection Agency (US EPA) also states that NA processes “include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention […]. These in situ processes include biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants.” (US EPA, 1999).

2.2 Natural attenuation processes

The processes involved in natural attenuation are varied and can be divided into two main categories, namely destructive and 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.

Natural source zone depletion (NSZD), which refers to natural collective processes of dissolution, volatilization, and biodegradation leading to contaminant mass losses over time, is a good example where NA processes are at stake.

A full understanding of these NA processes is needed in order to predict the fate of the various contaminants and to assess the potential exposure pathways for receptors. NA processes and their impact on contaminant fate and transport are described in greater detail in Table 1.

Table 1: Summary of main NA processes affecting contaminant fate and transport

Process	Description	Important factors	Effects
Advection	Movement of dissolved contaminant (solute) by bulk groundwater movement.	Dependent on aquifer properties, mainly hydraulic conductivity (K), effective porosity (Φ), and hydraulic gradient (i). Independent of contaminant properties.	Main mechanism driving contaminant movement in the subsurface.
Dispersion	Fluid mixing due to groundwater movement and aquifer heterogeneities.	Dependent on aquifer properties and scale of observation. Independent of contaminant properties.	Causes longitudinal, transverse and vertical spreading of the plume. Reduces contaminant concentration.
Diffusion	Spreading and dilution of contaminant due to molecular diffusion.	Dependent on contaminant properties and concentration gradients. Described by Fick's Laws.	Diffusion of contaminant from areas of relatively high concentration to areas of relatively low concentration. Generally unimportant relative to dispersion at most slow groundwater flow velocities.
Sorption	Reaction between aquifer matrix and solute, whereby contaminants become sorbed to organic carbon or clay minerals.	Dependent on aquifer matrix properties (organic carbon [foc] and clay mineral content, bulk density, specific surface area, and porosity) and contaminant properties (solubility, hydrophobicity, octanol-water partitioning coefficient [Kow]).	Tends to reduce apparent solute transport velocity and removes solutes from the groundwater via sorption to the aquifer matrix.
Recharge (simple dilution)	Reduction of concentration of a contaminant due to a feed of solvent, mainly water. Dilution is from recharge of the groundwater body.	Dependent on aquifer matrix properties, depth to groundwater, surface water interactions, and climate. NOTE: It is important to consider the potential for contaminants to reach a surface body of water from groundwater recharge as well as provisions of the Fisheries Act related to pollution prevention.	Causes dilution of the contaminant plume and may replenish electron acceptor concentrations, especially dissolved oxygen.
Volatilization	Transfer of contaminants dissolved in groundwater, and adsorbed to soils in unsaturated zones, from liquid phase to gaseous phase (i.e., soil gas).	Dependent on chemicals' vapour pressure and Henry's Law constant.	Removes contaminants from groundwater and transfers them to soil gas.
Biodegradation	Microbially mediated oxidation-reduction reactions that degrade contaminants.	Dependent on groundwater geochemistry, microbial populations and contaminant properties. Biodegradation can occur under aerobic and/or anaerobic conditions.	May ultimately result in complete degradation of contaminants. Typically, biodegradation is the most important destructive process to reduce contaminant mass.
Abiotic degradation	Chemical transformations that degrade contaminants without microbial facilitation, such as hydrolysis.	Dependent on contaminant properties and groundwater geochemistry.	Can result in partial or complete degradation of contaminants. Rates typically much slower than for biodegradation.
Partitioning from a non-aqueous phase liquid (NAPL)	Partitioning from NAPL into groundwater. NAPL plumes, whether mobile or residual, tend to act as a continuing source of groundwater contamination.	Dependent on aquifer matrix and contaminant properties, as well as groundwater mass flux through or past NAPL plume.	Dissolution of contaminants from NAPL represents the primary source of contamination in groundwater by dissolved substances.
Precipitation/ coprecipitation	Chemical forces partition dissolved contaminant into solid phase thereby reducing mobility and concentration in dissolved phase (liquid), but not in soils (solid phase).	Dependent on groundwater geochemistry and the thermodynamic relationships of contaminant with other constituents of the plume.	Contaminant is removed from plume (i.e., dissolved phase) and is bound as or within solid mineral. With coprecipitation remobilization of contaminant is dependent on dissolution of host mineral.
Radioactive decay	Applicable only to radionuclides. Decay of contaminant into less harmful daughter product(s).	Ten half-lives required for the loss of 99% of any given radionuclide. Radionuclides with intermediate half-lives pose the greatest threat due to their persistence and their radioactivity, which is sufficient to cause damage to living tissue.	Contaminant transformed into daughter product gradually over half-life. Daughter product either harmless or less harmful than original mother product.
Source: Adapted from UK Environment Agency (2000) and ITRC (2010).

2.3 What is monitored natural attenuation?

Monitored Natural Attenuation (MNA) is a remedial approach based on the demonstration that the naturally occurring processes at a contaminated site will either reduce contaminant levels within a reasonable time frame or prevent an exceedance of a remedial objective formerly established. MNA alone is not considered an appropriate remedial approach if the contaminants present an unacceptable current or future risk to human or ecological receptors. In that case, a mixed R/RM approach might be used and could involve combining MNA with enhanced MNA (EMNA), other risk management or source removal options, where possible. If modelling indicates the unacceptable risk as a contingency in the future, MNA could be considered feasible as long as appropriate triggers for action are well-established (see Section 6.3), such that corrective measures can be taken well before the contaminant(s) reaches the potential receptors. Under the FCSAP program, a 20-year time frame is considered as acceptable for MNA implementation and site closure. Should the data and supporting models indicate that the time of remediation may exceed the maximum 20-year MNA target, consideration should be given to taking a more proactive approach to reducing the contaminant mass, where appropriate, in order to meet the target remedial results in a shorter time frame.

MNA should not be seen as a “do nothing” or “walk away” approach. MNA does not imply that the usual activities (and costs) associated with investigating the site and developing the R/RM strategy (e.g., site characterization, risk assessment, comparison of remedial alternatives, performance monitoring, alternative solutions, confirmatory sampling and/or long-term monitoring ) are not necessary. These elements of the investigation and R/RM must be addressed as required under the FCSAP program, regardless of the remedial approach selected.

Text Box: Note: Unlike MNA, enhanced monitored natural attenuation (EMNA) involves human intervention. This approach seeks to accelerate the natural degradation of pollutants by supporting the activity of endogenous micro-organisms (e.g., through fertilization and aeration [WSRC, 2006]) or through other techniques that stimulate, reinforce or accelerate natural processes that break down contaminants in soil, sediment and groundwater (US EPA, 1999). This is not to be confused with bioaugmentation, a technique that involves adding micro-organisms to the contaminated media.

2.4 Advantages, disadvantages and important considerations for monitored natural attenuation

As with any other R/RM option, MNA is an appropriate method only where all the relevant requirements are met (see Section 4) and where it will be protective of human health, safety, and welfare, and the natural environment. It must also be capable of meeting the site remedial objectives and stakeholder priorities within a time frame that is reasonable compared to that of other remedial approaches.

In the majority of cases where MNA is proposed as a remedial option, it forms one component of the overall remedial action plan, in conjunction with active remediation and/or risk management approaches. It is necessary to develop a comprehensive conceptual site model (CSM) when using MNA as the main remedial method at contaminated sites.

If MNA is considered as a remedial option at Step 7 of the decision-making framework (FCSAP, in development), detailed site-specific information that demonstrates the potential effectiveness of this approach should be obtained. This information may have been collected in Steps 3 and 5, but additional site information may also be collected in Step 7.

Due to the uncertainty associated with the potential effectiveness of MNA to meet remedial objectives, source control (Section 4.2) and performance monitoring (Section 6) are fundamental components of the MNA approach. Progress monitoring of MNA should be carefully implemented using established sampling strategies and statistically reliable data for decision-making.

The advantages and disadvantages of MNA are presented in Table 2 below.

Table 2: Advantages and disadvantages of MNA

Advantages	Disadvantages
Minimizes disturbance of landscape and of receptors in the area	Longer time frames are usually necessary to attain R/RM objectives
Potentially lower implementation costs	Site characterization and data collection may be more demanding and more complex
Reduced risk of human or environmental exposure because contaminated media are left undisturbed (e.g., compared to excavation)	Additional education and awareness may be needed to gain public acceptance
Utilizes inherent natural processes and therefore may reduce energy use and produce less emissions. It is a more sustainable approach to remediation	Cannot be used for high concentrations of contaminants and may not reduce the mass of contaminant (due to non-destructive processes and possible reversibility of the process)
Limited impact on other operations that may be required for active remediation activities	Changes in hydrological and geochemical conditions may allow for resolubilization and remobilization of contaminants that were not removed by NA processes
Sources: US EPA (1999); ASTM International (2004); UK Environment Agency (2000); Reis et al. (2008); Sinke and Le Hecho (1999); Mulligan and Yong (2004).

3 Process for implementing monitored natural attenuation

Under the FCSAP program, MNA can be assessed alongside other R/RM options while site delineation is under way and evidence of contamination requiring further remedial consideration is confirmed (i.e., at Step 5 of the DMF). The transition between Step 5 and Step 7 may entail additional site work including human health and ecological risk assessments.

The MNA implementation first stage, i.e., the feasibility assessment (as described in Section 4), consists in investigating the opportunity of relying on MNA at a given site using a preliminary assessment of technical and practical constraints. This stage objective is to quickly retain or dismiss the possibility to consider MNA as a remedial option.

At the effectiveness demonstration stage, as described in Section 5, field and laboratory data are used, and in some cases modelling, to closely investigate NA processes. This stage includes the development of a comprehensive conceptual site model (CSM) including reduction processes of contaminant concentration and mass for the impacted matrix. In some cases, modelling can be used to quantitatively assess contaminant fate and transport as well as rates of NA. The intent is to document and support the use of MNA as a remedial strategy based on reliable scientific data. Modelling should produce scientifically defensible outputs to support the use of NA processes as a remedial method.

The performance monitoring stage, as described in Section 6, covers the environmental monitoring requirements to confirm whether NA is occurring at a rate consistent with predictions made during the effectiveness demonstration stage. 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. Furthermore, it includes a contingency plan that can be implemented in case MNA does not meet the expected objectives.

Finally, the confirmatory sampling and site closure stage involves confirming the attainment of R/RM objectives after implementing the R/RM strategy including MNA. A 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.

Collaborating with FCSAP expert support departments at each stage is recommended to verify all considerations were taken into account to determine whether MNA is an appropriate strategy for a given site. Appendix B summarizes considerations regarding the use of MNA as a R/RM strategy at federal contaminated sites.

4 Feasibility assessment of monitored natural attenuation

At feasibility assessment stage, it is assumed that environmental site assessment activities have been carried out in accordance with the DMF (i.e., Steps 2, 3 and 5; see Figure 1). The feasibility assessment consists of a review of available site data to determine whether the use of MNA, either alone or in conjunction with other remediation technologies, is a viable remedial option. More specifically, this stage aims to evaluate, in a general manner, whether the main requirements for the feasibility of MNA are likely to be met.

Table 3 contains a list of factors and criteria that should be taken into consideration when assessing the feasibility of MNA, and provides a qualitative rating of the feasibility of MNA. The flow chart presented in Figure 2 depicts another approach for the evaluation of MNA feasibility which requires similar information to that shown in Table 3. Both approaches should be used together.

Table 3: Criteria for the assessment of MNA feasibility

Contaminant and media properties

Assessment criteria	MNA feasibility: high	MNA feasibility: medium	MNA feasibility: low
Source of contamination	Removed, controlled, or ceased	Being removed or under control	Continuing or unknown
Chemical composition of contamination	Organic, non-persistent	Mixture of persistent and non-persistent organic contaminants	Inorganic, persistent

Contaminant and media properties (continued)

Assessment criteria	MNA feasibility: high to medium	MNA feasibility: medium to low
Dominant attenuation processes	Destructive/irreversible process	Non-destructive/reversible process
Extent and severity of the contamination (mainly refers to groundwater contamination)	Well defined	Poorly defined

Contaminant and media properties (continued)

Assessment criteria	MNA feasibility: high	MNA feasibility: medium	MNA feasibility: low
Surface area of the contaminant plume	Shrinking	Stabilized	Growing

Contaminant and media properties (continued)

Assessment criteria	MNA feasibility: high to medium	MNA feasibility: medium to low
Contaminant phase (e.g., presence of phase-separate hydrocarbons)	Contaminant immiscible phase is thin (a sheen only)	Contaminant immiscible phase is thick/measurable
Geochemistry of media	Well understood	Poorly understood
Potential for migration	Low potential for migration based on stabilization or sequestration mechanisms (including geochemical considerations such as pH, retardation, or partitioning coefficient of contaminants)	Potential for mobilization based primarily on geochemistry factors such as pH, retardation, organic carbon, etc.

Contaminant and media properties (continued)

Assessment criteria	MNA feasibility: high	MNA feasibility: medium	MNA feasibility: low
Rate of groundwater flow	Slow (<10 m/year)	Medium (10–100 m/year)	Rapid (> 100 m/year)

Contaminant and media properties (continued)

Assessment criteria	MNA feasibility: high to medium	MNA feasibility: medium to low
Direction of groundwater flow and location of discharges	Well known	Poorly known
Aquifer heterogeneity	Homogenous and isotropic	Heterogeneous and anisotropic

Potential receptors

Assessment criteria	MNA feasibility: high	MNA feasibility: medium	MNA feasibility: low
Surface water bodies1 (distance2 could be used as a preliminary assessment)	Are at more than 7 years travel time 10,000 m	Are between 2 to 7 years travel time 2,000 m	Are within 2 years travel time 500 m
Groundwater usage near the site	None, or groundwater wells mainly used for industrial supply of a non-potable nature/quality	Groundwater wells mainly intended for domestic non-potable uses (gardens, pool, etc.). Soil contamination monitoring should be carried out to ensure that contaminants in the water do not transfer to the soil matrix	Groundwater wells mainly used for drinking supply present in the capture zone
Species at Risk Public Registry (SARA Registry)	No species at risk present	Species at risk present, but do not come into contact with contaminated media	Species at risk present in area, with high chance of contact with contaminated media. In this case, a risk assessment should be done

Potential receptors (continued)

Assessment criteria	MNA feasibility: high to medium	MNA feasibility: medium to low
Protected or sensitive habitats	Site lies outside of a protected area	Site lies within a protected area. In this case, an environmental impact assessment should be carried out

Site context and custodian objectives

Assessment criteria	MNA feasibility: high to medium	MNA feasibility: medium to low
Level of confidence in data	High (>2 years of available seasonal data)	Low (only one study, one year or no seasonal data)

Site context and custodian objectives (continued)

Assessment criteria	MNA feasibility: high	MNA feasibility: medium	MNA feasibility: low
Objectives of custodian	Long-term interest in the site	Medium-term interest in the site	Short-term interest in the site
Financial and institutional provisions for monitoring and implementation of a contingency plan	Long-term, irrevocable budget provisions	Long-term, revocable budget provisions	No long-term budget provisions
Access to off-site monitoring locations (i.e., upstream and sentinel monitoring wells)	Long-term access secured	Long-term access possible	Limited or no access possible

Source: Adapted from Table 3.2 in Western Australia Department of Environment Protection [WA DEP] (2004).

1. The source document presents distances between the site and surface water bodies. Since hydrogeological conditions vary considerably between regions, the authors believe that travel time is a better representation of the risk in relation to the distance of water bodies from contamination sources.
2. To use distances as a criterion for preliminary assessment, some conditions must be taken into consideration. These conditions are presented in the Guidance Document on Federal Interim Groundwater Quality Guidelines for Federal Contaminated Sites (FCSAP, 2016).

Figure 2: Flowchart for assessing MNA feasibility

Several general requirements should be met for MNA to be specifically considered as a R/RM strategy, including:

• Destructive NA processes: Destructive NA processes should be the principal attenuation mechanism in place; however, NA processes are complex and depend on many factors (including contaminant properties, hydrogeological conditions, geochemical conditions, etc.) that should be considered before implementing MNA as a R/RM method.
• The available site assessment data should be reviewed from the beginning to determine whether destructive NA processes are currently occurring at the site, as evidenced by reductions in contaminant mass and/or concentration over time, or by decreasing or stabilizing contaminant plume. It is also important to determine whether the contaminants present on site are biodegradable or immobilized, or will decay (radionuclides), and to keep in mind that destructive NA processes are largely ineffective for inorganic contaminants. These considerations are discussed further in Section 4.1.
• Source control: The contaminant source must be exhausted (to cease releasing contaminants) and any NAPL bodies or dissolved phase plumes (if present or relevant to the CSM) must be stable or decreasing. If free product has not been removed or contained, removal or mitigation through active remediation techniques must be implemented, either before MNA is initiated or in conjunction with MNA (Suthersan, 2001). Control of the contaminant source is discussed in Section 4.2.
• Achievement of remediation objectives within a reasonable time frame: MNA should be capable of meeting the remediation objectives within a reasonable time frame. Under the FCSAP program, a reasonable timeframe for MNA is defined as 20 years. The same time frame has been suggested by other jurisdictions, including the BC Ministry of Environment (BC MOE, 2014). In some cases (i.e., radionuclides), this time frame could be difficult to meet, and consultation with stakeholders, including the FCSAP Secretariat and expert support departments, is recommended. Assessment of MNA feasibility requires a preliminary estimate of the time of remediation (TOR), which is discussed in more detail in Section 4.3.
• Protection of potential receptors: The use of MNA must not pose an unacceptable risk, or a risk that cannot be managed, to existing or future receptors. All potential exposure pathways should be assessed, and future changes in land or water use, as well as the potential impact of climate change on site conditions, should be taken into account. Adequate controls must be in place for both soil and groundwater contamination, and it is crucial to have a good understanding of and to carefully monitor potential contaminant migration.

Note: Remote sites such as those in the Arctic represent unique challenges when it comes to designing a MNA program. These sites require thorough planning of site characterization works in support of the development of the MNA program and its implementation. The costs and timeframes are often much greater for these sites than for sites in urban areas.

4.1 Contaminants best suited for monitored natural attenuation

By their nature, some contaminants are more amenable to the use of the monitored natural attenuation. According to the National Research Council (2000), MNA is mostly applied to organic contaminants and non-recalcitrant biodegradable substances. The use of MNA has typically been more successful for organic compounds, especially PHCs and chlorinated organic compounds. However, the Interstate Technology & Regulatory Council (ITRC) has published guidance indicating that certain radionuclides and inorganic compounds, such as metals, may also be susceptible to non-destructive NA processes, under specific geochemical conditions (ITRC, 2010).

Table 4 presents the likelihood of success of MNA for different classes of organic and inorganic compounds, as well as the specific NA process involved. The list of contaminants in Table 4 is not meant to be all-inclusive; other contaminants can be considered depending on their properties. This table does not apply to emerging contaminants for which there is insufficient knowledge of fate and transport processes. Professional judgment should be exercised when considering other contaminants.

Table 4: Likelihood of success of MNA for various compounds

Contaminant type	Dominant NA processes	Likelihood of success
Hydrocarbons
Benzene, toluene, ethylbenzene, and xylenes (BTEX)	Biotransformation1	High
Gasoline, fuel oil	Biotransformation	Moderate
Non-volatile aliphatic compounds	Biotransformation, immobilization	Low
Polycyclic aromatic hydrocarbons (PAHs)	Biotransformation, immobilization	Low
Creosote	Biotransformation, immobilization	Low
Oxygenated hydrocarbons
Alcohols, ketones and esters of low molecular weight	Biotransformation	High
Methyl tert-butyl ether (MTBE)	Biotransformation	Moderate to high
Chlorinated aliphatic
Perchloroethylene (PCE), trichloroethylene (TCE), carbon tetrachloride	Biotransformation	Moderate
Trichloroethane (TCA)	Biotransformation, abiotic transformation	Moderate to high
Methylene chloride	Biotransformation	High
Vinyl chloride (VC)	Biotransformation	Moderate to high
Dichloroethylene (DCE)	Biotransformation	Moderate
Chlorinated aromatics
Highly polychlorinated biphenyls (PCBs), pentachlorophenol (PCP), multichlorinated benzenes	Biotransformation, immobilization	Low
Less chlorinated PCBs, dioxins	Biotransformation	Low
Monochlorobenzene	Biotransformation	High
Nitroaromatics
TNT and RDX (explosives)	Biotransformation, abiotic transformation, immobilization	Low
Metals and radionuclides
Metals: Cr, Ni, Cu, As, Se, Cd, Pb	Abiotic processes influenced by microbial and geochemical conditions, immobilization, biotransformation	Moderate to low
Radionuclides: Sr, Tc, I, Cs, Rn, Ra, Th, U, Np, Pi, Am	Radioactive decay, immobilization	Moderate to low

4.1.1 Hydrocarbons

The contamination at a given site may often include a mixture of chemicals which may have different susceptibilities to NA processes. It is therefore important to identify all of the contaminants that are present at a site in order to determine the potential effectiveness of MNA. For example, under specific field conditions, benzene, toluene, ethylbenzene, and xylene (BTEX) may naturally degrade through microbial activity and ultimately produce non-toxic end products (e.g., carbon dioxide and water) (US EPA, 1999). Where microbial activity is occurring at a sufficient rate, the dissolved BTEX contaminant plume may stabilize (i.e., stop expanding), and contaminant concentrations may eventually decrease to levels that do not pose an unacceptable risk to human or ecological receptors.

Following the degradation of the lighter fractions of PHCs (such as dissolved BTEX), a residual fraction consisting of heavier PHCs of relatively low solubility and volatility may remain in the original source (spill) area (US EPA, 1999). Although this residual contamination may have relatively low potential for further migration, it still may pose a threat to human health, safety, welfare, or the environment, either from direct contact with soil in the source area or from contaminants that continue to leach to groundwater. In this case, MNA alone may not be sufficient to reach remedial objectives and source control measures may need to be implemented in conjunction with MNA.

4.1.2 Oxygenated hydrocarbons

Other petroleum products, including oxygenated additives such as methyl tert-butyl ether (MTBE) which are present in petroleum fuels, are more resistant to biological or other degradation processes and should be given special consideration when assessing the feasibility of MNA (McLaughlan et al., 2006). A plume in which the BTEX contamination has stabilized may still contain MTBE that has not yet reached a stable state, making remediation through NA a longer, more complex, and potentially less feasible option.

When the use of MTBE as a fuel oxygenate was phased out in the USA and Canada, ethanol became the preferred replacement. Several research studies have demonstrated the viability of MNA for managing some sites contaminated with MTBE and other fuel oxygenates such as tert-butyl alcohol (TBA), a key daughter product of MTBE degradation. Most fuel oxygenates, including MTBE, TBA and ethanol, are highly soluble and able to migrate rapidly in groundwater following dissolution from light non-aqueous phase liquid (LNAPL) sources. Several contaminant plume studies have shown that the majority of MTBE plumes are relatively short, are decreasing or both (US EPA, 2005; ITRC, 2005).

Both MTBE and TBA readily degrade under aerobic conditions. Anaerobic biodegradation of MTBE and TBA has also been demonstrated, which is an important factor to consider since at least a portion of most MTBE plumes is anaerobic (ITRC, 2005). A readily available pool of electron acceptors (e.g., sulfate, iron and manganese) appears to be one prerequisite for TBA degradation under anaerobic conditions, whereas in the case of MTBE, the initial degradation process can occur under strong reducing (methanogenic) conditions and thus does not require external electron acceptors (US EPA, 2005, 2007).

4.1.3 Chlorinated solvents

Implementing MNA as a remediation approach for chlorinated solvents may pose challenges since the individual processes of chlorinated aliphatic hydrocarbon biodegradation are fundamentally different from the processes involved in the biodegradation of fuel hydrocarbons. For example, biodegradation of fuel hydrocarbons, especially benzene, toluene, ethylbenzene, and xylenes (BTEX), is mainly limited by electron acceptor availability. Since there appears to be an adequate supply of electron acceptors in most, if not all, hydrogeologic environments, biodegradation will generally proceed until all of the contaminants biochemically accessible to the microbes are destroyed (Wiedemeier et al., 1998).

On the other hand, the more highly chlorinated solvents, such as perchloroethene (PCE) and trichloroethene (TCE), typically are biodegraded under natural conditions via reductive dechlorination, a process that requires both electron acceptors (the chlorinated aliphatic hydrocarbons) and an adequate supply of electron donors. Electron donors include fuel hydrocarbons or other types of anthropogenic carbon (e.g., landfill leachate) or natural organic carbon. If the subsurface environment is depleted of electron donors before the chlorinated aliphatic hydrocarbons are removed, biological reductive dechlorination will cease, and natural attenuation may no longer be protective of human health and the environment. For this reason, it is more difficult to predict the long-term behavior of chlorinated aliphatic hydrocarbon plumes than fuel hydrocarbon plumes (Wiedemeier et al., 1998).

The potential for generation of toxic metabolites (daughter products) during NA should also be evaluated to determine if the implementation of MNA is appropriate and will be protective in the long term. Toxic metabolites are more likely to occur in association with non-petroleum organic contaminants (e.g., chlorinated solvents or other volatile organic contaminants) than with other classes of contaminants.

4.1.4 Metals and radionuclides

Attenuation of metals and radionuclides involves more interdependent sets of processes than is the case for attenuation of organic compounds. With metals, since the attenuation processes do not destroy the contaminants and since the complex attenuation processes that are involved may be reversible for many metal contaminants, the affected sites may need continuous remediation and management for many years or even decades (ITRC, 2010).

Unlike organic compounds, inorganic contaminants such as metals cannot be transformed into harmless products through destructive NA processes. Attenuation mechanisms for such compounds involve rendering the contaminant immobile, which is bound either on or within mineral solids. The primary mechanisms of attenuation for metals in groundwater include sorption-desorption, precipitation-coprecipitation-dissolution and dilution-dispersion, and in the case of radionuclides, radioactive decay (ITRC, 2010).

Given that non-destructive NA processes ultimately leave the contaminant in place (or in the case of radioactive decay, convert it to a less harmful daughter product), and because spatial and temporal changes in site geochemistry may result in mobilization of previously stabilized metals and radionuclides contaminants, MNA should only be used for sites with low potential for contaminant migration (ITRC, 2010). The potential for migration will depend on a range of factors such as the presence of source material, recharge conditions, hydraulic conductivity, and other physical and biological NA mechanisms. In all instances, direct contact with soils in the source area or continued slow leaching of contaminants to groundwater may preclude the implementation of a fully passive R/RM strategy.

4.2 Control of contaminant sources

Contaminant sources generally persist for a long time, and those that remain unaddressed may continue to release contaminants into soils and groundwater or migrate through the environment. When MNA is used, the presence of ongoing or persistent sources may increase the time that is required to achieve remedial goals (typically 20-year deadline for FCSAP projects).

LNAPL, DNAPL and their associated dissolved phase plumes, must be demonstrated as stable before MNA can be implemented (FCSAP, in development). When evaluating plume stability, particular attention should be given to addressing the potential for vapour intrusion as a significant exposure pathway for both LNAPL and DNAPL plumes (FCSAP, in development). Measures to achieve plume stability include removal, treatment, containment, or a combination of these approaches (US EPA, 1999). It is important to note that residual hydrocarbon contamination in soils and groundwater is often a good source of carbon for endogenous microorganisms, and that bacteria that degrade this carbon will create geochemical conditions favorable to these other bacteria that have the ability to degrade chlorinated solvents (Suthersan, 2001).

There are several methods of reducing or containing a source of contamination (MDNR, 2007), including:

• extraction of volatile contaminants from the soil (i.e., soil vapour extraction);
• bioventing;
• air sparging;
• hydraulic containment;
• enhanced biodegradation;
• soil solidification/stabilization;
• in situ chemical destruction;
• permeable reactive barrier;
• recovery of free phase (i.e., non-aqueous phase recovery);
• extraction of groundwater in the source zone; and
• excavation of the soil.

4.3 Time of remediation

MNA generally entails lengthy remediation times. The estimated rate of contaminant biodegradation/attenuation is therefore a key determinant of the adequacy of MNA as a R/RM strategy for a contaminated site where destructive processes are at play. There are several methods for developing estimates of contaminant biodegradation and attenuation rates. These methods require different degrees of site characterization and produce estimates of varying accuracy depending on the specific site conditions. It is recommended that several techniques be used together and the results compared in order to develop a better overall estimate.

The information theoretically required to calculate the time of remediation (TOR) includes the initial contaminant mass (M_o), the contaminant mass threshold (M_threshold; targeted contaminant mass), and the rate of ongoing NA processes (R_NA) that are acting on the contaminant. However, obtaining reliable estimates for these parameters can be technically challenging, which ultimately affects the reliability of the TOR estimate. The R_NA values, and the rate of biodegradation in particular, may vary as a result of many different factors, making it very difficult to obtain reliable estimates of this parameter without an in-depth analysis. Tools and a range of factors to consider for the MNA of hydrocarbons are provided in the ITRC’s document Evaluating LNAPL Remedial Technologies for Achieving Project Goals (ITRC, 2009). This guidance is useful in establishing key factors that will influence site-specific LNAPL mass loss rates (empirical determination of time of remediation) through processes that consider LNAPL mass reduction via naturally occurring volatilization (in the unsaturated zone), aqueous dissolution (in the saturated zone), and biodegradation (in both zones).

Estimating the TOR for groundwater presents several challenges that can be divided into three distinct yet related components:

1. Distance of stabilization is the maximum distance a contaminant plume will migrate from a source zone of a given contaminant concentration and is heavily dependent on the NA capacities of groundwater systems and the soil sorption capacity (Chapelle et al., 2003).
2. Time of stabilization is the time it takes for a plume to shrink to a smaller configuration if a contaminant source is wholly or partially removed. It is affected to a fairly significant degree by the sorption capacities of the aquifer, which depend on the organic content of the soil present in the aquifer, as well as by the sorption properties of the different contaminants. As a rule, a high sorption capacity delays the growth or shrinking of the plume and increases the time of stabilization. The time it takes for a plume to achieve a stable state is independent of NA capacities (Chapelle et al., 2003).
3. Time of NAPL dissolution is the time it will take for the NAPL to dissolve and disperse to the point where existing groundwater quality standards are met. It should be noted that where there is a high potential of discharge for contaminated groundwater in aquatic or marine environment, care must be taken to ensure the protection of aquatic and marine receptors, in keeping with the Fisheries Act and other applicable laws. Source zone control measures may be required in these cases. NAPL dissolution depends largely on the mass, composition, and geometry of the plume, as well as on hydrogeological factors such as groundwater flow. For more information on LNAPL, readers should refer to FCSAP’s Guidance Document on the Management of Light Non-Aqueous Phase Liquids (LNAPLs) at Federal Contaminated Sites (FCSAP, in development). For dense non-aqueous phase liquids (DNAPL), subject matter experts should be consulted.

The box below provides a definition of the TOR and describes in theoretical terms how to estimate the TOR for contaminated groundwater.

Adapted from Methodology for Estimating Times of Remediation Associated with Monitored Natural Attenuation (Chapelle et al., 2003).

Overall, the time required for NA processes to reduce contaminant concentrations to levels that can ensure the protection of human health and the environment varies widely according to hydrogeological systems and types and concentrations of contaminants. It should be noted that variability in remediation time depends on the geological, biological, and hydrogeological attributes of groundwater systems which are themselves not random. These parameters can be measured and interpreted in a logically consistent way, thereby making it possible to estimate remediation times associated with NA processes (Chapelle et al., 2003).

For radionuclides, radiological decay follows first-order kinetics, which means that the rate of the decay is proportional to the number of nuclei present. This process gives rise to a characteristic half-life for each radionuclide. A half-life is the amount of time required for half of the atoms of a particular radionuclide to decay. Ten half-lives are required for the decay of 99.9% of any given radionuclide (ITRC, 2010). Radionuclides having very short half-lives typically decay too rapidly to affect the environment and do not have an adverse effect on most receptors at risk in groundwater (e.g., iodine-131 with a half-life of 8 days). Radionuclides with very long half-lives may be environmentally persistent but may be of sufficiently low activity that little environmental damage ultimately occurs, depending on the extent of the contamination (e.g., uranium-235 with a half-life of 7.04 x 10⁸ years). Radionuclides with intermediate half-lives, such as strontium-90 (28 years), present the greatest threat to human and ecological receptors, because they persist long enough to enter living systems and have sufficient radioactivity to damage living tissues (ITRC, 2010).

The uncertainties created by the factors outlined above will always pose a challenge when it comes to calculating the TOR. However, some of these challenges may be overcome through computer modelling, which is more amenable to the multi-factorial analyzes that are required in order to assess the TOR quantitatively (UK Environment Agency, 2000).

5 Monitored natural attenuation effectiveness demonstration

In order to fulfill the requirements for the acceptance of MNA as a R/RM strategy at Step 7 of the FCSAP Decision-Making Framework (FCSAP in development), several criteria must be clearly met using an evidence-based approach:

• Contaminant destruction is the principal NA process (for organic compounds);
• Effective immobilization of contaminants can be achieved if non-destructive processes are relied upon for NA;
• The TOR is acceptable (20 years); and
• The project can be staffed and supported to undertake MNA (i.e., at all stages that follow the demonstration stage, such as performance monitoring).

Furthermore, the demonstration stage serves to more precisely identify potential exposure pathways and potential risks to human or ecological receptors as well as the fate and transport of the contaminants present, and to identify the NA processes at stake.

Demonstrating the efficacy of MNA will typically be an iterative process that includes full and detailed characterization of the site, development of a CSM, and establishment of lines of evidence to determine whether MNA will be sufficient to achieve the remediation objectives. Demonstrating the effectiveness of MNA usually requires a higher level of site characterization than that needed to support more traditional remedial measures. For example, in the case of contaminants that do not readily respond to destructive NA processes, prediction of future site geochemistry may be needed to confirm that the contaminants will remain bound to solid phases (ITRC, 2010). Similarly, sites with contaminants that do not tend to biodegrade readily will usually require more site characterization data than those with more biodegradable contaminants. The extent of investigation necessary to adequately characterize a site is highly site-specific and will depend on the nature of the contamination, hydrogeological complexity, proximity of receptors, and other factors, such as geochemical conditions (MDNR, 2007).

5.1 Site characterization and modelling

The decisions to employ NA as a R/RM strategy should be thoroughly and adequately supported by site characterization data and analyzes. Site characterization should include the collection of data in three spatial dimensions over time such that an adequate understanding can be gained of the nature and distribution of the soil contamination, the contaminant source zones and the groundwater plume, as well as the potential behaviour of contaminants in the affected media and potential exposure pathways. The following quantitative parameters, adapted from the US EPA (1999), are required for this step, and they should have been determined in Steps 3 and 5 of the DMF process (Figure 1; i.e., in Phase II and III ESAs), and possibly enhanced in the course of development of the R/RM strategy at Step 7:

• Location, nature, and mass of the contaminant sources;
• Location, extent, and concentration of dissolved contaminants;
• Chemical properties of contaminant sources and plume:
  • Solubility;
  • Volatility;
  • Rates of biological and non-biological transformation; and
  • Presence of degradation or transformation products (e.g., TCE, DCE, VC);
• Contaminant phase distribution and partitioning between soil, groundwater, and soil gas;
• Climatic characteristics;
• Geochemical parameters of the soil and aquifer:
  • Saturation, porosity, permeability, density, soil temperature, pH, dissolved oxygen, organic matter, organic nitrogen, and redox potential;
  • Presence of appropriate nutrients; and
  • Microbiological analysis to determine the presence and viability of an appropriate microbial population;
• Hydrogeological parameters:
  • Hydraulic conductivity;
  • Hydraulic gradient;
  • Groundwater flow direction and velocities;
  • Lithological characteristics;
  • Soil granulometry;
  • Migration pathways for non-aqueous phase liquids;
  • Site/aquifer heterogeneity;
  • Preferential pathways;
  • Location and type of surface water; and
  • Recharge and discharge zones.

The temporal (both over time and between seasons) 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. An understanding of these processes is crucial before MNA can be appropriately applied at a site (US EPA, 1999). Ultimately, site-specific geochemical conditions determine the dominant pathways of metal and radionuclide fate and transport. The principal pathways for natural attenuation of metals and radionuclides in groundwater are determined largely by the particular contaminants of potential concern (COPC) present at the site. The MNA assessment for each contaminant may require consideration of contaminant- and/or site-specific data (ITRC, 2010). Therefore, building a CSM, or refining one that has been developed previously (at Step 5), will be a key component of any supplemental site characterization.

5.1.1 Conceptual site model

A conceptual site model (CSM) consists of a written or illustrated representation of the site-specific geological, hydrogeological, hydrological, biological, geochemical and climatic characteristics (listed in the above section) that influence the transport, migration and potential impacts of a contaminated site on human and/or ecological receptors. The CSM depicts what is known or suspected about the source, release mechanisms, migration and fate of the contaminants, and is used to gain an understanding of the contamination, potential for migration, and exposure pathways, as well as to identify the risks. The CSM serves to develop a hypothesis regarding the processes that are promoting NA, and to address how MNA processes are likely to perform in relation to the remediation objectives. Finally, the CSM serves as a foundation for the quantitative analyzes that will be carried out using computer modelling. The CSM should include a description of the source and the contamination (nature, location, distribution, concentration, extent and form/speciation), the characteristics of the contaminated media, and the receptors (location, type, and distance from site). In some cases, the CSM may indicate that further site characterization data are needed before MNA can be implemented.

Additional guidance on the development of a CSM can be found in the Guidance Manual for Environmental Site Characterization in Support of Environmental and Human Health Risk Assessment, Volume 1, Guidance Manual (CCME, 2016).

5.1.2 Mathematical modelling

Computer modelling or simulation can be used to understand current site conditions and predict future conditions on a site-specific basis. The main objectives are to measure the extent, concentration and migration of contamination over time; to evaluate the risk of exposure and impact to receptors; to assess the time required to achieve remedial objectives; and to predict the best locations for plume monitoring points.

Modelling is an important tool for evaluating MNA of metals and radionuclides, which may require CSM development, mass-balance and geochemical speciation calculations, and predictive fate and transport modelling. Modelling of some sort is pertinent to each of the four tiers presented in the US EPA (2007) approach to demonstrating MNA (ITRC, 2010), discussed further below.

The selection of the appropriate model for predicting the effectiveness of MNA depends on the modelling objective, the complexity of the site, the type or behaviour of the contaminant, the NA processes, the quality and quantity of site-specific data available, and the limitations of the model. Simple analytical computer models may be useful for developing preliminary estimates of contaminant migration and attenuation over time. Examples of these models include BIOCHLOR (US EPA, 2000) and BIOSCREEN (US EPA, 1996). Additional models referenced by the British Columbia Ministry of Environment Technical Guide on Contaminated Sites (BC MOE, 2014) are listed in Appendix A. These models can be fairly limited in their application due to the simplifications used to minimize the amount of input data required, which makes the models comparatively easy to use. It may also be necessary to employ more complex numerical models in order to better account for site complexity and improve estimates of attenuation rates.

The hypotheses and parameters used in modelling must be documented, described and justified based upon site-specific field testing, bio-laboratory verification, and/or sound technical assumptions. The documentation must include all sources for data taken from the literature. Where the time required to achieve contaminant criteria levels is estimated, supporting calculations must be submitted confirming that NA is a cost-effective and suitable remediation approach.

The models must be validated (using site-specific parameter values) and a sensitivity analysis should be carried out to determine which parameters have the greatest influence on model predictions. Modelling should be interpreted with care, and any uncertainties or limitations of the model should be clearly understood and described.

Computer modelling is not always useful. For example, it may be unsuitable or fail to add value in cases where NA can be verified through direct observation, NA processes cannot be adequately represented by a model, too many simplifying assumptions are required, or insufficient data are available to provide an adequate definition of the system or validate the model results. However, modelling is typically required to evaluate sites where complex biodegradation, sorption, and/or transport processes are identified.

The four-tiered modelling approach US EPA (2007) is summarized in Table 5 (adapted from Table 2-4 of ITRC, 2010). This iterative approach is designed to progressively reduce uncertainty as site-specific data are collected, while minimizing the overall costs where possible. Typically, the more complex the modelling approach, the more site-specific data are required, increasing costs but also mitigating site-specific uncertainty.

Table 5: Integration of modelling, characterization, conceptual model development and the tiered approach to assess MNA as a R/RM strategy

Elements of conceptual model	Characterization	Calculations/modelling
Tier I analyzes
Hydrogeological environment affected or potentially affected hydrogeological units definition of flow regime potentiometric surface defined	Definition of hydrogeological units from subsurface cores or geophysical logs Hydraulic head measurements in wells Stream base-flow measurements	Hydraulic gradient calculations/flow net generation Simple flow or transport modelling
Spatial distribution of contaminants	Groundwater analysis of contaminants Surface water analysis of contaminants	Aqueous speciation calculations
Chemistry of plume	Groundwater field measurements: pH, redox potential, alkalinity, dissolved oxygen Groundwater laboratory measurements: major cations and anions, total organic carbon
Distribution of contaminants between aqueous and solid phases	Bulk analysis of contaminant concentrations in aquifer solids	Mass-balance calculations (if natural background concentrations are known) Probabilistic models
Tier II analyzes
Definition of contaminant/aquifer solid interactions identify aquifer mineralogy identify dominant attenuation mechanisms measure attenuation mechanism rates define geochemical heterogeneity	X-ray diffraction, thermogravimetric analysis, etc. to analyze mineralogy Sequential extractions, X-ray spectroscopy, scanning electron microscopy / energy dispersive X-ray spectroscopy to identify specific attenuation mechanisms Laboratory studies, in situ tests to measure attenuation rates Use of geological knowledge coupled with chemical and mineralogical analyzes of aquifer solids to define geochemical heterogeneity	Mass-balance calculations Reaction path modelling
Detailed microbiology (if appropriate) appropriate when sufficient carbon source is available to support biological mechanisms appropriate when variation in nutrient chemical species (NO3, SO4 -2, O2) cannot be explained by flow	-	Reaction path modelling
Detailed hydrogeology measure key parameters define hydrogeological heterogeneity	Laboratory and/or field measurements of porosity and hydraulic conductivity Variation in key parameters, geophysics, and use of geological knowledge to define hydrogeological heterogeneity	Analytical models of contaminant transport Numerical flow models
Tier III analyzes
Measurement of attenuation capacity quantitative mineralogy flux of nutrients (if microbial processes are active) determine flux of contaminant from vadose zone	More spatially dense quantitative mineralogy or use of heterogeneity information and less dense quantitative mineralogy Lysimeter studies of nutrient concentrations in recharge	Numerical reactive transport models
Determine flux of contaminant from vadose zone	Lysimeter studies of contaminant concentrations in vadose zone Use of contaminant/tracer ratios and flow modelling	Reaction path modelling
Stability of contaminant stabilized zones stability at conditions that reflect the geochemical evolution of the waste site	Laboratory studies of contaminant stability In situ push-pull tests of contaminant stability in stabilized zone	Reaction path modelling Numerical reactive transport models
Tier IV analyzes
Determine performance monitoring program	-	Monitoring optimization models Numerical reactive transport models
Identify alternative remedy	-	Reaction path models Numerical reactive transport models

5.2 Lines of evidence

An evidence-based approach relies on the assumption that more than one line of convergent, independent evidence is necessary. Once the site characterization data have been collected and a CSM and mathematical model (if needed) have been developed, the efficacy of NA as a remedial approach can be evaluated. Three types of site-specific information or “evidence” should be used in such an evaluation, some of which have been compiled in earlier steps of the MNA process (i.e., when examining historical trends in contaminant concentration). The evidence is organized in primary, secondary, and tertiary lines (Table 6). The primary and secondary lines are essential, and the third is optional; the essential lines of evidence are required in order to use MNA as a remediation approach. Other optional lines of evidence are required if the essential lines of evidence are insufficient to provide a convincing demonstration of the effectiveness of MNA. These optional lines of evidence may also improve the interpretation of essential evidence. It should be kept in mind that cost and complexity increase progressively with the number of lines of evidence obtained.

In the case of metals, identifying precipitation or co-precipitation of contaminants can be difficult. The contaminants are typically present at such low concentrations that there is often only a small chance of actually observing the minerals in which they occur. Groundwater analyzes and saturation indices are valuable for indicating whether precipitation of co-precipitation of a contaminant is likely to be occurring, but the results are not definitive. Therefore, multiple lines of evidence are usually required as mentioned above for other contaminants. These include spatial and temporal trends in contaminant concentration in groundwater, saturation indices, and contaminant leaching experiments on aquifer solids. The leaching experiments can be designed to target the dissolution of specific minerals, or patterns in the concentration of contaminant in the leachate may indicate that certain minerals are present (ITRC, 2010).

A detailed list of data and analyzes that may be required for lines of evidence is presented in Appendix C.

Table 6: Group of lines of evidence

Group of lines of evidence	Description
Primary: Documented past reduction/stabilization in contaminant mass and concentration over time	Involves reviewing historical groundwater and soil chemistry data, in conjunction with site geology and hydrogeology, to demonstrate a clear and meaningful trend of decreasing and/or stabilizing contaminant mass and concentration over time at appropriate monitoring or sampling points. This loss of mass may occur in the source area and/or along the groundwater flow path. In the case of a groundwater plume, decreasing concentrations should not be solely the result of plume migration. The decay rate kinetics must be evaluated. In some cases, it is more important to estimate a biodegradation rate from field data. For example, chlorinated solvents do not biodegrade on their own as PHCs do, and thus their biodegradation rates are more site-specific (e.g., dependent on oxidation-reduction conditions and electron donor concentration), which requires more specific analyzes.
Secondary: Presence and distribution of hydrogeological, geochemical and biochemical indicators of NA	Analysis of hydrogeological and geochemical data that can be used to demonstrate the type(s) of NA processes active at the site, and the rate at which such processes will reduce contaminant concentrations to the required remediation levels. Should assess presence and quantify rates of aerobic and anaerobic degradation processes occurring at the site, including the presence of daughter and radioactive decay products. Microbial enumeration and analysis of the nutrients that are present can assist in calculating the theoretical biodegradation capacity.
Tertiary: Direct microbiological evidence of NA for destructive NA processes	Direct microbiological evidence is obtained through laboratory microcosm studies (conducted in or with actual contaminated site media) and is used to: Confirm specific biodegradation processes that cannot be conclusively demonstrated with field data alone (e.g., anaerobic vinyl chloride oxidation); and/or Estimate site-specific biodegradation rates that cannot be conclusively demonstrated with field/historical data alone. The need for the third line of evidence is evaluated on a case-by-case basis; this evidence is generally only required when field data supporting the first two lines of evidence are insufficient to adequately support NA.

6 Performance monitoring of monitored natural attenuation

Performance monitoring of MNA (implemented at Step 8 of the DMF) is important in the overall implementation of site R/RM, given the potentially longer remediation time frame, the possibility of contaminant migration, and the other uncertainties inherent in this approach. Therefore, when MNA is selected as a R/RM strategy, the design of a performance monitoring plan, including the development of a contingency plan, are key elements of the remedial approach.

Understanding the biogeochemical evolution of the system through monitoring is important in performance evaluation. The long-term system performance depends on the total contaminant mass that may need to be attenuated, and on the extent that its release from source areas can be controlled (ITRC, 2010). A common challenge with metal- and radionuclide-contaminated sites is that the contaminants are not destroyed, but are instead immobilized or detoxified, and these processes must be sustainable for long time frames, even for centuries to millennia (ITRC, 2010). It should be noted that contaminated sites where these conditions exist are outside the scope of the FCSAP program.

Monitoring should make it possible to:

• demonstrate that NA is occurring according to expectations;
• detect, in a timely manner, changes in site conditions that might reduce the efficacy of NA processes;
• identify any degradation and transformation products;
• verify the absence of unacceptable impacts on receptors;
• detect new releases of contaminants to the environment that could generate a risk or impact the effectiveness of MNA;
• verify the attainment of remediation objectives;
• check that the plume is not expanding, either downgradient, laterally or vertically;
• demonstrate the ongoing effectiveness of institutional controls put in place to protect potential receptors;
• support a decision to terminate activities or implement the contingency plan, if necessary; and
• implement suitable R/RM techniques, if MNA proves to be ineffective.

6.1 Development of a performance monitoring plan

The monitoring plan design is based on site-specific features and the characteristics of the contamination, which were identified at the MNA effectiveness demonstration stage and represented in a comprehensive CSM. The main components of the monitoring program include the number, type and location of monitoring points, frequency of sampling, and parameters monitored.

The goals and targets of the performance monitoring plan must be clearly defined, and institutional and financial mechanisms for maintaining the monitoring program must be clearly established in the R/RM plan or other site related documents, as appropriate.

The monitoring program should include the establishment of a network of wells that will:

1. provide adequate area and vertical coverage to verify that the groundwater plume remains static or shrinks; and
2. provide the ability to monitor groundwater chemistry throughout the zones where contaminant attenuation is occurring.

It is recommended that the performance monitoring program include the assessment of the groundwater flow regime (piezometry, hydraulic gradients and velocities), so that adjustments can be made to the monitoring network according to real time observations. The influence of potential changes will therefore be evaluated in the hydrogeological patterns initially established, including the aquifer recharge and the predominant flow direction within the contaminant plume, and the necessary adjustments will be made. In addition to monitoring of groundwater parameters associated with NA processes, periodic monitoring of parameters that allow tracking of non-beneficial changes in groundwater conditions is also recommended. Monitoring of the NA effectiveness will include continued verification of contaminant removal from groundwater, but will also include tracking trends for other parameters that participate in the NA processes (possible examples include pH, alkalinity, ferrous iron, and sulfate). For sites where contaminant immobilization is the primary attenuation process, periodic collection of soil samples from the aquifer may be warranted to verify consistency in reaction mechanisms (US EPA, 2007).

There are several important factors to consider when developing a monitoring network to support an attenuation-based remediation plan. If attenuation-based remediation is demonstrated to be a viable remedial option, then:

• contaminant concentrations should be at values approaching the remedial goals;
• the contaminant flux, if evaluated, should be decreasing; and
• concentration and flux changes should occur more slowly than in the source area (primary and secondary).

These conditions influence both the spacing of the monitoring points within the network, as well as the frequency of sampling (ITRC, 2010).

6.1.1 Monitoring points – type, number, and location

When establishing the types, number and location of performance monitoring points, consideration should be given to a number of factors such as:

• the distance to, and locations of, potential receptor exposure points;
• groundwater flow direction and velocity;
• types and distribution of contaminants;
• aquifer heterogeneity;
• surface water impacts;
• the effects of engineered remediation systems;
• site complexity;
• level of understanding of plume behavior, complexity and configuration; and
• access issues, property lines, and contaminant contributions from off-site sources.

In the case of soils, collecting a series of samples that can be compared with earlier samples may be challenging, since by their very nature, soil samples cannot be obtained repeatedly at exactly the same location. Therefore, samples must be obtained as near the previous sampling location as possible to ensure the results will be comparable (US EPA, 2004). If possible, composite samples from each sampling station, or incremental samples (ITRC, 2012) may be collected from the site in order to reduce variability in the sampling results. These composite samples may be collected during the MNA effectiveness demonstration stage (see Section 5.1) as the baseline (i.e., time zero) concentrations. Monitoring of non-contaminated soils and/or groundwater below contaminated soils must be conducted to ensure that an observed reduction in the contaminant concentration is not the result of their migration.

In the case of groundwater, the number of sampling points should be sufficient to assess the horizontal and vertical distribution of the plume, and to predict its migration and the spatial distribution of contaminants over time. The stratigraphy at the location of the monitoring points should be taken into account. In many cases, existing monitoring wells installed for site characterization may be reused for the monitoring program, subject to careful evaluation of their locations. In most cases where MNA is applied, additional wells are required. The sampling strategy should include wells in the following locations:

• upgradient and on the lateral periphery of the plume, in order to monitor water quality;
• in the source zone, or the most contaminated zone, in order to assess changes in contaminants and their concentration;
• downgradient from the source near the centre line of the plume;
• immediately downgradient of the front of the contaminant plume; and
• between the plume and potential receptors and above and under the contaminated strata (“sentinel” wells).

Monitoring of the upgradient well(s) for biogeochemical conditions, such as pH and alkalinity, can provide an early warning of changes to the more distal zone(s) in which attenuation-based remedies are being applied (ITRC, 2010).

6.1.2 Sampling timeline and frequency

Performance monitoring should continue until the remediation objectives have been achieved (i.e., as long as the contamination remains above target levels), in accordance with the site-specific goals and the CEQGs.

Where soil contamination exists, the first year of monitoring should include seasonal (quarterly) sampling to assess the effectiveness of the existing NA processes, provided weather and/or site access conditions allow this. Depending on the results, sampling frequency may be reduced to yearly or biannually, with sampling scheduled during the period of the year when NA processes are most active or when the site is accessible.

For groundwater, quarterly monitoring is recommended during the first two years. This activity could be carried out quarterly or biannually, depending on the risks involved, the logistics (e.g., remote site), and groundwater dynamics. Monitoring could be extended to subsequent years if deemed appropriate based on the annual variability of the results. Alternatively, the frequency of this activity may be reduced to once a year, during a period when concentrations of contaminants are generally higher or when the site is accessible.

The monitoring plan should allow for some flexibility in the sampling frequency, so that it may be reduced when the situation is stable, or increased when conditions change, depending on the situation.

Once performance monitoring has demonstrated that the level of contamination is stable below the target level, confirmatory sampling (i.e., at Step 9 of the DMF) is performed.

6.1.3 Analytical parameters

The parameters chosen for analysis must be capable of revealing the behaviour of the contamination and whether any human or environmental receptors are potentially exposed to unacceptable risk. The parameters to be assessed depend on the site and, in particular, on the types of contaminants present. Groundwater monitoring wells should measure groundwater flow direction(s), horizontal and vertical gradients and velocities, and trends in contaminant concentrations within the plume and source areas, and should make it possible to determine whether the plume is migrating or presents a threat to receptors. Changes in site conditions (hydrogeological, geochemical, microbial, etc.) can be measured to assess the efficacy of NA processes, and indirect indicators of contaminant migration can be quantified at downgradient wells in order to assess whether contamination is migrating.

Sites with chlorinated solvent, metal, or radionuclide contamination will likely require a more diverse suite of analytical parameters than sites contaminated with fuel hydrocarbons, given their respective biodegradation capacities (Wiedemeier et al., 2000) and their potential for sorption-desorption, or precipitation-coprecipitation-dissolution. Table 7 presents examples of parameters to analyze when monitoring MNA performance, in the case of contamination by chlorinated solvents or petroleum hydrocarbons.

In addition, common groundwater and soils analyzes for the MNA performance monitoring of metals and radionuclides are presented in Table 8 and Table 9. These tables can also be used to establish analytical requirements at other steps of the DMF (e.g., Step 7 during the R/RM strategy development and Step 10 for long-term monitoring considerations).

Table 7: Examples of parameters to analyze for performance monitoring¹ of natural attenuation of petroleum hydrocarbons and chlorinated solvents present in groundwater or soil

Monitoring frequency	Number and location of samples	Monitoring parameters
Groundwater
Quarterly for the first three years, and at least annually thereafter, until exit criteria have been reached. It could be quarterly or biannual depending on the risk, the logistics and the groundwater dynamic.	At least three perpendicular transects through the plume and one perpendicular transect upgradient from the plume.2 A wall of sentinel wells should be placed downgradient. Screens must be placed in order to prevent vertical groundwater migration. All monitoring wells (if present) should also be monitored.	Fuel hydrocarbons Substances of interest: BTEX Fuel hydrocarbons MTBE Other additives Geochemical indicators: Dissolved oxygen Redox potential Dissolved inorganic carbon pH SO42- NO3- Fe2+ CH4 CO2 Mn2+ Piezometric level Depth and thickness of free phase
		Chlorinated solvents Substances of interest: Chlorine compounds Degradation by-products Geochemical indicators : Dissolved oxygen Redox potential Total organic carbon pH SO42- NO3- Fe2+ CH4 CO2 Piezometric level Depth and thickness of free phase Chloride ions
Soil
First year of monitoring should include seasonal sampling to assess existing NA processes. Depending on the results, the sampling frequency may be reduced to once or twice a year, at the time of the year when the NA processes are most active.	Statistically significant number of core samples covering the entire contamination zone.3	Fuel hydrocarbons Substances of interest: BTEX Fuel hydrocarbons If VOCs are present: analysis of O2, CO2 and CH4 (to measure microbial activity) In the case of biodegradation: C, N and K (after 5 years)

Table 8: Examples of common groundwater analyzes for MNA of metals and radionuclides (from ITRC, 2010)

Analytes or purpose	Method	Comments
Field parameters
pH, Eh, dissolved oxygen, specific conductance, temperature	Electrode	Many available tools measure all of these either in a well or in flow-through cells during sampling
Alkalinity	Colorimetric titration kit	-
Metals
Most metals, including many contaminants	Inductively coupled plasma emission spectrometry (ICP ES)	Commonly used for obtaining concentrations of major cations in groundwater for aqueous speciation analysis
Many metals with poor detection limits by ICP-ES (As, Se)	Atomic absorption	-
Oxidation state of some metals	Ion chromatography (IC)	-
Major anions
F, Cl, Br, NO3-, NO2-, SO42-	IC	-
Microbiology
Physiological types and numbers, overall biomass, specific organisms	Most probable number (MPN) analysis, cell counts, molecular tools, microcosms	Microbial analyzes of soil samples are preferred over analyzes of groundwater samples, as groundwater may not be representative of all subsurface microbial activity; recommended only if a microbially mediated process is inferred to be relevant to the attenuation mechanism based on geochemical analyzes

Table 9: Examples of common aquifer solids analyzes for MNA of metals and radionuclides (from ITRC, 2010)

Analytes or purpose	Method	Comments
Partitioning of contaminant to soil
Adsorption or precipitation of contaminant	Chemical extractions followed by appropriate analysis (e.g., inductively coupled plasma emission spectrometry (ICP-ES))	Sequential extractions subject the sample to numerous extractions each targeting a particular type of partitioning; gives information on attenuation mechanism
Adsorption or precipitation of contaminant	Scanning electron microscopy with energy dispersive spectrometry	Can identify that contaminant is associated with particular minerals or has precipitated; useful only if contaminant is present at fairly high concentrations
Mineralogy
Identification of minerals in aquifer solids	X-ray diffraction	Generally, minerals must be present at concentrations near 5 wt.% to be detected; various sample preparation techniques can be used to concentrate and identify specific clay minerals; quantification can be achieved but is subject to many uncertainties
Identification of minerals in aquifer solids	Thermogravimetric analysis	Can distinguish minerals, typically clays, that contain water from one another; some quantitative information
Bulk chemical composition
Elemental analysis of aquifer solid	X-ray fluorescence	Can be used in conjunction with X-ray diffraction and thermogravimetric analysis to quantify mineralogy
Cation exchange capacity
Measures general adsorption capacity of aquifer solid	Various techniques that involve analyzing the amount of a common cation that adsorbs to a sample at a particular pH	Useful as a general indicator of adsorption capacity, but provides little information regarding specific contaminants
Microbiology
Physiological types and numbers, overall biomass, specific organisms	MPN analyzes, cell counts, molecular tools, microcosms	Recommended only if a microbially mediated process is inferred to be relevant to the attenuation mechanism based on geochemical analyzes

6.2 Performance assessment

Performance of NA at the site is assessed using monitoring data. On the basis of that assessment, a decision is made as to whether monitoring of NA should continue, the contingency plan should be implemented, or no further action is required at the site (achievement of remediation objectives and cessation of remedial activities).

As part of the performance assessment, performance evaluation results should be compared to predictions made in the demonstration stage, especially if those predictions are based on mathematical computer models enabling a quantitative comparison. A performance assessment should be conducted every year, at which time the use of MNA should be evaluated. This assessment should be documented in the performance monitoring plan and detailed in the decision rules and triggers for action. It may also be set out in the section of the plan dealing with management decisions.

6.3 Contingency plan

Although MNA is a well-established and effective R/RM strategy for many contaminated sites, it can include uncertainties due to site complexities and may not necessarily work according to predictions and models. The custodian must be prepared to implement a contingency plan, if necessary.

The contingency plan identifies a replacement technology, an alternative approach, or additional measures that can be implemented should MNA prove ineffective or insufficient to achieve the set objectives (US EPA, 1999). This plan should be developed before the R/RM strategy is implemented. The contingency plan should be re-evaluated during performance monitoring to ensure that it remains appropriate.

A contingency plan should be flexible and allow new information about site risks and technologies to be incorporated. A contingency plan may include options to change the remediation approach, including more extensive source removal or containment measures, more stringent institutional controls, or other remedial actions.

It is recommended that one or more criteria be established, as appropriate, in the MNA monitoring plan to be used as MNA performance indicators and as triggers for implementation of the contingency measures. Such criteria might include the following:

• Contaminant concentrations in soil or groundwater at specified locations exhibit an increasing trend;
• Near-source wells exhibit large concentration increases indicative of a new or renewed release;
• Contaminants are identified in sentry/sentinel wells located outside of the original plume boundary, indicating renewed contaminant migration;
• Contaminant concentrations are not decreasing at a sufficiently rapid rate to meet the targeted remediation objectives within the prescribed time frames; and
• Changes in land and/or groundwater use that adversely affect the protectiveness of the NA remediation.

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, anomalous spike(s) in dissolved concentration(s) at a well(s), which may set off a trigger, might not be a true indication of a change in trend.

It may be useful to construct a flow chart in order to guide the process of evaluating the efficacy of MNA and determining whether the contingency plan should be implemented.

7 Confirmatory sampling and site closure

Once target contaminant concentrations in groundwater have been achieved (either generic guideline values or SSTLs based on the exit criteria established in the MNA plan), confirmatory sampling (i.e., Step 9 of the DMF) should continue for a period of at least three years. This is done to ensure that concentrations remain stable and below acceptable levels (based on a decision rule). For soil, confirmatory sampling should demonstrate that the achievement of target contaminant concentrations is not due to soil heterogeneity. The frequency of sampling during confirmatory sampling should be established based on the performance monitoring results.

Site closure, is intended to provide custodians with consistent evaluation criteria for determining when it is appropriate to close sites. The site closure tool (SCT) of the FCSAP (FCSAP, 2012) provides a template for determining which critical information about site R/RM decisions should be documented and summarized in a site closure report, including information supporting the Tool for Risk Assessment Validation (TRAV) embedded in the SCT. Custodians are required to fill in certain mandatory sections and submit the SCT to the FCSAP Secretariat for all FCSAP-funded remediation/risk management sites that will be closed. Alternatively, custodians can submit a site closure report that has been developed for use within their organization and that has been deemed equivalent by agreement between the custodian and the FCSAP Secretariat (FCSAP, 2012).

8 Long-term monitoring

After successful demonstration of MNA at the confirmatory sampling step (section 7), long-term monitoring (LTM; i.e., Step 10 of the DMF) is usually required if there is potential for contaminant remobilization or if the assumptions used for risk assessment in the CSM may no longer be valid due to changes in environmental conditions. The use of LTM is typically required if non-destructive processes are key components of the R/RM strategy. The FCSAP Long-Term Monitoring Planning Guidance (FCSAP, 2013) provides additional information on the development of a LTM plan. Following successful LTM implementation, where appropriate, a SCT should be used to document and record all necessary site data and decisions.

9 Conclusion

Monitored natural attenuation (MNA) makes use, in a controlled manner, of the natural capabilities of micro-organisms to degrade contaminants as well as the geochemical immobilization and NA processes in the environment. MNA can be an efficient, green, and sustainable approach for contaminated soils and groundwater remediation when source of contamination is controlled, sound natural attenuation processes are already in place and when potential risks for human and ecological receptors can be managed in an acceptable manner. MNA is often associated with other remedial approaches and its use is best suited within vulnerable habitats where the implementation of other remedial activities might cause additional unacceptable environmental damages. 

MNA is part of Steps 7 to 9 of FCSAP decision-making framework, after site characterization and development of a comprehensive conceptual site model. Although costs associated with MNA may be more or less comparable to remedial method costs, this option requires deploying significant efforts for the site characterization as well as for the planning and implementation of performance and confirmatory monitoring. Generally, many data sources are being used to support the application of MNA for site remediation. During the MNA programme development, a contingency plan needs to be developed and implemented if remedial objectives are not met and, if needed, a long-term monitoring will also be required. The use of MNA as a remedial approach has been demonstrated in several case studies and more and more data indicate that this method can be used as part of an efficient land remediation approach.

This guide is intended to support the use of MNA as a R/RM approach for hydrocarbons, chlorinated organic compounds, metals, and radionuclides in soil and groundwater at federal contaminated sites. It includes information to estimate MNA effectiveness to meet remediation or risk management objectives as well as a summary of measures for MNA performance monitoring. The information provided can be used to guide the application of MNA, with respect mainly to soil and groundwater contamination. Lastly, MNA should not only provide protection for human health and the environment but should also be capable of meeting the site remediation goals within a reasonable time frame of less than 20 years. Appendix B provides a summary of considerations for the application of MNA as a R/RM strategy at federal contaminated sites.

10 References

ASTM International. (2004). Standard Guide for Remediation of Ground Water by Natural Attenuation at Petroleum Release Sites, E 1943-98.

British Columbia Ministry of Environment [BC MOE]. (2014). Technical Guidance on Contaminated Sites - Using Monitored Natural Attenuation and Enhanced Attenuation for Groundwater Remediation (#22), Version 1.0, Draft 15, November 2014.

Canadian Council of Ministers of the Environment [CCME]. (2016). Guidance Manual for Environmental Site Characterization in Support of Environmental and Human Health Risk Assessment - Volume 1 - Guidance Manual.

Chapelle, F. H., Widdowson, M. A., Brauner, J. S., Mendez III, E., & Casey, C. C. (2003). Methodology for Estimating Times of Remediation Associated with Monitored Natural Attenuation. U.S. Geological Survey. Water-Resources Investigations Report 03–4057.

Federal Contaminated Sites Action Plan [FCSAP]. (2012). Guidance for Site Closure Tool for Federal Contaminated Sites. Public Works and Government Services Canada.

Federal Contaminated Sites Action Plan [FCSAP]. (2013). FCSAP Long-Term Monitoring Planning Guidance. Her Majesty the Queen in Right of Canada.

Federal Contaminated Sites Action Plan [FCSAP]. (2016). Guidance Document on Federal Interim Groundwater Quality Guidelines for Federal Contaminated Sites - June 2016 (Version 4). Including the Federal Interim Groundwater Quality Guidelines Memo. Her Majesty the Queen in Right of Canada.

Federal Contaminated Sites Action Plan [FCSAP]. (2020). Ecological Risk Assessment Guidance - Module 5: Defining Background Conditions and Using Background Concentrations. Version 1.0, March 2020. Ottawa: Her Majesty the Queen in Right of Canada. ISBN 978-0-660-33974-0

Federal Contaminated Sites Action Plan [FCSAP]. (In development). Decision-Making Framework. Version 4.0.

Federal Contaminated Sites Action Plan [FCSAP]. (In development). Guidance Document on the Management of Light Non-Aqueous Phase Liquids (LNAPLs) at Federal Contaminated Sites.

Federal Contaminated Sites Action Plan [FCSAP]. (In development). Guide to Monitored Natural Recovery (MNR) at Federal Aquatic Contaminated Sites.

Interstate Technology & Regulatory Council [ITRC]. (2005). Overview of Groundwater Remediation Technologies for MTBE and TBA. IRTC Team on MTBE- and Other Fuel Oxygenates, Washington.

Interstate Technology & Regulatory Council [ITRC]. (2009). Evaluating LNAPL Remedial Technologies for Achieving Project Goals.

Interstate Technology & Regulatory Council [ITRC]. (2010). A Decision Framework for Applying Monitored Natural Attenuation Processes to Metals and Radionuclides in Groundwater.

Interstate Technology & Regulatory Council [ITRC]. (2012). Technical and regulatory guidance: Incremental sampling methodology.

McLaughlan, R. G., Merrick, N. P., & Davis, G. B. (2006). Natural attenuation: A scoping review. CRC CARE Technical Report no. 3, CRC for Contamination Assessment and Remediation of the Environment, Adelaide, Australia.

Missouri Department of Natural Resources [MDNR]. (2007). Monitored Natural Attenuation of Groundwater Contamination at Brownfields/Voluntary Cleanup Program Sites.

Mulligan, C. N. & Yong, R. N. (2004). Natural Attenuation of Contaminated Soils. Environment International, vol. 30, no 4, p. 587 to 601.

National Research Council [NRC]. (2000). Natural Attenuation for Groundwater Remediation. Washington, DC, National Academy Press.

Reis, E., Lodolo, A., & Miertus, S. (2008). Survey of Soil Remediation Technology. ICS-UNIDO, Trieste, Italy, 158 p.

Sinke, A., & Le Hecho, I. (1999). Monitored Natural Attenuation: Review of Existing Guidelines and Protocols. TNO-NICOLE Report R99/313, TNO-MEP, Appeldorn, the Netherlands.

Suthersan, S. S. (2001). Natural and Enhanced Remediation Systems. Boca Raton, Lewis publishers, CRC Press Company LLC.

United Kingdom Environment Agency [UK Environment Agency]. (2000). Guidance on the Assessment and Monitoring of Natural Attenuation of Contaminants in Groundwater.

United States Environmental Protection Agency [US EPA]. (1996). BIOSCREEN Natural Attenuation Decision Support System, User’s Manual, Version 1.3. Office of Research and Development, Washington DC 20460, EPA/600/R-96/087, August 1996.

United States Environmental Protection Agency [US EPA]. (1999). Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. Office of Solid Waste and Emergency Response, Washington.

United States Environmental Protection Agency [US EPA]. (2000). BIOCHLOR Natural Attenuation Decision Support System, User’s Manual, Version 1.0. Office of Research and Development, Washington DC 20460, EPA/600/R-00/008, January 2000.

United States Environmental Protection Agency [US EPA]. (2004). How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers. Office of Solid Waste and Emergency Response, Washington.

United States Environmental Protection Agency [US EPA]. (2005). Monitored Natural Attenuation of MTBE as a Risk Management Option at Leaking Underground Storage Tank Sites. Office of Research and Development, National Risk Management Research Laboratory, Cincinnati.

United States Environmental Protection Agency [US EPA]. (2007). Monitored Natural Attenuation of Inorganic Contaminants in Ground Water, Volume 1 - Technical Basis for Assessment, EPA/600/R-07/139.

Western Australia Department of Environment Protection [WA DEP]. (2004). Contaminated Sites Management Series - Use of Monitored Natural Attenuation for Groundwater Remediation.

Westinghouse Savannah River Company [WSRC]. (2006). Enhanced Attenuation: A Reference Guide on Approaches to Increase the Natural Treatment Capacity of a System. Prepared for the U.S. Department of Energy.

Wiedemeier, T. H., Lucas, M. A., & Hass, P. E. (2000). Designing Monitoring Programs to Effectively Evaluate the Performance of Natural Attenuation. Prepared For Air Force Center for Environmental Excellence, Technology Transfer Division, Brooks Air Force Base, San Antonio, Texas, USA.

Wiedemeier, T. H., Swanson, M. A., Moutoux, D. E., Gordon, E. K., Wilson, J. T., Wilson, B. H., Kampbell, D. H., Haas, P. E., Miller, R. N., Hansen, J. E., & Chapelle, F. H. (1998). Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-98/128 (NTIS 99-130023).

11 Appendices

Appendix A: Protocols and technical guides for assessing MNA feasibility

Protocols and technical guides for assessing MNA feasibility

No.	Title	Media	Targeted contaminant	Originator
1	Guidance on the Selection of Natural Attenuation as a Cleanup Alternative for the Restoration of Soil and Ground Water at Contaminated Sites (2000)	Soil and groundwater	All contaminants	Alaska Department of Environmental Conservation, United States
2	Technical Protocol for Evaluating the Natural Attenuation of MTBE (2007)	General	MTBE	American Petroleum Institute (API), United States
3	Standard Guide for Remediation of Ground Water by Natural Attenuation at Petroleum Release Sites (2004)	Groundwater	Petroleum products	ASTM International, United States
4	Guidance on the Assessment and Monitoring of Natural Attenuation of Contaminants in Groundwater (2000)	Groundwater	General	UK Environment Agency, United Kingdom
5	Natural Attenuation of Perchlorate in Groundwater: Processes, Tools, and Monitoring Techniques (2008) and Frequently Asked Questions about Monitored Natural Attenuation in Groundwater (2014)	Groundwater	All contaminants	Environmental Security Technology Certification Program (Lieberman and Borden, 2008), United States
6	Petroleum Cleanup Program Remediation Action Guideline (1998)	General	Petroleum products	Florida Department of Environmental Protection, United States
7	Applicability of Monitored Natural Attenuation at Radioactively Contaminated Sites (2006)	General	Radioactive contamination	International Atomic Energy Agency (IAEA), International
8	Technical/Regulatory Guidelines-Natural Attenuation of Chlorinated Solvents in Groundwater: Principles and Practices (1999) and A Decision Flowchart for the Use of Monitored Natural Attenuation and Enhanced Attenuation at Sites with Chlorinated Organic Plumes (2007)	Groundwater	Chlorinated solvents	The Interstate Technology and Regulatory Council (ITRC), United States
9	A Decision Framework for Applying Monitored Natural Attenuation Processes to Metals and Radionuclides in Groundwater (2010)	Groundwater	Metals and radionuclides	The Interstate Technology and Regulatory Council (ITRC), United States
10	Assessment of Natural Biodegradation at Petroleum Release Sites (2005)	Groundwater	Petroleum products	Minnesota Pollution Control Agency, United States
11	Monitored Natural Attenuation of Groundwater Contamination at Brownfields/Voluntary Cleanup Program Sites (2016)	Groundwater	General	Missouri Department of Natural Resources (MDNR), United States
12	Technical Guidance Document no4 -Evaluation of Monitored Natural Attenuation at Petroleum Release Sites (2015)	Groundwater	Petroleum products	Department of Environmental Quality, Montana, United States
13	Monitored Natural Attenuation Technical Guidance (2012)	Groundwater	General (PHCs, chlorinated solvents, inorganics and radionuclides)	New Jersey Department of Environmental protection, United States
14	Guidelines for the Assessment and Management of Groundwater Contamination (2007)	Groundwater	General	New South Wales Department of Environment and Conservation (NSW DEC), Australia
15	Monitored Natural Attenuation Demonstrations under TRRP (2010)	Groundwater (but may also apply to soils)	General	Texas Commission on Environmental Quality (TCEQ), United States
16	Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater Volume II (1995)	General	Petroleum fuels	U.S. Air Force Center for Environmental Excellence, United States
17	Implementing Monitored Natural Attenuation and Expediting Closure at Fuel-release Sites (2004)	General	Petroleum fuels	U.S. Air Force Center for Environmental Excellence, United States
18	Draft Protocol for Evaluating, Selecting and Implementing Monitored Natural Attenuation at Explosives-Contaminated Sites (1999)	General	Explosives	U.S. Army Corp of Engineers, United States
19	Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites (1999)	Soil and groundwater	General	US EPA, OSWER, United States
20	How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers, Chapter IX, Monitored Natural Attenuation (1994)	Soil and groundwater	Focus on petroleum products, leakage from underground tank	US EPA, United States
21	Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater (1998)	Groundwater	Chlorinated solvents	US EPA, United States
22	Monitored Natural Attenuation of Inorganic Contaminants in Ground Water. Volume 1 Technical Basis for Assessment (2007)	Groundwater	Some inorganic contaminants	US EPA, United States
23	A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA) (US EPA, 2008a)	Groundwater	Inorganic contaminants	US EPA, United States
24	Site Characterization to Support Use of Monitored Natural Attenuation for Remediation of Inorganic Contaminants in Groundwater (US EPA, 2008b)	Groundwater	Inorganic contaminants	US EPA, United States
25	Guidance on Remediation of Petroleum- Contaminated Ground Water by Natural Attenuation (WSDE, 2005)	Groundwater	PHCs	Washington State Department of Ecology (WSDE), United States
26	Use of Monitored Natural Attenuation for Groundwater Remediation. Contaminated Sites Management Series (WA DEP, 2004)	Groundwater	Emphasis on petroleum products	Western Australia Department of Environmental Protection (WA DEP), Australia
27	Guidance on Natural Attenuation for Petroleum Releases (WDNR, 2003)	Soil and groundwater	Petroleum products	Wisconsin Department of Natural Resources (WDNR), United States
28	Naturally Occurring Biodegradation as a Remedial Action Option for Soil Contamination Interim Guidance (WDNR, 2004)	Unsaturated soil	Emphasis on petroleum products	Wisconsin Department of Natural Resources (WDNR), United States
29	Guidance on Remediation of Petroleum- Contaminated Ground Water by Natural Attenuation (WDNR, 2014)	Groundwater	Petroleum products	Wisconsin Department of Natural Resources (WDNR), United States

Appendix B: Summary of recommendations for the main elements of the MNA process

Summary of recommendations for the main elements of the MNA process

Elements	Recommendations
MNA feasibility evaluation (Step 7 of the DMF)
Feasibility	MNA can be used when: the source of contamination has been controlled. site testing shows that NA processes are taking place and that the existing contamination is susceptible to destructive NA or immobilization. the risks to receptors can be managed. If this is not possible, MNA should at least be used in combination with other techniques. a long remediation time frame is acceptable in the circumstances surrounding the contaminated site (maximum target of 20 years).
Remediation timeframe	The necessary remediation time should be assessed before MNA is selected as the remediation method. MNA should be capable of achieving remediation objectives within 20 years.
MNA effectiveness demonstration (Step 7 of the DMF)
Site characterization	Detailed site characterization is required in any MNA assessment or demonstration.
CSM	The development of a detailed, site-specific CSM is required for the demonstration of MNA, in order to ensure that the contamination, migration and exposure pathways and receptors are well understood, and to identify risks. When modelling is done, site-specific data should be used, particularly for key elements such as biodegradation rates and geochemical parameters.
Mathematical/computer modelling	Mathematical modelling should also be done in order to quantitatively assess the NA processes taking place at the site and to more accurately predict the TOR.
Lines of evidence	Primary evidence (condition of the plume) and secondary evidence (NA process, geochemical data) are required to support the use of MNA. Tertiary evidence (microbiological studies) should be required only when primary and secondary evidence is insufficient to demonstrate MNA of if microbial degradation is seen as the key NA process.
MNA performance monitoring (Step 8 of the DMF)
Monitoring plan	The monitoring plan is specific to the site and to the contaminants that are present. The number of sampling points should be sufficient to provide a clear understanding of what is happening, and should cover the upgradient, periphery, downgradient and core of the contamination. Quarterly sampling should be required, at least in the first year, in order to assess seasonal variations; frequency may be reduced depending on results.
Performance assessment	The use of MNA should be reassessed when the results that are obtained differ from expectations. In general, the decision to terminate yield monitoring should be based on well-established exit criteria demonstrating achievement of remediation objectives, as recommended by the US EPA, and not just on the basis of a prediction that the objectives will be achieved. However, monitoring frequency may be greatly reduced pending achievement of remediation objectives at some sites, where justified.
Contingency plan	A contingency plan should always be developed when MNA is used as a remediation method, given its inherent uncertainties. The plan should be re-evaluated periodically.
Confirmatory sampling and site closure (Step 9 of the DMF)
Two (2) years of confirmatory sampling are recommended following the same sampling procedure as the performance monitoring. Once performance monitoring has demonstrated that the level of contamination is stable or below the remedial target level, then the site is ready for closure as outlined in the DMF (Step 9). This includes final reporting, completion of the site closure tool (SCT) and the tool for risk assessment validation (TRAV) as appropriate and updating of the Federal Contaminated Sites Inventory (FCSI). Note: Long-term monitoring (LTM; Step 10) is generally required only if non-destructive processes are a key part of the MNA strategy. This would apply for metals and radionuclides.

Appendix C: Data and analyzes supporting lines of evidence

Data and analyzes supporting lines of evidence

Line of evidence	Description
Primary: Documented past reduction or stabilization in contaminant mass and concentration over time	Historical Review: Groundwater flow direction Concentration data from a minimum of three monitoring wells along the direction of groundwater flow Groundwater velocity for calculation of conductivity (K) in the absence of a reliable velocity estimate of groundwater velocity (V), the K/V factor may be useful relative to comparable sites with available data Areal and vertical extent of contaminant plume over time to establish if plume is expanding, stabilizing or shrinking Establish decay rate kinetics as a function of time (shrinking plume) or distance (stable plume or limited historical data) for all chemicals of concern, and Groundwater and soil chemistry data (e.g., oxidation-reduction conditions, electron donor concentrations, etc.)
Secondary: Presence and distribution of hydrogeological, geochemical and biochemical indicators of NA	Use of monitoring wells, sufficient number of upgradient and downgradient wells; selection criteria for representative wells include: Concentration distribution, areas of potential aeration such as drainage structures Backfill areas and other heterogeneities Monitoring well construction, etc. Adsorption coefficient (Kd) Concentrations of ferrous iron, sulphate, nitrate, methane Biological indicators for aerobic degradation: dissolved oxygen (DO) pH carbon dioxide (CO2) potential metabolites These parameters must be analyzed in the field utilizing an appropriate sampling procedure that minimizes aeration of the groundwater sample Indicators for anaerobic degradation (if DO concentrations are too low): Sulfate (SO42-) Nitrate (NO3-) Iron (Fe2+) Methane (CH4) Manganese (Mn), Redox potential (including Eh-pH relationships) pH Potential metabolites (changes in ratio of parent-daughter products) Anaerobic degradation has significantly slower kinetics, and thus must be fully evaluated when determining whether NA is a suitable remediation alternative at sites with low background DO levels Total heterotrophic and specific bacterial counts (microbial enumeration and nutrients) Calculate the theoretical biodegradation capacity Determination of precipitation and coprecipitation processes
Tertiary: Direct microbiological evidence of NA for destructive NA processes	Laboratory microcosm studies confirm microbiological degradation processes and calculate rates of biodegradation: Mass balance of parent, intermediate and final products including daughter and radioactive decay products Measure same parameters as for second line of evidence