Fact sheet: Monitored natural attenuation

From: Public Services and Procurement Canada

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Description

Monitored natural attenuation (MNA) is not a technology in itself, but rather an approach. It consists of a range of biological, chemical and physical processes that contribute to reducing the concentration, mobility and/or toxicity of a contaminant. This process uses the indigenous microbial population to degrade or transform dissolved or residual contaminants within the soil’s saturated zone. MNA consist of demonstrating, documenting and monitoring biodegradation without biostimulation (addition of oxygen or nutrients) or bioaugmentation (addition of microorganisms).

MNA requires the monitoring of key indicators influencing the capacity for bioremediation within contaminated aquifers (such as pH, temperature, dissolved oxygen, etc.) as well as a detailed monitoring plan to avoid any risks to human and/or environmental receptors. Studies have shown that natural biodegradation of contaminants depends upon the indigenous microbial population in the aquifer, the properties and concentrations of the contaminants, as well as the geochemical and hydrogeological characteristics of the contaminated aquifer. The mechanisms influencing the transport and the biodegradation of organic contaminants are divided into two groups: destructive and non-destructive.

Destructive mechanisms transform the contaminants, which reduces their total concentrations. These include all biological, physical and chemical (photolysis, hydrolysis, chemical reduction, etc.) degradation processes.

Non-destructive mechanisms reduce the concentration of contaminants without their transformation. Examples of non-destructive mechanisms are advection, hydrodynamic dispersion, sorption, dilution and volatilization.   

Certain non-destructive processes, such as dispersion, are not accepted by some authorities as natural attenuation mechanisms.

Before selecting MNA as a remediation approach for a given project, the characteristics of the contaminated site, such as geology, hydrogeology and potential receptors, as well as the contaminant plume characteristics, including the concentration of each contaminant, must be determined and understood. Therefore, it can be useful to develop a conceptual model of the site and of the contamination present. The model must be able to determine if MNA can sufficiently reduce contaminant concentrations such that potential receptors will not be exposed to any risks. Mathematical modelling software (such as RT3D, FEFLOW, BIOPLUME III, etc.) are often used to evaluate if natural attenuation is a viable option for site restoration. Incomplete data or a lack of information may result in improper application of MNA mechanisms. A monitoring system must be implemented using monitoring wells upstream, within, and downstream of the contaminant plume. The monitoring system must also include warning wells downstream and between the contaminant plume and any potential receptors.

There are four methods to determine if natural attenuation and biodegradation are occurring on a site. During pilot tests, at least two of these methods should be performed:

  1. mass balance;
  2. analysis of geochemical parameters;
  3. microcosm laboratory studies;
  4. identification of biodegradation by-products.

A mass balance study can approximate the extent of biodegradation compared to other physical attenuation processes. This is done using compounds which have the same physical characteristics as the contaminants, but are resistant to biodegradation, such as trimethylbenzenes for mono cyclic aromatic hydrocarbons and methyl phenanthrene for polycyclic aromatic hydrocarbons.

The second method, analysis of geochemical parameters, can be used to establish the importance of various biodegradation processes on-site. Here, determining the concentrations of dissolved oxygen (O2), ferrous iron (Fe2+), sulfate (SO42-), nitrate (NO3-), methane (CH4) and carbon dioxide (CO2) in groundwater is essential.

The third method consists of producing microcosms in a laboratory or in situ. These trials can determine if the indigenous microorganisms are capable of degrading the contaminants and at what rate this degradation occurs under the site-specific conditions.

The last method is used to determine if there are any biodegradation by-product present. For example, benzene cis-dihydrodiol and catechol are produced during aerobic degradation of benzene; 1-hydroxy-2-naphthoic acid is produced during the anaerobic digestion of phenanthrene; and, vinyl chloride and cis-1,2-dichloroethylene are produced in the biodegradation of trichloroethylene (TCE) and tetrachloroethylene/perchloroethylene (PCE).

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Implementation of the technology

MNA can be described by a range of naturally occurring biological, physical and chemical processes, which reduces the concentration, toxicity and/or mobility of contaminants. This process uses the indigenous microbial population to degrade or transform dissolved or residual contaminants within the soil’s saturated zone.

Monitoring this attenuation (recovery) to verify that it is occurring and that risks are controlled, is accomplished by the systematic assessment of contaminant fate and transport as well as the factors influencing the attenuation rate (recovery).

As part of the MNA process, institution controls must be instated. Such as site access, occupancy, transfer and/or use limitations to protect human and environmental receptors while natural recovery takes place.

MNA and/or institutional controls may include:

  • pre-treatment pilot studies;
  • contaminant fate and transport modelling;
  • site fencing and signage;
  • site capping (in other words, covering the ground surface with asphalt pavement, soil, synthetic liners, concrete pavement or other types of cover materials);
  • prohibiting certain site activities (such as excavation) or land uses (such as residential use);
  • off-site interventions such as assisted relocation, provision of alternative drinking water sources, etc.;
  • surface water, groundwater, soil, surficial soil, soil vapour, plant tissue and/or animal tissue analyses;
  • ecological/community characterization and monitoring.

Within the subsurface, natural processes that influence the concentration, load, flux, toxicity and/or the bioavailability of contaminants include:

  • biodegradation, biosequestration or biosorption;
  • sorption;
  • advection, dispersion and diffusion (transport and dilution);
  • abiotic (non-biological) chemical reactions including abiotic oxidation, hydrolysis or precipitation;
  • volatilization (evaporation).

 Source control (removal of the source of the contamination as far as it is practicable) and/or physical barriers (caps, walls, etc.) may be incorporated into some MNA remedies. 

Materials and storage

None since this method relies on natural biological, physical and chemical processes.

Waste and Discharges

No new waste or discharge are created using this method. However, any discharges from the site occurring prior to remediation may continue. 

Recommended analyses for detailed characterization

Biological analysis

  • Total heterotrophic and specific bacterial counts (according to the contaminants of interest)

Chemical analysis

  • pH
  • Alkalinity
  • Oxidation reduction potential (Eh)
  • Conductivity
  • Metals concentrations
  • Chloride concentrations (chlorinated solvents)
  • Metabolite concentrations
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free
  • Nutrient concentrations including:
    • ammonia nitrogen
    • total Kjeldahl nitrogen
    • nitrates
    • nitrites
    • total phosphorus
  • Electron acceptor concentrations/reaction by-products including:
    • dissolved oxygen
    • nitrate
    • sulfate
    • ferric and ferrous iron
    • methane
    • dissolved manganese

Physical analysis

  • Dissolved oxygen concentration
  • Dissolved methane concentration
  • Temperature
  • Contaminant adsorption coefficient
  • Soil granulometry
  • Presence of non-aqueous phase liquids (NAPLs)
  • Evaluation of biological conditions and ecological factors
  • Adsorption coefficient (Koc)

Recommended trials for detailed characterization

Biological trials

  • Microcosm mineralization trial
  • Biodegradation trial

Hydrogeological trials

  • Tracer tests

Notes:

Tests examining the effect of temperature change on hydraulic conductivity and establishing the zone of freezing with a pilot scale tubing system are recommended to properly design the full-scale containment system.

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)
  • Presence of receptors:
    • presence of potential environmental receptors
    • presence of above and below ground infrastructure
    • the risk of off-site migration

Phase III

  • Soil stratigraphy
  • Identification of preferential pathways for contaminant migration
  • Conceptual site model with hydrogeological and geochemical inputs
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient

Applications

The MNA remediation technology is most effective with the following:

  • organic compounds;
  • residual and dissolved contaminants in the saturated zone;
  • biodegradable contaminants.

Applications to sites in northern regions

  • MNA and/or institutional controls are frequently very applicable to remote northern sites provided that the underlying risk assessment accounts for northern lifestyles, cultures and unique ecological systems.  
  • Remote and northern sites are prone to high mobilization and on-site monitoring costs, limited equipment availability and short seasonal work windows. Telemetry can be used for remote monitoring of site conditions.
  • Difficulties in procuring timely analytical results may necessitate reliance on field screening, staged interventions and/or risk management.
  • In less populated areas, risk management might require less intensive monitoring and control measures that those generally required in more densely populated areas. 
  • Northern systems require climate-appropriate design, including consideration of deep freezing, permafrost, spring melt and frost heave.
  • The cold weather will have a negative impact on the contaminant degradation processes—it will take much longer than it would take in warmer climates. However, it may slow down contaminant migration and the potential of volatilization of contaminants.
  • This technology can be used in isolated regions without services or electricity.
  • Enforcement of institutional controls and periodic monitoring of natural attenuation processes may be challenging.

Treatment type

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

State of technology

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

Target contaminants

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

Notes:

Biodegradation of aliphatic chlorinated hydrocarbons (PCE, TCE, dichloroethene [DCE], trichloroethane [TCA], dichloroethane [DCA], etc.) with more than two chloride molecules occurs mainly under anaerobic conditions.

Treatment time

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

Notes:

Depending on-site-specific characteristics, the time required for treatment can be years or even decades.

Long-term considerations (following remediation work)

If natural attenuation eventually meets remediation goals, there would be few to no post-remediation, long-term considerations.

Secondary by-products and/or metabolites

Mono cyclic aromatic hydrocarbons and petroleum hydrocarbons biodegradation doesn’t typically generate by-products or metabolites that are more toxic than the original compound, as it results in innocuous by-products including carbon dioxide and water.

The biodegradation of certain aliphatic chlorinated hydrocarbons may generate more toxic metabolites (such as biological transformation of DCE forms vinyl chloride). Also, certain metals may be mobilized from the anaerobic fuel biodegradation process.

Limitations and Undesirable Effects of the Technology

  • Long treatment time.
  • Negative public perception. may be perceived as a “do nothing approach”.
  • Required site characterization is complex and expensive.
  • Long-term monitoring required.
  • Potential for contaminant migration and plume expansion.
  • If free phase is present, a complementary technology should be used to remove it.
  • Not suitable for heavy metal remediation. During the natural attenuation process, heavy metals are immobilized within the aquifer. The total mass of heavy metals is not reduced and there is a risk of mobilization of the heavy metals if the conditions within the saturated zone change.
  • High heavy metal or contaminant concentrations can be toxic for certain microorganisms.
  • Contaminants must be available to microorganisms.
  • Metabolite toxicity may be an issue, in specific cases.
  • Presence of potential environmental receptors.’
  • Natural attenuation frequently takes place in conjunction with advective transport and dispersion or diffusion (in other words, dilution).
  • Issues to consider include, interference with the use of resources such groundwater, stakeholder policy or philosophy, neighbouring property value and/or short-to-medium term socio-economic impacts. 

Complementary technologies that improve treatment effectiveness

Biostimulation (nutrient, oxygen and carbon additions).

Bioaugmentation (addition of microorganisms).

Enhanced vacuum extraction (bioaspiration) in the presence of free phase hydrocarbons.

Required secondary treatments

None

Application examples

The following documents provide application examples:

Performance

This site remediation approach is less expensive than active/intrusive remediation technologies, but is a long-term solution that limits the future exploitation of the site for certain activities.

Measures to improve sustainability or promote ecological remediation

  • Use of renewable energy and energy-efficient machinery (such as geothermal or solar energy for site characterization).
  • Use of non-invasive methods for site characterization (for example Ground Penetrating Radar [GPR] for stratigraphic units).
  • Minimizing site visits by the use of telemetry for remote monitoring of site conditions.
  • Review of historical data to reduce amount of sampling.
  • Use of field analytical kits and probe monitoring when possible. 
  • Optimization of monitoring programs to reduce site visits and travel.

Potential impacts of the application of the technology on human health

Unavailable for this fact sheet

References

Author and update

Composed by : Magalie Turgeon, National Research Council

Updated by : Karine Drouin, M.Sc., National Research Council

Updated Date : March 1, 2009

Latest update provided by : Daniel Charette, P.Eng., eng., Jan McNicoll, M.Sc., P. Geo., exp Services Inc.

Updated Date : March 31, 2017

Version:
1.2.1