Fact sheet: Monitored Natural Attenuation

From: Public Services and Procurement Canada

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Description

Monitored Natural Attenuation is an environmental management approach that uses a natural process, or combination of natural processes, that results in the reduction of the mass, toxicity, mobility, volume, or concentration of contaminants and their degradation products in the environment.

Monitored Natural Attenuation processes include a range of physical, chemical, or biological processes that, under favourable conditions, act without human intervention. These in situ processes include: biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay and stabilization; transformation or chemical or biological destruction of contaminants.

Sources:

Implementation of the technology

Implementation of this environmental management approach may include:

  • mobilization, site access, and the establishment of temporary facilities, if necessary;
  • the establishment of a monitoring network based on potential contaminant transport pathways. Thus, the monitoring network typically includes the installation of observation wells to document MNA processes in groundwater. The monitoring network may also include environmental monitoring of other matrices, such as soil vapour, surface water, and sediment.

Materials and Storage

Controlled natural attenuation is a management approach. It does not require materials or storage on site.

Residues and Discharges

Controlled natural attenuation produces no residues or discharges other than those generated by natural processes. Drill cuttings could be generated during the installation of the follow-up wells.

Recommended analyses for detailed characterization

Biological analysis

  • Enumeration of the total heterotrophic bacterial population and the specific bacterial population.

Chemical analysis

  • pH
  • Alkalinity
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free
  • Nutrient concentrations including:
    • ammonia nitrogen
    • nitrates
    • nitrites
    • total phosphorus
    • organic nitrogen
    • potassium
  • Total organic carbon
  • Redox potential
  • Dissolved oxygen
  • Groundwater geochemical parameters (pH, oxidation potential, dissolved oxygen, temperature)
  • Groundwater biodegradation indicators (dissolved oxygen, nitrate, sulfate, dissolved iron, methane)

Physical analysis

  • Temperature
  • Electrical conductivity
  • Presence of light or dense immiscible liquids
  • Assessment of biological conditions and ecological factors such as habitat sensitivity, presence of protected areas, presence of species at risk

Recommended trials for detailed characterization

Biological trials

  • 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.

  • Tests to estimate:
    • Mass flow and/or volumetric flow of contaminants
    • In situ attenuation rates
    • Rate of in situ biodegradation

    • Other information recommended for detailed characterization

    Phase II

    • Presence of potential environmental receptors
    • Presence of above and below ground infrastructure
    • Nature of the contamination and the delineation of its extent
    • Lithologies and stratigraphy of the site

    Phase III

    • Soil stratigraphy
    • Identification of preferential pathways for contaminant migration
    • Characterization of the hydrogeological system including:
      • the direction and speed of the groundwater flow
      • the hydraulic conductivity
      • the seasonal fluctuations
      • the hydraulic gradient
    • Geochemical and/or hydrogeological modeling
    • Approximate volumes of contaminated soil and water
    • Mass balance and mass flow of contaminants

    Applications

    Natural attenuation applies to all contaminants for which effectiveness has been demonstrated, primarily organic contaminants. MNA has a greater potential for success based on the following key characteristics:

    • The source of the contamination has been removed or is under control;
    • Organic contamination;
    • Extent of contamination well defined and delineated across the impacted matrices;
    • Relatively homogeneous and isotropic aquifer.

    Applications to sites in northern regions

    • This management approach is possible in the northern regions, however, remote sites require greater mobilization, resulting in higher on-site monitoring costs. In addition, equipment availability is limited and work windows are relatively short.
    • This management approach can be used in remote locations without services or electricity.
    • Cold climate generally has a negative impact on the biodegradation processes of contaminants—the half-life of contaminants will be longer compared to the same process in a temperate climate.

    Treatment type

    Treatment type
    Treatment typeApplies or Does not apply
    In situ
    Applies
    Ex situ
    Does not apply
    Biological
    Applies
    Chemical
    Applies
    Control
    Applies
    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
    With restrictions
    Monocyclic aromatic hydrocarbons
    Applies
    Non metalic inorganic compounds
    With restrictions
    Pesticides
    With restrictions
    Petroleum hydrocarbons
    Applies
    Phenolic compounds
    Applies
    Policyclic aromatic hydrocarbons
    With restrictions
    Polychlorinated biphenyls
    With restrictions

    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 meets remediation goals, there will be little or no long-term considerations.

    Secondary by-products and/or metabolites

    The biodegradation of monocyclic aromatic hydrocarbons as well as petroleum hydrocarbons does not generally generate by-products or metabolites more toxic than the original compound. It generates harmless products such as carbon dioxide and water.

    Biodegradation of some chlorinated aliphatic hydrocarbons can generate more toxic metabolites. For example, the biological transformation of dichloroethene forms vinyl chloride.

    Biodegradation processes can alter the geochemical conditions of groundwater and promote the mobilization of certain substances such as metals. For example, a change in redox potential in groundwater, caused by biodegradation processes, can increase the solubility of some metals.

    Limitations and Undesirable Effects of the Technology

    • This process can take a relatively long time.
    • Negative public perception caused by a false reputation of MNA as a “do nothing” approach.
    • For some types of contaminants, the demonstration work required can be complex and expensive.
    • Potential for migration from the contamination enclave.
    • Cold temperatures significantly reduce efficiency.
    • Degradation by-products are sometimes more harmful and toxic than the original compounds.
    • Depending of site conditions and the nature of the contamination, the bioavailability of contaminants may be limited.

    Complementary technologies that improve treatment effectiveness

    • Biostimulation (addition of nutrients, oxygen or carbon).
    • Bioaugmentation (addition of microorganisms).
    • Enhanced vacuum extraction (bioaspiration) in the presence of a free phase of petroleum hydrocarbons (immiscible liquid).
    • Source control (removal of the source of contamination where possible) and/or physical barriers (covers, walls or partitions).

    Required secondary treatments

    Controlled natural attenuation is rarely used as the sole remediation strategy. It is combined with other source area treatment technologies. Examples are numerous and could include:

    • Bioventilation
    • In situ chemical oxidation
    • Air sparging
    • Excavation
    • Solidification/stabilization

    Application examples

    The following links provide application examples:

    Performance

    Controlled natural attenuation is a management option that is generally inexpensive compared to active or intrusive treatment technologies. However, it takes several years to complete, does not always achieve the remediation goal, and may limit the reuse of the land for other types of activities.

    Measures to improve sustainability or promote ecological remediation

    • Use of renewable energy and energy-efficient equipment for the implementation of technology.
    • Optimization of the schedule to promote resource sharing and reduce the number of mobilization days.
    • Limit the number of field visits by using telemetry for remote monitoring of secondary treatments if applicable.
    • Review of historical data and optimization of the monitoring program to reduce the number of samples required and the sampling effort.
    • The use of the TRIAD approach to planning and executing site characterization steps to optimize characterization efforts and reduce the environmental footprint of this work.

    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 : Nathalie Arel ing., M.Sc., Frédéric Gagnon CPI., Sylvain Hains ing., M.Sc., Golder Associates Ltd.

    Updated Date : March 21, 2022

    Version:
    1.2.6