From: Public Services and Procurement Canada
Monitored Natural Recovery (MNR)
Monitored Natural Recovery (MNR) is an environmental management approach to contaminated sediments that uses a natural process, or combination of natural processes, to reduce the mass, toxicity, bioavailability, mobility, volume and/or concentration of contaminants and their degradation products in the environment to an acceptable level to protect human health and the environment.
Natural recovery processes refer to various physical, chemical, and biological mechanisms that occur naturally without human intervention in favourable conditions. These in situ processes include:
Enhanced Natural Recovery (ENR)
Enhanced Natural Recovery (ENR) is a form of MNR in which a thin-layer cap of granular material or an amendment (generally between 10 cm and 20 cm) is placed to augment natural recovery processes. In this respect, the technique can be compared to capping. However, while capping is an immediate barrier to contaminant exposure, ENR progressively reduces the concentration of contaminants of concern.
Internet links:
Implementation of this technology may include:
MNR is considered a passive in situ environmental management approach. No equipment or material inputs are required for successful performance of MNR .
However, the ENR design integrates clean sediments or amendments to enhance natural recovery. In this case, they must be stockpiled and specialized equipment may be required for transport and placement. Please refer to the Materials and Storage section of the capping data sheet.
No significant waste discharge is created with the MNR approach, except with the use of ENR . The latter may generate waste and discharge. According to local waste disposal guidelines, excess materials (clean sediments or amendments) may be reused or disposed of.
Soluble and/or buoyant materials (sediments and amendments) may enter the water column during placement, increasing the loading into the water column and potentially migrating downstream. Resuspension of contaminants in the water column can occur during sediment capping.
Rainfall runoff from unprotected stockpiled material can enter the waterbody (if the storage has not been properly managed). Potential runoff should be captured and treated prior to discharge.
Notes:
The analyses, trials, and characterization recommendations are dependent on the type of recovery employed at the site: chemical transformation, physical isolation, reduction in bioavailability and mobility, and/or dispersion. Not all analyses listed will be necessary at every site. For more information on required analyses, trials, and characterization, see ESTCP (2009).
Natural recovery can generally be applied to most aquatic environments and contaminants. Natural recovery has a greater potential for success when the contaminated area is well defined, the source of contamination is removed and controlled, the erosion rate is lower than the rate of sediment deposition, and contaminants tend to biodegrade. The ideal context is one of naturally decreasing concentrations of contaminants in surface sediments and biota over time.
Restrictions involve contaminants that require the presence of specific natural processes (such as sedimentation) or that may require the use of ENR .
MNR has been successfully conducted on several contaminated site remediation projects and thus can be considered a commercialized technology. ENR is commercially available for certain amendments and in the testing phase for others.
The time required for MNR to remediate a contaminated site depends on site characteristics (type and volume of contaminants, size and depth of the contaminated area, type of sediment present, and physical conditions at the site). It may take years to decades to clean up a site using MNR .
If MNR achieves its remediation goals, there are few to no post-remediation long-term considerations.
Locations where sedimentation has buried contaminants under clean sediments are at risk of re-exposure. Institutional controls, such as restrictions on navigation, may be required to avoid disturbance of the overlain clean sediments. Additional monitoring following extreme weather events or turbulence (such as floods, earthquakes) should also be considered in sedimentation locations.
Another risk in MNR remediation is that contaminant dispersion may redistribute contaminants downstream, resulting in increased risk off-site while giving the appearance of moving toward site remediation objectives. Careful water and sediment quality monitoring during the remediation period will identify this risk early so measures may be taken to prevent further loss of contaminants off-site.
Biodegradation of Monocyclic Aromatic Hydrocarbons and Petroleum Hydrocarbons usually does not generate any deleterious secondary by-products or metabolites but generates harmless products such as carbon dioxide and water.
Depending on site conditions and contaminant characteristics, harmful intermediate degradation products may be formed. Furthermore, the biodegradation processes may lead to changes in the geochemical conditions and increase the mobility of some chemical parameters.
It has been shown that other technologies can improve the efficiency of MNR / ENR and reduce the time needed to achieve the remediation objectives:
No secondary treatment is required if risk reduction levels are achieved.
Application examples are available at these links:
MNR performance and recovery time vary depending on sedimentation rates, the dispersion of contaminants within the sediments, the degree of bioturbation, contaminant sequestration and biodegradation processes. For example, MNR occurs more quickly in net depositional environments, where a layer of clean sediments buries contaminated sediments. Performance will also be impacted by the use of complementary technologies and/or acceleration methods (such as introduction of a thin capping layer, presence of structures favouring sedimentation).
Natural recovery is a generally inexpensive management option compared to active or intrusive treatment technologies. However, it takes several years to complete, does not always meet the remediation objective and may limit the reuse of the land for other types of activity.
Site Managers should be prepared to adjust and readjust performance forecasting and consider alternative technologies if measured results deviate significantly from their original predictions.
MNR is inherently a more sustainable remediation technology than active treatment. However, the following points can be considered to improve the sustainability of the technology and/or promote ecological recovery:
The environmental and human health effects arising during remediation through MNR are the result of exposure to contaminants already in the system, which are allowed to continue as the site moves toward remedial objectives. When human exposure levels are unacceptably high, MNR may be combined with risk management techniques (such as institutional controls preventing site access) or barrier technologies (such as ENR or capping) to reduce exposure.
The placement of a thin-layer sediment cap may change the existing sediment concentration, which may lead to changes in the food supply and nutrients available to aquatic and benthic organisms. The added sediments may also change the existing habitat structure and protective cover. These changes may result in mortality and difficulty re-establishing habitats and colonies within the site.
Negative effects associated with ENR may be mitigated through a thorough site characterization and relocation of threatened or endangered species and associated habitats. Using materials similar to native sediments will encourage the re-establishment of the benthic communities and decrease the time required to restore biological activity at the site. In addition, including large debris, such as boulders, will provide cover for aquatic and benthic organisms and increase the likelihood of survival.
Composed by : Bruno Vallée M.Sc, LVM Inc.
Updated by : Ashley Hosier, P.Eng. Royal Military College of Canada
Updated Date : January 2, 2017
Latest update provided by : Juliette Primard, Frédéric Gagnon and Sylvain Hains. WSP Canada Inc.
Latest update date :March 31, 2024
