Monitored natural recovery (MNR) is defined as a remediation practice that relies on natural processes to protect the environment and receptors from unacceptable exposures to contaminants. Enhanced MNR (EMNR) applies material or amendments to enhance these natural recovery processes. The success of MNR/EMNR also depends on adequate control of contributing sources of contamination so that the recovery processes can be effective.
MNR can be used alone or in combination with active remediation technologies to meet remedial objectives. EMNR can use several technologies including, but not limited to, thin-layer capping and introduction of reactive amendments such as activated carbon (AC). Thin-layer caps (typically up to one foot) are often applied as part of an EMNR approach. These caps enhance ongoing natural recovery processes, while minimizing effects on the aquatic environment. The thin-layer cap provides a top layer of cleaner sediment, which reduces surface chemical concentrations so that benthic organisms can colonize the sediment. This layer also accelerates the process of physical isolation, which continues over time by natural sediment deposition.
Remedial approach
With MNR, contaminated sediments are left in place and monitored for ongoing physical, chemical, and biological processes that transform, immobilize, isolate, or remove contaminants until they no longer pose a risk to receptors. These natural processes in below, can reduce exposure to receptors (and thus reduce risk) and contribute to the recovery of the aquatic habitat and the ecological resources that it supports:
Physical processes (burial and dispersion): The primary process responsible for successful MNR is the deposition of cleaner sediment that buries and isolates the contamination. Reducing surface sediment concentrations or chemical bioavailability is thus the primary goal of sediment remediation processes due to greater risk of chemical exposure to benthic receptors and to humans through ingestion of contaminated fish.
MNR can be affected by periodic or episodic erosion events, which can disperse surface sediments across a larger area.
Chemical processes (sequestration and transformation): Attenuation of contaminants via sequestration (sorption, for example) is promoted through adsorption, complexation, and in situ precipitation (or co-precipitation). Transformation generally occurs through natural microbial processes that will either change a parent chemical into a less toxic metabolite (for example, Cr(VI) → Cr(III)) or degrade a constituent through metabolic reactions (phenol → CO2 + H2O).
Biological processes: Biological characteristics of sediments often govern site-specific MNR attenuation processes. The microbial community and the nutrients that sustain its metabolic processes are often key to the site specific attenuation process.
EMNR consists of an engineered amendment, such as placement of a thin-layer cap or injection of a carbon based sorbent into the surface sediments. The objective of EMNR is to accelerate the process of physical isolation, which is continued over time by natural sediment deposition.
Design consideration
If appropriate for the site conditions, MNR/EMNR offers a relatively low-cost, low-risk option that provides a high level of effectiveness and permanence. Typically, MNR/EMNR is used in concert with active remedial technologies that can quickly eliminate exposure, such as dredging and capping.
Unlike active remediation technologies (dredging and capping), MNR is noninvasive and does not disrupt or destroy biologically active zones. MNR is beneficial in wetland environments where rare or threatened endangered species exist, or where existing habitats would not recover from a disturbance for a long time. MNR, however, requires monitoring of the natural recovery process of an ecosystem over time.
Other benefits of implementing MNR/EMNR includes:
- Low implementation efforts results in lower costs
- Multiple risk reduction processes can occur concurrently
- Mixing (or bioturbation) of contaminated sediment with overlying clean sediment reduce contaminant concentration
- Applicable to locations where dredging and capping are not feasible
- Less disruptive to human activities
Data needs for MNR and EMNR
Data needed to evaluate the natural recovery processes at sediment sites fall into four general categories: physical site characteristics, sediment characteristics, contaminant characteristics, and land and waterway use characteristics (see summary table for site characterization needs respective to MNR and EMNR). If MNR or EMNR are expected to be used in the sediment site remedy, then the planning stage of the sediment transport evaluation/sediment erosion and deposition assessment (STE/SEDA), conducted prior to alternative evaluation and remedy selection, should address investigating potential mechanisms of the fate of COCs, such as transport, burial, and degradation.
The summary of site characterization needs for contaminated sediment sites: (link)
The summary of key site characteristics of remedial technologies: (link)
Evaluation process
While MNR/EMNR remedies do not immediately reduce risks, they also do not increase short term risks. The effects of contaminated sediment on the environment continue but gradually decline over time. Some risk reduction can be achieved through implementation of institutional controls.
The long-term effectiveness of MNR/EMNR remedies is high where site conditions are stable and the processes relied upon to achieve protection are unlikely to be reversed. Decreasing trends in contaminant concentrations, measured in the tissue of organisms collected at the site that can be linked to natural reductions in the bioavailability of contaminants in sediments, is strong evidence of the long-term effectiveness of MNR/EMNR.
MNR remedies are more easily implemented than other options. The implementability of long-term monitoring programs should be considered when evaluating MNR/EMNR. Detecting long-term reductions in sediment and tissue concentrations may be hindered by spatial heterogeneity, variations in bioavailability, and seasonal and climatic factors that may influence chemical concentrations in the media being monitored.
MNR is generally considered an attractive option due to the low cost involved. Costs incurred with MNR include: institutional controls, long-term monitoring to ensure that natural processes are working as predicted, and monitoring to ensure that, once protective levels are achieved, the conditions associated with those levels will be stable over time. EMNR options include these costs, as well as capital costs associated with thin-layer capping or addition of sorptive media.
Monitoring
Monitoring is a fundamental part of an MNR/EMNR remedy. Baseline monitoring establishes the current conditions and documents any natural recovery processes present at the site. For EMNR remedies, construction monitoring is implemented following the remedy implementation to determine whether design criteria have been achieved.
| Objectives | Measures | ||
| chemical | physical | biological | |
| construction phase | |||
| Construction monitoring is applicable to EMNR and typically includes monitoring during placement of thin layer caps to ensure turbidity standards established in the applicable permit are achieved. Construction monitoring also includes monitoring cap thickness during or immediately following implementation of the remedy to determine whether design criteria have been achieved. | NA | 1. thin-layer cap thickness; 2. turbidity 3. TSS |
NA |
| post-remediation phase | |||
| Performance monitoring is not applicable to MNR/EMNR. MNR/EMNR requires measurement of recovery over the long-term and not immediately following remedy implementation. |
NA | NA | NA |
| Effectiveness | |||
| Monitoring to determine whether COC concentrations in affected media meet RAOs, or continue to decrease and are expected to meet RAOs in an acceptable time frame. | COC concentration in: – surface sediment – pore water – fish/shellfish – benthos |
Sediment deposition or sediment/thin layer cap stability | 1. benthic reproductive growth, and survival toxicity tests; 2. benthic community survey |