3. Monitored Natural Recovery and Enhanced Monitored Natural Recovery

Monitored natural recovery (MNR) is defined by the National Research Council (2000) as a remediationThe act or process of abating, cleaning up, containing, or removing a substance (usually hazardous or infectious) from an environment. practice that relies on natural processes to protect the environment and receptors from unacceptable exposures to contaminants. This remedial approach depends on natural processes to decrease chemical contaminants in sediment to acceptable levels within a reasonable time frame. Enhanced MNR (EMNR) applies material or amendments to enhance these natural recovery processes (such as the addition of a thin-layer capA covering over material (contaminated sediment) used to isolate the contaminants from the surrounding environment. or a carbon amendment). Parallel natural or enhanced processes, taken together with observed and predicted reductions of contaminant concentrations in fish tissue, sediments, and water, provide multiple lines of evidencePieces of evidence are organized to show relationships among multiple hypotheses or complex interactions among agent, events, or processes. A weight of evidence approach includes the assignment of a numeric weight to each line of evidence. to support the selection of MNR/EMNR (Magar et al. 2009). The success of MNR/EMNR also depends on adequate control of contributing sources of contamination (see Section 2.3) so that the recovery processes can be effective. MNR is not viable as a stand-alone remedial technology if it does not achieve the RAOs.

3.1 MNR and EMNR Background Information

MNR can be used alone or in combination with active remediation technologies to meet RAOs. EMNR can use several technologies including, but not limited to, thin-layer cappingTechnology which covers contaminated sediment with material to isolate the contaminants from the surrounding environment. 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. Thin-layer caps are not intended to completely isolate the affected sediment, as in a conventional isolation capping remedy (see Chapter 5). Instead, 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.

Evaluation of MNR/EMNR during the FS step is highlighted in the Contaminated Sediment Remediation Guidance for Hazardous Waste Sites (USEPA 2005a). Using MNR as a remedy at a contaminated sediment site requires a thorough understanding of the sources, exposure pathways, and receptors in the CSM. Site managers must be able to predict, with some degree of certainty, that contaminant concentrations will decline or be effectively addressed within a specific time frame. Numerical modeling of sediment contaminant levels and biota tissue levels are thus essential tools for defining timely goals and tracking the effectiveness of MNR (Suter et al. 2000).

3.2 Approaches to and Objectives for MNR/EMNR

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. MNR relies on a natural decrease in sediment contamination and a reduction in bioavailabilityThe relationship between external (or applied) dose and internal (or resulting) dose of the chemical(s) being considered for an effect (NRC 2003). or toxicity of chemicals following accretion of clean suspended sediment. Natural processes that contribute to MNR may include sediment burial, sediment erosion or dispersion1) Pollutant or concentration mixing due to turbulent physical processes; 2) A distribution of finely divided particles in a medium., and contaminant sequestrationThe act of segregation. In environmental terms this usually refers to separation of materials by use of various technologies. Carbon sequestration refers to the capture and removal of of CO2 from the atmosphere through biological or physical processes. or degradation (for example, precipitation1) The formation of a solid in a solution or inside another solid during a chemical reaction or by diffusion in a solid; or 2) rain, sleet, hail, snow and other forms of water falling from the sky., adsorptionAdsorption is the adhesion of molecules of gas, liquid, or dissolved solids to a surface. The term also refers to a method of treating wastes in which activated carbon is used to remove organic compounds from wastewater. Additionally, Adsorption is defined as the process by which nutrients such as inorganic phosphorous adhere to particles via a loose chemical bond with the surface of clay particles., or transformation). These natural processes, discussed in detail 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.

3.2.1 Physical Processes (Burial and Dispersion)

Physical processes relevant to contaminated sediments include depositional or erosional processes, groundwater upwelling, and sediment transport events (such as scour, propeller wash, or tidal effects). These processes can help or hinder a sediment remediation project and must be considered prior to selection of MNR. The primary process responsible for successful MNR is the deposition of cleaner sediment that buries and isolates the contamination. Contaminants in surface sediments, especially in the BAZbiologically active zone (the upper bioturbation layer) often pose the greatest risk of chemical exposure to benthic receptors and to humans through ingestion of contaminated fish or shellfish or by direct contact. Reducing surface sediment concentrations or chemical bioavailability is thus the primary goal of sediment remediation processes.

A good example of physical burial by natural deposition and MNR is presented in the Koppers Barge Canal case study. Located on the Ashley River in Charleston, SC, the Koppers Barge canal has a shallow slope and the estuary is turbid. With each tidal cycle, suspended sediment is left behind. Mixing of residual COCs occurs through bioturbation by fiddler crabs. Yearly monitoring showed significant decreases in site-related COCs. This example shows that, with successful source controlThose efforts that are taken to eliminate or reduce, to the extent practicable, the release of COCs from direct and indirect ongoing sources to the aquatic system being evaluated., the deposition of cleaner sediments results in lower surface sediment contaminant concentrations over time. Additionally, the Lower Fox River case study and Twelve Mile Creek/Lake Hartwell case study present two examples in which dispersion and physical isolation were the primary physical processes for the natural recovery of large aquatic ecosystems contaminated with low levels of PCBs. Many sites often include some form of MNR in the remedy when either low zones of contamination are present or the sites are located in depositional areas. Other case studies documenting physical isolation through burial are presented in the ESTCP MNR technical guidance (ESTCP 2009).

MNR can be affected by periodic or episodic erosion events, which can disperse surface sediments across a larger area. Erosion can be a problem when COC concentrations are high and control of scour or erosion is desirable. For low-level contaminated sediments, however, dispersion can result in dilution of COCs and ultimately achieve the site-specific cleanup objectives.

3.2.2 Chemical Processes (Sequestration and Transformation)

Two categories of chemical processes can effectively reduce contaminant bioavailability and toxicity: sequestration and transformation. Attenuation of contaminants via sequestration (sorptionThe process in which one substance takes up or holds another; adsorption or absorption., 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). Transformation into a more toxic metabolite (such as methylated mercury or selenium) can also occur. Sequestration (Sorption and Precipitation)

Sorption is the partitioning of a dissolved contaminant from the aqueous phase onto the surface of a solid phase (adsorption) or diffusion of the contaminant into the sediment matrix (absorptionAbsorption is the assimilation or incorporation of a gas, liquid, or dissolved substance into another substance.). Partitioning of a contaminant from the mobile aqueous phase to the stationary sediment matrix is often quantified using the ratio of the concentration of the contaminant adsorbed to the sediment to the concentration of the contaminant dissolved in the surrounding water at equilibrium (the partition coefficient, Kd). The higher the Kd, the greater the percentage of contaminant mass partitioned to the solid. Use of Kd values is common, but these values are often measured in the laboratory and are more variable when measured in the field. For example, within a given site at any one time, multiple Kd values may be measured for a contaminant because of spatial variability in mineralogy and chemistry. More complex treatments of sorption require more characterization data. Ultimately, site managers must balance the level of complexity and data needs with the level of acceptable uncertainty. For organic compounds, Kd is normalized by dividing it by the sediment fraction of organic carbon to yield the Koc. The normalized value is a better indicator of how strongly an organic contaminant binds to the solid phase of a sediment.

Solids precipitation may lead to contaminant sequestration by three principal routes:

Precipitation occurs when the aqueous phase becomes saturated with either a metal or a metal and ligandComplexing chemical (ion, molecule, or molecular group) that interacts with a metal to form a larger complex (USEPA 2003a). which causes the formation of an insoluble phase (for example, the reaction of lead with phosphate to precipitate insoluble pyromorphite, or the reaction of mercury with sulfide to precipitate insoluble cinnabar; see ITRC CS-1, Section 2.1.2). In the process of precipitation, the metal contaminant is incorporated within the mineral matrix of the dominant solid phase and essentially substitutes for the major ion within the mineral matrix of the newly precipitated solid phase. In the case of co-precipitates, where the contaminant metal in question is a minor constituent of the mineral precipitation, the solubility of the metal contaminant in question depends on the solubility and dissolution of the dominant mineral matrix. Commonly occurring solid phases principally responsible for attenuation or sequestration of metals in sedimentary environments include, but are not limited to, hydroxide, carbonate, phosphate, and sulfide minerals. An example of this process is the co-precipitation of arsenic by iron hydroxide complexes as landfill leachate transitions from a reducing to an oxidizing environment. Transformation (Degradation)

Chemical reactions such as photolysis, hydrolysis, and oxidation/reduction are responsible for contaminant transformations in sediments (Schwarzenbach, Gschwend, and Imboden 2003). Microbes mediate many of these reactions. For example, MNR was the selected remedy for “Area A” in the Hackensack River, a 34-acre estuarine parcel which had received chromium ore processing residue for over a hundred years. The reducing nature of the sediments converted Cr(VI) to Cr(III), which transformed this potentially toxic element to a form that is not bioavailable to aquatic organisms.

In some cases, abiotic degradationProcess in which a substance is converted to simpler products by physical or chemical mechanisms; examples include hydrolysis and photolysis. can occur. Some organic contaminants, such as nitroaromatic compounds, can be rapidly transformed in sediments (such as abiotic reduction by ferrous iron). Other organic contaminants (PAHs, PCBs, and PCDDs/PCDFs) are resistant to degradation and therefore are extremely stable in the environment. These recalcitrant compounds, however, may still undergo chemical reactions such as electrophilic substitution, oxidation, and reduction. Chemical transformation alone may occur over time periods of years or decades; however, most of these chemical transformationabiotic or biotic chemical process (such as photolysis, hydrolysis, oxidation/reduction, radioactive decay) that transform an element (Cr(VI) - Cr III) or compound (phenol – CO₂+ H₂O) to a different element or chemical compound. reactions can be catalyzed by metabolic activity of microorganisms in sediments. Contaminant transformation should thus be considered in the context of biological mediation and the biological aspects fundamental to the reaction chemistry (benthic habitatThe benthic habitat is the ecological region at the lowest level of a body of water such as an ocean or a lake, including the sediment surface and some subsurface layers. and nutrient status). Transformation (Radioactive Decay)

Radioactive decay, the only process by which elemental contaminants are subject to transformation, is applicable to specific isotopes of certain contaminants. Radionuclides are subject to the same environmental attenuation processes related to sorption, precipitation, and redox reactions as described above; however, they also exhibit radioactive decayThe process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles (ionizing radiation). There are many different types of radioactive decay. characteristics resulting in their transformation. Radionuclide decay follows first order kinetics, which means that the rate of the decay is proportional to the number of nuclei present. Consequently, each radionuclide has a characteristic half-life. Five half-lives are required for the loss of greater than 95% of any given radionuclide, and ten half-lives for 99.9%. In contaminated sediment systems, the radiological half-life must be considered in the context of the time needed to achieve remedial objectives. For certain elements with short half-lives (seconds to years), the toxicity of the radiological decay products must be considered. For those elements with long half-lives (centuries to millennia), radioactive decay is not a viable MNR process.

3.2.3 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 (for example, mineralization of organic compounds or sulfate reducing bacteria (SRB) catalyzing metal sulfide precipitation). The indigenous benthic community, where sediment dwelling organisms mix oxygen-containing surface sediments with anoxic deeper sediments, also strongly affects bacterial population dynamics (for example, by sediment ingestion and production of fecal pellets).

Microbial process can directly or indirectly change the bioavailability and the toxicity of a contaminant (ITRC 2011a). Direct processes include degradation of an organic contaminant to innocuous products or changes to valence states of metals affecting speciation, solubility, and bioavailability (see Hackensack River case study). Indirect processes include changes in bulk pHA measure of the acidity or alkalinity of a solution, numerically equal to 7 for neutral solutions, increasing with increasing alkalinity and decreasing with increasing acidity. The pH scale commonly in use ranges from 0 to 14. and oxidation-reduction potential. ORP affects geochemical conditions and thus the disposition of redox sensitive metals. Metabolic processes, such as iron or sulfate reduction, can indirectly affect contaminant attenuation by producing or depleting geochemical reactants that may govern contaminant fate. An example of a metabolic process is the production of sulfide by SRBs. Sulfide can combine with divalent metals to form insoluble metal sulfides. Areas of microbial iron reduction can produce excess ferrous iron, which can facilitate the reductive transformation of organochlorines or nitroaromatics to less toxic compounds.

Phytoremediation is another approach for MNR of sediments, primarily in riparian zones and areas of shallow water. Plants may absorb contaminants directly or accelerate contaminant transformation as a result of their metabolic processes. Water hyacinth, which is a robust species (particularly in tropical wetlands), has been explored for the cleanup of shallow contaminated sediments. Furthermore, selective planting of indigenous species, adapted to the local climate, can enhance the MNR processes already under way at a site. Increasing aquatic vegetation through plantings decreases water velocity, thus encouraging deposition of suspended sediment and increasing the organic carbon content of local sediments. In some cases, it may be necessary to harvest the plants to remove contaminants from the system.

3.2.4 Enhanced Monitored Natural Recovery

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. EMNR also enhances ongoing natural recovery processes (such as burial and sorption) and minimizes more invasive effects to the aquatic environment (for example, aquatic habitat that would be lost with dredge and fill). These sediment amendments do not completely isolate chemically impacted sediment as in a conventional capping operation (Chapter 5). Instead, the sediment amendment speeds the development of a surface layer of cleaner sediment, which results in the reduction in surface chemical concentrations and facilitates the re-establishment of a healthy benthic community. Implementation of EMNR must be based on a demonstration that situ recovery can achieve RAOs in a reasonable time. For example, the EMNR solution implemented at the Ketchikan Pulp Company site (Merritt et al. 2009) achieved both an effective isolation of thick, organic-enriched sediments (containing elevated sulfide and ammonia) and a benthic substrate more conducive to the recolonization of a the benthic macroinvertebrateAny organism that will, after sieving out surface water and fine suspended matter, be retained on a 0.5 mm mesh (No. 35 Standard Sieve) screen. community. Like MNR, EMNR is validated over time through performance monitoring (ESTCP 2009).

3.3 Design Considerations

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. Selection of the optimal remedy for a specific site, however, requires consideration of multiple factors, some unrelated to the environment (such as budgetary constraints and public opinion). Typically, MNR/EMNR is used in concert with active remedial technologies that can quickly eliminate exposure, such as dredging and capping.

3.3.1 Design Advantages

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. In these cases, the value of sensitive habitats outweighs the benefits of removing or capping the contamination. MNR, however, requires monitoring of the natural recovery process of an ecosystem over time.

MNR also avoids the contaminant resuspensionA renewed suspension of insoluble particles after they have been precipitated. that commonly occurs during capping or dredging. These more invasive technologies may create conditions that hinder rather than help efforts to attain RAOs. Dredging, for instance, can cause resuspension of sediments, release of bound contaminants, exposure of residual concentrations associated with the dredge cut, and additional ecological and human health risks generated by greater contaminant levels following re-exposure (NRC 2007b). Postdredging monitoring data collected at a number of sites have also demonstrated temporary spikes in water column1) The basic habitat and the medium through which all other fish habitats are connected; 2) a conceptual column of water from surface to bottom sediments. This concept is used chiefly for environmental studies evaluating the stratification or mixing (such as by wind induced currents) of the thermal or chemically stratified layers in a lake, stream or ocean. Some of the common parameters analyzed in the water column are: pH, turbidity, temperature, salinity, total dissolved solids, various pesticides, pathogens and a wide variety of chemicals and biota. Understanding water columns is important, because many aquatic phenomena are explained by the incomplete vertical mixing of chemical, physical or biological parameters. For example, when studying the metabolism of benthic organisms, it is the specific bottom layer concentration of available chemicals in the water column that is meaningful, rather than the average value of those chemicals throughout the water column. and fish tissue levels following construction activity. If higher COC concentrations are buried below the biologically active zone, dredging can re-mobilize the contaminant back into the ecosystem, re-exposing the biological community to COCs. As a result, risks to the biological community are increased and site cleanup may be prolonged. Contaminant mass reduction may thus not be an optimal solution if it results in an increase in net risk.

Capping also has potentially negative effects. For instance, unconsolidated native sediments may not have sufficient load bearing capacity to support the capping material. The placement of capping material also results in destruction of habitat. Furthermore, contaminants contained in the pore waterWater located in the interstitial compartment (between solid-phase particles) of bulk sediment. of unconsolidated sediment can be released to the cap and surface water (USEPA 2005a). Additional concerns arise from the availability of a suitable capping material, minimum required water depth, water body uses, ebullitionThe act, process, or state of bubbling up usually in a violent or sudden display., and groundwater advectionBulk transport of the mass of discrete chemical or biological constituents by fluid flow within a receiving water. Advection describes the mass transport due to the velocity, or flow, of the water body. It is also defined as: The process of transfer of fluids (vapors or liquid) through a geologic formation in response to a pressure gradient that may be caused by changes in barometric pressure, water table levels, wind fluctuations, or infiltration. conditions adjacent to the site.

Secondary advantages of MNR/EMNR addressed elsewhere in this chapter include the following:

3.3.2 Design Limitations

MNR is not a viable remedy when the physical, chemical, and biological processes are not expected to achieve RAOs within a reasonable time. MNR may not be viable when sediment deposition rates are inadequate for timely burial, when sediment erosion (such as ice scour) is likely, or where advection may be a substantial source contribution. Areas with stable sediments but inadequate deposition rates in terms of achieving RAOs in an acceptable time frame, however, may be good candidates for EMNR.

A major drawback for MNR is that contaminated sediment is left in place and could be reintroduced into the environment. This shortcoming must be considered in light of potential degradation rates. Leaving the contaminated sediment in place also results in a public perception that MNR is a "do-nothing" approach. At sites where this misconception exists, public education is critical.

Another limitation of MNR, which affects all remedial alternatives to some degree, is the uncertainty associated with the data, the site CSM, and model predictions. Uncertainty can result from unexpected disturbance to the sediment, changes in sedimentation and resuspension rates, bioavailability, and abiotic or biotic transformation rates. Confidence in MNR as a remedial solution is gained by developing multiple lines of evidence to minimize uncertainty by defining declining trends in contaminant concentrations in fish tissue and sediment through consistent monitoring of the site over time. Providing routine updates to the stakeholders on the outcome of the remedy also builds confidence in this remedial approach.

MNR also requires long-term monitoring to verify that the RAOs are met. Because of the difficulty of meeting some RAOs (for instance, the removal of a fish advisory), some monitoring programs can be overly burdensome. Eagle Harbor, for example, has been monitored for over 18 years.

Natural groundwater or surface water discharges, if related to the site, can make MNR infeasible. For example, significant quantities of dissolved and particulate phase contaminants may pass into ponds or lakes through surface runoff. The long-term transport of low levels of bioaccumulative substances must therefore be regarded as a confounding variable when making MNR decisions in any watershed.

Secondary limitations of MNR (some of which also apply to more invasive remedial technologies) that are addressed elsewhere in this chapter include the following:

3.3.3 Additional Considerations for Implementation

Before implementing an MNR or EMNR design, several factors should be taken into account to avoid unnecessary delays and subsequent cost, including: Institutional Controls

Most remedial alternatives include institutional controls until long-term monitoring indicates risk reduction has been achieved and the RAOs have been met. Remedies that include MNR frequently require institutional controls, such as fish consumption advisories, to limit human exposure during the recovery period. Institutional controls often require public education programs and postings of warning signs. Time Frame to Reach Cleanup Objectives

The time frame for natural recovery is often longer than that predicted for dredging or capping. Time frames for various alternatives may overlap when uncertainties are taken into account. In addition, realistic estimates of the longer design and implementation time for active remedies should be factored into the comparison. For example, when a single RAO for unlimited fish consumption exists, the time required for MNR, capping, and dredging alternatives may not differ greatly because the active remedial measures can initially result in a spike in fish tissue levels. These possible outcomes should be communicated to the public and other stakeholders before a remedial option is finalized. Public and Community Stakeholder Acceptance

Public and community stakeholder acceptance is one of the two modifying criteria under the NCP; the other is state acceptance (USEPA 1998). Remedies such as MNR may have poor public acceptance at the outset. If disruption due to off-site transport and disposal is not an issue, communities typically prefer that contaminated material be dredged and removed from the area. Stakeholders should be made aware, however, that in general no remedial technology can remove all contaminants from a sediment site.

Remedies that leave site contaminants in place have some risk of continuing exposure or re-exposure of buried contaminants. When MNR is based primarily on natural burial, some risk exists for buried contaminants to be re-exposed or dispersed if the sediment bed is significantly disturbed. A disturbance can result from unexpectedly strong natural forces (ice scour or flooding), through human activities (boating, dredging, or construction), or by groundwater advection. Public acceptance often hinges on a clear CSM, a logical analysis of remedial alternatives, and a robust long-term monitoring program. Informing the public about the tradeoff between risks and benefits associated with contaminants that are left in place, capped, or removed, is key in creating support for the MNR decision. Multiple lines of evidence are necessary to establish the expected permanence of an MNR remedy and to achieve remedy acceptance.

3.3.4 An Example CSM in Support of MNR – Sediment Contamination by Groundwater-Surface Water Interaction

The CSM should call out data needs and lines of evidence necessary to evaluate the various complex physical and biogeochemical factors required to evaluate MNR/EMNR as viable remedial alternatives. At a minimum, the CSM should address the following: source(s), nature and extent of contamination, sediment transport pathways and mechanisms, sediment deposition rateThe amount of material deposited per unit time or volume flow.; exposure pathways associated with chemical contamination, and the potential for in situ degradation (see following example). The RIremedial investigation CSM identifies which major processes must be evaluated and investigated using a sediment transport evaluation or sediment erosion and deposition assessment for the site.

Discharge of contaminated groundwater to surface water is gaining more attention as a mechanism of sediment contamination, particularly for organic chemicals. A former dye manufacturing plant that used chlorinated solvents offers one example of this mechanism.

Sediment Contamination by Groundwater-Surface Water Interaction at a Dye Manufacturing Facility

During a drought, the water level in a freshwater canal was low, and purple water was observed seeping from the canal sidewall into the water. An initial round of sediment samples revealed that chlorobenzene concentrations in the sediment were above ecological screeningThe comparison (by ratio, usually the environmental medium concentration divided by a benchmark, standard, criterion, or similar value) of site conditions to a screening value. Often this is synonymous with “compare to a list that is readily available.” criteria. An extensive round of groundwater, surface water, sediment, and soil sampling was then performed to identify the source of the seep and the extent of affected sediment. Water levels in the canal and in neighboring wells were also monitored to establish the hydraulic connection between groundwater and surface water. The principal elements of the resulting CSM are described and illustrated below:

1. A groundwater plume originates from a dense nonaqueous phase liquid (DNAPL) zone in the nearby manufacturing area.

2. The seep observed during the drought is the groundwater plume discharging from the upper portions of the shallow aquifer.

3. The sediment is contaminated because volatile organic compounds (VOCs) in groundwater sorb to the organic-rich sediment as the plume migrates upward through the sediment.

Comparison of measured groundwater concentrations beneath the sediment to sediment pore-water concentrations supports this model. Groundwater chlorobenzene concentrations (μg/L) are shown below in blue.

Since the discharging plume was the cause of the sediment contamination, sheet piling was installed to prevent further discharge. Compound specific isotope analysis indicated that degradation of the VOCs was occurring in groundwater. Anaerobic degradation in the sediment was also expected because of the anaerobic environment. Thus, source control (removal of the source by stopping groundwater discharge) and biodegradation provided the means to initiate an MNR remedy. Samples have been collected since the sheet piling was installed and are being evaluated to assess the effectiveness of this coupled source control and biodegradation remedy.

After the CSM is developed, study questions and problem statements can guide the plan in addressing specific data needs. Sufficient data should be gathered to answer the following questions for each identified sediment zone:

The preliminary CSM and the preceding questions form the basis for developing Data Quality Objectives that are used to plan field investigations and environmental studies (for example, to support sediment transport evaluations and sediment erosion and deposition assessments) needed to evaluate whether MNR and EMNR are viable alternatives. The following sections describe the data needs and lines of evidence necessary to evaluate whether MNR/EMNR should be selected as a remedy at sediment sites.

3.4 Data Needs for MNR and EMNR

An evaluation of natural recovery and sediment transport processes must be completed prior to fully developing either MNR or EMNR as viable remedial alternatives. Data needed to evaluate the natural recovery processes at sediment sites fall into four general categories (see Table 2-1): physical site characteristics, sediment characteristics, contaminant characteristics, and land and waterway use characteristics. Data needs are most often addressed during RI field activities and by performing a sediment transport evaluation or sediment erosion and deposition assessment, as described in the User’s Guide for Assessing Sediment Transport at Navy Facilities (Blake et al. 2007). NAVFAC's Technical Guidance for Monitored Natural Recovery at Contaminated Sediment Sites (ESTCP 2009) provides a framework for MNR and EMNR data needs specifically for contaminated sediment programs. 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.

3.4.1 Physical Site Characteristics

Data regarding the physical and hydrodynamic processes occurring at a sediment site are critical for evaluating MNR/EMNR remedies. Measures of the forces (discharge, waves, currents, tides) that drive the major sediment transport processes (erosion, water column transportMovement within a water column due to changes in certain parameters (see water column)., deposition) are necessary to effectively evaluate MNR/EMNR remedies (Blake et al. 2007). Sediment Stability

Sediment bed stability can be assessed by using calculated estimates or literature values based on sediment properties. Surficial critical shear stressThe shear stress at which a small but measurable rate of erosion occurs (related to strength of the sediment). and resuspension potential can be obtained for cohesive sediments (such as by using a shaker/annular flume) from core samples. Sediment erosion profiles with depth can be characterized for cohesive sediments using Sedflume or other similar methods. Another line of evidence that demonstrates sediment stability is the vertical profile of contamination in the sediment, which reflects the history of contaminant releases and source control efforts in highly stable sediments. If natural burial processes indicative of stable sediments have occurred at the site following cessation or reduction of contaminant releases, then contaminant concentrations should be lower at the surface. Additionally, the contaminant concentration profile should be trending from a peak concentration at depth toward the background concentrations at the surface. Sediment Deposition Rate

Sediment deposition rate can be established by evaluating historical bathymetric differences in conjunction with reviewing dredging records, coring followed by radioisotope analysis, sediment traps, and pin/pole surveys. For MNR/EMNR, the annual sedimentation rates should be greater than erosion or resuspension rates (annual net deposition). For MNR/EMNR technologies that rely on burial, the annual sedimentation rates should be greater than erosion or resuspension rates (positive net deposition). Sites with annual net deposition much greater than annual erosion and resuspension and with annual net deposition rates greater than roughly 0.5 cm/yr are prime candidates for MNR/EMNR.

Although sediment deposition rate is a critical data need for those MNR/EMNR remedies that rely on burial as a primary recovery mechanism, deposition rates outside of this stated range may also be acceptable depending upon the specifics of the CSM (including vertical extent of contamination, sediment stability, and erosion potential). These metrics, as well as others discussed in Chapter 3, should also be evaluated to determine MNR/EMNR viability. An example calculation illustrating the interdependency of these metrics is provided below.

Target Risk Reduction Example

Where risk reduction depends primarily on burial from deposition, the bioactive zone can be represented as a completely mixed zone if the burial is the result of annual events (not infrequent large episodic events). The decay in bioactive zone concentration can be represented by the decline in the concentration in a completely stirred reactor by steady flushing.

The achievable target risk reduction by burial can be estimated as follows:

Co/C = 1 / e-Qt/T


Q = deposition rate, cm/yr (net deposition rate plus erosion/resuspension rate; resuspension rates typically range from 0.1 to 1 cm/yr in slow moving water bodies, increasing with velocity and decreasing with water depth)

T = bioturbation depth, cm

t = maximum allowable recovery time, years

Co = existing bioavailable concentration in the bioactive zone, ppm

C = target bioavailable concentration in the bioactive zone, ppm

For example, if the bioturbation depth were 10 cm and the deposition rate were 1.1 cm/yr (net deposition rate of 0.6 cm/yr and resuspension rate of 0.5 cm/yr), the predicted concentration reduction factor in 30 years would be

Co/C = 1/e-(1.1 cm/yr *30 yr / 10 cm) = 27

If the bioturbation depth were 5 cm and the deposition rate were 0.4 cm/yr (net deposition rate of 0.2 cm/yr and resuspension rate of 0.2 cm/yr), the predicted concentration reduction factor in 25 years would be

Co/C = 1/e-(0.4 cm/yr * 25 yr / 5 cm) = 7.4

If the bioturbation depth were 15 cm and the deposition rate were 1.8 cm/yr (net deposition rate of 1.0 cm/yr and resuspension rate of 0.8 cm/yr), the predicted concentration reduction factor in 20 years would be

Co/C = 1/e-(1.8 cm/yr *20 yr / 15 cm) = 11

EMNR can be evaluated using the same approach, except that Co should be adjusted to reflect the initial dilution or partial burial of the bioactive zone by the material applied.

Note: Target risk reduction equation is based on a sediment mass balance without degradation presented in Boyer et al. 1994, Chapra and Reckhow 1983, and Jacobs, Barrick, and Ginn 1988. Erosion Potential

Sediment erosion properties must be defined to determine the potential for removal of protective sediments during extreme events. Non-cohesive sediment behavior can generally be predicted from grain size and bulk density information. Cohesive sediment behavior may require the use of other tools to evaluate erodibility. STEs address hydrologic and hydraulic processes that influence the erodibility of sediments and the probability of episodic hydrodynamic events, which may result in the loss of the protective sediment layer and increase the potential exposure to COCs in underlying sediments. The erosion potential of the sediments should be evaluated with consideration of site-specific recovery mechanisms, estimated recovery time, and the expected effect of episodic hydrodynamic events. If the critical shear stress of the sediments below the bioactive zone is lower than the shear stress that may be produced under episodic high energy events, then further evaluation is required to confirm the stability of the protective sediment layer throughout the recovery period. Note that a high suspended sediment load also may indicate a high erosion potential in some areas. Water Depth and Bathymetry

Water depth can be assessed using maps, NOAA bathymetric charts, aerial photographs, and other available regional and site-specific data (current and historical). Detailed bathymetric surveys using single or multi-beam mapping systems can also be conducted. A basic level of bathymetric, topographic, and historical information is needed to characterize a site because physical boundaries often define the relevant zone of influence. A bathymetric/shoreline change analysis can yield information on long-term depositional or erosional characteristics of the system (sediment sources and sinks) and help quantify rates of change. Water depth is not a critical consideration for MNR. EMNR, however, may have depth limitations similar to in situ treatment (see in situ treatment TAG, Section The literature indicates that accurate delivery and placement methods are improving, thus expanding the application of EMNR for a wide range of aquatic environments. In-water and Shoreline Infrastructure

Information describing current or historical in-water and shoreline infrastructure can be obtained from local agencies and or developed from site specific data collected while visually inspecting the site. In-water and shoreline infrastructure is usually not an issue when considering MNR. For EMNR, however, structures may limit accessibility or require specialized equipment to be used for amendment application. Delivery and placement methods are improving, making EMNR a more viable remedial option even where access is limited. Presence of Hard Bottom and Debris

The presence of a hard bottom or debris in sediments is typically not a constraint for MNR that target contaminants in surficial sediments. EMNR on the other hand, requires placement of the treatment amendment on the sediment surface, in which case the presence of debris must be considered. Hydrodynamics

Hydrodynamic information can be obtained from regional or site-specific flow data. Site-specific measurements are necessary, however, to characterize the hydrodynamicsThe branch of science that deals with the dynamics of fluids, especially that are incompressible, in motion. of the area within, and immediately upstream, of the site. These measurements include the following:

Seasonal hydrodynamics generally control the erosion potential of the site sediments (Section The dominant seasonal hydrodynamic forces should be identified and quantified because these forces drive sediment transport. When these data are combined with suspended sediment measurements, directions and quantities of sediment transport can be determined. Additionally, analysis of water column transport properties is necessary to determine sediment flux on site and off site and to determine settling properties of sediments. Slope and Slope Stability

The weight of material placed for EMNR (thin-layer sand covers) imposes a new load on the underlying sediment. When the sediment surface is sloped, this weight produces a force that pushes the cover and underlying sediment downslope. The force pushing downslope is resisted by the shear strength of the underlying sediment. In slope stability calculations, the ratio of the force available to resist sliding to the force pushing downslope is called the factor of safety. The minimum factor of safety for permanent slopes under static loads is generally 1.5, based on guidance documents such as Design Manual 7.2, Soil Mechanics (NAVFAC 1986). For EMNR slope stability, the factor of safety under static loads should be greater than 1.5. The factor of safety decreases as the sediment shear strength decreases, as the thickness of the cover material increases, or as the slope angle increases. Slope stability calculations are recommended when the slope is greater than 5% or when the sediment shear strength is less than 1 kPa (20 psf). Thin-layer cap placement may require special design and placement methods when the slope is greater than 15%. As discussed in Section, the sediment must have sufficient strength to support the weight of EMNR cover material without lateral displacement (mud waves) of the sediment under the cover. Groundwater-Surface Water Interaction

Seasonal groundwater flow data, groundwater and chemical data, and pore-water data are needed to understand the potential groundwater and surface water interaction at a site. Data can be collected using piezometers, groundwater modeling, infrared surveys, salinity gradient surveys, flux chamber measurements, and seepage meter measurements. A variety of passive and active pore-water samplers are available also (ITRC 2011a, Appendix C).

Groundwater must be characterized as part of the CSM, both as a potential source of chemical contamination and as a physical transport mechanism (advection). Effects of groundwater advection on dispersion of sediment contaminants can be identified using pore-water chemistry, which characterizes surface sediment dissolved chemical concentrations. Groundwater springs and heavy discharge areas may also cause sediment to be unstable and contribute to long-term dispersion of particulate bound contaminants, as well as dispersion of dissolved-phase contaminants in certain site-specific environments. Sediment stability (Section and contaminant contributions from groundwater discharge must also be considered when evaluating MNR/EMNR.

Long-term contaminant migration rates by groundwater advection upwards through the newly deposited sediment should be substantially less than the long term burial rate. Contaminant flux rates are generally much lower than the groundwater flux rate due to the adsorptive capacity of the sediment. Long-term monitoring and verification of assumptions are recommended to assure site conditions are consistent with the input parameters of the flux rate calculation.

The contaminant flux rate is calculated by dividing the pore-water velocity (Darcy flux divided by the porosity of the sediment deposition) by the retardation coefficient, R. The retardation coefficient is calculated as follows:

R = 1 + (ρb Kd / n)


R = retardation coefficient

ρb = dry bulk density, kg/L

Kd = partition coefficient, L/kg = foc Koc

foc = fraction organic carbon

Koc = organic carbon/water partition coefficient, L/kg

n = porosity

As an example, a burial rate of 0.5 cm/yr for a sediment with 3% TOC, a specific gravity of 2.6 and a porosity of 0.7 (ρb = 0.78 kg/L) would require a groundwater flux less than 585 cm/yr (1.6 cm/dy) for a contaminant having a Koc of 100,000 L/kg. (Kd = 3,000 L/kg for the deposition; R = 3344).

3.4.2 Sediment Characteristics

In addition to understanding the physical, biological, and geochemical characteristics present at a contaminated sediment site, a site-specific evaluation of sediment characteristics is necessary prior to implementing MNR/EMNR. The data needs for specific sediment characteristics required to evaluate the feasibility of MNR/EMNR are summarized in the following sections. Geotechnical Properties

Geotechnical parameters strongly affect the physical disposition characteristics of the sediment bed and therefore affect the fate and transport of contaminants over space and time.

Initial estimation of bulk density, shear strength, and cohesiveness can be measured based on preliminary sediment characterization. Surficial critical shear stress and resuspension potential for cohesive sediments, using shaker/annular flume and sediment erosion profiles with depth, can be estimated using Sedflume. These measurements are useful in determining the potential for sediment erosion and potential depths of erosion during extreme events. Non-cohesive sediment behavior can generally be predicted from grain size and density information. Grain Size

Grain size data can normally be obtained from typical RI or equivalent information. Sieve analysis should be obtained for sediments greater than 63 μm and laser diffraction methods for high resolution less than 63 μm. Generally for MNR/EMNR, a high percentage of fines is indicative of a low energy (potential depositional) environment. Sediment bed property data can be used to infer the sediment transport characteristics based on distributions and sorting of sediment grain sizes and densities. Resuspension, Release, and Residuals

Resuspension or release of COCs is not a concern for MNR or EMNR, as long as the physical site characteristics and geotechnical parameters are well understood. Sediment Consolidation (Pore-water Expression)

Sediment consolidation is evaluated using percent solids data, which should be available from sediment analytical data in the RI or equivalent study. Centrifugation of a sediment sample is typically performed to determine the fraction that consists of pore water. Additional data collection, such as sediment consolidation tests, can provide engineering properties necessary to evaluate the potential application of EMNR. This data may be needed because settlement of the sediments can cause contaminant flux into newly deposited material or material used for EMNR.

Sediment and pore-water geochemical data (including TOC, DOC, and POC) can normally be obtained from an RI (or equivalent) or supplemental sediment and pore-water sampling. Geochemical constituents related to contaminant binding (bioavailability) or decay (transformation/degradation) should be targeted. The effectiveness of MNR/EMNR typically increases with increasing natural sorption capacity (for example, with the presence of organic carbon, including highly sorptive black carbon) of sediment and suspended sediment in the waterway. Sorption of contaminants by organic carbon reduces bioavailability, which reduces exposure even if total contaminant bulk sediment concentrations are not reduced (ITRC 2011a).

Sediment data should also be used to determine the concentration, source, and spatial distribution of geochemical constituents (such as sulfide or manganese) responsible for contaminant attenuation and sequestration. Measurement of acid-volatile sulfide/simultaneously extracted metals (AVS/SEM) helps to assess the bioavailability of divalent metals. Benthic Community Structure and Bioturbation

Literature data on benthic community characteristics (such as species inventory, habitat evaluation, burrowing depths, and bioturbation rates) should be reviewed. When evaluating benthic habitat, sediment profile imaging can identify the presence and types of burrowing organisms, indicate the depth of redox zones, and measure the bioturbation depth. Metrics such as abundance and diversity of the benthic community can also be measured following taxonomic evaluation of organisms preserved from conventional sediment grabs (ESTCP 2009). Sediment sites with a relatively deep BAZ (greater than 10 cm) may not be remediated as quickly as those with a shallower zone (less than 5 cm), but MNR can be used at sites with deeper BAZ if given enough recovery time. The acceptable length of the recovery period is a site-specific decision. EMNR is most effective when the emplaced layer thickness exceeds the BAZ depth.

Recolonization of the benthic community typically follows the placement of the enhancement layer. The bioturbation depth influences the rate of change in surface sediment chemical concentrations. Benthic mixing can affect the rate of physical isolation of the contaminated sediment below. Benthic bioturbation depths also indicate how to define surface sediments (sediments to which organisms may be exposed). Without site-specific data, 10–15 cm depth as an average may be assumed. Benthic community structure may be used to evaluate the recovery of the community.

3.4.3 Contaminant Characteristics

The types, properties, concentrations, and distribution of contaminants present at a site and their potential to be transported or transformed must be understood when considering MNR/EMNR. Table 2-2 presents some of the data that help to better define the factors that affect the disposition of COCs for MNR/EMNR. A key objective of any sediment remediation is the reduction in bioavailability, toxicity, and volume of COCs, which in turn directly reduces site risk. For MNR/EMNR, these reductions are best accomplished through physical isolation (natural burial) and degradation (such as reduced half-life). Natural sedimentation provides further reductions in chemical mobility and leads to reduced contaminant concentrations in surface sediment through natural dilution and burial.

Although most sediment guidance calls for an assessment of bioavailability, this process is often inadequately addressed or even ignored. Bioavailability can be a key factor in the decision to use MNR (for example, low bioavailability is a favorable line-of-evidence) and EMNR (for example, the use of sorptive media can markedly reduce bioavailability of bioaccumulative compounds) (ITRC 2011a). At a minimum, TOC should be measured in all samples to estimate partitioning behavior of COCs. AVS/SEM data are also valuable, particularly in estuaries or marine environs, for predicting bioavailability of and risk from divalent metals (Hammerschmidt and Burton 2010). Horizontal and Vertical Distribution

Sediment chemistry data typically collected from the RI (such as high-resolution horizontal and vertical sediment contaminant distribution data) can be used to evaluate the contaminant extent. If contaminant sources and loading history are known, then sediment transport patterns can be inferred from the horizontal and vertical contaminant distribution. Some sediment constituents (aluminum, iron, and others) can act as a tracer for the transport of contaminants away from the site, to normalize site-specific contamination (metal ratios), and to identify potential off-site sources contributing to sediment contamination. MNR remedies are most effective when contaminant concentration increases with depth, indicating that a natural burial process occurs at the site. Lower surface concentrations, over time, translate to a lower degree of risk. Contaminant Type

A detailed evaluation of the nature and extent of contaminants and their potential to migrate or be transformed is essential to understanding risks posed by a site over time and whether natural recovery mechanisms that rely on transformation are viable. A literature review of typical fate and transport behavior of chemicals (such as metals, chlorinated organics, pesticides, and UXO) in sediment should be conducted. This review and testing should include speciation and valence state data, partition coefficients, typical half-life in sediments, and factors that control migration such as organic carbon, sulfides, sediment geochemical data, and pore-water data. USEPA’s online EpiSuite program can assist in predicting many fate and transport parameters, including biodegradation probability, octanol-water partition coefficient (Kow), organic carbon partition coefficient (Koc), bioconcentration factorThe ratio of the steady-state COPC concentration in an aquatic organism (CB) and the COPC concentration in water (CW) determined in a controlled laboratory experiment where the test organisms are exposed to chemical in the water (but not the diet). In the subscript, the numerator (N) is the basis of the tissue phase (L for lipid-normalized, WW for wet weight, and DW for dry weight bases) and denominator (D) is the basis for the water phase (FD for freely dissolved, T for total, and D for dissolved/filtered water). Commonly used BCF expressions are as follows: • BCFL/FD = where concentrations in tissue and water are on a lipid and freely dissolved basis, respectively • BCFWW/T = where concentrations in tissue and water are on a wet weight and total basis, respectively • BCFDW/T = where concentrations in tissue and water are on a dry weight and total basis, respectively (BCF), and bioaccumulation factor (BAF)The ratio of COPC in tissue to the COPC concentration in an external environmental phase (water, sediment, or food) (Spacie, Mccarty, and Rand 1995). The BAF is typically assumed to be measured or expressed on a steady-state basis. For applications to the water phase, the BAF is best determined from field data where sampled organisms are exposed to chemical measured in the water and their diet. For applications in reference to the sediment and food phases, the BAF is expressed using concentrations in the tissue and environmental phase on a wet weight basis or dry weight basis, for example, (µg/g of w/w tissue)/(µg/g of w/w food), (µg/g of d/w tissue)/(µg/g of d/w food), and (µg/g of d/w tissue)/(µg/g of d/w sediment). This definition of BAF is used for metals, organometallic compounds, and organic compounds. For clarity, the BAF is expressed with the units in subscripts. For the concentration in the tissue phase, the numerator (N subscript) is the basis of the tissue phase (L for lipid-normalized, WW for wet weight, and DW for dry weight bases). For the environmental phase, the denominator (D subscript) is the basis for the water (FD for freely dissolved, T for total, and D for dissolved/filtered water), food (WW for wet weight and DW for dry weight), or sediment (WW for wet weight, and DW for dry weight) phases. Some commonly used BAF expressions are as follows: • BAFL/FD = where concentrations in tissue and water are on a lipid and freely dissolved basis, respectively • BAFWW/T = where concentrations in tissue and water are on a wet weight and total basis, respectively • BAFDW/DW = where concentrations in tissue and sediment are both on a dry weight basis. Contaminant Concentration

A review of historical site information and a literature review of chemical data and reference or background data is helpful in understanding the distribution of contaminants present at the site. Key exposure routes and receptors for these contaminants should have already been identified during the development of the site CSM and risk assessment. Reducing risks from these contaminants often depends on changes in site-specific factors and conditions that can be used to make a decision for MNR/EMNR. These factors include sediment deposition rates, degradation rates of COCs, the recovery of the benthic community, and the acceptable time period in which to achieve the remediation goals. These site-specific factors can be used to determine the concentrations of COCs that are amenable to MNR and the concentrations of COCs that are amenable to EMNR (as illustrated in the Target Risk Reduction Example in Section

For sediments that have characteristics (such as sediment stability) suitable for MNR or EMNR, the risk reduction is primarily achieved by reducing the bioavailable contaminant concentration in the BAZ, where significant sediment mixing occurs by bioturbation.The displacement and mixing of sediment particles  and solutes  by fauna (animals) or flora (plants). This zone is typically the top 6 inches (15 cm) of the sediment profile in freshwater systems and the top 12 inches (30 cm) of sediment in estuarine and marine systems. The target risk reduction factor can be expressed as the ratio of the existing bioavailable contaminant concentration in the BAZ to the remediation goal (RG). This factor may be estimated by the ratio of the existing dissolved contaminant concentration in the BAZ to the target dissolved contaminant concentration.

For example, at a site with a deposition rate of about 1.1 cm/yr, a BAZ thickness of 10 cm, and the desire to achieve the remediation goals within 30 years, concentrations of COCs 27 times or less than the remediation goals are amenable to MNR and concentrations of COCs 100 times or less than the remediation goals are amenable to EMNR (see the Target Risk Reduction Example in Section for applicable equations and examples). Exposure Pathways

MNR and EMNR can control exposure pathways to the aquatic food web that involve direct or indirect exposure to the available chemicals in the sediments. These pathways may include a direct exposure to biota, bioaccumulationThe accumulation of substances, such as pesticides, or other organic chemicals in an organism. Bioaccumulation occurs when an organism absorbs a toxic substance at a rate greater than that at which the substance is lost. Thus, the longer the biological half-life of the substance the greater the risk of chronic poisoning, even if environmental levels of the toxin are not very high. into benthic invertebrates with subsequent transfer to higher trophic levels, and contaminant flux to the overlying water column. Natural burial processes can eliminate direct exposure to contaminants in the sediments through physical isolation of the contaminated sediments from the overlying water column biota. Chemical transformation and sequestration (immobilization) can reduce or eliminate the bioavailability of contaminants to the bioactive zone and subsequent food web.

The CSM should clearly determine whether natural sediment chemical and biological processes and net deposition are capable of controlling the exposure pathways to the aquatic environment. The immediate threat from direct exposure of the aquatic environment to the contaminants in the sediment can be reduced by the addition of sediments to the natural sediment surface. This enhancement should be designed to accommodate any erosional effect, thus preventing a re-occurrence of direct exposure to contaminants. Amendments may be added to the sediment surface to enhance the degradation and immobilization capabilities of the surface sediment. Source Material

Identifying the sources of contamination are especially critical for MNR/EMNR because continued loading from in-water sources may prevent MNR/EMNR from achieving RAOs. Examples of source material include NAPL, sand blast grit, slag, and areas of highly contaminated sediment that are ongoing sources of contamination through sediment transport, advective groundwater transport, or other transport mechanisms (Section 2.3). In general, NAPL and other source materials should not be present when considering MNR/EMNR.

Understanding and controlling sources of sediment contamination allows MNR mechanisms to reach cleanup goals. Source material present in surface sediments may migrate through sediment erosion and deposition, thus limiting the effectiveness of MNR. Although natural biodegradation of PAHs and other degradable contaminants has been documented in sediments overlying NAPL deposits, the potential upward mobility of source material constituents into the BAZ through groundwater and ebullition mechanisms must be characterized. Ebullition can be a potential pathway for oil/sheen migration from subsurface sediments; see Section Mobility

Mobility of contaminants, such as metals or NAPLs, is generally controlled by the solubility of the contaminant (USEPA 2005b). MNR/EMNR may be amenable when site-specific factors or modeling reveals low natural contaminant mobility or bioavailability (see Section of contaminants. Increased mobility, however, does not necessarily result in increased risk. When evaluating contaminant mobility, consider vertical extent of contaminant concentrations, redox conditions at various depths, deposition rate, and the exposure pathwayThe channel or path followed by pollutants from their source, via air, soil, water, and food to humans, animals, and the environment.. For example, chemical degradation of PCBs is more likely to occur in deeper sediments rather than in the BAZ. Therefore, although this degradation may result in slightly increased mobility, it may not result in increased risk in the BAZ if the sediment bed is stable. Bioavailability and Toxicity

Two categories of chemical processes can effectively reduce contaminant bioavailability and toxicity: 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 (Cr(VI) → Cr(III)) or degrade a constituent through metabolic reactions (phenol → CO2 + H2O). The possibility of transformation into a more toxic metabolite such as methylated mercury or selenium, should also be considered. Bioaccumulation and Biomagnification

To understand the potential for bioaccumulation and biomagnification at the site, conduct a thorough literature review of BSAFs or BCFs for contaminants of potential concern (COPCs). Based on this review, evaluate the potential for contaminant migration into biota through sediment, pore water, and the water column. Biota tissue residue data may also be available for the water body of interest (such as in state or federal databases). Literature values are not site-specific, however, so testing of tissues and environmental media should be performed to develop site-specific accumulation factors. COPC and TOC concentrations in sediment, pore-water and surface water can be used to develop site-specific BSAFs and BCFs as a line-of-evidence to support a case for MNR/EMNR. Higher trophic level receptors are often an endpoint of MNR/EMNR monitoring activities to show progress toward recovery. This should not preclude monitoring shellfish, which can also illustrate more localized trends. In general, MNR may be more applicable if site-specific partition coefficients strongly favor partitioning into the sediment matrix (see ITRC 2011a).

Higher, trophic-level receptors are often an endpoint of MNR/EMNR monitoring to show progress toward recovery (for example, removal of a fish advisory). As a precaution, contaminant concentrations may often fall below analytical detection limits before the endpoint is regarded as met. Both the public and other stakeholders, however, may still perceive levels of concern for downstream aquatic organisms (benthos and fish) due to contaminant transport from regional watersheds. Transformation and Degradation

Data in the literature should be thoroughly reviewed for information regarding contaminant transformation pathways and biological or geochemical conditions under which these pathways occur. Physicochemical data (such as Eh-pH, redox/ORP, sulfides, AVS/SEM, divalent metals, TOC, DOC, and POC) should be collected to determine the presence or absence of parent compounds and transformation by-products in situ. Generally, for those COCs known to degrade, contaminant degradation rate versus recovery time should be compared. The time required for a contaminant to degrade below an acceptable level of risk should be less than the stipulated period of recovery. Processes reduce risk when the transformation product is less toxic or less bioavailable than the parent compound. Source Identification and Control

See Section Ebullition

If ebullition is occurring at a site, caution should be used when selecting MNR or EMNR. A clear understanding of the potential contaminant mixing in the surficial sediment caused by ebullition is required in order for MNR or EMNR to be successful at the site. Background

As discussed in Section 2.2 and in Section, background refers to substances, conditions, or locations that are not influenced by the releases from a site, and are usually described as either naturally occurring (consistently present in the environment but not influenced by human activity) or anthropogenic (influenced by human activity but not related to specific activities at the site). RAOs should account for background conditions and MNR progress should be measured against RAOs.

3.4.4 Land and Waterway Use

The land and waterway use characteristics described below include a variety of interrelated technical and nontechnical issues. In general, the collection of land and waterway use data is not required for MNR. Implementing EMNR may require this data, however, because EMNR has an active remedy component. Watershed Characteristics

Watershed sources must be identified and controlled, if possible, for successful restoration, because these sources may be the limiting factor for the effectiveness of the remedial technology selected. Even though the on-site characteristics may indicate that MNR/EMNR are viable, uncontrolled off-site sources can contribute additional contaminants to the remediated site. The accumulation of watershed-derived COCs can negate the effectiveness of MNR/EMNR. Conversely, the lack of watershed sources would suggest that clean material will deposit within the site, thus increasing the effectiveness of MNR/EMNR. Cultural and Archeological Resources

A review of cultural and archeological resources should include consideration of cultural influence, loss of traditional cultural practices by Native American tribes, or effects on historic or archeological landmarks such as grave sites. These issues fall under the items covered under the National Historic Preservation Act of 1966, Archaeological Resources Protection Act of 1970, and the Native American Graves and Repatriation Act of 1990. Since MNR does not disturb the natural environment, cultural and archeological issues are not a concern. EMNR, however, may have a component of active remediation that requires upland access to the site during implementation. In this case, cultural and archeological issues may need to be considered. Site Access

Site access is key but is typically not an issue after remedial measures have been implemented. Since MNR does not require active remediation, this data category is mainly applicable for regular monitoring activity. Information on how the area will be used, such as anticipated recreation activities, may be warranted. For EMNR, a thin-layer cap in a shallow waterway would require temporary access to stage equipment along the shore to monitor the long-term efficacy of the remedy. Current and Anticipated Waterway Use

The current use of the waterway does not affect the selection of the MNR remedy. EMNR may have a short-term influence on waterway use during mitigation and may slightly change bathymetryThe measurement of or the information from water depth at various places in a body of water.. Future uses with respect to navigation, recreation, and habitat are generally not an issue, but may need to be considered if the MNR/EMNR remedy requires that the sediment remain undisturbed. Sediment could be scoured and contaminants released if, for example, the waterway was open to heavy navigation. Current and Anticipated Land Use

Non-invasive remedies such as MNR are not expected to affect current and anticipated land use. EMNR may have a short-term influence during mitigation activities. Endangered Species and Habitat

Endangered species and habitat are not considered if the MNR remedy does not involve disturbance of the environment. Unique and sensitive species may need to be considered for EMNR. For example, a sensitive wetland habitat or species present in the affected area could be smothered by placement of a thin-layer cap. At the Johnson Lake site in Portland, OR (see Case Study D-17 in ITRC 2011a ), a portion of the lake with the lowest concentrations of sediment contaminants was left uncapped to provide a means for threatened mussels to repopulate the newly covered portion of the lake.

As indicated in Chapter 2, this document assumes that RAOs have been established for the site. The decision-making steps described in Section 2.1 apply broadly to the remedy evaluation process and are critical to establishing the framework within which MNR/EMNR is evaluated. The following discussion focuses on the standard remedy evaluation criteria established by USEPA (1988), which, with some variations, many state cleanup programs have adopted. Relevance of remedy performance criteria to MNR/EMNR is also discussed in the following sections.

3.4.5 Protection of Human Health and the Environment

All alternatives considered in the detailed analysis stage of the FS must demonstrate that they provide protection of human health and the environment. MNR/EMNR achieves protection by allowing natural processes to reduce contaminants to protective levels. Protection should be documented by describing the mechanism pertinent to the COCs and estimating the time that will be required to adequately reduce contaminant concentrations (ITRC 2011a). Institutional controls used to reduce exposures during this time should be described as part of the protection determination.

Estimating the time required for various processes can be difficult and subject to uncertainty. These estimates generally include modeling the primary process involved based on deposition rates in the area of concern or chemical degradation kinetics. In many cases, MNR/EMNR is identified as an alternative for consideration based on data trends over time or implications derived from the contaminant distribution. For example, recent studies may indicate elevated concentrations of contaminants are already being covered by less contaminated sediment. Monitoring MNR/EMNR remedies should generally include contingencies for evaluating more active measures if the processes relied on do not have the anticipated result. Reasonable time estimates are site-specific and depend on how critical and feasible it is to control exposures during the time that natural mechanisms require to reduce risks.

3.5 Evaluation Process

3.5.1 Compliance with Applicable or Relevant and Appropriate Requirements

ARARs for MNR primarily arise with respect to chemical specific RAOs. These may include ambient water quality criteria (AWQC); however, the media to which AWQC apply (pore water, surface water, back-calculated sediment value) will vary depending on the exposures of concern (food chain versus direct toxicity) and the availability of other sediment cleanup criteria established by the state that may take precedence. Sediment sampling may require a permit or documentation that substantive requirements are met and tissue sampling typically requires a scientific collection permit from the applicable state or federal agency. EMNR options using placement of thin-layer caps require permits or documentation that substantive requirements are met and possibly local planning agencies. In some cases, equivalent cuts must be made in another location within the waterway to compensate for fill placed at the cap.

3.5.2 Short-term Effectiveness

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, though these mechanisms typically offer no benefit to ecological receptors.

The time required for natural processes to reduce contaminant levels should be estimated and the rate of risk reduction considered in evaluating the effectiveness of MNR/EMNR remedies. An approach based on net deposition should consider the sedimentation rate of clean sediments, whereas an approach based on degradation requires an estimation of the half-life of the COCs in sediment.

The nature and extent of the risk posed by contaminants is also a factor. For MNR/EMNR, it is viable to allow contaminants that bioaccumulate or biomagnify to remain at low levels for short periods of time if they do not pose a risk to the food chain. Depending on the risks, sites involving bioaccumulative contaminants may include some enhancement (such as thin-layer capping) to reduce exposures while the natural processes take effect. Note that EMNR alternatives may incur short-term risks associated with placement of a thin-layer cap, which can increase turbidity.

Potential effects on large regional ecosystems should also be considered, particularly for MNR options that could result in some dispersion of contaminants to sensitive areas. Bioaccumulative contaminants in trace quantities can accumulate to levels of concern in downstream areas if the rate of turnover in the receiving water body is extremely long.

3.5.3 Long-term Effectiveness

Long-term effectiveness is perhaps the key evaluation criteria for MNR/EMNR due to the lack of short-term impacts, the relative ease of implementation, and the low cost. 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. Episodic events (flooding or seismic activity) that disturb sediment at a site must be considered for remedies that rely on natural burial. The long-term stability of physical, chemical, or biological transformations that form the basis for some MNR/EMNR remedies must also consider seasonal changes. Changes in physical processes, such as groundwater gradient or flow rate, must also be considered where advection of contaminants through overlying sediments may be an issue. As discussed in Section , potential ramifications of leaving contaminants in place include effects on downstream resources where any contamination that migrates may accumulate. Potential effects of releases occurring during episodic events and dissolved phase transport through overlying sediment should be considered in terms of the regional ecosystem.

3.5.4 Implementability

MNR remedies are more easily implemented than other options and generally do not require construction other than signage and public outreach activities associated with institutional controls. EMNR remedies require some active measures during placement of a thin-layer cap or while modifying the sediment environment. As with standard capping, the placement of a thin-layer cap can disturb underlying contaminated sediment. Furthermore, methods used to gently place thin-layer caps can create significant turbidity, especially when the cap material includes some proportion of organic material; even levels lower than 0.5% TOC can be problematic.

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 (see Section 3.6 for additional discussion regarding monitoring). Reliability of MNR/EMNR options can be uncertain when rates of natural processes are not well defined or environments are unstable. More intensive monitoring may be required in these cases.

Water depth and future site uses that may reverse the containment of contaminated sediment should be considered. For example, the ability to restrict activities that will disturb sediment covers (such as recreational watercraft) must be considered and used to develop adequate institutional controls where warranted.

Unlike some capping options, MNR/EMNR remedies do not preclude implementing alternative approaches if monitoring indicates the processes selected are not effective.

3.5.5 Reduction of Toxicity, Mobility, or Volume through Treatment

Since no active treatment occurs with MNR/EMNR remedies, reduction of risk through active treatment is generally not applicable. Where a significant toxicity or mobility reduction is achieved through natural degradation processes (or additional sorptive material), however, some treatment credit can be given. Typically at sites, or portions of sites, where principal threats are present and where high-level risk is indicated, MNR/EMNR remedies will generally not be appropriate on their own. These remedies may be appropriate, however, in combination with active remedial measures.

3.5.6 Cost

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.

3.5.7 State Regulatory Acceptance

State regulatory acceptance for MNR/EMNR actions can be critical as states generally own submerged lands. Many states prefer that sediment be actively remediated so activities are not restricted in the area. States may be concerned about associated economic impacts and reduced property values if contaminants remain at levels that present an unacceptable risk for several years. Coordination with the appropriate state land and natural resources departments early in the project is necessary to identify and address their concerns.

3.5.8 Tribal Regulatory Acceptance

As with state regulatory agencies, it is important to coordinate early with local tribes who often rely on fishing resources to a greater extent than other populations. With bioaccumulative COCs, acceptable concentrations in fish may be lower because tribal fish ingestion rates may be higher than those used to estimate risk for recreational fishing. The time estimated for achieving protective levels estimated for this scenario will thus be much longer. See Chapter 8 for additional information on tribal stakeholder issues.

3.5.9 Community Acceptance

Cleanup actions that involve little more than monitoring are often difficult to justify to communities that want resources restored more quickly or may suspect that MNR is merely a form of doing nothing. If disruption due to off-site transport and disposal is not an issue, communities typically prefer that contaminated material be dredged and removed from the area; however, no remedial technology can remove all contaminants from a sediment site. Any remedy that leaves site contaminants in place has some risk of continuing exposure or re-exposure of buried contaminants.

When MNR is based primarily on natural burial, some risk exists for buried contaminants to be re-exposed or dispersed if the sediment bed is significantly disturbed by unexpectedly strong natural forces (such as ice scour or flood events), through human activities (boating, dredging, or construction), or by groundwater advection. Informing the public about the tradeoff between risks and benefits associated with the contaminants if a) they are to be left in place; b) they are to be capped; or c) they are to be removed using invasive methods is key in creating support for the MNR decision. Multiple lines of evidence are necessary to establish the expected permanence of an MNR remedy in order to achieve remedy acceptance.

Project managers should devote adequate time to explaining the processes that are at work to reduce contaminant levels naturally and the associated benefits over more invasive methods. Watershed councils and fishing groups are particularly interested, and focused outreach to these groups is helpful in gaining community support.

3.5.10 Green and Sustainable Remediation

MNR/EMNR is likely to be the greenest and most sustainable alternative evaluated for sediment sites because it involves minimal equipment and no hauling or treatment of contaminated material. Releases associated with periodic sampling events are minimal and are likely required to some extent for other remedial options as well (see ITRC 2011b for more information on green and sustainable remediation).

3.5.11 Habitat and Resource Restoration

MNR, and to a large extent EMNR, are conducive to restoring habitat because they rely on processes that occur naturally in the system and do not destroy existing habitat. The time required for restoring resources such as fisheries, however, will likely be longer for these options than for other alternatives.

3.5.12 Future Land and Waterway Use

As discussed in earlier sections, MNR/EMNR options that rely on deposition of clean material over contaminated sediments are not feasible in waterways where a particular channel depth that would extend into the contaminated layer must be maintained. MNR/EMNR alternatives generally require that site use be relatively stable and uses of adjacent upland properties would be unlikely to change depositional characteristics in the affected area.

3.6 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. Future data trends are compared to baseline conditions during long-term or post-remediation monitoring. Post-remediation monitoring evaluates natural recovery or enhanced natural recovery performance, and verifies the effectiveness in attaining remedial goals. Table 3-1 summarizes the monitoring used for MNR or EMNR.

Table 3-1. Monitoring phases for MNR and EMNR







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.


  • thin-layer cap thickness
  • turbidity
  • TSS


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.





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.

Depends on RAOs, but may include COC concentrations in:

  • surface sediment
  • pore water
  • fish/shellfish
  • benthos


Bathymetry survey to demonstrate sediment deposition or sediment/thin-layer cap stability

Depends on RAOs, but may include:

  • Benthic reproductive, growth, and survival toxicity tests
  • Benthic community survey

Note: NA = Not applicable


3.6.1 Baseline Monitoring

Baseline monitoring (Section 7.1) is used in the characterization of pre-remedy conditions and processes. Baseline conditions might be established as part of the sampling conducted during the RI/FS. This information can also be complemented with historical data or additional sampling to establish a complete data set. Baseline data can be compared to past conditions to determine historical trends, and can be used to develop model predictions of future site conditions. The baseline study is used as a benchmark to compare against contaminant levels measured during post-remediation monitoring, and must be qualitatively comparable to future data sets and model predictions.

3.6.2 Construction Monitoring

Baseline and performance monitoring apply to both MNR and EMNR; however, construction monitoring only applies to an EMNR remedy. Construction monitoring typically takes place during or immediately following implementation of the remedy to determine whether design criteria have been achieved. For example, if a thin-layer cap is placed as part of an EMNR remedy, the thickness of the placed cap will be measured. These measurements may be conducted through sediment cores that are collected following the placement of the thin-layer cap, or through the use of sediment pans. Sediment pans are used prior to cap placement, and following cap placement the pans are retrieved and the thickness of the collected material is measured. In addition, any potential effects from remedy implementation, such as an increase in turbidity of the water column, may also be measured as part of construction monitoring for an EMNR remedy.

3.6.3 Post-remediation Monitoring

For MNR/EMNR remedies, post-remediation monitoring is conducted to determine rates of recovery and if contaminant levels have or will reach cleanup goals in an adequate time frame. Post-remediation monitoring should be continued until remedy stability and permanence is confirmed, or the risk reduction is certain. Monitoring data should be collected over many years and, if possible, several seasons per year. Given significant uncertainties in the data, substantial spatial and temporal data sets may be needed to establish reliable trends (USEPA 2005a). Sediment profile imaging is an ideal tool to use for post-remediation monitoring because it allows direct visualization of both physical parameters (such as grain size, sediment accretion, and redox zone) and biological recovery (bioturbation zone, benthic organisms). Once remedial goals are met, monitoring might be reduced to low-frequency, disturbance-based monitoring. If it is determined that the remedy is permanently protective of human and ecological health, the site may be closed. It will likely be necessary to include institutional controls to ensure that future activities do not adversely impact the intended recovery.

Post-remediation monitoring is used to demonstrate success of an MNR/EMNR remedy. Typical trends used to determine success are listed below. Elements of these trends are further discussed in the next section.

If post-remediation monitoring demonstrates that remedial goals will not be met in an acceptable time, an alternative remedy should be considered. In addition, other aspects of the monitoring plan may need to be adjusted if it is determined that the data are not sufficient to establish trends with sufficient certainty.

3.6.4 Post-remediation Monitoring Program Design

The Monitored Natural Recovery Technical Guidance Document (ESTCP 2009 and SPAWAR 2010) identifies specific elements of the monitoring design process for an MNR remedy. These elements can also be applied to an EMNR remedy. Monitoring elements and examples from the MNR guidance document are summarized below:

3.6.5 Monitoring Elements

The media and elements monitored as part of an MNR/EMNR monitoring plan depend on the site-specific RAOs and the physical, chemical, and biological processes that have been identified to achieve the remedial goals and cleanup levels. Monitoring elements as part of an MNR/EMNR remedy may include the following:

3.7 Case Studies for MNR and EMNR

The following table summarizes case studies that describe the use of MNR or EMNR as a primary treatment remedy. Appendix A includes more details on remedies (Table A-1) and specific contaminants (Table A-2).

Table 3-2. Case studies using MNR or EMNR

Case Study, Appendix A


Site Description


Hooker Chemical, Niagara Falls, NY




Bellingham Bay, WA

Hg,4 methylphenol, phenols

Marine Embayment


Columbia Slough, OR

Stormwater, DDT/DDE, dieldrin, dioxins, PCBs, Pb

Freshwater Slough


Commencement Bay, WA

Metals, PCBs, PAHs, VOCs, phthalates

Marine Embayment


Koppers Co. Former Barge Canal, Charleston, SC

PAHs, arsenic, dioxin, PCP, metals

Marine Embayment


Fox River & Green Bay, WI

PCBs, dioxins, furans, pesticides, metals (Hg)

Freshwater River and Embayment


Hackensack River, NJ




Lavaca Bay, TX

Hg, Methylmercury, PAHs,

Estuarine embayment


Manistique River & Harbor, MI


Tidal River Environment


Milltown Reservoir, MT


Freshwater Reservoir


Sheboygan River & Harbor, WI


River and Harbor


Shiawassee River, MI




Torch Lake Superfund Site, MI

Metals, PAHs, PCBs, phthalates, coal tar, nitrates, ammonia compounds, explosives contaminants

Freshwater Lake


Twelve Mile Creek/Lake Hartwell, SC


Freshwater Lake


Vineland Chemical, NJ




Wyckoff-Eagle Harbor, Bainbridge Island, WA

Creosote, PCP, PAHs, metals

Subtidal and Intertidal


Zidell – Willamette River, OR

PCBs, metals, PAHs, TBT



Bremerton Naval Yard OU B, WA

PCBs, Hg

Marine Embayment


Ketchikan Pulp, AK

Arsenic, metals, PCBs, ammonium compounds, 4 methylphenol, H2S

Marine Cove


Publication Date: August 2014

Permission is granted to refer to or quote from this publication with the customary acknowledgment of the source (see suggested citation and disclaimer).


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