4. In Situ Treatment

In situ sediment treatment involves applying or mixing of an amendment into sediments. Mixing may be achieved either passively, through natural biological processes such as bioturbation, or actively through mechanical means (using augers, for instance). For the purposes of this guidance, in situ treatment includes only those technologies that mix amendments into sediments. This approach differs from cappingTechnology which covers contaminated sediment with material to isolate the contaminants from the surrounding environment., in which treatment amendments are placed as a distinct layer above the sediment and the contaminants are treated as they migrate upwards through the treatment zone (see Chapter 5). In situ treatment technologies can achieve risk reduction in environmentally sensitive environments such as wetlands and submerged aquatic vegetation (SAV) habitats, where sediment removal or containment by capping might be harmful. Treatment amendments typically reduce concentrations of freely dissolvedThe concentration of the chemical that is freely dissolved in water, excluding the portion sorbed onto particulate and dissolved organic carbon (kg of chemical/L of water). Freely dissolved concentrations can be estimated with an empirical equation with knowledge of the Kpoc and Kdoc and can be measured with passive samplers, such as POM, SPMD, SPME, and PE. chemicals (termed "CCarbon (Free)free") that are available for exposure to organisms or that may be mobilized and transferred from sediment to the overlying 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.. Reducing Cfree in sediment pore waterWater located in the interstitial compartment (between solid-phase particles) of bulk sediment. through sorptionThe process in which one substance takes up or holds another; adsorption or absorption. (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 lowers exposure and risk.

4.1 In Situ Treatment Background Information

In situ treatment, when viable, has emerged as an improvement over the remedial performance of MNR/EMNR and removal technologies. Thus, many of the site factors evaluated when selecting these technologies are also relevant to in situ treatment. Treatment amendments may be preferred in areas with higher contaminant concentrations, where MNR/EMNR cannot achieve risk goals in an acceptable time or where immediate risk reduction is needed. In situ treatment is also a means of managing exposures associated with residuals that remain following the removal of sediments.

While various amendments can target different types of contaminants in sediment, AC is one of the most widely used for in situ immobilization (Ghosh et al. 2011). Bench-scale data suggests that pore-water concentrations and bioavailabilityThe relationship between external (or applied) dose and internal (or resulting) dose of the chemical(s) being considered for an effect (NRC 2003). of hydrophobic contaminants can be reduced between 70% and 99% at AC doses similar to the native organic carbon content of sediment. Based on these results, over 25 field-scale demonstration projects spanning a range of environmental conditions are now underway or nearing completion in the United States and Norway (Patmont et al. 2013). These projects have demonstrated the efficacy of full-scale in situ sediment immobilization treatment technologies to reduce the bioavailability and mobility of a range of organic and metal contaminants, including PCBs, PAHs, dimethyl dioxane, dioxins/furans, chlorinated benzenes, tributyltin (TBT), and mercury. A wide range of AC placement options has been demonstrated at the field scale, including:

In situ immobilization treatment can be a permanent sediment cleanup remedy that rapidly and sustainably addresses key exposures (such as 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. in fish) and may become more effective over time, since sorption does not reach equilibrium immediately and complete mixing of amendments with the sediment may take time. In situ treatment can be less energy intensive (less material used and transported), less disruptive to the environment (certain in situ technologies do not damage the habitat, whereas capping and dredging always do), and less expensive than conventional remedial technologies such as dredging and capping. This technology can also significantly reduce ecosystem exposure by binding contaminants to organic or inorganic sediment matrices.

Through 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., in situ treatment with AC reduces biota and human exposures to many contaminants. AC can adsorb PCBs, which are one of the most common contaminant groups driving risk at sediment sites. AC can also be mixed with other amendments such as organophilic clayClay minerals whose surfaces have been ion exchanged with a chemical to make them oil-sorbent. Bentonite and hectorite (plate-like clays) and attapulgite and sepiolite (rod-shaped clays) are treated with oil-wetting agents during manufacturing. Quaternary fatty-acid amine is applied to the clay. Amine may be applied to dry clay during grinding or it can be applied to clay dispersed in water., zeolitesMicroporous, aluminosilicate minerals commonly used as commercial adsorbents., bauxite, and iron oxide/hydroxide to bind additional contaminants in the sediments. Other amendments, such as apatiteName given to a group of phosphate minerals, usually referring to hydroxylapatite distributed widely in igneous, metamorphic, and sedimentary rocks, often in the form of cryptocrystalline fragments. Hydroxylapatite is used in chromatographic techniques to purify proteins and other chemicals., nutrients or ozone (for biostimulationModification of the environment to stimulate existing bacteria capable of bioremediation.), KB-1 (for bioaugmentationUse of (microbes) to clean up oil spills or remove other pollutants from soil, water, or wastewater.), and zero valent iron (ZVI), are specifically designed to degrade chemicals or transform them into less toxic forms (O'Day and Vlassopoulos 2010).

Theoretically, once molecules of chemicals such as PCBs are bound to a sorbent such as AC, the exposure potential of that chemical is negligible. Unlike organic carbon, the sorbent AC is not readily broken down in the environment and the binding remains strong, based on thermodynamic principles, resulting in a high confidence in the short-term and long-term fate of the bound chemicals. The chemicals are expected to remain bound whether the sorbent and bound chemicals remain in the sediment bed or are resuspended and transported away from the area. Studies may be needed on a site-specific basis, however, to confirm that this theoretical assumption holds true in the field. Currently, few long-term studies on in situ effectiveness are available because the technology is still relatively new.

Other amendments such as cement and cement with lime or fly ash can physically solidify or stabilize contaminants (see Table 4-1). This in situ solidificationTo make solid, compact, or hard, to make strong or united, or to become solid or united. approach can be applied to higher concentrations of contaminants, but is considered a more active and invasive form of treatment. Treatment amendments that immobilize or degrade contaminants within the sediments address concerns that may be raised about leaving contaminants in place.

With a growing emphasis on sustainability, in situ treatment remedies offer an opportunity to realize significant environmental benefits while avoiding the environmental damage associated with more invasive remedial technologies. Three key benefits of sustainability associated with in situ treatment include:

  1. Environmental. In situ treatment can accomplish destruction of contaminants in some cases, which is typically preferable to nontreatment alternatives. Alternatively, in situ treatment can achieve near-immediate reduction of the bioavailable fraction of contaminants (thus reducing exposure to contaminants) with minimal effects on habitat, leading to a potentially shorter ecological recovery time as compared to other alternatives. In situ treatment often requires less energy and material usage and results in lower emissions (carbon and other) than other active remedies.
  2. Economic. In situ treatment is typically a cost-effective way to rapidly return the system to economic and ecological productivity (such as restoring tourism and fisheries). The costs associated with implementation are likely to be lower than capping or dredging.
  3. Social. In situ treatment results in reduced risk to workers and fewer effects on the community (compared to capping and dredging). The potential also exists for faster restoration of recreational and aesthetic resources than occurs with MNR. In situ treatment also reduces adverse effects on the community associated with long-term remedial projects, such as noise, truck traffic, and fumes.

Finally, while in situ treatment is commonly used for treating contaminated soil and groundwater (USEPA 2006a), the use of in situ treatment for sediments is still an emerging technology. The success and promise of this technology, particularly in situ immobilization treatment using AC, has been demonstrated primarily through a number of bench-scale treatability studies and field-scale pilot projects (Patmont et al. 2013). A limited number of full-scale implementations of in situ treatment have been applied at relatively small sites, but larger-scale applications are being planned or are currently underway in the U.S. and Norway. In situ demonstration projects are underway in several USEPA regions and in situ projects are gaining interest and funding from USEPA, state agencies, DOD, the Superfund Research Program, and private industry.

The following sections provide information necessary to evaluate in situ treatment as a remedial technology on a site-specific basis. Some of the information included in these sections is considered theoretical because some types of treatment have not yet been applied to sediments in the field. Information available from real-world applications is included where it is available. For a summary of the some of the most promising in situ treatment technologies, see Use of Amendments for In Situ Remediation at Superfund Sediment Sites (USEPA 2013a), which provides information on the state of the practice for this technology and presents three case studies describing sites where amendments have been used.

4.2 In Situ Treatment Objectives and Approaches

The design of any in situ treatment application must address two key issues: treatment amendments (materials) and delivery system (method). The following section summarizes general types of treatment amendments and delivery methods and provides information on the development status of each method.

4.2.1 Materials for Treatment Amendments

In situ treatment approaches can be grouped into the categories listed below; see Table 4-1 and Table 4-2 for the development stage of each of these technologies (bench, pilot, or full scale):

Biological

Chemical

Physical

Combination

4.2.2 Delivery of Amendments

In order to be successful, in situ sediment treatment must achieve adequate contact between treatment amendments and the contaminants. Factors involved in achieving this contact include:

  1. Sediment stability. Sediment stability information helps site managers to judge whether an in situ remedy will be effective and what additional design is needed to secure the treatment amendments in place. For in situ treatment, low-energy environments are generally more suitable than high-energy environments. Examples of suitable environments include wetlands, vernal pools, ponds, embayments, and harbors that are depositional with low likelihood of highly erosive events. The stability of an in situ treatment can be increased for high-energy environments by modifying the physical characteristics of the amendment and by incorporating the amendment into an EMNR technology designed to withstand higher shear stress.
  2. Amendment placement location. Amendments can be either mechanically dropped into place at the surface of the water column or sprayed onto the surface. Amendments then settle through the water column to the sediment surface. Alternatively, some delivery systems use a boat or barge to drag a machine that injects amendments directly into the sediment. Key delivery issues include achieving the desired treatment dose over the required area while minimizing losses to adjacent areas outside of the treatment zone. Water depth, waves, and currents are key hydrodynamic characteristics that must be accounted for in order to achieve the desired placement (for instance, by designing amendments with adequate density to settle through the water column). To some extent, these same factors must be considered when implementing an EMNR or a capping technology. Experience and expertise with those technologies can be applied to in situ treatment technologies.
  3. Mixing method. Mixing of the amendment and sediment can be accomplished actively and mechanically (for example, by using augers) or passively by relying on natural biological process (for example, bioturbation by benthic organisms) and physical processes (such as gravity).

A summary of some in situ treatment technologies (amendments and delivery systems) that have been implemented in the field at pilot or full scale is provided in Table 4-1. These technologies are relatively mature and are likely to be effective. Table 4-2 provides information about treatment techniques that have been tested only in laboratory studies to date. These techniques may require more in-depth study (such as additional bench-scale tests or field pilot tests) before selecting them as a remedy.

Table 4-1. Use of in situ technologies for sediments (field demonstrations at full or pilot-scale)

In situ Technology and References

Treatment

Technical Basis

Contaminant Applicability

Application

Development Stage

Comments

Amendments

Biostimulation (oxidation)

(Golder Associates 2003)

Biological - Biostimulation

Aerobic degradation of organic contaminants through introduction of oxidants such as calcium nitrate or sodium nitrate

PAHs, BTEX compounds and TPH

Marine and Freshwater

Several pilot scale and full scale projects implemented

 

AC Amendments

(Ghosh, Zimmerman, and Luthy 2003; Cho et al. 2009; Beckingham and Ghosh 2011; Ghosh et al. 2011; Patmont 2013)

 

Physical – Sorption

Deployment of various carbon options including AC, coke, black carbon/charcoal that strongly sorb organics and inorganics

Hydrophobic organics and metals: PCBs, PAHs, dioxins, pesticides, mercury

Marine and Freshwater

Laboratory studies and field pilots; several full-scale applications currently underway

 

Organophilic clay

(Knox Et al 2011; Arias-Thode and Yolanda 2010)

Physical - Sorption

Sorbing amendment for organic compounds and organically complexed metals

Sorption of organics and organically complexed metals (such as methylmercury)

Marine and Freshwater

Laboratory studies;  has been incorporated into sediment caps full scale; may also be used as an amendment in situ

 

Apatite (calcium phosphate mineral)

(Knox et al. 2008 ; Williams et al. 2011; Scheckel et al. 2011)

Chemical Reaction - Transformation

Apatite reaction with metals to form phosphate minerals that sequester the divalent metals and reduce toxicity to aquatic organisms by reducing bioavailability

Cd, Co, Hg, Ni, Pb, Zn, and U

Marine and Freshwater

Pilot test in Chopawamsic Creek, VA, sediments. multiple successful laboratory  studies

Short reaction time (on the order of weeks);

can enhance desorption of As, Se, and Th; reactions sensitive to redox conditions.

Delivery systems

Limnofix In situ Sediment Treatment Technology

(Golder Associates 2003)

Mechanically mixed/injected

Amendments introduced through a series of tines and nozzles on an injection boom

Generally used to apply oxidative amendments (calcium nitrate) for biodegradation of PAHs, BTEX, TPH or to mitigate acute sulfide toxicity

Freshwater and Marine

Full scale applications and Field Pilots

Has been used to treat sediment to a depth of 0.5 meters (into the sediment) with water depths of 3 to 24 meters.

SediMite (Menzie-Cura and UMBC)

(Menzie, personal communication 2011; Ghosh et al. 2009)

Surface placement/biologically mixed

Pelletized AC with a binding amendment tailored to contaminant of concern. Binding adds weight for emplacement on sediment bed. Benthic organisms and natural processes mix  SediMite into sediments where binding eventually breaks down increasing surface area of AC

PCBs, methylmercury and other hydrophobic chemicals

Freshwater and Marine particularly in areas of sensitive environments or in hard to reach areas such as around pier structures.

Small full scale, Field Pilot Scale, and Laboratory Studies

Initial thickness of application is generally less than 1 cm.

AquaGate

(AquaBlok patented)

(ESTCP program, Aberdeen Proving Ground, Canal Creek, Bremerton Naval Shipyard)

Low impact AC, organoclay and other mineral delivery system

Composite particle of powder AC or other coating material tailored to a contaminant of concern. Coating materials are delivered to sediments by a high density core. Density of particle provides for mixing with sediments (mixing occurs due to gravity).

Used to date on PCBs, range of PAH, pesticides, and a range of metals.

Freshwater and Marine

Laboratory Studies and Field Pilot Scale. Full Scale applications of materials as component of active cap design.

Allows for placement of materials at greater depths; proven full-scale placement

 

Table 4-2. Use of in situ technologies (laboratory demonstrations only)

In situ Technology and References

Type

Technical Basis

Contaminant Applicability

Application

Stage

Comments

Amendments

Ozonation (biostimulation)

(Hong 2008)

Biological - Biostimulation

Chemical - Degradation

Introduction of ozone to sediments may degrade organic compounds through first abiotic and then aerobic degradation mechanisms.

PCBs and PAHs

Marine and Freshwater

Laboratory Studies

Pressure-assisted introduction of ozone appears to be more effective than conventional ozonation.

Zero Valent Iron (ZVI)

(Hadnagy and Gardener, personal communication, 2011)

Chemical - Transformation

Reductive dehalogenation using zero valent iron usually with a bimetal catalyst. Mg or Zn instead of Fe has also been shown to be effective.

Abiotic destruction of halogenated aromatic organics (such as PCBs, PCDD/F and chlorinated pesticides)

Marine and Freshwater

Laboratory Studies

Achieves destruction of contaminants.

Incomplete reactions could potentially produce compounds that are more toxic than parent compounds.

Zeolites

(Knox et al. 2008)

Physical - Sorption

Hydrated aluminosilicate minerals with a large open framework that forms large “cages” in the mineral structure. Cages can trap cations and even molecules.

Pb, Cu, Cd, Zn, Cr, Co, Ni

Freshwater

Laboratory Studies

 

Preferential exchange with Na ions over metals occurs.

Bauxite/ Bauxite Residues/“Red Mud”

(Lombi et al. 2002; Gray et al. 2006; Peng et al. 2009)

Physical - Sorption

Bauxite residue (red-mud) contains both Al oxides and Fe oxides. Experiments suggest chemisorption of heavy metals to Fe oxides in the red-mud.

Heavy metals and metalloids Cd, Cu, Pb,Ni, Zn

 

Laboratory Studies and Soil Pilot Study

 

Iron Oxides/Hydroxides

(Lombi et al. 2002)

Physical - Sorption

Fe minerals such as limonite and goethite adsorb metals reducing bioavailability

Heavy metals

Cd, Cu, Zn, and As

Marine and Freshwater

Laboratory Studies

 

Cement with Lime or Fly Ash

(Gray et al. 2006; Peng et al. 2009)

Physical-  Solidification/Stabilization

Physical solidification of the media and 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. of metal carbonates or increases 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. to allow oxide formation onto which metals can sorb (stabilization)

Heavy metals Cd, Cu, Ni, Pb and Zn

 

Laboratory Studies and soil pilot study

 

 

4.3 Design Considerations

If appropriate for the site conditions, in situ treatment offers a relatively low-impact remedial option that provides a high level of effectiveness. The following section describes the advantages and disadvantages of in situ treatment, as well as the design factors that should be considered for this approach.

4.3.1 Design Advantages

One primary advantage of in situ treatment is that it can accelerate sediment cleanup using low-impact methods, either on its own or when paired with MNR/EMNR. In situ treatment may, in some situations, be preferable to EMNR, capping, and removal because it may be able to achieve similar or better results with less impact. Some in situ approaches can degrade or destroy contaminants; for these treatments, the remedy evaluation should quantify the amount of contaminants that are likely to be removed from the system (similar to estimates prepared for removal by dredging).

Because in situ treatments add an otherwise foreign element to sediments (such as AC), the acceptance of this approach depends on demonstrating that the benefits of adding amendments clearly outweigh any potential negative effects. Based on evidence to date, AC shows little or no long-term negative effect on sediments, thus its benefits usually outweigh possible harm. Other low-impact in situ treatment technologies such as SediMite, Limnofix, and AquaGate also deliver treatment agents without disturbing the physical characteristics of the sediments and water column (as occurs in dredging and some capping alternatives). Because physical sediment characteristics are the predominant factors influencing the community structure of benthic invertebrates, leaving these characteristics generally unchanged is a distinct advantage over remedies that add materials which change the physical characteristics of the sediment (such as some EMNR and capping technologies). In addition, low-impact in situ technologies allow for management of sediment adjacent to retaining and support structures, which are often aged and require structural analysis and support prior to dredging or removal activities. Substantial costs, which often do not directly benefit the environment, can be associated with infrastructure management on dredging projects, thus management in place may be preferred.

In situ treatment also offers the potential to provide better long-term protectiveness from recontamination than dredging or capping, because excess treatment capacity can be built into the initial sediment treatment. The long-term effectiveness of any treatment may be reduced if treatment capacity is overwhelmed by contaminants in the sediments, by new contaminants freshly mobilized from untreated sources, or by other components of the sediment system. If treatment efficacy is compromised or overwhelmed, repeat treatment or application of another remedial technology may become necessary. With in situ treatment, additional amendments can be added in the first application at a sufficient dosage to provide excess treatment capacity. This excess capacity protects the sediments from recontamination that may occur from uncontrolled sources.

In situ treatment technologies that destroy contaminants also achieve mass reduction, which is an advantage over other available sediment remediationThe act or process of abating, cleaning up, containing, or removing a substance (usually hazardous or infectious) from an environment. technologies. In situ immobilization treatments that use sorbents (such as AC) act on contaminants in place, but do not degrade them and do not, on their own, achieve mass reduction. These remedies are similar to MNR/EMNR in that leaving contaminants in place is often viewed as a disadvantage relative to removal technologies. Some evidence, however, suggests that natural biodegradation can be enhanced by sorbing contaminants to AC because, although AC does not directly degrade contaminants, the carbon substrate provides a surface for microbial growth. The low biodegradation rates of recalcitrant compounds such as PCBs may result in long predicted time frames for complete degradation.

One-way Degradation Processes

Most degradation processes are one-way processes. Once a treatment agent degrades a chemical molecule, the molecule cannot be re-created and is no longer available for exposure. Treatment thus reduces the overall inventory of chemicals present. Future resuspensionA renewed suspension of insoluble particles after they have been precipitated. and transport of contaminants from the treated sediment is not a concern because of the high degree of confidence in the short-term and long-term fate of chemicals degraded through these one-way processes.

In situ treatment can be more cost effective and less environmentally damaging than dredging or capping for areas that have the requisite site and contaminant characteristics and where the concerns regarding deeper contamination (see Section 4.3.2) can be addressed. In situ treatment is especially favorable over dredging or capping for sensitive environments and where disturbances must be minimized. In these situations, in situ remedies also reduce exposures more quickly than MNR alone.

In situ treatment approaches may be selected to reduce toxicity, mobility, or volume of contaminants for select areas and may be favored over other approaches for specific remedial zones. For example, access, water depth, or habitat-related issues may preclude some treatment alternatives. Dredging under bridges, piers, or against bulkhead walls may leave areas where significant residual contamination may exist after, or as a result of, remediation activity. In situ treatment may provide a means to enhance the overall remediation effort for these residual areas.

4.3.2 Design Limitations

One challenge to gaining acceptance for in situ treatment is the lack of full-scale, completed projects using this technology. While the results from numerous pilot studies are encouraging, remedies that have been used in full-scale applications are more readily accepted. This situation arises for many new technologies and should not preclude the use of in situ treatment, especially given the many potential advantages that this approach offers.

Another design limitation is that some in situ technologies treat only surficial sediments, leaving deeper contamination untreated. While this approach is not a limitation if sediments are stable, it is possible for contamination remaining in deeper sediments to become exposed following a storm or as a result of contaminant migration to the surface. This issue, which also arises with MNR/EMNR, can be addressed during the design phase with estimates of long-term performance and design adjustments as needed.

Uncertainty about future site activities, such as construction projects or navigational dredging, can also lead to concerns about leaving deeper contamination in place. These concerns can be addressed through institutional controlsNon-engineered instruments, such as administrative and legal controls, that help minimize the potential for human exposure to contamination and/or protect the integrity of the remedy. and through memoranda of understanding regarding actions that must be taken if conditions change. Concerns about long-term performance can lead to requirements for intensive long-term monitoring programs, which can be costly and may offset some of the savings that would otherwise be realized with in situ treatment. Site owners or other potentially responsible parties may also be concerned about the future liability associated with buried, untreated sediment.

4.3.3 Additional Design Considerations

Several in situ pilot studies and full-scale applications of soil and sediment remediation have been conducted in the field (see Table 4-1). These studies have evaluated feasibility in a wide range of environmental conditions including marine mudflats, freshwater rivers, estuarine marshes, tidally influenced creeks, and open ocean harbors (Patmont et al. 2013). The following sections provide a summary of key design factors developed in these applications.

4.3.3.1 Selection of Appropriate Criteria for Success.

An in situ treatment option that leaves contaminants behind requires monitoring to confirm the effectiveness of the remedy. The monitoring methods must evaluate the effectiveness of the remedy and should discriminate between exposure from the treated site versus exposure from untreated, off-site sources. This differentiation is challenging at sites where uncertainty remains regarding the extent and contribution from different sources of exposure. For example, at many sites high levels of bioaccumulative chemicals (such as PCBs and mercury) in fish are the primary risk drivers for the remedy. Tracking effectiveness based on pollutants in animals at the top of the food chain, however, may be difficult if ongoing sources of pollution contribute to the exposure. In this case, selecting a success metric that narrowly targets specific uptake pathways to fish from the treated sediments may be more appropriate. Additional examples of effective criteria include measurement of pollutants in native benthic animals, pore water of treated sediments, and flux from sediment into the water column.

4.3.3.2 Accumulation of New Sediment Deposits

Sites contaminated with legacy chemicals are typically in historically depositional environments, thus deposition of new sediments is expected to continue after the remedy is implemented. Planning for post-remedial monitoring must consider these new sediment deposits. For example, if an amendment is placed on surface sediments and is tracked over time, a gradual decline in the levels of the amendment on the surface may be observed. The interpretation of this observation, however, must account for new sediment deposition at the site, especially from major weather events, which can potentially deposit several inches of new sediment in a short time. Accurate bathymetryThe measurement of or the information from water depth at various places in a body of water. measurements are useful in keeping track of sediment deposition. As with other technologies, if the newly deposited sediments are contaminated (for example, with PAHs from urban runoff), the effectiveness of the remedy may appear to decrease with time.

4.3.3.3 Site Heterogeneity 

Heterogeneity in site conditions and contaminant levels can sometimes confound monitoring results. Adequate density of sampling should be performed to capture site heterogeneity and inform remedy design. The sampling plan for effectiveness monitoring should have sufficient statistical power to adequately track changes over time.

4.3.3.4 Application Heterogeneity

Application of in situ amendments is typically at a low dosing rate and results in actual surface coverage that is often less than 1 inch. At this application rate, a uniform surface coverage in the field can be difficult to achieve. AC placement at uniform levels has now been demonstrated using a wide range of conventional equipment and delivery systems. Uniform AC placement has also been demonstrated in relatively deep and moving water (Patmont et al. 2013). Other innovative application methods that have not been demonstrated at other sites should be tested in advance to show that uniform surface coverage can be achieved. Potential approaches for achieving uniform coverage include:

4.4 Data Needs for In Situ Treatment Design

Data collected during the development of a CSM, specifically in the RI process, are fundamental in assessing the applicability and potential performance of any sediment remediation technology. Four general categories of data are typical of CSM investigations (see Section 2.4.1): physical site characteristics, sediment characteristics, the contaminant characteristics, and land and waterway use.

4.4.1 Physical Site Characteristics

Physical site characteristics define the physical ability of the bed to support a uniform amendment application. The bed must have uniformity and stability sufficient to result in a uniform distribution and adequate mixing of the amendment. The amended sediment bed must also remain in place for an adequate time to complete and maintain the treatment. The following sections describe the key physical characteristics to consider when evaluating the potential performance of in situ treatment.

4.4.1.1 Advective Groundwater Transport

Data regarding contaminant fluxes due to advective groundwater transport are key to in situ treatment design. Advection into and through the sediment can transport contaminants into the treatment zone, either from contaminated groundwater entering the sediment system or from initially uncontaminated groundwater carrying deeper sediment contamination into shallower zones. This mechanism creates a potential ongoing source to the treatment zone and may reduce treatment efficacy over time. Note that in tidal areas, tidal oscillations can cause advective fluxes that are orders of magnitude higher at the sediment-water interface relative to the average regional flux.

Contaminant flux via groundwater is chemical specific. Site investigations conducted prior to selecting remedies typically provide information necessary for assessing additional contaminant loads expected from advective groundwater transport. When the contaminant flux can lead to unacceptable exposures if the additional contaminants are left untreated, sufficient amounts of amendments must be added to treat both existing contamination and the predicted contaminants from groundwater advective flow. Contaminant treatment capacity must exceed the supply of contaminants from groundwater.

4.4.1.2 Sediment and Pore-Water Geochemistry

Sediment geochemistry1) Science that deals with the chemical composition of and chemical changes in the solid matter of the earth or a celestial body (as the moon); 2) The related chemical and geological properties of a substance. can be an important consideration for amendments that are designed to degrade contaminants. For example, reductive dechlorination requires anaerobic (low oxygen), highly reducing conditions to be present, while degradation of petroleum compounds typically occurs under aerobic (high oxygen), oxidizing conditions. Certain treatments are sensitive to other aspects of sediment geochemistry, including the sediment organic carbon content, sulfide concentrations, and pH.

Amendments such as AC adsorb persistent hydrophobic chemicals and can be used under a variety of geochemical conditions. The dosage needed, however, may be influenced by specific geochemical conditions that dictate the availability of contaminants. These amendments typically adsorb several orders of magnitude more contamination than natural organic carbon. A typical amendment dosage is approximately equal to Foc in existing sediment, which will decrease contaminant availability by several orders of magnitude. The sorbent must be applied in an amount sufficient to out-compete natural carbon in the sediments.

Site-specific geochemical conditions must also be well defined in order to select an in situ treatment technology that relies on geochemical reactions or biodegradation. In addition, these treatments may change geochemical conditions, which can affect both target and nontarget contaminants. Increases in biological activity, for example, can reduce pH and thus mobilize certain metals. Similarly, metals are often bound in the sediment by sulfides, but if the treatment method selected reduces sulfide concentrations, then the metals can become more bioavailable and potentially increase the direct toxicity of the sediment. Furthermore, many contaminants can be strongly bound to organic and inorganic carbon in sediment. If the in situ treatment consumes carbon (such as addition of amendments that cause chemical oxidation of organic compounds), then certain contaminants may become more bioavailable. Sediment geochemistry also influences the native state of binding and availability of target chemicals with which in situ treatment agents (especially sorptive amendments) compete. For example, sediments with strongly sorbing native black carbon may need a higher dose of AC amendment, compared to sediments without native black carbon, to achieve the same degree of effectiveness.

4.4.1.3 Hydrodynamic Characteristics

Hydrodynamic characteristics, such as water depth and flow, influence the design and implementation of in situ treatment. In more energetic areas, in situ treatment may be used to augment EMNR, but a mechanical placement or injection method might be needed (rather than a gravity settling method) to deploy the treatment amendment. Binder and weighting agent amendments can also be added to improve gravity settling of AC through the water column (Patmont et al. 2013). Treatment performance is influenced by the energy level and bottom shear stress, and in general, less energy and bottom shear stress is preferred for effective in situ treatment.

Some sediment environments in embayments and tributaries can experience flash flooding following storms, which can mobilize treatment materials. In situ treatment design must consider not only average conditions, but also these periodic erosional events. For in situ treatment, the water depth affects whether equipment can be brought to the treatment area over water (if a land-based application is not selected). Water depth affects physical delivery when the water body has a flow component. For example, when treatment materials are sprayed onto the surface of the water and allowed to settle to the bottom, the materials move with the flow of the water body. If the depth to sediment is too great, treatment amendments may be dispersed beyond the targeted sediment area before they can settle (Cornelissen et al. 2012). These conditions may require delivery using subsurface delivery systems or binder and weighting agents.

4.4.1.4 Sediment Depositional Rate

Depositional rate data can indicate potential long-term recovery conditions. Ideally, in situ treatment of contaminants in sediment is an irreversible process capable of reducing contaminant concentrations to protective levels. Once this treatment is complete, the deposition of additional clean sediment serves as an additional element of recovery, but is not necessary for achieving protection goals. A positive annual net deposition rateThe amount of material deposited per unit time or volume flow. improves the long-term effectiveness of in situ treatment, but is not a prerequisite for the use of in situ treatment. Note that sediment stability (Section 4.4.1.5) and erosion potential (Section 4.4.1.10) can also affect depositional rates

When in situ treatment is used for mercury contamination, deposition can eventually remove source sedimentary mercury from the zone of potential methylation. The deposited sediment layer, along with the sediment's capacity to adsorb methylmercury and ionic mercury, provides long-term remediation. If the treatment is focused on only the bioactive zone, and contaminated sediment is left untreated below, then the potential for future erosion must be evaluated to determine whether deposition can sufficiently protect underlying materials.

4.4.1.5 Sediment Stability

Sediment stability data can indicate whether the sediments are stable enough to remain in place until the treatment is complete. The efficacy of in situ treatment remedies increases with increasing sediment stability, because a minimum contact time is usually needed to achieve treatment. In situ treatments work best in low-energy environments, where the potential for erosion is minimal. While in situ treatment can work in less stable sediments, additional design features may be needed to secure the treatment in place long enough to remediate the target contaminants.

Water velocity determines the shear stresses that affect sediment stability and scour potential. Data regarding the frequency and magnitude of potential high-velocity flooding can help to predict the associated hydrodynamic effects on in situ treatment. For example, high shear forces may prevent the in situ treatment amendment from remaining in contact with the contaminated sediments. Flooding may diminish treatment effectiveness and cause treated sediments to be resuspended and transported away from the treatment area. Treated sediments that are resuspended may be deposited in the floodplain or downstream. While no specific data suggests that these sediments could pose a risk if deposited in the floodplain, site managers should be aware of this potential issue. For treatment processes that achieve complete destruction of contaminants (or irreversible transformation to a nontoxic form), there is little concern for future remobilization if treatment is complete at the time sediments are eroded. On the other hand, when contaminants are only sequestered, it is preferable for sediments to remain stable over time.

Future movement of treated contaminants does not necessarily lead to unacceptable risks. For example, sequestration using AC is believed to be irreversible under normal conditions, so there is little concern over the sediment stability for this treatment. On the other hand, if future erosion leads to exposure of deeper contaminated sediments that have not been treated, then additional treatment may be required.

The treatment amendments themselves can potentially affect the sediment stability. For example, mechanical mixing while adding amendments may cause sediments to be less cohesive, and therefore more subject to erosion in the short term. Conversely, in situ solidification and stabilization of sediment can increase sediment stability, in which case the stability prior to treatment is relatively unimportant (see also slope stability, Section 4.4.1.8, and resuspension potential, Section 4.4.2.4).

4.4.1.6 In-water and Shoreline Infrastructure

Information describing current or historical in-water and shoreline infrastructure can be obtained from local agencies, as well as developed from site-specific data collected while visually inspecting the site. In situ treatment can be an effective alternative in some cases for contaminated sediments located adjacent to or beneath structures such as piers. Because in situ treatment does not remove sediment, this approach does not compromise support for structures relying on sediment for their stability. By comparison, accessing sediment beneath piers, for example, can be time consuming and costly if dredging or directly injecting or mechanically mixing sediment amendments (such as auger mixing for stabilization/solidification). Additionally, most in situ treatments do not change the existing bathymetry, and thus lessen influences on currents and waves. Although in situ treatment may require less access than other technologies (such as removal) some direct access is needed (either for placement of amendments or for monitoring/sampling activities). Implementability of in situ treatment decreases as the amount of in-water and shoreline infrastructure increases, unless the infrastructure does not hamper placement of amendments on or into the sediment. 

Infrastructure data can also help to guide in situ treatment applications that spray amendments onto the surface of the water and use gravitational settling to the bottom to place amendments. These treatments can sometimes reach sediments beneath, and immediately adjacent to, in-water structures where dredging and capping are difficult. Accurate delivery and placement methods are improving, and in situ treatment is becoming applicable to a wider range of environments where infrastructure is present.

4.4.1.7 Hard Bottom and Debris

The presence of a hard bottom or debris in sediments is typically not a constraint for in situ remedies that target contaminants in surficial sediments. Usually, the treatment amendment is placed on the sediment surface and mixing occurs naturally; a hard bottom or debris has little effect on this process. Some applications, however, rely on shallow mixing of sediment or injection of amendments directly into the subsurface and debris or a hard bottom can interfere with these processes. When bedrock, cobble, or other forms of hard bottom exist beneath the sediment to be treated, evaluate the amount of mixing required in order to determine whether objectives can be achieved.

4.4.1.8 Slope Stability

Slope stability data is needed because placing treatment materials on slopes may result in instability (see Section 3.4.1.8). The slope stability factor of safety should be greater than 1.5. 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). For in situ treatments, these loads are relatively light compared to thicker containment caps. Placement of amendments on the surface of the sediment, for passive incorporation/mixing into the sediment, may not be effective if amendments do not remain in place due to poor slope stability. AC has been effectively placed at slopes as steep as 50% or 2H:1V (Patmont et al. 2013). The sediment must have sufficient strength (bearing capacity) to support the weight of amendment material without lateral displacement (mud waves) of the sediment under the cover (see Section 5.4.1.6).

4.4.1.9 Water Depth and Bathymetry

The water depth and specific bottom bathymetry data are necessary for the selection and design of in situ sediment treatment. Most in situ treatment studies in the United States have been conducted in shallow waters (less than 3 m) and wetlands, but trials in Norway have applied in situ treatment agents to sediments under water depths of up to 100 m in contaminated fjords (Cornelissen et al. 2012). If conventional mechanical equipment is used to deliver treatment amendments and to mix the sediment and amendments together, then the length of the equipment and desired thickness of sediment to be treated dictate the maximum water depth at which sediment treatment can be achieved. If treatment amendments are being applied at the water surface and are allowed to settle by gravity to the bottom of the water column (for example, using Aquagate and SediMite), then the total water depth and the water velocity determine how far downstream the amendments travel before settling onto the sediment. If the water depth is so great that amendments must be placed at a significant distance from the area where treatment is required, then the reliability of treatment may be lower. Bathymetry data are also needed because irregular sediment surfaces may cause challenges for mechanical delivery systems.

Modeling alone may not be sufficient to predict amendment transport well enough to design the delivery system to target the treatment area. Therefore, the water depth data is required not only in the exact location where treatment is required, but also along the river channel upstream. In all cases the delivery mechanism must be able to deliver amendments to the targeted sediment areas. Accurate delivery and placement methods continue to improve, thus expanding potential application of in situ treatment to a wider range of aquatic environments.

4.4.1.10 Erosional Potential

Erosion data is needed because erosional potential is directly related to sediment stability. In general, surface-applied or thin-layer in situ treatment amendments (passively mixed) are not well suited to high energy environments. It is difficult to place amendments in areas with large erosion or scour potential because erosion may expose deeper contaminated sediments or may cause an amendment to erode before it can be naturally mixed in to the sediment.

Conversely, in situ solidification/stabilization of sediment, which is achieved through active injection of amendments and mechanical mixing, reduces erosional potential. In this case, the erosion potential of existing sediments is relatively unimportant.

4.4.2 Sediment Characteristics

Data regarding characteristics of the sediment bed help to define the geotechnical properties necessary to support the application and mixing of an in situ treatment amendment. While the size, sorting, and orientation of the physical grains provide sediment stability, the benthic community contributes mixing of the contaminants as well as natural mixing of amendments. During the application of the amendment, the ability of the sediment bed to support the amendment prior to mixing can result in temporary release of contaminants due to surface pressure or can allow a slight penetration of amendment into the sediment bed due to density differences.

4.4.2.1 Particle (Grain) Size Distribution

Data regarding sediment particle size and distribution is necessary because in situ treatment tends to be most appropriate for fine-grained depositional sediments. Particle size distribution also affects sediment properties such as the depth of the BAZ, stratification (layers of coarse and fine sediment), and adsorption. Methods for measuring particle size include the sieve with hydrometer method (ASTM D422) and sand-silt-clay content by pipette (PSEP) method. 

Particle size distribution in sediments affects various aspects of in situ treatment design. Significant differences in particle size or densities between amendment materials and sediment can cause problems with mixing, which can reduce the effectiveness of in situ treatment. Some studies have shown a direct relationship between particle size and reaction rates when treating sediment, although the mechanism is not well understood. Additionally, the percentage of silt and clay particles present in bottom sediments determines the composition of the biological community, the adsorption of contaminants to sediment particles, and the exposure of organisms to contaminants. The biological community and exposure are relevant for in situ treatment because the sediments will not be removed or covered, so the biological community is expected to remain in direct contact with the sediment after treatment. The adsorption of contaminants (and potentially treatment amendments as well) can be influenced by the specific grain size distribution. Clays, for example, have a permanent negative surface charge and often provide a sorption surface (and mechanism) for metal anions (positively charged ions).

4.4.2.2 Geotechnical Parameters

The efficacy of in situ treatment increases with increasing sediment stability, cohesiveness, shear strength, and bulk density. Data regarding these geotechnical test parameters help to define sediment stability and the fate of sediment and amendments that are added. These factors also determine the potential for resuspension and release of sediments and contaminants. If sediments are stable with high shear strength and cohesiveness, then amendments that are added are likely to stay in place long enough to be effective, especially for many promising applications that do not involve mechanical mixing of amendments into the sediment. On the other hand, the method of amendment addition can cause changes in the stability conditions. Mechanical mixing with sediment, for example, can reduce cohesiveness and bulk density in a way that reduces shear strength and stability (with the possible exception of stabilization/solidification, which would actually increase shear strength and stability after treatment).

The presence of a nepheloid layerA layer of water, above the bed or floor, that contains significant amounts of suspended sediments. sediment zone makes mechanical treatment, capping, or removal processes difficult because any disturbance of the zone can potentially cause the sediment to simply move rather than be treated or removed. Even placement of sediment capping materials can cause the nepheloid materials to be pushed aside into neighboring areas. Relatively light (low density) in situ treatment amendments can be applied from the surface and, on passing through the nepheloid layer, could mix with suspended sediments to achieve some treatment. Nepheloid layer data can help to determine whether this layer is driving risk levels at the site and whether treatment may be effective in this zone.

4.4.2.3 Pore-water Expression

Most in situ treatments apply a thin layer of amendments that adds little additional pressure on sediments. Pore-water expression, however, may be a factor if greater amounts of in situ treatment amendments are applied to the sediment bed. The influence of this pore-water generation on the effectiveness of the in situ treatment amendment depends on the treatment method and site conditions. In general, treatment amendments are applied at a rate of about 1 to 5% by mass of the sediments being treated, so pore-water expression is unlikely. Any expressed pore water that is generated would additionally pass through the treatment amendment materials, thereby being attenuated in the process.

4.4.2.4 Potential for Resuspension, Release, and Residual During Implementation

Data and modeling that provides insight on potential for resuspension under a range of foreseeable conditions are valuable for judging whether in situ treatment will be effective. The potential for resuspension from in situ sediment treatment depends on the type of treatment technology being used. Mechanical mixing, for example, can cause resuspension of sediments and release of contaminants into the water column (similar to, but more limited than, resuspension from dredging). The degree and speed of mixing controls the magnitude of resuspension and release. Because sediments are not lifted up through the water column, the resuspension and release are less extensive than that from dredging, but may be greater than that from capping. For sediment treatment that places amendments by gravity-settlement through the water column, resuspension and release are expected to be minimal because only a small amount of material is placed and the density of the materials is similar to the existing sediment. Resuspension data can also be used when in situ treatment is evaluated for treating the resuspended contamination from dredging (see Section 6.3.5.2).

Residual contaminated sediments can be generated by in situ treatment if resuspension occurs as described above. Treating sediment from upstream to downstream minimizes generated residuals. Subsequent treatment applications also capture a portion of the generated residuals from upstream. In situ treatment can also leave untreated sediment residuals if the amendment application is not fully effective (for example, if the mixing mechanism cannot reach into corners or cannot achieve the required depth).

4.4.2.5 Benthic Community Structure and Bioturbation Potential

Data regarding the benthic community structure is relevant because the benthic community determines the bioturbation potential, the BAZ, and the type of acceptable final substrates (if a remedial goal is to achieve a particular community structure or to maintain the current structure). The presence of a healthy, high-quality community may support the selection of low-impact treatment that does not destroy the existing habitat and community. Where surface application of an amendment is used, the benthic community should include worms and other organisms that provide bioturbation (on the order of 5–15 cm is typically sufficient), which will provide natural mixing of amendments into the sediment.

Certain types of treatment amendments rely on the activity of the benthic community to provide adequate mixing. Current in situ treatments using AC often rely on gravity settling through the water column and mixing of the carbon into the surface sediment by bioturbation.The displacement and mixing of sediment particles  and solutes  by fauna (animals) or flora (plants). If this treatment approach is used, adequate bioturbation potential must be available to achieve mixing; a depth on the order of 5–15 cm is generally sufficient to reduce the bioaccumulation of PCBs, for example. Bioturbation depth information can be obtained from chemical and radioisotope profile cores and from vertical profiling cameras, sometimes referred to as sediment profile imaging (SPI). Bioturbation rate information typically requires radioisotope analyses (such as beryllium-7).

Additional factors affect the benthic community. For example, the potential toxicity of the treatment amendment should be considered for the specific benthic community present. Amendment toxicity can affect the quantity of material that can be safely used (application rate or dosage), as well as the method of placement. The depth of the BAZ is another critical factor. SPI cameras can be used to determine the depth of the BAZ to help achieve treatment throughout the entire BAZ. 

In situ treatment can change the physical characteristics of the sediment surface, which can also affect the benthic community. Mechanical mixing of sediment, for example, may make the substrate looser, which can increase the potential penetration depth for benthic organisms. On the other hand, a soft sediment surface may be converted into a hard substrate if solidification is used. The types of benthic organisms that can use the new substrate may be different from those that were present before treatment, or the depth of bioturbation (the BAZ) may be changed. Finally, the relative recovery rate of the community structure should be evaluated and estimated. This value may help to determine the relative applicability or desirability of various treatment materials and methods. Several field implementation projects have shown that adding up to 4% (by weight) AC to sediment, by gravity settlement and passive mixing into the surficial (bioactive) layer, does not cause unacceptable adverse effects to the benthic community.

4.4.3 Contaminant Characteristics

Characteristics of the contaminants are particularly valuable in assessing in situ treatment. Contaminants must either be able to be absorbed on amendments such as AC or be degradable. The contaminants must be accessible with current amendment application and distribution systems and distributed in concentrations that can be treated. Mobility of the contaminant may contribute to exposure and may require an amendment to reduce mobility. Contaminant mobility may be increased unintentionally by the addition of an amendment. In either case, the assessment of contaminant species determines the most effective in situ treatment.

4.4.3.1 Contaminant Type - Forensics/Speciation

Data regarding contaminant type determines whether treatment is possible and what type of treatment can be used. For example, in situ treatment should be considered for sites where hydrophobic organic contaminants (such as PCBs) or methylmercury are the primary COCs because enough experience with in situ treatment using AC and other amendments is available for these contaminant classes to warrant consideration. When these chemicals are the risk drivers, in situ treatment can be a promising low-impact alternative. In situ degradation of hydrocarbons has also been demonstrated by injecting oxidants and in situ solidification/stabilization has been used to some extent for a variety of contaminant types including metals and hydrocarbons. In situ treatment experience is not as extensive for other contaminants and if these contaminants are the risk drivers, then literature searches and an extensive laboratory testing program are needed to assess whether in situ treatment is viable.

If the primary risk driver is a metal, then the metal speciation may be important if the treatment contemplated is only effective on one form or species of the contaminant. An example of a contaminant that exists in various forms is arsenic, which may be present as inorganic arsenate (AsV), inorganic arsenite (AsIII), methylated arsenic, or organoarsenic. Speciation data in this situation can help to determine contaminant mobility, toxicity, and treatment potential.

4.4.3.2 Vertical and Horizontal Distribution of Contamination

Because most in situ treatment technologies target surface sediments, information on the vertical distribution of contamination is key to treatment design. In situ treatment may be preferred at sites where concentrations are higher in deeper sediments and within zones where surficial contaminant concentrations are fairly uniform. If the entire depth of contamination is to be treated, then the depth must be within the practical implementation limits of the in situ treatment technology selected. When required treatment depths exceed several feet, in situ treatment may become difficult to implement. An exception is in situ stabilization/solidification (ISS), which has been performed at greater depths. ISS is an aggressive treatment technology that may involve installing a sheet pile wall or cofferdam, removal of overlying surface water, and mechanical mixing of amendments with augers or other devices to reach greater treatment depths.

Surface applications of in situ treatment amendments are unlikely to have significant effect on deeper sediments. It may not be necessary, however, to treat deeper contamination if the sediment is considered stable or the area is a depositional environment. If it is necessary to treat deeper contamination, consider whether the available in situ treatment technologies can penetrate to the necessary sediment depth. If the highest concentrations are below the surface, then clean sediment is most likely depositing on the surface and mixing with the bed sediments, thus naturally reducing exposure concentrations. Note that high concentrations at depth can potentially migrate to the surface, either by 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., diffusion through pore water, biological activity, or other mechanical processes such as gas ebullitionThe act, process, or state of bubbling up usually in a violent or sudden display.. If deeper contamination has the potential to essentially recontaminate surface sediment, then the contaminant flux must be quantitatively assessed and treatment adjusted to accommodate the additional contaminant load.

The horizontal distribution of contaminants also informs design choices. While a large lateral area may be affected, that area may contain certain hot spots with elevated concentrations surrounded by areas with lower concentrations of contaminants. In these cases, it may not be necessary to use in situ treatment across the entire area if exposure to the target receptors is primarily from higher contaminant concentration areas. Similarly, the concentrations within the hot spot zones may be too high to be treated effectively by the available in situ technologies. An effective remedy for these areas may involve a combination of removal of the hot spots, followed by in situ treatment of the less contaminated areas.

4.4.3.3 Contaminant Concentrations

The dose of an in situ treatment amendment needed to reduce risks to acceptable levels is typically proportional to the contaminant concentration. As contaminant concentrations increase, the dosage required also increases up to a certain level, above which it is no longer feasible to consider in situ treatment with amendments. For example, if AC is to be added to the sediment, but calculations or bench-scale tests indicate that carbon must be added at a dosage of 20% of the sediment mass to be treated, then this additional mass would lead to significant alterations of the sediment substrate itself and may cause unacceptable damage to the habitat. The increase in concentration may also require such a high dose (or multiple doses) of amendments that the treatment would be cost prohibitive. Upper bounds on contaminant concentrations are site-specific determinations based on site-specific risk estimates and risk management goals. Bench-scale and in situ treatability and pilot testing may be required to determine whether risks can be adequately reduced using in situ treatment.

Contaminant concentrations are less relevant when ISS is used. An upper bound on the concentrations that can be treated may exist, but because ISS is a predominantly physical process that affects the sediment matrix, the limitation on concentrations may not be as significant as it is for other in situ treatment techniques (see Section 4.4.3.2).

4.4.3.4 Contaminant Mobility

If the goal of treatment is to reduce contaminant mobility, then data about the specific conditions affecting mobility are needed in order to select an appropriate treatment. Conversely, if the treatment itself could increase contaminant mobility, the impact on site risks must be evaluated prior to selecting this technology. In general, treatment is likely to be most effective for contaminants that are not highly mobile or that will not be mobilized by the treatment itself.

4.4.3.5 Bioavailability

Most in situ remedies treat contaminants that are bioavailable because these contaminants are the primary source of risk potential. Information on the bioavailability of the chemicals is a useful design parameter and site-specific bench-scale tests should be used to confirm that bioavailability will be reduced by the selected in situ treatment method.

In some cases, reliance on existing experience and literature may be sufficient to confirm that treatment would be effective at reducing bioavailability. Risk reduction with respect to bioavailability is pathway specific. Thus, while treatment typically works on freely dissolved chemicals, exposures involving pathways such as incidental ingestion of sediment by humans might not be adequately reduced by in situ treatments.

4.4.3.6 Bioaccumulation/Biomagnification Potential

The predominant current approach to in situ treatment uses AC to bind hydrophobic chemicals, such as PCBs, that bioaccumulate and biomagnify in food webs. Thus, for bioaccumulative chemicals that can be treated by a sorbent, in situ treatment with AC offers a viable option for reducing exposures. For bioaccumulative compounds, an adequate reduction in exposure (either through sequestration, reductions in bioavailability, or through destruction/transformation of contaminants) must occur in order to meet site-specific remedial objectives. Because exposure areas for higher trophic levels may be different from the exposure areas under consideration for in situ treatment, the degree of treatment is not necessarily correlated with reduction in the tissue concentrations of chemicals, especially when uncontrolled sources of these chemicals are present.

4.4.3.7 Transformation or Degradation Potential (Biotic and Abiotic)

The transformation or degradation potential (both biotic and abiotic) is essential information to gather before evaluating in situ treatment if the intent of treatment is to transform or degrade the contaminants. The specific biotic and abiotic pathways by which the contaminant degrades or is transformed is used to select an appropriate treatment amendment. Contaminants that have high potential for transformation or degradation to nontoxic forms are amenable to in situ treatment.

4.4.3.8 Nonaqueous Phase Liquids (Presence of Source Material)

The presence of nonaqueous phase liquids (NAPLs), such as petroleum products or chlorinated solvents, in sediment can be a potential problem for some in situ treatment technologies. For example, if AC becomes saturated with NAPL, then the treatment becomes less effective in controlling dissolved constituents. Other amendments, however, such as organo-clay can be used to achieve treatment where NAPLs are present. In general, it is difficult to treat all NAPLs in situ because of mass-transfer limitations (slow dissolution and reaction of free product). Therefore, NAPL can continue to act as a source of contamination long after treatment amendments are applied, especially if groundwater flux, diffusion, or gas ebullition cause upward movement of deeper NAPL. The nature and extent of any NAPL that may be present should be incorporated into the evaluation of effectiveness of in situ treatment. The estimated contaminant flux from the NAPL should be less than the long-term treatment capacity of the treatment amendments applied.

4.4.3.9 Source Identification and Control

Sources of contamination in the system must be identified and controlled prior to implementing in situ treatment (see Section 2.3). Potential continuing sources can result from groundwater flux, stormwater and process water outfalls, and nonpoint sources such as runoff and atmospheric deposition. If ongoing sources are well defined and predictable, it may be possible to provide for future treatment by increasing the initial dose of treatment amendments.

4.4.3.10 Ebullition

Ebullition, the migration and release of gases from sediment, may enhance transport and provide preferential pathways for groundwater and NAPL transport of contaminants from depth into or through the in situ treatment zone. Ebullition can also disturb the vertical stratification of sediment contaminants or the stability of sediments, thus preventing adequate contact between contaminants and treatment amendments and resulting in reduced treatment effectiveness. Ebullition is of particular significance for solidification because it can adversely affect the integrity of the solid matrix formed.

In addition, if ebullition causes upward movement of buried contamination, then the treatment amendment dosage and anticipated long-term effectiveness are affected. As with other sediment processes, it is important to determine the quantitative extent and magnitude of ebullition and how the additional flux resulting from that process may affect the remedy (see USEPA 2013a).

4.4.3.11 Background Concentrations

Background concentrations indicate the contaminant concentrations of material that will deposit on the sediment bed over time. Background concentrations should be taken into account during the design of application estimates for in situ treatment material (see Section 2.2).

4.4.3.12 Exposure Pathways

In situ treatments work best for controlling exposure pathways to the aquatic food web that involve direct or indirect exposure to the available chemicals in the sediments. These pathways could include a direct exposure resulting in toxicity to biota, bioaccumulation into benthic invertebrates with potential transfer to higher trophic levels (including wildlife and humans), and flux to the overlying water column with subsequent exposures to water column biota (algae, zooplankton, and fish). Therefore, a CSM that includes these pathways can help target where in situ treatment may be most appropriate. The effectiveness of in situ treatment in situations where a high likelihood for direct sediment contact or incidental sediment ingestion by humans exists is less well understood and would require a consideration of how such exposures are influenced by the bioavailability of the chemicals (either incidentally ingested or that come into contact with the skin).

4.4.4 Land and Water Use Characteristics Data Needs

Current and future use of the land above and adjacent to the waterway, and the waterway itself, may be limited due to the resources that require protection such as cultural resources, critical habitat, and sensitive species. These concerns are sometimes balanced by the anticipated use of the waterway during implementation and after remediation. In situ treatment, like all other treatments, is susceptible to recontamination from sources that are unrelated to the site but continue to contribute contaminants to the site. Understanding these additional contaminant contributions is essential before selecting in situ treatment or designing the treatment.

4.4.4.1 Watershed Sources and Impacts

As with any sediment remedy, the presence of ongoing sources also affects the potential efficacy of in situ treatment technologies. Watershed characteristics also influence sediment loading and deposition, potential for flashinessFrequency and rapidity of short term changes in stream flow, especially during runoff. and erosive events, and the biological productivity of the system. The biological productivity of the system is affected by agricultural runoff (nutrients such as phosphorous and nitrogen) and wastewater overflow or posttreatment releases (such as biological oxygen demand or nutrients).

Watershed inputs can enhance or reduce treatment effectiveness. For example, if nutrients and organic carbon are being added to the system and are necessary for treatment reactions such as bioremediation to occur, then the watershed effects can increase treatment effectiveness. On the other hand, if the added constituents change the biochemistry of the sediment environment in a way that impedes treatment, adverse effects on treatment occur. In general, for in situ treatment to be effective, the ongoing sources of both target contaminants and other constituents must be anticipated and the information must be used in design of the in situ treatment.

4.4.4.2 Cultural and Archeological Resources

Because of their low-impact nature, in situ treatment technologies should not pose a significant threat to cultural and archeological resources, unless the treatment will be implemented using aggressive mechanical mixing of sediment. Nevertheless, determining the nature of cultural or archeological resources in the contaminated area is important for any remedial technology and should be communicated to all interested parties. In situations where cultural or archeological resources have been identified, in situ treatment may be a preferred remedial alternative for reducing exposures. For example, if AC is added at the surface, sinks to the sediment surface, and is passively mixed in by benthic organisms (a typical in situ treatment approach), cultural or archeological resources are not disturbed.

4.4.4.3 Site Accessibility

A safe, efficient means to deliver and place treatment amendments is required to successfully implement this technology. Some of the considerations for evaluating site accessibility include:

4.4.4.4 Current and Anticipated Waterway Use

Current and anticipated waterway uses can affect both the implementation of in situ treatment and the long-term effectiveness of treatment. The disruption of sediment during treatment should be minimized and the treatment itself should not interfere with current or reasonably anticipated future uses (or use can be postponed during treatment).

Placement of a thin layer of material, a common form of in situ treatment, may not interfere with waterway use even in navigation channels; however, the current and anticipated waterway uses must be considered on a site-by-site basis. Consider the following when evaluating waterway use:

4.4.4.5 Current and Anticipated Land Use

In situ treatment has relatively little effect on land use because once the treatment has been performed, little need exists to retain structures or other operations on land. The primary concerns with current land use are accessibility and the potential for re-treatment if long term monitoring indicates that treatment effectiveness has been reduced.

4.4.4.6 Unique or Sensitive Species

In situ treatments that are low-impact may be more appropriate when unique or sensitive species are present than more invasive remedies. Data on these species are particularly relevant for determining whether the site is appropriate for a low-impact treatment remedy. While most of the current in situ remedies tend to be low-impact, some in situ treatment methods (such as solidification) could transform the habitat or directly injure stationary organisms such as mussels. Potential negative effects of the amendments on the species present must be considered in selecting the type and dose of treatment amendment. Bench-scale or pilot testing may be required to estimate potential effects on these species and to evaluate if the effects are short term or potentially long term. A wide range of field-scale pilot studies have shown that potential effects of AC amendments on the ecological community are limited, particularly at AC doses of less than roughly 4% (Patmont et al. 2013). At many contaminated sediment sites, the positive effects of AC reducing toxicity generally outweigh the potential negative ecological effects of AC, and therefore lead to substantial improvement of habitat quality (Kupryianchyk et al. 2012).

4.5 Evaluation Process

The sections below provide some of the information necessary to evaluate in situ treatment and compare it to other alternatives. Before selecting in situ treatment as a final remedy, one or more of the following types of studies will likely be required and may be necessary during remedial design or prior to the start of construction.

  1. Literature review. Demonstrate through literature review and calculations that the proposed treatment approach can be effective at reducing the risks at the site. If sufficient literature documentation is available to support the use of in situ treatment, then the following two steps may not be needed.
  2. Bench-scale (laboratory) treatability studies. If the literature review suggests that in situ treatment may be possible, then bench-scale (laboratory) treatability testing using a variety of mixtures and doses of amendments can be implemented. If the literature review indicates that in situ treatment is possible, but not well documented, then the bench-scale testing would likely be conducted as part of the remedy selection process (perhaps during preparation of a feasibility study). On the other hand, if sufficient evidence indicates that in situ treatment is effective, then the bench scale testing may be performed after remedy selection to determine the appropriate amendments and doses for delivery.
  3. Field pilot studies. If a new or innovative delivery system is to be used or if there are unique site conditions that could affect implementation, then pilot studies using one or more methods of amendment delivery are appropriate. Pilot studies may be needed as part of the remedy selection if significant uncertainty exists regarding the ability to deliver amendments to sediments in situ, or if there are concerns regarding site-specific treatment effectiveness (for example, significant heterogeneity is present). It is more common, however, for pilot studies to be performed as part of remedial design or just prior to implementation to confirm and refine the methods used. Pilot-scale tests help establish which delivery mechanism will be most effective, and whether treatment of the site sediments can provide the targeted reductions of risks. Note that AC placement has now been demonstrated using a wide range of conventional equipment and delivery systems; uniform AC placement has also been demonstrated in relatively deep and moving water (Patmont et al. 2013). Therefore, field pilot studies for AC placement should not be needed prior to selecting this technology as part of the remedy.

4.5.1 Protection of Human Health and the Environment

Protection of human health and the environment is typically considered a threshold criterion for any remedial alternative. In situ treatment approaches must adequately meet this criterion to be considered. The design process should determine whether in situ treatment technologies are likely to reduce current and future risks to levels consistent with remedial objectives for the site. This assessment is generally based on either a reduction in mobility or availability of contaminants, or actual degradation of the contaminants. The assessment of whether a treatment technology can meet remedial goals related to human health is typically based on literature and site-specific bench-scale or pilot tests.

4.5.2 Short-term Effects

The acceptability of an in situ remedy depends in part on the potential short-term adverse effects from implementation of the remedy. Other issues related to recovery rates are also important considerations. Some of the relevant issues include:

4.5.3 Long-term Effectiveness

The acceptability of an in situ remedy also depends on how well the remedy performs over the long term. Some considerations for long-term effectiveness include:

4.5.4 Implementability

Implementability of in situ treatment depends on the following factors:

4.5.5 Cost

The total cost for in situ treatment can vary widely depending on amendment quantity and cost, delivery system cost, and the cost of placement and implementation (including monitoring and verification). It is often not possible to determine amendment quantities until preliminary laboratory treatability studies have been performed and objectives for contaminant reductions are determined. The primary factors that drive in situ treatment costs include:

4.5.6 Reduction in Contaminant Toxicity, Mobility, or Volume through Treatment

In situ amendments target different types of contaminants in sediment and function through different mechanisms to reduce the availability or effects of contaminants in the environment. AC is widely used as a treatment amendment because it is proven to reduce mobility and bioavailability (and thus exposure to contaminants) through adsorption and immobilization. Additionally, organophilic clay, zeolites, and iron oxide/hydroxide can bind contaminants in the sediments through adsorption, thus reducing mobility and exposure to biota and humans. Other amendments designed to degrade the chemicals or transform them into less toxic forms (reduction in toxic contaminant volume) include apatite, biostimulation (ozone) and bioaugmentation amendments, and ZVI compounds. Additional amendments such as cement, with or without lime or fly ash, can physically solidify or stabilize contaminants (see Table 4-1).

4.5.7 ARARs

Few ARARs relate specifically to contaminant levels in sediments. ARARs that apply are typically associated with the overlying surface water and, for these, a relationship between flux of contaminants from sediments and surface water concentrations may exist. Thus, a surface water ARAR may result in a remedial objective for contaminants in sediments.

Other action- or location-specific ARARs, however, may apply for in situ sediment treatment. For example, a state may have restrictions regarding what materials can be added to a public waterway. Similarly, the U.S. Army Corp of Engineers (USACE) regulates navigable waterways, so certain permitting requirements may be triggered by in situ treatment. In general, in situ treatment is not likely to have more difficulty achieving ARARs than capping or dredging.

4.5.8 State Acceptance

Little experience is available regarding state acceptance of in situ treatment alternatives. Several states support using in situ treatment and no state is known to explicitly reject this technology. Additionally, many state cleanup statutes encourage treatment remedies over containment or removal technologies. Both states and communities are more likely to find in situ treatment a preferred option for minimizing environmental disturbance and reducing exposures.

4.5.9 Community and Stakeholder Involvement

In situ treatment, especially with sorbent/reactive amendments, is a relatively new approach for contaminated sediment management and presents some concerns for stakeholders, since contamination is left in place. Communities often favor removal as the preferred remedy for sediment remediation. This preference generally results from a lack of effective communication on alternatives that can reduce contaminants and risk with less disruption to the habitat and environment. In evaluating in situ treatment, recognize that an active program of outreach and education is necessary to inform the community and gain acceptance for a treatment that does not actively remove the contaminants. Discussions with stakeholders about remedy selection should include detailed analysis of application methods and the expected mode of risk reduction.

Engage stakeholders early. Unless stakeholders have an existing preference for minimally invasive remedial approaches, the evaluation of in situ treatment should include early discussions with key stakeholders to evaluate the level of acceptance for the approach. The support of key stakeholders has been proven to significantly influence both community and regulatory acceptance of in situ treatment approaches.

4.5.10 Other Applicable State or Tribal Requirements

No known applicable state or tribal requirements exist for in situ sediment treatment. Some tribes, however, may object to foreign materials being placed in the environment, especially in areas that the tribes consider sacred. See Chapter 8 for additional information on tribal stakeholderAffected tribes, community members, members of environmental and community advocacy groups, and local governments. issues.

4.5.11 Green and Sustainable Technologies

In situ treatments offer several favorable and environmentally sustainable features, including: low energy costs, low emissions, low community disturbance, small footprints, and preservation of habitats. Additionally, ongoing work with biocharsBiomass that has been carbonized under thermal conditions less intense than those that are used to form activated carbon., such as AC, is promising and may offer a sustainable source of treatment amendments. These biocharsBiomass that has been carbonized under thermal conditions less intense than those that are used to form activated carbon. can be produced from waste wood or other carbon sources including invasive species of plants such as Phragmites (common reeds). Biochar production for in situ treatment offers a waste disposal alternative, a means of managing invasive plants, and a method of carbon sequestration (through growth of the plants prior to harvesting). Finally, ITRC offers additional guidance on green and sustainable remediation approaches that may support in situ treatment (ITRC 2011b).

4.5.12 Habitat and Resource Restoration

A number of in situ treatment remedies are designed to have low environmental impact. These approaches can lower chemical exposures without compromising the habitat or species using the habitat. This low-impact footprint accelerates habitat and resource restoration and can potentially lower natural resource damages relative to other remedial alternatives.

4.5.13 Watershed Considerations

In situ remedies can be used in parts of the watershed where acceptable physical requirements are met. Treatment generally does not adversely affect the physical or hydrological characteristics of the watershed and is generally compatible with habitats and resources. Unless used as a temporary measure, in situ treatment is usually not applied to areas where deep contamination exists and where navigation or construction projects are planned.

4.6 Monitoring

Monitoring of stream and sediment conditions is essential to confirm that adequate amendment and distribution for treatment has been achieved. During the construction phase of an in situ treatment program, the sediment bed and associated contaminants may be resuspended during mixing and distributed downstream to an uncontaminated area. Similarly, valuable amendment may be lost or unevenly distributed along the sediment bed surface, depending on the hydrodynamicsThe branch of science that deals with the dynamics of fluids, especially that are incompressible, in motion. of the waterway, depth of water, delivery mechanism, and amendment used. During implementation, the stability of the sediment bed containing the amendment and the thickness of the treatment zone as well as the concentration of the amendment must be monitored to confirm that adequate treatment capacity exists (vertically and horizontally).

While construction monitoring confirms that the remedy has been properly implemented, monitoring of stream and sediment conditions after implementation evaluates the overall performance of the remedy. Performance monitoring results must be evaluated to determine whether the treatment has successfully reduced exposures to acceptable levels.

4.6.1 Construction and Implementation Monitoring

Constructions and implementation monitoring generally measures the relative success in achieving the designed delivery or placement of treatment agents to the sediments. The design goal is to establish contact, or near contact, between the treatment materials and the contaminants that are to be treated (in either the BAZ or a thicker sediment interval). For example, if site-specific bench-scale tests indicate that the desired amount of AC is 5% of the dry weight of the top 10 cm of sediment, then this value becomes the design basis for the application and the method of delivery. This design specification and any others developed for additional treatment agents become metrics for construction monitoring.

Treatment effectiveness is influenced by the degree of contact between the treatment agent and the contaminants and by the degree of horizontal and vertical mixing over the desired treatment area. Uneven distribution, loss of treatment agent in the water column, or poor mixing can reduce the effectiveness of the treatment. Construction and implementation monitoring measures the characteristics of the physical placement that can confirm delivery and mixing of treatment materials. These aspects of treatment performance are monitored by evaluating the horizontal and vertical distribution of treatment agent and the small-scale variability in treatment efficacy for reducing exposures.

Implementation of in situ treatment is similar to capping in several aspects that can affect monitoring (Palermo et al. 1998). For example, achieving distribution or placement of materials depends on the physical properties of the material being placed, the sediment on which it is being placed, and the flow characteristics and depth of the water body. These factors should be considered when developing a placement and construction monitoring plan. Evaluation can be performed through measurements such as thickness (immediately post placement through core samples or other means), composition (such as carbon content) of the completed installation, visual means (SPI camera), or a range of other physical or chemical methods (bathymetric, tray samples, or diver assist).

Variability and uncertainty often occur in placement and measurement approaches. Typically the thickness and composition should be specified on a statistical basis, such as 95% upper confidence limit on the mean or a reasonable tolerance to a target. Note that actual performance, as well as individual measurement methods, may vary from areal average values without a substantial impact on the overall performance of the treatment.

The construction and implementation monitoring for other in situ treatment approaches may vary substantially. For example, monitoring for in situ solidification may use chemical and physical targets (such as achieving final hydraulic conductivity values) that will limit the ultimate migration of contaminants contained within the solidified mass of treated sediment.

Construction and implementation monitoring methods for in situ treatment remedies are specific to the materials and techniques used. Because many of the materials and methods are relatively new or experimental, the design stage should include a careful selection of metrics to define success for construction and implementation. 

4.6.2 Post-remediation Performance Monitoring

Performance monitoring for in situ treatment assesses treatment efficacy over time and monitors potential environmental effects from the treatment. While most in situ treatments are relatively low- impact, some in situ approaches have a greater effect on the surrounding area and must be monitored to confirm that the remedy does not cause harm to the environment.

The efficacy of most treatment technologies can be judged by how well they reduce short-term and long-term exposures. Most assessments of efficacy measure the degree to which concentrations of dissolved contaminant (Cfree) are reduced in surficial sediments, but may also include demonstrating that contaminants are being transformed to nontoxic degradation products. Remedial goals are typically expressed as either a percent reduction in exposure over current levels or as specific target concentrations. Target concentrations are typically expressed as an average over the remediation zone, but in some situations might include single-point maximum allowable concentrations. In most cases, performance monitoring measures both bulk chemical concentrations in sediment and Cfree either on a composite or point-by-point basis or as a composite over a set exposure level. For most full-scale in situ treatment projects, biological metrics may also be needed to provide assurance that the treatment is performing as expected. Performance monitoring can be adjusted as information is gathered over time and across the treatment area.

Some common performance monitoring methods include the following:

At some sites, differences between real and perceived changes in performance may be evident. It is possible to have effective treatment, but still observe an apparent decrease in treatment efficacy over time. For example, if a previously unknown source is releasing contaminants into the system, fish tissue concentrations may stop decreasing or begin increasing again in the future. Higher fish tissue concentrations could lead to a perception that the treatment is no longer effective. The real versus perceived performance of an in situ treatment alternative is affected by the following factors:

If performance appears to decline over time (or if treatment appears to be reversed in the long-term), then post-implementation monitoring may also include additional sampling or testing to determine the cause of the poor performance.

4.7 Case Studies for In situ Treatment

Table 4-3. Case studies describing in situ treatment

Pilot Study

Contaminant and Amendment

Site Description

Application

Reference

Hunters Point, San Francisco Bay, CA, 2004 and 2006

PCB/AC

Tidal Mud Flat

1. Slurry injection

2. Tiller

Cho et al. 2007

Cho et al. 2009

Grasse River, NY 2006

PCB/Granulated AC

River

Tiller

Beckingham and Ghosh 2011

Trondheim Harbor, Norway, 2007

PAHs, PCB/Powdered AC and AC-bentonite

Harbor

Slurry application

Norwegian Research Council 2011

James River, VA

Hydrophobic contaminants/ AC

Estuarine Wetland

SediMite

Menzie 2012

Deep Fjord, Grenlandfjords, Norway, 2009

PCDD/F/AC mixed with clays

Fjord

Thin-layer cap

Cornelissen et al. 2012

 

Publication Date: August 2014

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