In Situ Sediment Remediation
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). 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.
While various amendments can target different types of contaminants in sediment, activated carbon (AC) is one of the most widely used for in-situ immobilization of contaminants in sediment. 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:
- Direct application of amendments, with or without binder and weighting agents
- Mixing amendments with sediment or sand prior to placement
- Placement of amendments below covers or caps
Other amendments, such as apatite, nutrients or ozone (for biostimulation), KB-1 for bioaugmentation, and zero valent iron (ZVI), are specifically designed to degrade chemicals or transform them into less toxic forms.
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 solidification approach can be applied to higher concentrations of contaminants, but is considered a more active and invasive form of treatment.
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.
Biological amendments consist of three categories:
- Bioaugmentation is the addition of cultured microorganisms directly on or into the sediment to degrade and transform specific contaminants. (such as KB-1)
- Biostimulation is the enhancement of rate-limiting sediment conditions in order to stimulate the indigenous microorganisms to degrade and transform specific contaminants
- Inhibition occurs when amendments are added to inhibit biological processes that would normally cause contaminants to be transformed into more toxic forms under existing conditions.
Chemical amendment is mainly the mixtures used to decompose the contaminant to less toxic or bioavailable forms (ZVI).
Physical amendment can be applied in order to achieve two effects:
- Sorption results from the addition of chemicals or other materials (such as activated carbon, organophilic clay, zeolites, bauxite, and iron oxide/hydroxide) that physically or chemically bind (adsorb) contaminants to reduce their bioavailability. Application of AC is the most widely used and tested of these techniques
- Stabilization/solidification involves the addition of chemicals or cements (such as Portland cement, quicklime, and fly ash) to encapsulate contaminated sediments into a solidified mass that reduces contaminant mobility and bioavailability.
Delivery of Amendments
In order to be successful, in-situ sediment treatment must achieve adequate contact between treatment amendments and the contaminants in the sediment.
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.
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).
Data needs for In Situ Treatment Design
Check the summary table for key site characteristic data for in-situ treatment.
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.
- Feasibility review
- Bench-scale treatability studies
- Field pilot studies
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 acceptability of an in-situ remedy depends in part on the potential short-term adverse effects from implementation of the remedy, including effects on habitat and resident biota; release, resuspension, and untreated residuals; resource consumption and sustainability and time to achieve protection.
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: potential for chemical release from treatment zone; the extent of treatment depth; treatment capacity; recontamination.
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). The primary factors that drive in situ treatment costs include:
- Amendment materials
- Implementation methods
- Performance 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. 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.
Coordinator: EnvGuide Team