DNAPL Site Remediation
Many technologies have been developed for source and plume treatment, all with specific advantages and limitations. Selecting a treatment technology requires evaluating several factors, including technical site features (e.g., geology, hydrogeology, and contaminant levels), regulatory requirements, sustainability, and community stakeholder interests. Traditionally, treatment technologies are applied individually at a site, with the expectation that one technology can achieve all objectives. More comprehensive approaches have gained favor recently for chlorinated-solvent sites because complete restoration can be difficult. These comprehensive approaches involve integrating several technologies in time and space.
Setting realistic objectives is critical when developing an integrated DNAPL site strategy. Objectives may be absolute (objectives based on broad social values, such as protection of public health) or functional (steps or activities taken to achieve absolute objectives, such as supplying bottled water to affected residents).
Functional objectives are established to demonstrate attainment of absolute objectives and have often been missing, difficult to measure, or unattainable. A key concept is that functional objectives should be specific, measurable, attainable, relevant, and time-bound (SMART).
Selecting objectives that reflect SMART attributes makes subsequent decisions more valid and remedial approaches more successful. It is often necessary to develop SMART functional objectives for different locations, phases, and alternative end points of an overall site cleanup. Given the unique perspectives of different stakeholders and the practical and economic limitations that exist, defining the SMART functional objectives appropriate for a given site requires cooperation, consensus, and often some compromises.
Typical time frames involved in remediating chlorinated-solvent sites may be long (decades to centuries), but functional objectives should have relatively short time frames—years to less than one generation—to encourage accountability for specific actions and to make it easier to measure progress toward the objectives.
Treatment technology selection requires evaluating a number of different factors, including technical site considerations (e.g., geology, hydrogeology, and contaminants), regulatory requirements, sustainability and community stakeholder interests.
Many technologies exist for chlorinated-solvent site remediation. It is useful to categorize available technologies according to the primary mechanism by which they impact individual chlorinated solvent phases (e.g., DNAPL, sorbed, dissolved, and vapor phases).
Physical removal technologies
Contaminants in the source zone are removed by excavation. The excavated material then is treated or managed, for example by on-site treatment or off-site disposal. Excavation is very effective for mass removal from near-surface source zones contaminated by strongly sorbed and/ or essentially immobile organic liquids (e.g., high-viscosity coal tars) and for recent release sites.
Excavation was applied at 11 of 118 sites (9%) in a survey of DNAPL source zone treatment results (Geosyntec Consultants 2004). Ex situ source treatment technologies for which excavation is necessary as part of implementation (e.g., composting, off-site thermal treatment, soil washing, land farming, stabilization etc.) was a component of 104 of 230 (45%) source treatment decision documents at Superfund sites from fiscal years 2005–2008.
Figure 1: Excavated soil is treated on site with soil washing process
MPE combines groundwater P&T with SVE. Pumping extracts contaminated groundwater and draws the water table down to facilitate volatilization and removal of DNAPL and sorbed-phase chlorinated solvents through SVE. This approach can effectively remove all contaminant phases but becomes more costly and difficult with increasing depth and extremely high or low aquifer permeability. MPE preferentially removes contaminants from high-permeability intervals and has the potential to leave contaminant mass in lower-permeability intervals, depending on total time of operation.
MPE was applied at 13 of 118 sites (11%) in a survey of DNAPL source zone treatment results (Geosyntec Consultants 2004).
Figure 2: DNAPL is extracted along with aqueous phase or soil gas for aboveground treatment
Thermal conductivity and electrical resistance heating
Thermal technologies are included in the physical removal category because the primary removal mechanism is volatilization coupled with vapor extraction; however, other mechanisms, such as pyrolysis and hydrolysis, can destroy contaminant mass in situ if the soil is heated sufficiently.
Two of the main thermal technologies used in IDSS applications are thermal conduction heating (TCH) and electrical resistance heating (ERH), with steam treatment being used less frequently. TCH technology uses heating elements in direct contact with the soil, resulting in heat transfer to the matrix. ERH technology applies electrical energy among electrodes, and the electrical resistance of the matrix to the flow of the electricity results in heating the formation. Thermal technologies are more demonstrated compared with other technologies for treatment of low-permeability media and for time-critical remediation.
Thermal technologies were applied at 27 of 118 sites (23%) in a survey of DNAPL source zone treatment results (Geosyntec Consultants 2004).
In situ thermal conductive heating
Figure 3: In pile thermal conductive heating project in Vietnam
In situ chemical oxidation
ISCO is the injection of oxidant and amendment solutions into the source zone and/or downgradient plume to destroy contaminants, primarily through chemical reactions. Oxidants include catalyzed hydrogen peroxide (Fenton’s reagent or modified Fenton’s reagent), ozone, permanganate, and persulfate. The oxidants react with the contaminants to produce nonhazardous intermediate and final products, such as carbon dioxide, carboxylic acids, and chloride from organic compounds, as well as iron, sulfate, and other ions from the catalyst amendments or the oxidants which may remain dissolved in groundwater or precipitate or react further with naturally occurring constituents of the soil or groundwater.
ISCO can be applied over a wide range of concentrations to address dissolved, soil-sorbed, and DNAPL phases. However, different oxidants have different ranges of applicability. For example, catalyzed hydrogen peroxide is generally most applicable at higher concentration ranges and for DNAPL, while permanganate and ozone are generally more applicable at the lower concentrations found in dissolved plumes. Radical-based oxidants such as ozone, catalyzed hydrogen peroxide, and activated persulfate are applicable to a wide range of contaminants, including mono- and polynuclear aromatic compounds, saturated (ethane) and unsaturated (ethene) chlorinated aliphatics, while permanganate is applicable to unsaturated chlorinated aliphatics and certain other compounds, such as phenols.
ISCO was applied at 25 of 118 sites (21%) in a survey of DNAPL source zone treatment results (Geosyntec Consultants 2004), at which permanganate was used for 15 sites (60% of the ISCO sites), catalyzed hydrogen peroxide for nine sites (36% of the ISCO sites), and ozone at one site (4% of the ISCO sites).
Figure 4: Array of chemical injection wells for at chlorinated solvent site
In situ chemical reduction
As with ISCO, a wide variety of approaches and treatment reagents is available. For example, direct treatment of source areas using nanoscale ZVI is a recent development, although there are relatively few case studies or long-term performance evaluations. In an alternative approach, ZVI and clay can be blended into soil, resulting in direct treatment (via reaction with ZVI) as well as reduced mass flux (due to reduced hydraulic conductivity from the clay). ISCR also can be implemented as a containment technology (see Section 188.8.131.52).
ISCR was applied at 6 of 118 sites (5%) in a survey of DNAPL source zone treatment results (Geosyntec Consultants 2004). Soil mixing with ZVI and clay had a median percent reduction in soil concentration of 98% (1.7 OoMs) based on data from four sites (Olsen and Sale 2009).
Figure 5: Prepare ZVI slurry for injection
In situ bioremediation
In situ bioremediation (ISB), often referred to as “engineered bioremediation,” involves biological transformation of contaminants, preferably (but not always) to less harmful intermediate and final compounds. Microbes may use contaminants as electron acceptors or as electron donors under a wide variety of conditions.
ISB may involve injection of bioremediation substrates to supply bacteria with fermentable sources of carbon and other nutrients to produce hydrogen for dechlorinating bacteria (biostimulation) or injection of nonnative microbes (bioaugmentation).
ISB is commonly applied as a plume remediation technology coupled with a more aggressive source remedy, such as ISCO, thermal treatment, surfactant enhanced extraction and excavation. ISB is also commonly used in residual source areas after more aggressive source remedies are implemented.
ISB was applied at 25 of 118 sites (21%) in a survey of DNAPL source zone treatment results (Geosyntec Consultants 2004).
Figure 6: Substrate can be added into subsurface to promote growth of microbial responsible for the degradation of chlorinated compounds
Surfactant/cosolvent flushing is a DNAPL-removal technology involving the injection and subsequent extraction of chemicals to solubilize and/or mobilize DNAPLs. The surfactants are injected into a system of wells positioned to sweep the DNAPL source zone within the aquifer. The chemical flood and the solubilized or mobilized DNAPL is removed through extraction wells, and the produced liquids are then either disposed (usually off-site treatment) or treated on site to remove contaminants, and then reinjected to the subsurface to remove additional DNAPL mass.
The primary appeal of surfactant/cosolvent flushing is its potential to quickly remove a large fraction of the total DNAPL mass as compared to other technologies. As an in-situ technology, it eliminates the need to excavate, handle, and transport contaminated media. It is applicable as a stand-alone technology or as a component in a “treatment train” consisting of several remedial technologies, depending on site-specific cleanup objectives.
Technical challenges to the successful use of surfactant/cosolvent flushing include locating and delineating the DNAPL source zone and obtaining an accurate estimate of the initial DNAPL mass and its spatial distribution. Additional requirements include characterizing the hydraulic properties of the aquifer, delivering and distributing the injected chemicals to the targeted zone, and designing the optimum chemical formulation for a given DNAPL composition and soil type. The implementability of surfactant/cosolvent flushing will depend on site-specific geologic conditions and on the type of DNAPL present at the site.
Figure 7: Surfactant is injected to the subsurface to re-mobilize the NAPL
Monitored natural attenuation
Natural attenuation includes physical, chemical, or biological processes that act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater. Examples of these in situ processes include biodegradation, dispersion, dilution, sorption, volatilization, and chemical or biological stabilization, transformation, or destruction of contaminants. MNA implies that remediation progress is periodically assessed to ensure that the remedy is operating as planned, for example, that plumes are not expanding and that there are no new or increasing threats to human health and the environment. Active remediation technologies rarely achieve complete remediation of all contaminant mass; thus, in effect, MNA is typically a component of every chlorinated-solvent site remedy.
MNA was a component of 116 of 627 (19%) groundwater plume treatment decision documents at Superfund sites from fiscal years 2005–2008 (USEPA n.d.)
The objective of containment technologies is to prevent or reduce mass discharge from a source. Containment remedies should consider the possibility of back-diffusion from silt and clay zones downgradient of the containment system location, which could result in sustained VOC concentrations downgradient from the contained source area for some time. Major containment technology would include the following:
- Pump and treat – Extraction wells are typically installed with their screen intervals intersecting the contaminant plume. Pumps extract groundwater, which is treated above grade using various technologies (e.g., air stripping, carbon absorption, condensation, or oxidative destruction). P&T is generally effective for removal of aqueous-phase contamination and thus is commonly used as a plume capture and containment remedy because extraction wells positioned downgradient from sources intercept a migrating plume and prevent further downgradient contaminant migration.
- Permeable reactive barriers – PRBs are typically constructed of a material that directly or indirectly destroys dissolved contaminants passing through the barrier due to groundwater advection. PRBs typically contain a chemically or biologically active treatment medium and thus may also be considered a chemical or biological treatment technology. PRBs are commonly constructed with ZVI.
- Low-permeability barrier walls – Low-permeability barrier walls are designed to prevent or control migration of fluids into and/or from the contained area. A common complement to low-permeability barriers is extraction of groundwater from within the contained area to maintain an inward hydraulic gradient. Examples of low-permeability walls are sheet pile barriers and slurry walls. Sheet piling can be driven into the subsurface. Individual sheets are joined together. Slurry walls are a trench in which a slurry of soil-bentonite or soil-cement is placed.
Source area containment and vertical engineered barriers were a component of 80 of 362 (22%) source treatment remedies (including source remedies with treatment and without treatment).
Rationale for coupling technologies
More recently, the remedial paradigm has shifted to accept that more than one technology is almost always necessary over the treatment life cycle or at different locations within a source area or plume. Remedial designers and chlorinated solvent site managers recognize that coupling technologies into sequences, combinations, or treatment trains, can more effectively reduce contaminant concentrations and project duration than individual methods, particularly at complex source zones containing DNAPL or multiple contaminants. Treatment trains require an assessment of the compatibility of individual technologies.
|Technology applied first or in upgradient area||
|Surfactant/cosolvent flushing||Thermal technologies||Other extractive technologies||In situ chemical oxidation||In situ chemical reduction||Bioremediation||Monitored natural attenuation||Permeable reactive barriers|
|Other extractive technologies|
|In situ chemical oxidation|
|In situ chemical reduction|
|Monitored natural attenuation|
|Permeable reactive barriers|
Might be compatible