 |
| Resource Library > Technology Transfer > Programs and Initiatives > Source Zone Treatment > Background > Performance |
|
Performance
Consideration of the physical and chemical properties of petroleum fuels and chlorinated solvents is critical in evaluating the movement, distribution, and fate of these chemicals in the environment, developing methods for identifying and delineating source areas, and assessing the possible range in performance of various remedial alternatives. Two characteristics of common non-aqueous phase liquids (NAPLs) -- low aqueous solubilities and high interfacial tensions with water -- result in the persistence of a non-aqueous phase and highly irregular distribution of NAPL in the subsurface. These circumstances in turn present significant difficulties for site characterization and subsequent remediation.
In 1994, the National Research Council (NRC, 1994) published a detailed report describing the inherent difficulties in site remediation and made recommendations to the USEPA regarding the general failure of pump-and-treat technology and the need for new approaches. The NRC created a Table to categorize sites according to the ease of cleanup. As expected, the presence of NAPL and heterogeneous soils increases the difficulty of site cleanup.
At sites where all NAPL residuals are situated above the water table, a high percentage of the NAPL generally can be removed, although long timeframes may be required for in-situ methods in low-permeability soils. Remediation strategies for vadose-zone contaminants can include removal of contaminated soil by excavation, removal of contaminant mass using in-situ extraction methods, or preventing groundwater contamination by placing an impermeable cover over the source area. At many chlorinated solvent sites, the DNAPL mass causing the groundwater plume may be situated below the water table. DNAPL source zones below the water table greatly complicates site remediation and reduces the potential for significant DNAPL reductions and groundwater cleanup. A summary of several proven and emerging technologies with potential application at NAPL source areas is presented in the linked Table. A brief description of each technology follows; technologies are described in greater detail on the Design Page of this Roadmap.
Bioremediation/Enhanced Biodegradation Biodegradation of most common NAPL components has been demonstrated to be effective in treating dissolved-phase contaminants. Biodegradation processes can be enhanced by the addition of inert soluble organics which supply a surplus of electrons. Vegetable oil and other food-grade organics are typically injected into the soil volume containing NAPL residuals and the site then is monitored to track the conversion of contaminants into byproducts. Enhanced biodegradation may provide an inexpensive method of containing dissolved contaminants and slowly destroying NAPL source zones. As with any in-situ technology, its success depends upon the complete contact between DNAPL residuals and the injected solution. Additional studies are required to determine the long-term cost-effectiveness of this technology.
Bioventing/Biosparging Bioventing is a promising technology that stimulates the natural in-situ biodegradation of any aerobically-degradable compounds in NAPL within the soil in the vadose zone by providing oxygen at low flow rates to microorganisms that are present in the soil. Biosparging has demonstrated ability to reduce the concentrations of volatile contaminants in the dissolved phase (beneath the water table) to target levels. Due to the tendency of low-permeability lenses to divert airflow, and the tendency of NAPL to accumulate on such lenses, it is unlikely that an entire NAPL source zone would be remediated effectively using bioventing or biosparging technologies, although significant success has been reported in some field trials. Therefore, extended treatment times at NAPL sites may be anticipated. Successful bioventing usually can only be applied to degradable organic compounds; and biosparging requires volatile contaminants and moderate permeability of the subsurface. Performance of these technologies suffers in low- and high-permeability units and at heterogeneous sites. Sparging typically is coupled with SVE to treat the vadose zone and saturated zone simultaneously.
Chemical Oxidation In-situ oxidation could potentially provide rapid contaminant destruction for readily oxidized contaminants. Performance may be limited by the presence of oxidation-resistant contaminants, by the large amounts of oxidizable material in the soil, by low permeability and by soil heterogeneities. Some oxidants also require maintenance of acid conditions and thus performance may be limited in carbonate-rich units. Field data are insufficient for determination of clean up levels or the range of conditions under which the technology is suitable; however, additional field tests are in progress.
Cosolvent/Surfactant Extraction/Flushing Cosolvent/surfactant extraction and flushing can provide rapid mass removal of NAPL at sites having good to moderate permeability. Extraction efficiency will be lower in low-permeability zones and at heterogeneous sites. High concentrations of reagents have been used in cosolvent flushing, and recycling of solvents has not yet been demonstrated, hence reagent cost may be high; however, recycling of surfactants has been demonstrated. All cosolvents and surfactants reduce the interfacial tension of water with NAPL, raising the potential for mobilization and downward movement of the NAPL. Although removal of nearly all NAPL has been accomplished in field tests using surfactants, some NAPL has remained in every case. The technology has potential for removal of NAPL mass from permeable units, under appropriate hydrogeologic conditions.
Groundwater Extraction and Treatment (Containment) Low-permeability barriers or hydraulic containment technologies (groundwater pump-and-treat systems) may be used to isolate NAPL source zones. Although they usually do not address removal of the NAPL, they may provide risk reduction and isolation of the source from the dissolved phase plume, thereby allowing treatment of the dissolved-phase plume. Barriers also may allow more aggressive remediation of the source zone by other technologies. However, because conventional groundwater extraction and treatment systems cost millions of dollars to install and operate, innovative remediation strategies should be pursued, when possible.
Natural Attenuation Natural attenuation refers to the reliance on natural processes to achieve site-specific remedial objectives. Natural attenuation processes include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or ground water. These processes include biodegradation, dispersion, dilution, sorption, volatilization, and chemical or biological stabilization, transformation, or destruction of contaminants. Under the proper conditions at some sites, natural attenuation can contribute significantly to remediation of dissolved fuel hydrocarbons and chlorinated solvents and may accomplish site remediation goals at a lower cost than conventional remediation technologies, within a similar timeframe. Because most natural-attenuation processes (other than dissolution) do not directly affect NAPL source zones, natural attenuation should be regarded primarily as a containment technology, addressing the downgradient, dissolved-phase contaminant plume.
Permeable Reactive Barriers/Reactive Zones Reactive barriers or engineered zones in the subsurface, using zero-valent iron or mulch, have proven effective for treatment of dissolved phase plumes of some chlorinated solvents, notably TCE. Other reactive units may be used to extend the technology to other compounds. Reactive barriers treat only dissolved-phase contaminants; thus, they do not directly address NAPL source zones, and must be considered a containment technology. The effective life of treatment barriers/zones remains to be determined.
Physical Removal (Excavation) and Treatment/Disposal of Soil If a discrete source zone containing NAPL can be identified and isolated, it is a candidate for remediation by excavation of contaminated soil. Soil excavation methods are well established; after soil containing DNAPL has been removed from a source zone, it may be properly disposed, or treated in a number of different ways. Excavation has a large capital cost but no operation and maintenance (O&M) cost and may be capable of achieving over 99 percent NAPL removal at smaller sites having contaminants in the upper 40 feet of the soil column. An excavation remedy should be considered seriously if the site is not covered with high-value buildings or mission critical facilities.
Soil Vapor Extraction (SVE) SVE is a proven technology for NAPL mass removal in the vadose zone, and is capable of providing rapid, relatively inexpensive mass removal of volatile components of NAPL in permeable, low water content soils. Performance is limited by low permeability, high soil water content and heterogeneities. SVE may be augmented by thermal techniques, potentially extending its applicability to semivolatile compounds and increasing its effectiveness for low-permeability zones. It may be coupled with biodegradation ("bioventing") to enhance remediation of hydrocarbons and other aerobically degraded compounds. SVE has excellent potential for mass removal of NAPL in permeable, relatively homogenous soils.
Thermal Heating/Thermal Enhancements Several in-situ heating methods, including steam or hot-water heating, resistive heating, and radio-frequency (RF) heating, are being tested and applied as means of reducing soil moisture and enhancing soil-vapor extraction (SVE) effectiveness by extending SVE influence into shallow aquifers. In-situ heating technologies have the potential to increase the permeability of silt and clay soils and increase the removal efficiency of SVE. However, heating technologies have limited application for DNAPL zones below the water table. Below the water table, groundwater and DNAPL are in close proximity. The heat capacity of water is much greater than the heat capacity of most chemicals. Much of the heat introduced to the subsurface is absorbed by ambient groundwater, and is not available to volatilize DNAPL chemicals. The use of thermal heating methods in heterogeneous soils must be monitored carefully to ensure that heating does not result in undesirable lateral migration of contaminants beyond the influence of the collection system.
Conclusions
Due to the lack of carefully controlled field tests at NAPL sites (in particular, DNAPL sites), the ultimate level of cleanup attainable for most technologies has not yet been adequately documented. Indeed, due to the difficulty in determining NAPL distribution in the subsurface, the level of cleanup achieved even in controlled field tests has seldom been well established.
Based on existing data, it may be expected that a combination of contaminant distribution, geologic heterogeneities and technological limitations will cause at least some NAPL to remain after remediation by any available technology. The mass remaining will depend upon the site-specific characteristics of the subsurface, the properties of the particular NAPL, the technology utilized, and the time of treatment. The implications of the residual NAPL must be assessed before determining if source zone remediation is appropriate. Coupling of technologies may be employed to increase remediation effectiveness, but there has not been adequate testing of the treatment-train approach to quantify achievable endpoints.
The task of removing or destroying contaminant mass to a degree sufficient to achieve the complete restoration of a NAPL source zone is formidable. The petroleum industry has spent billions of dollars on research to enhance the recovery of petroleum (an LNAPL) from oil fields. Oil companies consider a removal of 40 percent of the formation NAPL to be an exceptional success. By contrast, if groundwater is to be restored to drinking-water quality, at least 99 percent of the NAPL source must be remediated (Freeze and McWhorter, 1997). The limitations on identification, removal, or control of NAPL source represent immense obstacles for complete restoration of groundwater.
Groundwater restoration is much different from plume containment. Complete restoration of groundwater requires removal of the source(s) of chemicals from the subsurface, as well as removal of dissolved-phase chemicals to a degree sufficient to allow the original beneficial use of the groundwater. As a consequence of the difficulties of identifying and remediating residual NAPL, continued dissolution and migration of chemicals from a residual source may necessitate perpetual hydraulic containment at some sites.
|
|
|
|
|
|
