In situ flushing involves flooding a zone of contamination with an appropriate solution to remove the contaminant from the soil. Water or a liquid solution is injected or infiltrated into the area of contamination. The contaminants are mobilized by solubilization, formation of emulsions, or a chemical reaction with the flushing solutions. After passing through the contamination zone, the contaminant-bearing fluid usually is collected and brought to the surface for disposal, recirculation, or on-site treatment and reinjection. Traditional flushing techniques rely on the ability to deliver, control the flow, and recover the flushing fluid (EPA 2006) via a pump-and-treat system.

As with pump and treat, the effectiveness of flushing with water can be limited by the solubility of the contaminant, rate-limited desorption (i.e., when desorption of the contaminant from the solid phase to the aqueous phase is slow), and the presence of low-permeability zones and other subsurface heterogeneities. Chemically enhanced flushing solutions often can be tailored to address recalcitrant contaminants, and treatability studies are conducted to determine the feasibility of the approach; however, subsurface heterogeneities not detected during characterization or considered in implementation can still limit flushing effectiveness.

Flushing solutions can be water, acidic aqueous solutions, basic solutions, chelating or complexing agents, reducing agents, cosolvents, or surfactants. Water will extract water-soluble (hydrophilic) or water-mobile constituents. Acidic solutions can be used to remove metals or basic organic materials. Basic solutions can be used for some metals, such as zinc, tin, or lead, and some phenols. Chelating, complexing, and reducing agents are used to recover some metals. Cosolvents are usually miscible and are effective for some organics. Surfactants can assist in the removal of hydrophobic organics. Heating the flushing solution also can help to mobilize organic contaminants (ITRC 2003, ITRC 2009).

Chemical flooding is used primarily to target the removal of source areas of contamination and is less well suited for the remediation of dissolved, sorbed, or volatilized plumes; however, surfactant/cosolvent flooding can be used in conjunction with other technologies, such as multi-phase extraction, bioremediation, or chemical oxidation, to bring a contaminated site to closure (ITRC 2003, ITRC 2009). Coupling chemically enhanced flushing with in situ chemical oxidation (ISCO)—either concurrently or sequentially—is an emerging concept for effective cleanup of DNAPL-contaminated sites. Surfactants are effective for DNAPL mass removal but not useful for dissolved plume treatment, whereas ISCO is effective for plume control and treatment but can be less effective in areas where large masses of DNAPL are present. Coupling the two technologies offers new opportunities for source-zone treatment. Dugan et al. (2010) conducted a review of literature that discusses the use of oxidants and surfactants/cosolvents, either concurrently or sequentially, for DNAPL mass removal. Surfactants or cosolvents also can be used to recondition the water that is reinjected to stimulate physico-chemical processes (such as desorption at lower pH) or in situ biodegradation in the saturated zone (Christ et al. 2005, ITRC 2005).

Conceptual Design of a Surfactant/Cosolvent Flushing System (NAVFAC 2002)

The remediation timeframe for in situ flushing will vary depending upon the flushing technique, contaminant type, and specific site characteristics, such as the size of the contaminated area and its geochemistry. Chemically enhanced flushing is an aggressive technique that is expected to achieve its goal fairly rapidly, ranging from months to a few years (ITRC 2009), whereas the timeframe for natural water flushing can range dramatically from months to many years, as is illustrated by the 1988-2004 operation of the soil flushing system installed at the United Chrome Superfund site (EPA 2011). Flushing processes can be slowed when heterogeneities such as soil layers or lenses of less permeable soil (less than 10-5 cm/s) or organic materials are located within the soil horizon (Reddy 2008).

Soil flushing is considered a mature technology because of its use for decades in oil field applications to enhance oil recovery (EPA 2006). While both organic and inorganic contaminants have been addressed using flushing technology, the survey of the literature from which case studies were selected suggests that surfactant and cosolvent flooding for fuels and chlorinated solvents are the flushing techniques most frequently employed. EPA's 2010 Superfund Remedy Report (Thirteenth Edition) shows that in situ flushing was selected as a remedy at 19 Superfund sites between 1982 and 2008, with zero to two selections annually.

A 2010 Remediation editorial review of remediation industry trends indicated a decline in the number of presentations on surfactant flushing delivered between 2006 and 2010 at the large biannual conference hosted by Battelle, the International Conference on Remediation of Chlorinated and Recalcitrant Compounds. The reviewer concluded that surfactant flushing, "while enticing in concept, is difficult to apply due to distribution issues in the subsurface" (Simon 2010). Only two case studies of surfactant flushing were presented at the 2010 conference, but five field studies of combined technologies that included natural water flushing were given, along with non-field presentations on cosolvent, alkaline, and super-saturated water flushing. Newell (2011) also suggested that the use of surfactant technology for chlorinated VOC sites seems to have decreased, possibly due to high cost and the complexity of the injection/treatment equipment.

The emphasis in this focus area is on practical implementation of flushing techniques in the field. The search for case studies to populate these pages revealed that while experimental studies of flush-enhancing chemicals are abundant in the peer-reviewed literature, recent examples of field-based flushing implementation appear infrequently. The majority of field application studies from the last decade were found in non-peer-reviewed conference proceedings, government-sponsored publications, and site cleanup documentation posted by environmental regulatory agencies.

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Technology Advantages

• Elimination of the need to excavate, handle, and transport large quantities of contaminated soil.
• Relatively small surface footprint for use in space-limited areas.
• As enhancement to pump and treat, may speed up site remediation and closure.
• Applicable to a wide range of contaminants in both vadose and saturated zones.
• Potential for use in combined remedies.

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Technology Limitations

• Not effective in low permeability or heterogeneous soils.
• Generally not applicable to dissolved-phase plumes.
• Potential for spreading contaminants beyond the capture zone laterally or vertically if the extraction system is not properly designed or constructed, hydraulic control is not maintained, or a groundwater discharge zone captures flow from the treatment zone.
• Regulatory acceptance issues due to the potential for spreading contaminants (Lee et al. 2007, AFCEE 2012); a greater degree of site characterization possibly required to satisfy both technical and regulatory requirements (GAO 2005).
• Uncertainties involved in predicting performance and duration to achieve cleanup goals.
• Performance potentially limited by site geology (e.g., low permeability; high clay or organic content; high degree of heterogeneity or secondary permeability; and close proximity to sensitive recharge areas or potable aquifers).
• Selection of inappropriate flushing solutions that reduce effective soil porosity by adhering to soil, accelerating biogrowth, or causing precipitation or other reactions with ambient soil or groundwater.
• Complex interdisciplinary technology when using chemical enhancements, requiring site- and contaminant-specific design, optimization, and control for successful implementation.
• Can require extensive laboratory testing to optimize chemical formulation.
• Generally requires recovery and treatment of flushing solution to remove contaminants as well as contaminant disposal.

Many terms have been used to describe in situ flushing techniques, such as soil flushing, water flooding, chemically enhanced solubilization for aquifer remediation, surfactant-enhanced aquifer remediation, and in situ soil washing. Soil flushing is sometimes confused with soil washing because both use aqueous solutions to mobilize contaminants, but flushing is conducted in situ, whereas soil washing is an ex situ process. Results from studies of the performance of chemical mixtures for removing contaminants from soils often are applicable to both ex situ washing and in situ flushing.

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General Resources

Technology Overview Report: In Situ Flushing
Root, D.S.
TO-97-02, 1997

This report prepared by the Ground Water Remediation Technologies Analysis Center (GWRTAC) provides an introduction to the general principles of in situ flushing technology, its criteria for application, performance, advantages and limitations to use, and references.

Surfactant Enhanced Aquifer Remediation
Pope, G.A., Center for Petroleum and Geosystems Engineering, University of Texas at Austin, 2003.

In Situ Flushing with Surfactants and Cosolvents
Strbak, L., National Network of Environmental Management Studies Fellow, 36 pp, 2000

This paper outlines the major principles associated with the use of surfactants and cosolvents, and briefly summarizes surfactant and cosolvent studies, demonstrations, and full-scale implementation.

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