“That land doesn’t mean anything to you?” In “Gone with the Wind,” Scarlett O’Hara learned that “land’s the only thing in the world worth working for, worth fighting for, worth dying for, because it’s the only thing that lasts.” What if a vacant, blighted swath of land, regardless of ownership, could be transformed into a dream home, a beautiful park, or a business? In this article, we are going to walk you through state-of-the-art remediation strategies and technologies that clean up contaminants and make brownfield redevelopment possible.

Redevelopment may increase local tax bases, facilitate job growth, utilize existing infrastructure through the improvement of the environment; consequently it provides greater protection of human health. This process also faces key challenges including management of environmental liability, and technical and financial barriers for cleanup.

Drivers for Remediation Decision & Planning
For a brownfield redevelopment project, a technically and financially feasible remediation plan is essentially determined by the property’s historic use and a reuse plan agreed upon by all stakeholders. Historic use determines the types of contaminants present at the site and the level of contamination. The reuse plan determines the required cleanup levels that fit the future land use needs and the required pace for remedial activities. There is no doubt that forming the reuse plan involves negotiations among all stakeholders, and the decision-making process needs to consider the current site status and evaluate all of the possible future use plans.

Driven by past, current and proposed property use, remediation technologies are then selected based upon the types of contaminants, contaminated media, degree of contamination, cleanup goals and cleanup schedule. For example, manufactured gas plants (MGP) were constructed nationwide from the early 1800s through the mid-1990s. An estimated 3,000 to 5,000 former MGP sites exist across the country (EPA, 1999). The gas manufacturing processes at these plants typically yielded wastes and by-products such as coal tar, polycyclic aromatic hydrocarbons (PAHs), petroleum hydrocarbons, and metals that contaminated soil, groundwater, surface water and/or sediments at the sites. The contamination levels vary site by site. Depending upon future use of the land (e.g. residential, industrial, or commercial), and therefore the corresponding cleanup goals, remediation technologies could be co-burning, thermal treatment, bioremediation, or soil vapor extraction, etc. For different reuse schedules, more aggressive approaches (e.g. thermal treatment) may be used to accelerate the remediation process, while more passive approaches (e.g. bioremediation) may be used to save remediation costs for less demanding reuse schedules.

Site Photographs Provided by Tetra Tech, Inc.

Permeable reactive wall installed to block a plume from a nearby lake allowing site redevelopment in Florida.

Project Strategy
Remedial goals for a brownfield site are established through a series of organized steps including:

Development of Conceptual Site Model (CSM)
The general strategy for developing remedial goals and evaluating potential innovative technologies for brownfield redevelopment begins with the refinement of a conceptual site model (CSM). This is an adaptive process, where historical data are evaluated to identify possible baseline data gaps. The CSM needs to be iteratively refined as more data are available until adequate accuracy and details of the CSM can meet project purposes.

Development of Remediation Action Objectives (RAO)
The RAOs specify the contaminants and media of interest, exposure pathways, and preliminary remediation goals that permit the consideration of a range of remediation technologies. Assuming the property end use and regulatory agency acceptance, Risk-Based Corrective Action (RBCA) is one of many important tools to be considered in development of RAOs. RBCA has an advantage over traditional corrective action: it applies risk assessment to remediation efforts. Every site is different in the level of risk posed to human health and ecologic environment, depending on the future land use, toxicity of the contaminants, and exposure pathways. It applies risk assessment to remediation efforts because every site is different in the level of risk posed to human health and ecologic environment, depending on the future land use, toxicity of the contaminants, and exposure pathways. Application of RBCA can determine site-specific RAOs and remediation activities that are defensible and potentially cost saving.

Understanding Challenges and Limitations
The actual results of remediation efforts are always subject to uncertainty, contributed by potential undiscovered contaminants and exposure pathways, unexpected difficulties in implementation, and/or regulatory changes. For example, it wasn’t until investigative work of the Massachusetts Department of Environmental Protection (MassDEP) in the 1990s that regulators began to understand that vapor intrusion could be a significant exposure pathway for contaminated groundwater. To overcome the uncertainties associated with environmental cleanup, performance-based contracting or remediation with cost-cap insurance can be an effective tool in remediation planning. We have a Guaranteed Fixed-Price Remediation (GFPR) program to shift the financial risk of site remediation and associated liabilities from clients to the remediation consultant/contractor. With cost certainty for site owners, the risk in cleaning up properties is reduced through clearly defined liabilities, reduced risk of legal action, and protection from regulatory changes.

Setting Realistic Expectations
As described above, the beneficial reuse of a property is determined by the site status, cleanup cost, and the uncertainty associated with remediation efforts. Although remedial activities will certainly increase property values, cleanup costs should not exceed the property value. For example, restoring dense nonaqueous phase liquids (DNAPLs) contaminated groundwater to drinking water standards is technically and financially prohibitive within a reasonable time frame.

Selection of a Technology
The recent boom in innovative technologies has created opportunities for successful site remediation in terms of both available technologies and favorable remediation cost and time. Table 1 summarizes the most common innovative technologies in the market and the key issues relates to brownfield redevelopment, such as land reuse timeframe. However, innovative technologies may not be the “silver bullet” for every site. The applicability of each technology, and the trade-offs in cost and time must be considered when selecting the appropriate technology. Conventional technologies may outperform innovative technologies in certain cases. Moreover, a broader concept of innovative technologies should include evolving/optimized conventional technologies.

The technologies listed in Table 1 are described below with applicable case studies.

Bioremediation
Bioremediation is a dynamic remediation technology that can be used to treat numerous contaminants including chlorinated solvents petroleum hydrocarbons, (PAHs), metals and some pesticides in both the source zone and the resultant plume. This technology is one of the more commonly used in situ approaches. Bioremediation involves the stimulation of indigenous microorganisms or the inoculation of laboratory-grown microorganisms to consume and reduce the contaminants into harmless compounds such as carbon dioxide and water. There have been tremendous advances in the last several years where numerous “food sources” such as food-grade vegetable oil, molasses and cheese whey have been shown to effectively stimulate the reduction of chlorinated solvents. However, perhaps more interesting is the industry’s advanced understanding of the specific microbes responsible for efficient biodegradation. Where these microorganisms are absent or present in low numbers, injecting these microbes into the subsurface can create more efficient biodegradation rates. Bioremediation has been shown to be effective for contaminant concentrations ranging from 10 times the drinking water criteria to cases where source zones contain DNAPL.

Bioremediation in Florida
At a site in Florida, we implemented bioremediation by injecting food-grade lactate and a consortium of microbes including Dehalococcoides to reduce trichloroethene (TCE). The implementation created a favorable geochemical environment so that the microbes were able to transform the TCE from concentrations of nearly 1,000 times the drinking water standards into ethene, carbon dioxide and water over a 12-month period. This was an effective case where bioremediation was implemented without an impact to above-ground operations at the same time reducing the risk to acceptable levels for potential receptors.

Chemical Reduction Using Zero Valent Iron
Zero valent iron (ZVI) is primarily used to break down chlorinated solvents in the subsurface to innocuous byproducts such as carbon dioxide, ethene or methane. These processes occur very quickly when the chlorinated solvents are in the presence of ZVI; however, as with all in situ technologies, the proper delivery of the ZVI to create this contact is the most challenging aspect of remediation via ZVI, especially in low permeability and/or heterogeneous soil types. Several approaches and techniques have been utilized recently to increase this contact, including permeable reactive barriers (PRBs), soil fracturing, soil mixing, and the use of nanoscale particles.

Often, source area treatment is a very difficult and costly proposition at large brownfield sites, and the remedial goal specifies protection of receptors (e.g., surface water and drinking water wells). In these cases, a PRB may be utilized. In a PRB, ZVI and sand are emplaced in a trench installed perpendicular to the groundwater flow direction encompassing the width and depth of the contaminant plume. As the groundwater flows through the high permeability area, the chlorinated solvents come in contact with the ZVI and are destroyed. As the groundwater exits the PRB, it is “clean” and downgradient receptors are protected.

PRBs in Texas
At a site in Texas, we planned to install two PRBs to protect downgradient receptors. The first is installed at the property boundary to prevent flow of chlorinated solvents beneath a residential neighborhood, to eliminate the potential of vapor intrusion into the households. The second protects a surface water body. In this case, the ZVI PRB is being utilized to reduce hexavalent chromium to trivalent chromium, a much less toxic and mobile form of the metal.

A method of obtaining source zone treatment in low permeability aquifers involves creating fractures in the subsurface via hydraulic or pneumatic fracturing. The ZVI can then be injected to fill the fractures. Groundwater in the area will then preferentially flow through the high permeability fractures, contacting the ZVI and expediting remediation. This method was utilized at another site in Texas, obtaining greater than 90 percent reductions of chlorinated solvent concentrations in an extremely low permeability matrix.

Soil Mixing in Virginia
A more aggressive approach for source zone treatment in heterogeneous soil is soil mixing with ZVI addition. Large diameter augers mix the source zone soil using clay as a drilling fluid. In this manner, low permeability soils (which tend to tightly hold a large mass of absorbed contaminants) are broken up and mixed with higher permeability soils to create a homogeneous treatment area. As the soils are mixed, ZVI is injected in the subsurface through the large diameter augers to provide intimate contact between the ZVI and the contaminant. The addition of the clay creates a low permeability zone that increases contact time between the ZVI and the contaminant and significantly reduces migration of contaminants from the source area. This technology has been utilized at less than ten sites thus far. At a closed military property in Virginia, the technology was used to great effect in a small high-concentration area. Soil and groundwater concentrations were reduced to levels that allowed unrestricted re-use of the treatment area.

In situ soil mixing destroys chlorinated solvent contamination to reach drinking water standards in Virginia.

Nanoscale ZVI in Florida
The use of nanoscale ZVI (nZVI) particles is an emerging approach to treatment. This technology involves using ZVI particles 1000 times smaller than the diameter of a human hair for better mobility and a higher reactivity in the subsurface. In 2005, we implemented one of the first full-scale applications of nZVI in a source area containing DNAPLs. Overall concentration was reduced between 65 and 99 percent across the impacted area. This aggressive approach coupled with long-term monitoring was effective in mitigating the risks associated with the plume, reducing some wells below drinking water criteria. While this technology has shown considerable promise, research continues to increase particle mobility for improved distribution in the subsurface.

Thermal Treatment
Thermal treatment is a very aggressive technology that can significantly reduce contaminant concentrations in the subsurface in less than one year of operation. Two of the most common applications of thermal treatment include, electric resistance heating (ERH) and thermal conductive heating (TCH). In the case of ERH, steel electrodes in the form of well points or driven plies, are installed in the subsurface. Electricity is introduced through the electrodes and travels through the soil column. The natural resistivity of the soil causes it to heat. As the soil heats and the boiling points of the contaminants are reached, the contaminants are transferred from the soil and groundwater to the vapor phase, where they can be recovered and treated via standard soil vapor extraction techniques.

ERH in Illinois
ERH was used to great effect at a site in Illinois. Concentrations of chlorinated solvents in the soil averaged greater than 445 parts per million. After operation of an ERH system for approximately four months, concentrations in the soil were reduced to an average of fewer than four parts per million. All soil sample reductions like these significantly increase the value of the treated area to site operators.

Chemical Oxidation
Remediation of groundwater contamination using in situ chemical oxidation (ISCO) is most successful when oxidants and other amendments are injected directly into the source zone and downgradient plume. The oxidant chemicals commonly used with ISCO include potassium and sodium permanganate, sodium percarbonate, hydrogen peroxide and ozone. The oxidants react with the contaminant, producing innocuous substances such as carbon dioxide (C0₂), water (H₂0), and in some cases, inorganic chloride. Some examples of contaminants that are amenable to treatment by ISCO include BTEX (benzene, toluene, ethylbenzene, and xylenes), tetrachloroethylene (PCE), trichloroethylene (TCE), dichloroethylenes, vinyl chloride (VC), MTBE (methyl-tert-butyl-ether), PAH (polyaromatic hydrocarbons) compounds.

ISCO in California
An industrial property in California had a short time frame for remediation due to a pending real estate transfer. As a result, ISCO was the chosen remedial alternative to address both petroleum and chlorinated hydrocarbon contamination. Several chemical oxidants, surfactants and cosolvents were tested during a bench scale study using soil, groundwater, and petroleum non-aqueous phase liquid (NAPL) samples. Chemical oxidation testing was performed on collected samples that simulated field conditions and generated treatment parameters. The study showed that persulfate activated with sodium hydroxide, iron, or H₂O₂ results in rapid oxidation of the chemicals of concern. Plans are currently being prepared to implement full-scale remediation to remove both the remaining NAPL and contaminant concentrations.

Use of Electrical Heating (ERH) to remediate elevated chlorinated solvent contamination in Michigan.

Monitored Natural Attenuation
Monitored Natural Attenuation (MNA) relies on natural processes to cleanup or attenuate contamination. MNA is an attractive approach because it can be used for numerous contaminants including volatile organics, semivolatiles and metals. MNA can be performed in a “passive manner” that allows for site development concurrent with the monitoring program. A common misconception regarding MNA is that it only includes physical and biological processes. However, significant research has shown that MNA also includes abiotic processes that may include chemical changes in the subsurface. These processes are often difficult to observe, but if characterized carefully they can be very effective in plume management. For example, a project in Minnesota involved the characterization of abiotic degradation as part of a long-term plume management approach that offset the cost and effort of implementing an active remedy.

Vapor Barriers and Mitigation
In many cases, site redevelopment can occur during the period of active remediation or in lieu of aggressive remediation. In these cases, land-use controls may be necessary to mitigate risk associated with the residual contamination on site. In addition to prohibitions for use of groundwater and contact with subsurface site soil, LUCs may be necessary to prevent vapors present due to the subsurface contamination to enter on-site buildings (i.e., vapor intrusion). Due to the importance of this issue, new guidance on vapor intrusion has recently been issued by the Interstate Technology and Regulatory Council and the U.S Army, Navy and Air Force.

An example of how this issue can effect remedial decisions for both industrial and residential re-use is found at an industrial complex containing significant chlorinated solvent contamination in the site soil and groundwater. Vapor intrusion monitoring indicated that volatile organic compounds were present in some areas of the operating facility above applicable OSHA standards. A vapor mitigation system was installed to capture and treat the vapors before they entered the work space.

Additionally, part of the facility being prepared for sale and residential development is located along a waterfront. Vapor intrusion issues that would be present during active remediation, and an expected extended period of site monitoring must be considered in remedial decisions and divestment/ development schedules. One option is to accelerate the site development by instituted LUCs that require all structures be installed with appropriate vapor barriers and/or mitigation systems.

Sustainable Remediation
Sustainable remediation considers the impacts of remediation activities on the environment. Selection of a remedy involves evaluation of energy consumption, off-gas and wastewater discharge, treatment system efficiency, and possible recycling of extracted chemicals. Renewable energy sources such as solar energy, wind energy, landfill gas and recycled vegetable oil may be used to supply power to the remediation system. Currently, most sustainable remediation projects involve pump-and-treat or soil vapor-extraction systems.

Pump-and-Treat System in Oklahoma
At an Air Force base in Oklahoma, a pilot-scale bioreactor is being used to remove TCE from groundwater. The site uses pumps powered by solar energy to extract contaminated groundwater. The water is circulated through a bioreactor at a rate of approximately 2-3 gallons per minute. Flow rates average nearly 1,000 gallons per day. As is the case with most small pump-and-treat systems, the energy-specific operational cost savings are relatively small. However, significant savings were achieved during remediation system installation by avoiding construction of power lines from the existing utility grid to the remediation system’s remote location.

Landfill Gas Systems across the U.S.
Consolidation and subsequent decomposition of solid waste in landfills results in the emission of gas that typically contains about 50% methane and about 50% CO₂, both of which are considered greenhouse gasses. Landfill gas collection systems are in place at many locations throughout the United States to capture the gas before it is released by the landfill. For example, we’re capturing methane from a landfill in Kentucky. In addition to reducing greenhouse gas emissions and reducing the carbon footprint, the landfill gas is being utilized for conversion to electricity and thermal energy. In other cases the gas can be converted to alternative fuels.

‘Treatment Trains’ and Conclusions
There are many conventional and innovative technologies that provide effective solutions for the remediation of brownfield sites. However, a combination of these technologies is often necessary to meet the project objectives. This is commonly referred to as a “treatment training,” whereby a combination of technologies are implemented in either a time sequence, or are selected based on their specific applicability to treat different areas within a single site. Treatment trains may combine an aggressive treatment such as bioremediation or chemical oxidation with a passive treatment such as MNA. In some states’ regulatory environments, such as Florida, natural attenuation cleanup criteria are established that allow an aggressive treatment process to be implemented until contaminant concentrations are reduced to levels at which MNA can be utilized. Another treatment train that has been shown to be effective is the combination of thermal treatment and bioremediation, whereby the rise in subsurface temperature increases the activity of the microbes, thus destroying the contaminants more efficiently.

Selecting the correct technology to meet the objectives at a given site is not a trivial matter. However, understanding the site conditions, setting appropriate expectations, and evaluating the “toolbox of technologies” will lead to the proper selection and implementation of the correct technology.

Author Biographies
Keith Henn, P.G., is the manager of the Remediation and Carbon Management Group for Tetra Tech, Inc. in Pittsburgh.

Chris Pike, P.E., is the lead engineer for Tetra Tech’s Navy CLEAN Program.

William Wright, P.E., is the manager of the Environmental Engineering Department for Tetra Tech, Inc. in Pittsburgh.

Betty Li, PhD, joined the Remediation Group of Tetra Tech, Inc. in 2007 as an environmental engineer.

References
Dellens, Amanda D., Green Remediation and the Use of Renewable Energy Sources for Remediation Projects, Case Western Reserve University for United States Environmental Protection Agency, www.clu-in.org, August 2007.

Interstate Technology Regulatory Cooperation, Technical and Regulatory Guidance for In Situ Chemical Oxidation of Contaminated Soil and Groundwater, June 2001.

U.S. EPA (1999), A Resource for MGP Site Characterization and Remediation: Expedited Site Characterization and Source Remediation at Former Manufactured Gas Plant Sites, Office of Solid Waste and Emergency Response (5102G), EPA 542-R-99-005, Washington, DC 20460