by Corey Lynch & Kathryn Liddell
In 2007 the U.S. Environmental Protection Agency estimated that “294,000 sites will need to be cleaned up over the next 30 years. This includes 77,000 known sites and an estimated 217,000 sites yet to be discovered. Total clean-up costs are estimated to be $209 billion” (Coker, 2007, p. 28). With a task of this scale, efficient and cost-effective soil remediation techniques are essential. Implementing these remediation techniques involves thorough research of the contaminated site’s characteristics and the remediation technique’s abilities, advantages, and disadvantages. For soil remediation technologies to be most effective and applicable, biological, chemical and physical remediation technologies must be combined based upon contaminated sites’ characteristics.
Bioremediation Techniques
In the past, the only option for the remediation of toxic chemicals was the incineration of the contaminated areas. Today, more responsible techniques, such as bioremediation, are being employed. Bioremediation is the use of biological agents, such as bacterium or plants, to convert dangerous organic compounds into more stable carbon dioxide, methane, water or inorganic salts. As stated by the EPA in the Superfund annual report in 2001 “[b]ioremediation is being used with increasing frequency to remediate contaminated media at hazardous waste sites…” (Environmental Protection Agency, 2001, p. 1).
Brian Timmins, director of Environmental Technologies, states that, “…if it is soluble, it is likely biodegradable and it is just a function of establishing the right microbial conditions” (Personal Communication, 2009). To create these conditions, you can either follow the in situ, above ground, or the ex situ, underground, techniques. In situ techniques are utilized on site, and are far less costly to conduct. This approach involves five steps. First is bio-venting, when contaminated unsaturated soils are aerated as to stimulate biodegradation. Second is the slurry lagoon aeration phase, in which contaminated soils come into contact with natural air in a lagoon to promote biological degradation. Next is biosparging, in which forced air is added to a ground water system to improve biodegradation and volatilization of contaminants. The final phase occurs under both aerobic and anaerobic conditions to complete the breakdown of the contaminant.
Unlike in situ techniques, ex situ methods can be conducted off site, but can be far more costly, not only in dollars and cents but in risk of further site contamination. Because of the great risk, the ex situ method is still often conducted on site. The ex situ method is done in four phases. The first phase is general land treatment, in which the first excavation of contaminated soils happens. The contaminated soil is removed and deposited in lined beds and periodically turned over to ensure aeration. The next phase is composting, in which the contaminated soils are mixed with “bulking agents” to add surface area to the mixture. Next is the bio-pile phase, in which the bulked soil is placed in above ground enclosures. The last phase is the slurry-phase treatment. According to the EPA in 2001, “[a]n aqueous slurry is created by combining soil, sediment, or sludge with water and other additives. The slurry is mixed to keep solids suspended and microorganisms in contact with the contaminants. Treatment [of the contaminants] usually occurs in a series of tanks” (Environmental Protection Agency, 2001, p. 4).
Chemical and Physical Remediation Techniques
Other remediation technologies include chemical remediation and physical processes, which, among numerous options, can also be divided into in situ and ex situ techniques. In situ is performed at the contamination site, minimizing the exposure pathways. Ex situ techniques involve the removal of contaminated soil. The soil is either treated on-site or moved to another location for treatment (Sparks, 2003).
In situ techniques include volatilization and isolation and containment. Volatilization is based upon air venting through the soil by injecting or inducing a draft fan in the soil. This results in airflow through the soil. Soil particle movement is restricted but air movement can freely occur. The volatilized contaminant is then recovered using treatments such as activated carbon. Volatilization is limited to volatile organic compounds, but is relatively low-cost (Sparks, 2003). When considering volatilization it is important to note that “[volatilization] of hazardous chemicals is both a public health and air quality concern” (Coker, 2007, p. 29). In isolation and containment, subsurface physical barriers are installed to minimize lateral migration of contaminates. These barriers can be things such as clay liners and slurry walls (Sparks, 2003). The addition of surfactants to clay minerals is another method of minimizing contaminant movement. Surfactants can enhance the retention of organic pollutants, further minimizing the mobility of pollutants (Sparks, 2003).
Ex situ techniques include solidification/stabilization, excavation, and chemical extraction. In solidification/stabilization, an additive is added to the removed contaminated soil, encapsulating the contaminants. The soil mixture is then landfilled so the contaminants cannot move freely (Sparks, 2003). The accepting landfills are usually lined to minimize the mobility of the contaminants and are located in a region of low soil permeability. Excavation and disposal costs are high, and can include long-term liability (Sparks, 2003).
Remediation of soils contaminated with heavy metals can be expensive therefore the immobilization of a heavy metal can be a cost-effective solution (Alpaslan & Yukselen, 2001). A study by Alpaslan and Yukselen (2001) showed that lime and cement are effective in lead immobilization, while activated carbon and clay are not very efficient in lead immobilization. Determining the best chemical additive for the contaminant(s) present is important in developing an efficient remediation plan. In the chemical extraction technique, “the soil is mixed with a solvent, surfactant, or solvent/surfactant mixture to remove the contaminants” (Sparks, 2003, p.28). Lead, “the most frequently found metal at U.S. hazardous waste sites,” can be extracted using Water-Soluble polymers (WSP) (Sauer, Ehler, & Duran, 2004, p.585). When lead is bound to the WSP it can easily be concentrated by ultrafiltration to allow for the recycling of the extraction agent and the disposal of lead (Sauer, et al. 2004). In a study by Sauer, Ehler, and Duran (2004), WSP removed more than 97% of lead from contaminated Superfund soil samples. As this study demonstrates, chemical extraction can be an effective technique for the remediation of soils contaminated with heavy metals.
Other newly developed chemical and physical remediation technologies have only been proven on a laboratory scale. Ultrasound remediation has shown to be successful on a laboratory scale, but applying ultrasound in industry requires considering the costs of larger scale application (Shrestha, Duong, & Sillanpaa, 2009). In a study by Shreshtha, Pham, and Sillanpaa (2009) ultrasonication was shown to reduce concentrations of persistent organic pollutants (POPs) such as hexachlorobenzene (HCB) and phenanthrene (PHE). High-frequency ultrasound has also been shown to effectively remove DDT from sand slurries (Thangavadivel, et al., 2009). Intensity limitations in currently available equipment and low volume coverage make the practicality and affordable application of high-frequency ultrasonication currently only possible in laboratory settings (Thangavadivel, et al., 2009).
Superfund Overview
In the 1970s there were many abandoned toxic sites scattered across the country with no one organization, governmental or private, willing to take charge of the cleanup actions. In the late seventies to early eighties, Congress passed the Comprehensive Environmental Response, Compensation and Liability Act (also known as Superfund) in the wake of the discovery of many of the toxic sites, and the EPA began organizing the cleanup of 114 of the worst sites across the country. Superfund sites range from national disaster areas such as Love Canal in Niagara, New York, to smaller, yet equally important sites such as the Buzzards Bay PCB cleanup and the Hatheway & Patterson site, located in Mansfield Massachusetts, which is a 56 year old time capsule of largely unknown contamination. The EPA, with Superfund as its back, now has the power necessary to “compel responsible parties to perform cleanups or reimburse the government for… [these] cleanups” (Environmental Protection Agency, 2009, p. 1).
Combining Remediation Techniques for More Effective Results
Understanding the biological, chemical and physical remediation techniques discussed above is important in determining the most effective remediation plan for a contaminated site. As stated by Mark Brusseau, a Professor at the University of Arizona in the Department of Soil, Water and Environmental Science, in 2009, “[e]ach technology is suitable for only a subset of site types…[the] most effective [technologies] are very site specific depending on the nature of the site and the remediation objectives. … [N]o single technology will be the perfect solution. At most sites, multiple technologies are used, either simultaneously or sequentially” (personal communication). The combination of remediation techniques is exemplified in the Superfund site Hatheway & Patterson, in Mansfield, Massachusetts. Hatheway and Patterson Company is a former wood preserving facility comprising approximately 40 acres. It is bordered by residential properties, forested and wetland areas, and a welding and masonry supply company. Releases of chemical compounds have impacted fisheries and wetlands and threaten ground water of municipal and private drinking water wells (Environmental Protection Agency, 2009). Hazardous compounds located at the site include dioxins, furans, arsenic, chromium, copper, PCP and PAHs (Environmental Protection Agency, 2009). The Record of Decision, signed in 2005, states that remediation components will include excavation, stabilization/solidification, and off-site disposal of soils contaminated with dioxin and oily material (Environmental Protection Agency, 2009). Clean backfill will replace the excavated soil. The excavated soils containing PCP, semi-volatile organic compounds, and arsenic will be tested for leachability then stabilization/solidification agents will be used to immobilize pollutants. The treated soil will then be disposed of on-site under a low-permeability cover (Environmental Protection Agency, 2009). The remediation approach to this site includes excavation, stabilization/solidification, containment, and off-site landfill disposal. These techniques are used for particular contaminates within the site, and combined to create the most effective clean-up of the contaminated site. “Several factors are considered when deciding which technology to use at a particular site. An initial critical factor is the feasibility of a particular technology for that specific situation. … then several other factors are part of the evaluation – cost, long-term effectiveness, regulatory, and public acceptance” (Mark Brusseau, personal communication, November 13, 2009). Approaching site remediation with collective techniques can be more effective in extensive site remediation than the use of a single technique.
Unlike the Hatheway & Patterson Superfund site, the Buzzards Bay clean up could have employed more responsible techniques. Before being banned by the EPA in 1979, polychlorinated biphenyls, or PCBs, along with hundreds of other industrial and municipal wastes, were dumped directly into the Acushuet River and deposited in the bay. Today the sediment beneath Buzzards Bay is contaminated with layers of industrial waste and PCBs. PCBs were known to be above allowable concentrations all through the 1970s, and in 1979, the same year the bay was closed to commercial and recreational fishing and lobstering, they were found in concentrations “high as 100,000 ppm” (Environmental Protection Agency, 2001, p. 1) (the maximum allowable concentration is 50 ppm). Instead of applying the more widely accepted chemical, physical, or bio remediation techniques, the established group simply dredged the bay, and incinerating the contaminants. Instead of incinerating the PCBs, which reintroduces them into the environment in a different form, ex-situ remediation could have been performed.
Examination of remediation techniques reveals the wide availability of biological, chemical, and physical technologies that may be utilized to create an efficient clean-up process. The Hatheway & Patterson Superfund site is an example of combining techniques for responsible cleanup, while sites such as Buzzards Bay exemplify weak remediation strategies. “The innovation… to interconnect physical, chemical, and … biological technologies into a unified site-specific, coordinated system” (p. 36) avoids the “standard industry practices” (p. 36) in which “these technologies would normally have been deployed as standalone systems and prone to significant deficiencies or failure” (Vigneri, Adams, Scrudato, 2007, p. 36). The remediation challenges facing Americans require the re-evaluation of available remediation technologies in order to create the most efficient, cost-effective plan for each contaminated site.
References
Alpaslan, B. & Yukselen, M.A. (2002) Remediation of lead contaminated soils by stabilization/solidification. Water, Air, and Soil Pollution, 133(1-4), 253-263. doi: 10.1023/A:1012977829536
Buzzards Bay National Estuary Program (1992) Comprehensive Conservation and Management Plan the 1991 CCMP (8/91 version approved by EPA in 1992) (Chapt. 6)
Coker, C. (2007). Clean-up methods without chemicals. In Business, 29(1), 28-29. Retrieved from ebscohost.com
Sauer, N.N., Ehler, D.S., & Duran, B.L. (2004). Lead extraction from contaminated soil using water-soluble polymers. Journal of Environmental Engineering, 130(5), 585-588. doi:10.1061/(ASCE)0733-9372(2004)130:5(585)
Shrestha, R. A., Pham, T. D., & Sillanpää, M. (2009). Effect of ultrasound on removal of persistent organic pollutants (POPs) from different types of soils. Journal of Hazardous Materials, 170(2), 871-875. doi:10.1016/j.jhazmat.2009.05.048
Sparks, D.L (2003). Environmental Soil Chemistry, 2nd ed. Boston: Academic Press.
Thangavadivel, K. et al. (2009). Application of high frequency ultrasound in the destruction of DDT in contaminated sand and water. Journal of Hazardous Materials, 168(2-3), 1380-1386. doi:10.1016/j.jhazmat.2009.03.024
U.S. Environmental Protection Agency. (October 2009). Waste Site Cleanup & Reuse in New England: Hatheway & Patterson. Retrieved from epa.gov
U.S. Environmental Protection Agency. (September 2001) Use of Bioremediation at Superfund Sites. Retrieved from epa.gov
Vigneri, M., Adams, R., & Scrudato, R. (2007). Remediation for those hard-to-reach places. Pollution Engineering, 39(6), 36-40.