Timely and cost-effective restoration of contaminated soil and groundwater continues to be one of the most challenging problems facing environmental scientists and engineers. Remediation of contaminated subsurface environments is a multi-billion dollar business owing to years of improper waste management and disposal practices at tens of thousands of sites around the globe, and the current absence of technologies to effectively clean many of these sites.

Several factors contribute to our inability to remediate contaminated subsurface environments. First, subsurface heterogeneities often create regions of low water permeability. Over long periods of contaminant release to the subsurface, pollutants can diffuse into these low-permeability zones. When a conventional remediation technology such as pump-and-treat is applied, pollutants must diffuse back out of these low-permeability zones before they can be captured by the pump-and-treat system. Second, pollutants may desorb slowly from natural soil, or, in cases of gross subsurface contamination, pollutants may be present as nonaqueous phase organic liquids (NAPLs). For these cases, remediation efforts may be further limited by pollutant desorption and/or dissolution rates. Many organic pollutants are highly resistant to biodegradation or chemical transformation under natural conditions, and usually cannot be transformed unless present in the aqueous phase. Oftentimes, groundwater is contaminated by a variety of organic and inorganic pollutants, and a remediation technology that works well for one pollutant may not be appropriate for another. When pump-and-treat remediation systems are used, they are rarely designed for optimum performance.

Most of us would agree that much work remains to be done. As we transition into the 21st century, it is also apparent that this is an exciting time for engineers and scientists studying remediation technologies. Our research activities of the past and next 10 years will have a dramatic impact on the quality of the subsurface environment for the next century. In 20, or even 10 years from now, our approach to subsurface remediation will probably be vastly different than it is today. Many of the emerging technologies presented here will form the basis of standard remediation practices.

The first two chapters of this book address the design of pump-and-treat systems for remediation and/or containment of contaminated groundwater. Using powerful optimization algorithms built on top of conventional flow and solute-transport models, groups from the University of Massachusetts and the University of Virginia demonstrate the optimal design of pump-and-treat remediation systems. Using appropriate constraints and penalty functions, the optimization algorithms efficiently search for the optimal system design with regard to variables such as well pumping rate, well location, and management periods. These relatively inexpensive simulations can significantly reduce the cost and improve the effectiveness of pump-and-treat remediation technologies.

Chapters 3 through 5 present emerging technologies for chemical, electrochemical, and biochemical remediation processes. Munakata and Reinhard from Stanford University discuss the advantages and limitations of palladium catalysts for the reductive dehalogenation and hydrogenation of a variety of contaminants in wastewater. Researchers from Northeastern University and the U.S. Environmental Protection Agency discuss the transport and degradation of groundwater pollutants subjected to electric fields and include a discussion of the mobilization of pollutants from low-permeability media.

Chapters 6 through 8 focus on the use of sorptive and reactive in situ treatment walls. Alan Rabideau and his co-workers begin this series with an overview of sorptive vertical barrier technologies. They explore both low-and high-permeability systems and present two case studies of barrier performance. Baolin Deng and Shaodong Hu from New Mexico Tech discuss the reaction rates of chlorinated solvents on zero-valent iron surfaces. Their analyses will help guide future designs of these reactive treatment walls. Robert Bowman and his co-workers describe a pilot-scale test of a permeable, sorbing treatment wall composed of a surfactant-modified zeolite at the Large Experimental Aquifer Facility of the Oregon Graduate Institute. In this study, they observe and analyze problems associated with permeability reductions in the treatment zone.

Surfactant-enhanced aquifer remediation for both sorbed pollutants and nonaqueous phase liquids is addressed in Chapters 9 through 13. Chapters 9 and 10 discuss the effects of surfactants on the sorption of organic contaminants to natural soil. Seok-Oh Ko and co-workers present data on the equilibrium distribution of pollutants in the presence of surfactants, whereas James Deitsch and Elizabeth Rockaway discuss how surfactants can increase a pollutant's desorption rate by increasing the desorption mass-transfer coefficient and the desorption concentration gradient. In Chapter 11, Wu and co-workers from the University of Oklahoma discuss surfactant selection for separate-phase oil removal from the subsurface. They demonstrate that surfactant hydrophobicity should relate to oil hydrophobicity, and they describe a method to quantify the hydrophobicity of multicomponent nonaqueous phase liquids. In the next chapter, Kibbey and co-workers examine the use of surfactant/alcohol mixtures to reduce the density of nonaqueous phase liquids prior to their mobilization. This approach prevents downward migration of the nonaqueous phase liquid and possible complications with its extraction from the subsurface. In Chapter 13, Taylor and Pennell from Georgia Tech report on the use of surfactant/ethanol formulations to increase the solubilization of nonaqueous phase tetrachloroethene.

Finally, Chapters 14 and 15 address the remediation of the unsaturated zone. In Chapter 14, researchers from the University of Virginia study the effects of natural atmospheric pressure variations on the flow of air into and out of the unsaturated zone at the Picatinny Arsenal in northern New Jersey. This "barometric pumping" contributes to the natural remediation of the shallow, trichloroethylene-contaminated groundwater. In the last chapter of the book, Richard Meixner and co-workers present a detailed field study documenting the effectiveness of soil-vapor extraction to remediate gasoline hydrocarbons in the unsaturated zone.

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