| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Natural attenuation is a topic of great practical importance that has gained widespread acceptance by regulatory agencies and entities responsible for site remediation. Despite its increasingly common implementation, many gaps remain in our knowledge of the underlying processes and their collective action; the use of natural attenuation might be far greater if the constituent processes and their interrelationships were more broadly understood and disseminated. Yong and Mulligan clearly had these issues motivating them as they produced a comprehensive text about natural attenuation processes occurring in soils. The treatment is ambitious, covering both metals and organic contaminants in roughly equal detail, and ranges from relatively fundamental to quite applied. The book is also coherent, unlike many in the environmental genre, reading as if it was written for the present purpose (rather than being a hodge-podge of retrofit material from other articles or books) and prepared by a single author (rather than the two who actually contributed). The authors take an expansive view of natural attenuation, covering topics from monitored natural attenuation to various engineered approaches, such as barriers intended to contain or reactively transform the target contaminants.
The book is divided into three sections of approximately equal length comprising eight chapters. The first section (Chapters 1–3) introduces the topic and provides background on water and solute transport in soils. The second section (Chapters 4–6) is concerned with more detailed exposition of the operative abiotic and biotic attenuation processes for metals and organic compounds. The final section (Chapters 7–8) attempts to integrate the foregoing process-level information to produce an overall understanding of the attenuation process and its potential pitfalls.
Chapter 1 introduces the concept of natural attenuation and sets the process within the context of available techniques for restoring land to different sustainable uses. Soil structure and composition and their impacts on water transport are the focus of Chapter 2. The chapter provides a brief overview of soil formation processes and soil size classification, including the role of microscale soil texture in establishing water flow according to the Poiseuille and Karmen–Cozeny equations. The third chapter makes the transition between the more general material of the first two chapters and the more specific material to follow. It includes a discussion of surface charge on soil particles and its role in ionic solute uptake as described (qualitatively) by surface complexation models, although the use of these models in predicting metal uptake is not specifically outlined. The remainder of the chapter examines water retention in unsaturated soils, sorption mechanisms, and chemical reactions in pore water and introduces the governing equations for unsaturated and saturated solute transport.
The second section of the book delves into the specific processes governing the attenuation of metals and organic contaminants in soils. The reaction and partitioning of metals in soil solutions is covered in Chapter 4. Topics include the general properties of metallic contaminants, the effects of acid–base and redox reactions on their speciation, batch and column tests for assessing adsorption strength, and the effects of solution composition (e.g., competing adsorbates or ligands) on the sorption process. The chapter concludes with an extensive and helpful review of sequential extraction methods for assessing metal association in soils.
Chapter 5 outlines abiotic processes governing organic contaminant mobility parallel to that presented in Chapter 4 for metals. The treatment is primarily oriented toward petroleum hydrocarbons and chlorinated solvents, but is sufficiently general to be relevant to a broader spectrum of contaminants (i.e., emerging contaminants could be captured under the ideas presented, but are not explicitly considered). The advection-dispersion model for solute transport is introduced, but like much of the coverage of theory in the book, it is a bit cursory. For example, the distinction between molecular diffusion and dispersion is blurred, providing little feel for the origin and nature of the parameters. The hydrophobic organic compound "partitioning model" is introduced and explained, followed by a brief treatment of the importance of nonaqueous phase liquids (NAPLs) in site remediation. Biotransformation of both organic and metallic contaminants rounds out this section in Chapter 6. The chapter begins with a review of organism classification (bacteria, protozoa, plant, etc.) and briefly introduces models for growth and substrate utilization kinetics. Processes that limit or alter biotransformation are qualitatively discussed. The chapter concludes with a series of generalizations about the biodegradation of various classes of organic contaminants, metals, nitrogen, and sulfur. Given the critical role of biotransformation in all natural attenuation processes and the greater complexity in anticipating the success of this component of the process, a greater emphasis on the measurement and prediction of intrinsic bioremediation potential of soils would have been warranted. For example, there is almost nothing in the book regarding current approaches to conducting biostimulation or bioaugmentation.
The final section of the book attempts to integrate the individual flow and transport processes described in Chapters 3 through 6 into a more holistic view of natural attenuation. Chapter 7 focuses on methods of assessing natural attenuation, including procedures for monitoring the process, components of monitoring networks, and geotechnical techniques for probing subsurface conditions. Appropriate consideration is given to the importance of monitoring byproduct formation in addition to parent compound disappearance and to the need to evaluate whether contaminant attenuation observed in the field is sustainable in the longer term. Mathematical models that have been used to predict and/or rationalize attenuation results are then reviewed and critiqued, followed by a review of established natural attenuation assessment protocols (e.g., those from ASTM, USEPA, DOE, or others). Case studies on natural attenuation of chlorinated solvents and MTBE complete the chapter, but these are brief and provide few specifics. Future directions in natural attenuation application and research are considered in the concluding chapter. Improvements in the assessment protocols described in Chapter 7 are suggested, including expanded or enhanced laboratory and/or field measurement of site specific attenuation factors. A discussion of varied natural attenuation approaches (beyond monitored natural attenuation) such as permeable reactive barriers and containment strategies concludes Chapter 8. An extensive and well-organized index is also provided; my "spot-checks" of its utility found no errors and no omissions, which is appreciated in a reference of this type.
Throughout the book a proper emphasis is placed on the need for mechanistic understanding of the underlying processes and the application of site specific (experimental) information; however, the promise of the book is never fully realized as it does not provide useful specifics about how to do the things it suggests that practitioners need to do. Although the authors should be commended for their extensive coverage of natural attenuation processes and applications, the failure to fully link the process descriptions in Chapters 3 through 6 to the more holistic information in Chapters 7 and 8 represent a missed opportunity. The authors are explicit about the fact that the book is not a "fundamental principles" book, stating in the preface that it was not their "intent to develop or present extensive basic theories in any one discipline area of this multidisciplinary problem..." Notwithstanding the preceding comments, the book delivers much of what it promises and certainly is an advance over state-of-the-art "cookbooks" currently available to assess natural attenuation. The authors never clearly define their target audience, but many readers may find it fully satisfactory for use in practice, for example, as a complement to more cookbook approaches prepared by agencies, or as a textbook for a college class. However, its use for this latter purpose is hampered by the absence of any example problems or potential homework questions.
Civil & Environmental Engineering, University of California, Davis, Davis, CA 95616
(tyoung{at}ucdavis.edu)
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
