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Department of Crop and Soil Sciences, Washington State University Pullman, WA, 99164
bittelli{at}mail.wsu.edu
Panel of Conceptual Models of Flow and Transport in the Fractured Vadose Zone, National Committee for Rock Mechanics, Board on Earth Sciences Resources and National Research Council. National Academy Press, 2101 Constitution Avenue, Washington, DC 20055. 2001. Soft cover, 392 p. $69.00. ISBN 0-309-07302-2
Conceptual Models of Flow and Transport in the Fractured Vadose Zone is a thought-provoking collection of invited presentations originally delivered during a two-day workshop, convened by the National Research Council in March 1999, by a renowned group of water research specialists.
The book consists of the Panel's report, followed by 10 technical papers by scientific experts. The book covers an array of vadose zone research and includes an exhaustive survey of schematics, graphs, and photos. It is a thorough academic contribution. As noted in the preface of the book, the presentations not only describe the current state of conceptual models for flow and transport in the fractured vadose zone, but also provide guidance for government regulatory agencies and contribute knowledge and experience of related disciplines.
The book is arranged in 11 chapters. The first is the Panel's report, which consists of an overview of the general problem and modeling concepts, developing, and testing. Chapter two by Flint and coauthors is a summary of the hydrology assessment framework for the high-level radioactive nuclear repository at the Yucca Mountain site in Nevada. The authors describe the evolution of theoretical and experimental approaches to assess water flow in the highly heterogeneous, deep, unsaturated zone. The chapter provides descriptive references of the different conceptual models proposed by several investigators in the last 15 years. Particularly noteworthy is how the authors describe the evolution of the modeling concepts resulting from the need to address the expanding body of experimental evidence showing unexpectedly large transport rates, not predicted by the existing models. Chapter three by Jardine and coauthors addresses the problem of fractured weathered shales in a low-level radioactive waste repositories at the Oak Ridge Laboratory in Oak Ridge, Tennessee. The authors focus primarily on experimental techniques at different scales (laboratory, pedon, and field scale) to quantify physical nonequilibrium and to differentiate between matrix and fracture flow. The chapter provides an overview for readers interested in experimental methods, but gives little detail about actual modeling concepts, relying instead on references.
In Chapter 4, Berkowitz and co-authors discuss certain assumptions and theoretical treatments of phenomena related to flow and chemical transport in fractured media. The authors address, for example, non-Gaussian chemical distribution of the plume and an alternative approach to describe it, such as the continuous time random walk framework. The authors also stress the importance of using three-dimensional models for fracture flow quantification due to the strong three-dimensional nature of the flow patterns. This stimulating critical analysis emphasizes the necessity of developing new conceptual frameworks.
The use of different conceptual models depending on the scale of investigation of the system remains a problematic issue in hydrological science. In Chapter five, Hendrickx and Flury address this, and classify preferential flow based on the scale of its occurrence. Liquid flow in porous media can be described by different concepts, mathematical formalisms, and parameters, depending on the scale of investigation. The authors discuss evidence showing that preferential flow occurs at pore, field, and aerial or large scales, and they discuss modeling approaches for each of these scales. The chapter offers valuable material and references for readers interested in the "scaling problem" and its consequences for flow and transport. In similar fashion, Chapter 11 by Bodvarsson and coauthors addresses the application of "up-scaling" concepts for the parameterization of hydraulic properties of fractured media at the Yucca Mountain site. Bodvarsson and co-authors conclude that "up-scaling" methods conventionally used in homogeneous media cannot be used for fractured rocks because fractures may affect matric permeability on a larger scale.
A flow model for agricultural soils is described in Chapter 6 by Jarvis and Larsson. MACRO, a model for macropore flow, is currently under development for pesticide regulation in the European Union. The model solves the Richards' equation and the convection–dispersion equation, and it divides the flow domain into micro and macropores, with different hydraulic properties. Alternative concepts to describe water flow trough fractures are proposed by Dragila and Wheatcraft in Chapter 7 and by Doe in Chapter 8, where film flow and flow of discrete drops are noted as potentially significant factors in determining liquid transport in fractured media.
Chapter 9 by Phillips is mostly a summary of the use and limitations of different environmental tracers as tools for transport studies, where application to soils and rocks is considered and discussed. In Chapter 10, Neuman and co-authors presented a collection of results from experiments conducted at the Apache Leap Research site in Arizona, as well as conceptual models applied to the compelling study. The authors adroitly present the challenges of characterizing the hydrology of the site, such as potentials and limitations of using gaseous tracers rather than liquid, suspended or dissolved tracers. They also provide a numerical inverse interpretation of the results, and a thorough discussion of the results.
Overall, the book is a valuable collection of high-quality review papers that address pressing environmental water resource problems such as groundwater contamination from nuclear waste repositories, agricultural systems, and other sources. The book offers a diversity of approaches and ideas for addressing this problem, but the fragmented nature of our current knowledge about flow and transport in the fractured vadose zone is also apparent here. The authors recognize that there are still many unanswered questions, and they appropriately stress the need for additional research. A significant contribution has been provided in this collection of papers, however, and those interested or involved in vadose zone research will find it a stimulating, rewarding read.
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