**Book Review**:

*Gravitational Systems of Groundwater Flow: Theory, Evaluation, Utilization*, edited by József Tóth (2009), is published by Cambridge University Press (ISBN 978-0-521-88638-3).

__Laura L. Sanders, Book Editor__

Nearly five decades ago, József Tóth noticed a discrepancy between his field observations and conditions predicted by M.K. Hubbert's seminal work on groundwater flow. By Hubbert's model, streambeds consistently should be discharge areas and thus constantly supplied with water. However, in Tóth's study area in Central Alberta, Canada, streams frequently were intermittent or dry.

Perplexed, Tóth applied the Laplace equation to a hypothetical groundwater basin of simple geometry and boundary conditions, coarsely modeling his field setting. In a basin of undulating topographic surface with a linearly sloping water table, his results showed development of local, intermediate, and regional flow systems, with some streambeds situated in recharge areas—contrary to what Hubbert had proposed. The discovery explained a number of hydrogeological puzzles and inspired early basin-scale numerical modeling efforts. Tóth himself published more than two dozen subsequent papers exploring the topic.

Now Tóth has consolidated the work on gravity-driven systems into a single book. Six chapters comprise the slim, compact volume. The first introduces the basic concepts and terminology describing groundwater flow and reviews the mathematics describing flow systems, including the Laplace and diffusion equations.

Chapter 2 establishes the concept of the “unit basin,” the basic building block of the analysis. The hypothetical unit basin is isotropic, homogeneous, axially symmetric and bounded by impermeable vertical sides, an impermeable horizontal base, and a linear, sloping water table. A handy table at the end of this very short chapter summarizes hydraulic conditions in the recharge, midline and discharge portions of such a flow system.

In Chapter 3, Tóth explores more complex basin configurations, focusing on effects of basin geometry, including a water table developed with undulating topography, variations in basin depth, and major regional landforms. This chapter also describes effects of basin geology, including stratification, lenses, faults, and anisotropy, and effects of temporal changes and transient flow conditions.

In Chapter 4, Tóth explores groundwater as a geologic agent, reviewing 15 chemical, physical, and kinetic processes that perform geologic work. The material appears in other sources, and therefore may not be particularly revelatory, but Toth's placement of it in the context of flow-system hydrogeology is thought-provoking.

Chapter 5, by far the longest, gives case studies demonstrating application of gravity-driven flow principles to practical problems. The case studies individually are illuminating, but this chapter is particularly enlightening in its breadth. First illustrating how flow systems have been portrayed on regional hydrogeologic maps, Tóth then gives brief analyses of studies on municipal water supply development, waste disposal, mine dewatering, contaminant transport, salinization, geothermal energy, liquefaction, slope stability, land-use planning, nuclear waste isolation, and minerals and energy exploration. Nearly any field practitioner will find a situation relevant to their own work, and any one case would make a fine investigation for a graduate seminar.

In Chapter 6, a brief epilogue, Tóth contemplates the historical development of the science of hydrogeology. He describes hydrogeology as evolving from two separate sciences: natural and engineering, the former mainly occupied with explanations of springs and runoff, and the latter focused on water supply. He traces the development of thought in each area, shows them converging with the founding of modern hydrogeology, and predicts a future divergence of subfields. This material does not necessarily enlarge the reader's understanding of flow-system hydrogeology, but it is a thoughtful epilogue to a book presenting the life's work of an influential scientist.

**Gravitational Systems of Groundwater Flow**is thoroughly supported with citations to the relevant literature, both historical and current. Generously illustrated with clear diagrams and adequate photos, it includes an extensive bibliography. Two appendices give mathematical derivations of the most important equations. A wide-ranging 242-term glossary is provided.

The book is engaging reading for anyone interested in basin-scale flow. Scientists who create or interpret numerical models of groundwater flow will find it particularly relevant. The book is mainly written in standard scientific language, but occasionally Tóth switches to first person while describing a project particularly important in his development of the theory. This inflects the tale with an immediacy reminding the reader of the process by which science advances.

Source: Ground Water, Volume 48, Issue 6, page 786, November/December 2010.

**Book Review**: Gravitational Systems of Groundwater Flow: Theory, Evaluation, Utilization.