Ground-Water Microbiology
and Geochemistry

Preface

This book is about microbial processes and how they affect the chemical composition of ground water. Ten or fifteen years ago, this topic was considered to be merely an interesting novelty. Since then it has become clear that microbial processes do more to abate and detoxify human-induced contamination of ground-water systems than any other natural or human-contrived mechanism. Furthermore, it has become clear that the chemistry and water-quality of pristine ground-water systems-which produce about half of the world's drinking water-is largely determined by microbial processes. For these reasons, understanding the unique ecology of ground-water systems has gone beyond being a novelty, to being an important component of water-resources engineering and management.

But understanding the microbiology of ground-water systems, and using this understanding to solve practical water-quality problems, is not easy or straight-forward. Microbiologists are trained to understand the complexity and diversity of microbial processes, but seldom have a background in geology or hydrology. Thus it is often difficult for them to appreciate how geologic and hydrologic factors mold and shape the expression of microbial processes in subsurface environments. Geoscientists, on the other hand, understand the complexities and uncertainties inherent to ground-water systems, but generally lack formal training in microbiology. Thus it is often difficult for them to appreciate how the metabolism and physiology of microorganisms can affect hydrogeologic systems. This book is specifically designed to bridge this gap between applied microbiology and applied hydrogeology.

This book is also designed for civil and environmental engineers. Engineers are trained to solve problems, and thus are often called upon to find practical solutions to human-induced and natural water-quality problems. By giving an overview of how geologic and microbial processes interact, and by giving numerous examples of how water-quality problems can be addressed using these principles, this book can serve as a resource for engineers dealing with ground-water quality issues.

This book is divided into three parts. Part I is an overview of basic microbiology. Many geoscientists and engineers have not had formal training in microbiology and often are not familar with microbiologic techniques and nomenclature. Although not intended as an exhaustive treatise or as a substitute for formal training, this lays the groundwork for how the growth and metabolism of microorganisms affects mineral dissolution processes, redox geochemistry, and contaminant biodegradation. In particular, this section shows how new methods of molecular ecology can be applied to ground-water systems.

Part II focuses specifically on microbial processes in pristine ground-water systems, and shows how these processes are shaped by geologic and hydrologic factors. In particular, this section shows how the microbial ecology of subsurface environments-and particularly how carbon, oxygen, hydrogen, sulfur, and iron cycling-affect ground-water chemistry. Because these well-known biogeochemical cycles are often truncated by isolation from solar energy, it is often possible to predict certain water-quality trends in subsurface environments. In addition, this part contains a systematic evaluation of how microbial processes affect redox geochemistry, and provides methods for identifying the distribution of microbial redox processes in ground-water systems.

Part III deals with the biodegradation of human-introduced contaminants in ground-water systems. There have been rapid advances in our understanding of how microbial processes contribute to the degradation of contaminants in ground-water systems in the last ten years. This section gives an up-to-date overview of the physiology and biochemistry of important biodegradation processes. The treatment of chlorinated ethenes shows how redox conditions affect biodegradation efficiency, and includes an overview of how reductive, oxidative, and cometabolic processes contribute to biodegradation. This section gives several examples of how biodegradation processes affect the mobility of petroleum hydrocarbons, chlorinated solvents, pesticides, and herbicides in different ground-water systems.

There are many reasons for studying the microbiology of ground-water systems. One reason is that they are unique ecosystems containing unique microorganisms and microbial processes. Thus, understanding these systems will contribute our overall understanding of microbial life on earth and possibly on other planets as well. Another reason is that naturally occurring water-quality problems-such as excessive concentrations of dissolved sulfide or well-clogging by iron-oxidizing bacteria-can be solved or mitigated by understanding the underlying microbial processes. Finally, subsurface microorganisms can help abate many kinds of environmental pollution. The common ground for each of these issues is the chemical quality of ground water. This, therefore, is the focus of this book.

Acknowledgments

This book was funded by the Toxics Substances Hydrology Program of the U.S. Geological Survey. Special thanks are extended to Herbert T. Buxton, the coordinator of this program, who has always recognized the importance of microbial processes in contaminant hydrogeology, and whose support has produced some of the finest research in this field. This book could not have been written without the help and administrative support provided by Marjorie S. Davenport, the U.S. Geological Survey District Chief of South Carolina. The author would like acknowledge James Ray Douglas for preparing the illustrations for this book, and to thank him for his exceptional talent, admirable work ethic, and commendable patience.

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