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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