Hydrogeology Journal (2000) 8 : 2±3  Springer-Verlag Preface ± Groundwater and microbial processes Barbara Bekins, Guest Editor The subsurface below the soil zone harbors a large but poorly understood variety of microorganisms whose activities are important to both geologic pro- cesses and human endeavors. With the expansion of interest in this field, new techniques and strategies have been formulated to facilitate studies of subsur- face microbial processes. These include collection of uncontaminated deep cores, analyses of microbial biomarkers, and interdisciplinary collaborations between hydrogeologists, geochemists, and microbiolo- gists. These techniques are beginning to provide some insights into the subsurface biosphere. The guiding philosophy in organizing this theme issue on ªGroundwater and Microbial Processesº has been to provide an overview of the current state of this rapidly changing field. The articles are organized around three main themes: populations of microorgan- isms in the subsurface; the role of subsurface microor- ganisms in controlling chemical reactions; and methods of studying subsurface microorganisms. The discovery of viable populations of microorgan- isms in the subsurface at unexpected locations has been one of the major findings at the close of this cen- tury. Documentation of microbial activity at increas- ingly deeper, hotter, and nutrient-limited locations challenges current understanding of survival mech- anisms, maintenance energy, and growth rates. The article by Krumholz provides an overview of the strategies used by microorganisms to survive in the deep subsurface. With more than two-thirds of the planet covered by oceans, the subsurface below the seafloor comprises an enormous volume of sediments and oceanic crust that is now recognized as an impor- tant microbial habitat. Parkes and his colleagues pro- vide a summary of recent observations of bacterial populations and activities below the seafloor. Similar to the surprising survival of microbial populations in extreme subsurface conditions, the persistence and transport of viable pathogenic microorganisms in shal- low aquifers used for drinking water is also greater than previously thought. Macler and Merkle estimate that a significant fraction of water-borne disease out- breaks in the U.S. is associated with well water. In their paper they describe pathogenic microorganisms of concern for groundwater and methods for their control. The effect of subsurface microbial processes on chemical reactions in aquifers has been the subject of a large body of research. Chapelle notes that microor- ganisms were first recognized as an agent in control- ling groundwater chemistry around petroleum deposits in the early 1900's. He describes the importance of microorganisms in mediating reactions in aquifers that affect both geologic processes and human endeavors. The paper by Bennett and colleagues shows that microbially-driven reactions in aquifers can be due to either macroscale changes in aqueous chemistry or microscale perturbations around a single microorgan- ism or colony. Four papers in this issue pertain to microbial degradation of subsurface contaminants. Haack and Bekins summarize observations of micro- bial populations in contaminant plumes. They discuss how variations in populations and associated degrada- tive capabilities result from biological, chemical, and hydrologic factors. Lovley and Anderson provide an overview of the role of dissimilatory metal reduction on the fate of groundwater contaminants. They explain the importance of this process to natural attenuation of both organic and inorganic contami- nants. Hoyle and Arthur comment that, although pes- ticides are increasingly being detected in groundwater, most studies have focussed on their fate in soils and the unsaturated zone. In their summary of the existing literature on biotransformation of pesticides below the water table, they note that subsurface physical and chemical heterogeneity may cause confusing and inconsistent results in biotransformation studies. Brad- ley describes the known biodegradation reactions of chlorinated ethenes and their implications for natural attenuation. He suggests that a number of newly doc- umented anaerobic oxidation reactions may affect fate of chlorinated ethenes in groundwater under anaero- bic conditions. Many of the papers in this issue describe how phys- ical and chemical variations in the subsurface result in changes in microbial populations. The implication for Barbara Bekins U.S. Geological Survey, MS 496, 345 Middlefield Rd., Menlo Park, CA 94025 Fax: +1 650-329-5590 e-mail: babekins@usgs.gov