When an organic substance, either natural or anthropogenic, infiltrates into an aquifer it becomes a component of a dynamic bio-geochemical system. From the perspective of the subsurface microbe, these compounds may be benign, toxic, or a rich source of carbon in an otherwise carbon-poor environment. Microbes consume this carbon, producing energy, cell mass, and geochemically reactive byproducts. The transformation of organic toxicants by native microorganisms, sometimes known as intrinsic bioremediation, is considered one of the most promising remediation approaches for contaminated ground water. From a geologic perspective, however, rapid metabolic transformation of organic substances also results in dramatic changes in the geochemical ecology of that aquifer, changing the native microbial consortia, aquifer mineralogy and permeability, vadose-zone gas composition, and water chemistry. This lecture will examine the geology and geochemistry of microbial transformation of hydrocarbons using laboratory experiments, geochemical modeling, and field observations of contaminated aquifers, including results from the collaborative U.S. Geological Survey's Bemidji research program. Hydrocarbon degradation produces bicarbonate, acidity, and organic waste products, potentially changing the bulk geochemistry of the aquifer over wide areas, or more subtly, producing reactive micro-environments near attached microbes. How does oil degrade in ground water, what are the degradation byproducts, and what is the nature of the micro-environment created around an attached microbe? Are these biogeochemical reactions a significant contribution to subsurface mineral diagenesis? Is mineral weathering enhanced only by surface colonizing microbes, or do microbes affect equilibria by altering "bulk" pore-water chemistry? Do microbes colonize mineral surfaces in order to leach necessary nutrients, or is colonization controlled by surface charge and surface roughness? The goal of this lecture is to examine the geochemical consequences of subsurface microbial processes.

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