Biota can occur in all parts of a waste-injection system. A review of the literature and the results of field studies show that bacteria are the most important organisms in the biologic communities found in natural groundwaters. Bacteria are thus expected to be the principal inhabitants of formations in the vicinity of waste-injection wells. Versatility in adaptation to unusual environments suggests that bacteria will be excluded from waste-injection wells only under the most extreme conditions. The composition, size, and activity of a bacterial population depends on many factors, including temperature, pH, salt content, concentration and types of nutrients available, and oxygen concentration. Because groundwaters normally contain little oxygen and are under reducing conditions, anaerobic species are expected to predominate in this environment.
Bacteria are responsible for some types of corrosion in waste-injection wells, and their cell masses can clog formation faces. Bacterial travel in confined aquifers is negligible, and survival time is short; hence, danger to public health from pathogenic organisms in wastes is slight. Exceptions may occur in highly permeable strata. Microbial growth has been observed in the vicinity of wells into which reclaimed sewage is injected. Microbial growth can be controlled by addition of biocides to the injectant. Numerous chemicals are effective biocides. Biocidal efficiency may be lost by dilution with native formation water or by reaction with aquifer materials.
Ability of a given waste injectant to support microbial growth should be tested under conditions similar to those of the receiving formation. In particular, temperature, pressure, and oxidation-reduction potential during the test should match those in the receiving formation. Biocides should also be tested under comparable conditions.
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This publication consists of papers based on oral presentations at a symposium of the same name co-sponsored by the United States Geological Survey and the American Association of Petroleum Geologists. A wide range of technical issues are covered, as well as regulatory and liability concerns. Documentation of two areas in Colorado where earthquakes had resulted from subsurface fluid injection set the stage for modern debates regarding possible similar results elsewhere. A wide range of fluid compositions are subject to subsurface waste disposal. The largest volumes are brines separated during the production of oil and gas wells, but acid-water and industrial wastes of all types can be disposed in significant quantities in local areas. Large hydraulic fracture treatments never recover all of the injected fluids, and the chemical additives in the fluid that remains underground can be a concern. The subsurface injection of radioactive waste is a topic for three of the papers. The possible need for sequestration of carbon dioxide was not a significant concern at the time and was not covered, but many of the papers provide insight into the issues related to modern proposals. When fluids are injected under pressure into subsurface aquifers, they interact in numerous ways. The fluids can potentially migrate for long distances and potentially interfere with other uses for the native aquifer fluids. If the aquifer cannot transport all of the fluids away, the buildup in pressure can cause fracturing of the rock. Differences in composition between the injected and native fluids can cause chemical reactions to occur; in some cases these can be desirable in that they can immobilize certain solutes in mineral form. The long-term environmental consequences are a common theme in many of the papers because of the recognition that the disposed fluids would become a permanent fixture in subsurface aquifers and could have long-term consequences for their future utilization.