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 cond tions, 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.