Natural and Enhanced Biodegradation in the Field

Natural attenuation by itself, however, often is not sufficient to achieve a desired extent or rate of contaminant removal from an aquifer. In these instances, one remedial option may be to enhance the natural rate of biodegradation of pollutant chemicals in the aquifer. This strategy, called in situ bioremediation, is considered to be one of the most attractive remedial techniques from a cost perspective, because many of the high costs associated with pumping and treating groundwater or excavating contaminated aquifer material are avoided. Furthermore, the potential exposure of cleanup workers to pollutant chemicals is reduced if many of the contaminants are mineralized while still in the aquifer. 

Of course, no such changes or enhancements will increase the biodegrada- 

A related approach is to develop a dense community of microorganisms that, although they may be unable to derive energy from and grow upon a particular pollutant may have the ability to degrade the chemical via cometabolism. To induce the growth of such a bacterial community it is necessary to 

Although the subsurface environment may be somewhat simpler than surface waters, it is less well understood, perhaps because it cannot be observed directly and is accessible only at considerable expense through excavation and drilling. 

Transport in the subsurface environment is slow compared with the other environmental media. Contaminants may move only tens of meters per year by advection, contrasting sharply with surface waters, which travel this far in minutes or hours, and air, which may travel this far in seconds (as discussed in the next chapter). Similarly, Fickian transport coefficients are rarely higher than thousandths of a square centimeter per second and are often no larger than a fraction of the molecular diffusion coefficient in free water. Many organic compounds that would rapidly volatilize into the atmosphere from surface waters may reside in groundwaters for decades or longer. 

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