Microorganisms detected in the subsurface can profoundly alter geochemical conditions along groundwater flowpaths. In addition to indirectly creating conditions hindering contaminant mobility, many microorganisms are known to directly biotransform contaminants to innocuous or immobile forms. This is the basis for several in situ bioremediation technologies and natural attenuation mechanisms and may also play a role in the effectiveness of some in situ barrier systems. However, the sustained manipulation of subsurface microbial communities to affect contaminant transport and/or degradation is still largely an empirical exercise. Likewise the microbially-mediated mechanisms of natural attenuation processes and potential microbial involvement in other more physical/chemical in situ remediation techniques remain poorly understood. Much remains to be learned about the identity and functioning of subsurface microbial communities relevant to contaminant biotransformation processes. Of particular concern for in situ remediation and natural attenuation processes is a mechanistic understanding of how microbial growth and activity quantitatively relate to mineral and contaminant biotransformation. This requires a mechanistic understanding of how microorganisms access/obtain essential nutrients, electron donors and electron acceptors in order to sustain activity. Also, interactions among groups of active microorganisms need to be better understood in order to more fully explain competitive processes and shifts in community structure. Additional techniques are needed to evaluate the distribution of active microbial communities in the contaminated subsurface as well as identification of novel mechanisms of microbially mediated contaminant transformation.
The emphasis of this Science Element is on understanding the functioning of subsurface microbial communities and how their growth and activity affects contaminant fate and transport. Successful proposals for research will address microbial communities involved in metal and radionuclide immobilization/stabilization processes in environments of relevance to DOE.