Using transport model interpretations of tracer tests to study microbial processes in groundwater

It has long been known that microorganisms affect the geochemistry of groundwater. But despite this recognition, little detailed information is available regarding the rates and the factors controlling microbial processes in groundwater. Part of the reason stems from the relatively inaccessible nature of most groundwater and the difficulties encountered in obtaining representative samples of groundwater and subsurface sediments. At the same time, most groundwater systems are nutrient poor or oligotrophic environments in which the resident microorganisms are severely stressed and often nearly inactive. These populations are functioning so slowly that many types of activity measurements designed to assess microbial processes in more productive environments are ineffective for groundwater. However, because groundwater is by far the largest reservoir of freshwater in the world (Freeze and Cherry 1979), our lack of knowledge about groundwater microorganisms and their processes represents a significant void in the study of microbial ecology and in our ability to predict the outcome when these reserves are compromised by contamination.