Groundwater contaminated with trichloroethylene (TCE) is particularly difficult and costly to remediate. Injection of nanoscale zero valent iron particles (NZVI) into the aquifer is a promising solution as trapped TCE can be degraded quickly in-situ. These highly redox active particles also have the potential to cause large changes in the geochemistry. While NZVI particles can be highly effective at degrading TCE in soil systems, it is not yet known how the injection of nanoiron and the associated geochemical changes will impact the soil microbial community. These impacts could be harmful if NZVI were bactericidal and significantly decreased microbial diversity, or they could be beneficial if NZVI addition selected for organisms that enhance TCE degradation. The corrosion of zero valent iron produces hydrogen, which is the preferred electron donor for several species of bacteria that can dechlorinate TCE (e.g. Dehalococcoides sp.). In addition, the reducing conditions created by NZVI corrosion are favorable for microbial TCE reduction.

Microcosm studies were used to determine how the microbial diversity in a soil system changes after the addition of NZVI, as well as to determine if NZVI can create more favorable conditions for microbial dehalogenation. We observed rapid changes in geochemical conditions (decrease in oxidation and reduction potential and increase in pH) that caused significant changes in the microbial diversity over a period of several months as indicated by denaturing gradient gel electrophoresis (DGGE) analysis of microbial community DNA. Sulfate reducing and methanogenic populations, which generally thrive in reducing conditions, were enhanced, indicating that many of the diversity changes were caused by the changes in geochemical conditions.