Hexavalent chromium is a widespread contaminant found in groundwater. In order to stimulate microbially mediated Cr(VI)-reduction at site 100H at Hanford., a poly-lactate compound was injected into Cr(VI)-contaminated aquifers. An Integrated approach combining high-density 16S rDNA based microarray, functional gene microarray, MDA clone libraries as well as enrichment based culturing techniques were employed to investigate the bacterial community composition and to reveal dominant microbial metabolism at the site. The 16S rDNA based Phylochip revealed a stimulation of Pseudomonas, Desulfovibrio and Geobacter species amongst others. Enrichment of these organisms coincided with continued Cr(VI) depletion. Functional gene-array (Geochip) analysis of DNA from monitoring well indicated high abundance of genes involved in nitrate-reduction, sulfate-reduction, iron-reduction, methanogenesis, chromium tolerance/reduction. Clone-library data revealed Psedomonas was the dominant genus in these samples. Anaerobic enrichments in selective media resulted in isolation of an iron-reducing, a sulfate-reducing and a nitrate-reducing isolate among several others. Preliminary 16S rDNA sequence analysis identified the isolates as Geobacter metallireducens, Pseudomonas stutzeri and Desulfovibrio vulgaris species respectively. Further experimentation demonstrated the ability of these isolates to reduce Cr(VI) to non-toxic Cr(IV) enzymatically. These results demonstrate that Cr(VI)-immobilization at Hanford 100H site could be mediated by direct microbial metabolism in addition to indirect chemical reduction of Cr(VI) by end-products of microbial  activity.