The geochemistry and microbiology of a uranium-contaminated subsurface 
environment that had undergone two seasons of acetate addition to stimulate 
microbial U(VI) reduction was examined. There were distinct horizontal and 
vertical geochemical gradients that could be attributed in large part to the 
manner in which acetate was distributed in the aquifer, with more reduction of 
Fe(III) and sulfate occurring at greater depths and closer to the point of 
acetate injection. Clone libraries of 16S rRNA genes derived from sediments and 
groundwater indicated an enrichment of sulfate-reducing bacteria in the order 
Desulfobacterales in sediment and groundwater samples. These samples were 
collected nearest the injection gallery where microbially reducible Fe(III) 
oxides were highly depleted, groundwater sulfate concentrations were low, and 
increases in acid volatile sulfide were observed in the sediment. Further 
down-gradient, metal-reducing conditions were present as indicated by 
intermediate Fe(II)/Fe(total) ratios, lower acid volatile sulfide values, and 
increased abundance of 16S rRNA gene sequences belonging to the dissimilatory 
Fe(III)- and U(VI)-reducing family Geobacteraceae. Maximal Fe(III) and U(VI) 
reduction correlated with maximal recovery of Geobacteraceae 16S rRNA gene 
sequences in both groundwater and sediment; however, the sites at which these 
maxima occurred were spatially separated within the aquifer. The substantial 
microbial and geochemical heterogeneity at this site demonstrates that attempts 
should be made to deliver acetate in a more uniform manner and that closely 
spaced sampling intervals, horizontally and vertically, in both sediment and 
groundwater are necessary in order to obtain a more in-depth understanding of 
microbial processes and the relative contribution of attached and planktonic 
populations to in situ uranium bioremediation.