TIME-DEPENDENT CHANGES IN VIABLE NUMBERS AND ACTIVITIES OF AEROBIC HETEROTROPHIC BACTERIA IN SUBSURFACE SAMPLES

Vadose and saturated zone sediment cores from depths to 212 m were obtained from the U.S. Department of Energy's Hanford Site in south-central Washington by cable tool drilling, and volcanic ashfall tuff samples were obtained from tunnels 400 m beneath the surface of Rainier Mesa at DOE's Nevada Test Site (NTS) in southern Nevada. Numbers of viable aerobic heterotrophic bacteria were determined by plate counts and metabolic activities were determined by [C-14]glucose radiorespirometry at t=0 and at various post-sampling time points up to 154 d in order to assess the influence of sample storage on microbiological properties of subsurface samples. Increases in post-sampling populations of viable bacteria were observed in all samples, although the magnitude of the increase and time after sampling at which the maximum population size was reached varied with the sample type. The greatest post-sampling increases in viable counts and [C-14]glucose mineralization occurred in a high-organic carbon lacustrine sediment. The population of aerobic heterotrophs increased from below detection at t=0 to > 10(6) CFU g(-1) after 139 d. Significant increases in culturable counts were shown to occur within 24 h for tuff samples from NTS. These results indicate that precautions are necessary in the post-drilling handling of subsurface sediments and rock for microbiological analysis. In addition, these results suggest that even low biomass subsurface environments may be readily stimulated for applications such as in situ bioremediation.