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 obt ained from the U.S. Department of Energy's Hanford Site in south-centr al Washington by cable tool drilling, and volcanic ashfall tuff sample s 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 me tabolic activities were determined by [C-14]glucose radiorespirometry at t=0 and at various post-sampling time points up to 154 d in order t o assess the influence of sample storage on microbiological properties of subsurface samples. Increases in post-sampling populations of viab le bacteria were observed in all samples, although the magnitude of th e increase and time after sampling at which the maximum population siz e was reached varied with the sample type. The greatest post-sampling increases in viable counts and [C-14]glucose mineralization occurred i n 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 subsu rface sediments and rock for microbiological analysis. In addition, th ese results suggest that even low biomass subsurface environments may be readily stimulated for applications such as in situ bioremediation.