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An antibiotic‐resistant bacterium was tested for transport through volcanic tuff and sandstone cores. Tuff cores were representative of the geology of Rainier Mesa located on the Nevada Test Site (NTS). Rapid bacterial transport occurred in some of the tuff cores and all sandstone cores under the hydraulic heads used (5–500 cm). Hydraulic conductivity of the tuff cores ranged widely, 9.6 × 10‐5 to 7.2 x 10‐3 cm h‐1. A much narrower range was observed for sandstone cores, 1.6 × 10‐2 to 5.9 X 10‐2 cm h‐1, which served as experimental controls. The percentage of the initial bacterial inoculum recovered within 3 pore volumes from tuff and sandstone cores ranged from 9.4 to 54.7% and 0.20 to 2.9%, respectively. Bacterial recovery appeared to be controlled by the structure of the flow paths in rock cores and not by overall hydraulic conductivity. Saturated clay‐infiltrated and unfractured zeolitized tuff cores were impermeable to water flow, and therefore bacterial transport was not detected. Three routes of ba...

The isolation of viable microorganisms from deep volcanic rock formations (50–450 m) within Rainier Mesa, Nevada Test Site has posed questions regarding the origin of in situ bacteria. One hypothesis under investigation is that microbial transport to depth has been facilitated via recent natural water flow from surface bacterial populations. Recoverable microbiota from surface topsail, paleosol, rock, and outflow spring water were compared to those from subsurface rock and fracture flow water to determine if relationships existed between microbial communities along an elevational gradient. Direct counts were higher than culturable counts in all samples. Top‐soil samples had higher culturable counts and numbers of nitrogen‐fixing and sulfur‐reducing bacteria than samples from the subsurface rock. Heterogeneity in microbial communities from the different samples (measured by diversity and evenness indices and by comparison of morphological and physiological tests of representative isolates) was demonstrated...