Bacterial transport in volcanic tuff cores under saturated flow conditions

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 bacterial transport were discerned in permeable rock cores by comparison of the breakthrough patterns of bacteria and tracer solution (chloride ions) in cores of differing lithologies. In sandstone cores, where water flowed evenly through the matrix, bacteria were transported in a dispersed manner throughout the sandstone, whereas bacteria were transported primarily along preferred flow paths (fractures or macropores) in permeable tuff cores.