Heterogeneity of deep subsurface microorganisms and correlations to hydrogeological and geochemical parameters

Nineteen samples were obtained from a 21‐m³ section of zeolitized volcanic ash‐fall tuff, 390 m below the surface of Rainier Mesa, Nevada. Rock mined aseptically from the walls of deep subsurface tunnels provided pristine samples for microbiological and geochemical analyses. Microbiological parameters measured on all samples included direct counts and the abundance, diversity, morphology, and metabolic traits of culturable organisms. Physical and chemical parameters measured included ionic and nutrient chemistries, mineralogy, porosity, moisture content, and permeability. The results indicate that the culturable microbiological community size and composition exhibit random spatial variability within the geologically/geochemically homogeneous rock section. The relative abundance of microorganisms testing positive for nitrate reductase demonstrated a spatial trend along the vertical and front‐to‐back axes of the rock section by gradient analysis. The porewater concentration of nitrate correlated with numbers of bacteria testing positive for nitrate reductase and indicates that these bacteria may exist as dormant forms in situ.

Keywords:

• hydrogeology
• mining
• spatial variability
• subsurface microbiology