Hydrogeologic parameters affecting vadose‐zone microbial distributions

Abstract

The vadose zone and its contaminant-attenuating processes are physically interposed between surface contamination and groundwater supplies. Given the potential role of microorganisms in mediating vadose-zone chemical processes, it is vital to understand vadose microbial distributions and factors controlling those distributions. Vadose and shallow saturated zone sediments obtained from cores drilled to approximately 8 m below the surface at two hydrogeologically contrasting sites, named Dalmeny and Washington State University (WSU), were examined for culturable heterotrophic bacteria, total organic carbon (TOC), and sediment texture. Porewater elutions were analyzed for dissolved organic carbon, sulfate, and inorganic nitrogen species. Numbers of cultured bacteria (10 3 -10 7 g -1 ) generally decreased with depth at both sites. The TOC decreased uniformly with depth at WSU where soil processes are the sole carbon source; at Dalmeny, where both soil and kerogen carbon are present, TOC was higher and relatively constant with depth. Numbers of distinct colony types at Dalmeny did not decline below the solum. Bacteria at Dalmeny were more numerous, exhibited greater numbers of colony types, and were metabolically more flexible than those at WSU. The smooth decline of numbers with depth at WSU paralleled and may be caused by the TOC decline with distance from a solum source. Sediment permeability and pore-water flux did not control bulk populations as suggested in previous studies; this may be explained by bacterial residence on fracture surfaces in low-permeability materials. Psychrotolerant organisms did not appear to be as abundant as mean ambient temperatures might suggest.

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

“…Researchers who have investigated the spatial variability of microorganisms in deep environments usually acquired samples from boreholes and investigated vertical variability, or addressed lateral variability utilizing multiple adjacent boreholes Beloin et al, 1988;Colwell, 1989;Davis, 1967;Frederickson et al, 1991;Ghiorse & Wilson, 1988;Kuznetsov et al, 1963;Madsen & Ghiorse, 1993;McNabb & Dunlap, 1975). Correlations have been demonstrated between microbial activity and abundance to sediment texture, porewater pH and E h , and concentration of metals and nutrients in porewater (Beloin et al, 1988;Davis, 1967;Fredrickson et al, 1989Fredrickson et al, , 1991Severson et al, 1991;Sinclair & Ghiorse, 1989;Whitelaw &Rees, 1980). An alternative to drilling has been used at Rainier Mesa, Nevada Test Site, where samples have been mined from the walls of subsurface tunnels (Amy et al, 1992;.…”

“…Other researchers have determined relationships between microbial activity and physical parameters within a particular subsurface environment (Beloin et al, 1988;Frederickson et al, 1989;Severson et al, 1991;Sinclair & Ghiorse, 1989;Whitelaw & Reese, 1980). Generally, these studies have determined correlations on a much larger scale, vertically within boreholes, between boreholes, or between geological formations.…”

Review: Microbial biocenoses in pristine aquifers and an assessment of investigative methods

“…However, as shown above, the bacterial metabolism is highly flexible, and bacteria can rapidly adapt to changing environmental conditions. One bacterial species may thus use different metabolic patterns in different habitats, while different species may have a very similar metabolic pattern (Fernandez et al 1999;Severson et al 1991).…”

Soil respiration and georespiration distinguished by transport analyses of vadose CO2, 13CO2, and 14CO2

“…Water contents at the Dalmeny site were measured only once during the study period, in October 1993, and once during a previous study in July 1989 [Severson et al, 1991]. Gravimetric water contents were determined [Bowles, 1986] on material collected for organic carbon analysis (detailed in this section) and convened to volumetric water contents using previously measured porosity and solid density measurements [Keller, 1985].…”