PNNL

Abstract

Microorganisms associated with fractured basalt core were compared to those suspended in groundwater pumped from the same well in the eastern Snake River Plain Aquifer (Idaho, USA). Two wells influenced to different degrees by a mixed-waste plume in the fractured basalt aquifer were examined. In one well, an array of dialysis cells filled with either deionized water or crushed basalt was equilibrated to compare the microorganisms collected in this fashion with those from core and groundwater samples collected in a traditional manner from the same well. Analyses were performed to characterize these samples and to provide a basis for comparison. These included total cell counts by microscopy; total biomass by phospholipid fatty acid analysis; enumerations of viable aerobic heterotrophs, groups of putative aerobic co-metabolic TCE-degraders and aerobic H2-oxidizing bacteria; mineralization of 14C- labeled acetate; and enrichments for dissimilatory iron-reducing bacteria. The 16S rDNA sequences associated with DNA directly extracted from the samples were determined in selected samples following either clonal analyses or separation of PCR products by denaturing gradient gel electrophoresis. The ground water well most influenced by the waste plume had higher populations of organisms in both the groundwater and core samples. In both wells, significant differences were found between bacteria associated with the basalt core and those suspended in the groundwater. Microbial parameters measured in core samples showed high variation throughout the depth profile. Higher populations were found in the basalt- and water-filled dialysis cells incubated in the open well compared with core and groundwater samples, respectively. For a given parameter, the variation among dialysis cells incubated at different depths was much less than that observed in the core samples. Given knowledge of cell physiological changes associated with attachment and potential differences between attached and unattached communities in aquifers, consideration should be given to the type of sample media (i.e., core, groundwater, dialysis cells) used to represent a subsurface environment.