Geochemical and Microbial Community Attributes in Relation to Hyporheic Zone Geological Facies.

The hyporheic zone (HZ) is the active ecotone between the surface stream and groundwater, where exchanges of nutrients and organic carbon have been shown to stimulate microbial activity and transformations of carbon and nitrogen. To examine the relationship between sediment texture, biogeochemistry, and biological activity in the Columbia River HZ, the grain size distributions for sediment samples were characterized to define geological facies, and the relationships among physical properties of the facies, physicochemical attributes of the local environment, and the structure and activity of associated microbial communities were examined. Mud and sand content and the presence of microbial heterotrophic and nitrifying communities partially explained the variability in many biogeochemical attributes such as C:N ratio and %TOC. Microbial community analysis revealed a high relative abundance of putative ammonia-oxidizing Thaumarchaeota and nitrite-oxidizing Nitrospirae. Network analysis showed negative relationships between sets of co-varying organisms and sand and mud contents, and positive relationships with total organic carbon. Our results indicate grain size distribution is a good predictor of biogeochemical properties, and that subsets of the overall microbial community respond to different sediment texture. Relationships between facies and hydrobiogeochemical properties enable facies-based conditional simulation/mapping of these properties to inform multiscale modeling of hyporheic exchange and biogeochemical processes.

Physical versus chemical effects on bacterial and bromide transport as determined from on site sediment column pulse experiments.

Twenty-eight bacterial and Br transport experiments were performed in the field to determine the effects of physical and chemical heterogeneity of the aquifer sediment. The experiments were performed using groundwater from two field locations to examine the effects of groundwater chemistry on transport. Groundwater was extracted from multilevel samplers and pumped through 7-cm-long columns of intact sediment or repacked sieved and coated or uncoated sediment from the underlying aquifer. Two bacterial strains, Comamonas sp. DA001 and Paenibacillus polymyxa FER-02, were injected along with Br into the influent end of columns to examine the effect of cell morphology and cell surface properties on bacterial transport. The effects of column sediment grain size and mineral coatings coupled with groundwater geochemistry were also investigated. Significant irreversible attachment of DA001 was observed in the Fe oxyhydroxide-coated columns, but only in the suboxic groundwater where the concentrations of dissolved organic carbon (DOC) were ca. 1 ppm. In the oxic groundwater where DOC was ca. 8 ppm, little attachment of DA001 to the Fe oxyhydroxide-coated columns was observed. This indicates that DOC can significantly reduce bacterial attachment due electrostatic interactions. The larger and more negatively charged FER-02 displayed increasing attachment with decreasing grain size regardless of DOC concentration, and modeling of FER-02 attachment revealed that the presence of Fe and Al coatings on the sediment also promoted attachment. Finally, the presence of Al coatings and Al containing minerals appeared to significantly retard the Br tracer regardless of the concentration of DOC. These findings suggest that DOC in shallow oxic groundwater aquifers can significantly enhance the transport of bacteria by reducing attachment to Fe, Mn and Al oxyhydroxides. This effect appears to be profound for weakly and strongly charged hydrophilic bacteria and may contribute to differences in observations between laboratory experiments versus field-scale investigations particularly if the groundwater pH remains subneutral and Fe oxyhydroxide phases exist. These observation validate the novel approach taken in the experiments outlined here of performing laboratory-scale experiments on site to facilitate the use of fresh groundwater and thus be more representative of in situ groundwater conditions.

Ferrographic tracking of bacterial transport in the field at the narrow channel focus area, Oyster, VA.

The first results from an innovative bacterial tracking technique, ferrographic capture, applied to bacterial transport in groundwater are reported in this paper. Ferrographic capture was used to analyze samples during an October 1999 bacterial injection experiment at the Narrow Channel focus area of the South Oyster site, VA. Data obtained using this method showed that the timing of bacterial breakthrough was controlled by physical (hydraulic conductivity) heterogeneity in the vertical dimension as opposed to variation in sedimentsurface or aqueous chemical properties. Ferrographic tracking yielded results that compared well with results from other tracking techniques over a concentration range of 8 orders of magnitude and provided a low detection limit relative to most other bacterial tracking techniques. The low quantitation limit of this method (approximately 20 cells/mL) allowed observation of transport of an adhesion-deficient bacterium over distances greater than 20 m in the fine sand aquifer underlying this site.

Use of quantitative models to design microbial transport experiments in a sandy aquifer.

A suite of numerical models was applied to the problem of designing field tracer and bacterial injection experiments in a sandy surficial aquifer near Oyster, Virginia. The models were constructed based on the integration of diverse characterization data including hydrologic, geophysical, geological, geochemical, and biological information. A one-dimensional particle-tracking model was used to analyze laboratory transport experiments conducted using intact core samples to prescribe transport parameters describing solute dispersion and bacterial fate. A geostatistical model of three-dimensional hydraulic conductivity variations was developed, conditioned on in situ measurements of hydraulic conductivity and interpretations of geophysical data, and used to generate alternative aquifer descriptions. A regional-scale, two-dimensional flow model was used to design pumping rates of a forced-gradient hydraulic control system. Information from these various models was then combined into a high-resolution, three-dimensional flow and transport model for the prediction of field-scale solute and bacterial transport. Model predictions were used in an iterative experimental design process to specify: (1) the locations of multilevel samplers for monitoring transport; (2) frequency and timing of sample collection during bromide tracer injection experiments; and (3) frequency and timing of sample collection during a bacterial injection experiment. At each stage of the design, information gained during the previous stage was used to refine the model and target subsequent experimentation.