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[1] The effect of retention time on redox sequences along the hydrological flow path of groundwater discharging through low-relief coastal stream sediments and the subsequent impact on the fate of NO 3 À carried in the groundwater was examined in two intact cores. Rates of denitrification were determined for the organic-rich streambed sediments, and a macroscopic, multispecies, reactive transport model based on multiple Monod kinetics was developed to interpret and extend the experimental results. Regionalized sensitivity analysis and parameter estimation were used to determine a set of parameters that best describe the experimental data for one column. The calibrated model successfully replicated the spatial profiles of nitrate under both steady and transient conditions in the second column operated under different conditions. A dimensionless form of the model was used to examine how coupled biogeochemical reactions and hydrological transport processes operate within the stream sediments could be understood in terms of Peclet (ratio of advection to dispersion) and Damkohler numbers (the ratio of the characteristic time of transport to the characteristic time for reaction). At the study site, the Peclet number and the Damkohler numbers for both oxygen and nitrate are high (Pe = 25, Da N = 47.5, and Da O = 40). When Pe > 5, Damkohler numbers explain observed variations in nitrate removal rates; as the flow rate increases, the solute residence time in the reactive zone is shortened resulting in a lesser extent of reaction, such that more NO 3 À is delivered to the stream water.

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Resting-cell suspensions of bacteria isolated from groundwater were added as a pulse to the tops of columns of clean quartz sand. An artificial groundwater solution (AGW) was pumped through the columns, and bacterial breakthrough curves were established and compared to test the effects of ionic strength of the AGW, cell size (by using strains of similar cell surface hydrophobicity but different size), mineral grain size, and presence of heterogeneities within the porous media on transport of the bacteria. The proportion of cells recovered in the effluent ranged from nearly 90% for AGW of a higher ionic strength (I = 0.0089 versus 0.00089 m), small cells (0.75-I,m-diameter spheres versus 0.75 by 1.8-,um rods), and coarse-grained sand (1.0 versus 0.33 mm) to <1% for AGW of lower ionic strength, large cells, and fine-grained sand. Differences in the widths of peaks (an indicator of dispersion) were significant only for the cell size treatment. For treatments containing heterogeneities (a vein of coarse sand in the center of a bed of fine sand), doubly peaked breakthrough curves were obtained. The first peak represents movement of bacteria through the transmissive coarse-grained vein. The second peak is thought to be dominated by cells which have moved (due to dispersion) from the fine-grained matrix to the coarse-grained vein near the top of the column and thus had been retarded, but not retained, by the column. Strength of effects tests indicated that grain size was the most important factor controlling transport of bacteria over the range of values tested for all of the factors examined. Cell size and ionic strength were about equal in importance and were lower in importance than the grain size. The results indicate that significant numbers of bacteria can move through porous media, even when the percentage retained is very high, and the data suggest that manipulation of groundwater environments to control the transport of bacterial cells may be feasible.

The thctors that control the transport of bacteria through porous media are not well understood. The relative importance of the processes of dispersion, of immobilization of bacterial cells by various mechanisms (deposition), and of subsequent release of these trapped cells lentrainment) in describing transport has not been elucidated experimentally. Moreover, the variability of the phenomenological coefficients used to model these processes, given changes in such primary factors as grain size, organism, and ionic strength of the water, is unknown. We report results of fitting solutions of an advection-dispersion equation, modified to account for deposition and entrainment, to breakthrough curves from packed sand columns using two sizes of sand, two ionic strengths of the carrier solution, and two organisms with different sizes. A solution to the advection-dispersion equation including three processes, that is, dispersion, deposition, and entrainment, provides a match to the data that is superior to that achieved by solutions ignoring one of the processes. Fitted values of the coefficient describing deposition vary in a consistent manner with the control variables (organism, grain size, and ionic strength) and are generally within one order of magnitude of those predicted on the basis of theory. 317; 1982. The influence of mineralogy and solution chemistry on attachment of bacteria to representative aquifer materials, J. Contaminart• drol., 6, 321-336, 1991.

Abstract. Miscible displacement experiments were performed on intact sand columns ranging from 15 to 60 cm in length to determine whether bacterial deposition varies at the centimeter scale within aquifer sediments. A 1-pore-volume pulse of radiolabeled cell suspension was introduced into the columns followed by a 2-pore-volume flush of artificial groundwater. The columns were then drained and dissected along the axis of flow. At -1-cm intervals, nine samples were removed for the enumeration of sediment-associated bacteria. Concentrations of sediment-associated (deposited) bacteria varied by up to 2 orders of magnitude in the direction perpendicular to flow demonstrating that bacterial deposition cannot be described mechanistically by a single rate coefficient.

Evidence that fine particles mobilized and transported in soils and aquifers can have a profound influence on contaminant migration has spawned much interest recently in understanding colloid transport in natural materials. Repeated infiltration experiments on an initially dry field soil were conducted to evaluate rates of mobilization of fine particles over time and to investigate the importance of transient-flow events on particle transport. Water flow was measured in zero-tension lysimeters at 25 cm depth. For repeated infiltration events and for all plots, water flow sharply increased shortly after initial ponding of water at the soil surface, maintained a relatively steady level during the period of ponding, and decreased gradually thereafter. Particle concentrations measured in the pan lysimeters ranged from 7 mg L -1 to 265 mg L -1 and were typically on the order of 10 to 100 mg L -1 . Greatest particle mass flux was observed during the initial infiltration experiment on each plot. During four subsequent infiltration experiments, all conducted within 250 min of the first event, steady mass fluxes were observed that were approximately 70% of the average value seen in the first flush of water through a dry soil, indicating that the supply of mobile soil particles is only sparingly reduced over closely spaced infiltration events. All peak particle concentrations and mass fluxes occurred near either the rising limb or the falling limb of the water flux hydrograph, presumably reflecting the movement of air-water interfaces during imbibition and drainage. ES991099G FIGURE 5. Particle mass fluxes and measured water flow rates for the 20-cm infiltration event in Plots 4 and 6. The solid circles (-•-) represent particle mass fluxes, and the lines represent the flow rates. These two individual experiments are illustrative examples selected from the entire set of results shown in Figure 4.