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Information about the microbial removal efficiencies of subsurface media is essential for assessing the risk of water contamination, estimating setback distances between disposal fields and receiving waters, and selecting suitable sites for wastewater reclamation. By analyzing published data from field experiments and large intact soil cores, an extensive database of microbial removal rates was established for a wide range of subsurface media. High microbial removal rates were found in volcanic soils, pumice sand, fine sand, and highly weathered aquifer rocks. Low removal rates were found in structured clayey soils, stony soils, coarse gravel aquifers, fractured rocks, and karst limestones. Removal rates were lower for enteroviruses than for other human viruses; for MS2 phage than for other phage species; for waste-associated microbes than for those cultivated in the laboratory; and for contaminated media than for uncontaminated media. Microbial removal rates are inversely correlated with infiltration rates and transport velocity. The assumption of first-order law, or a constant removal rate (when the transport scale reaches a representative elementary volume), is appropriate for most of field data analyzed. However 30% of the datasets (26 out of 87 pairs) are better described with the power law, implying reduced removal rates with transport distance. The latter is most prominent for organically contaminated media, especially in relatively fine aquifer media. The presence of organic matter, heterogeneity in microbial properties, change in solution chemistry, detachment, and physical straining, may have caused the discrepancies from the first-order law traditionally used in transport models for describing microbial removal.

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Strongly sorbing chemicals (e.g., heavy metals, radionuclides, pharmaceuticals, and explosives) in porous media are associated predominantly with the solid phase, which is commonly assumed to be stationary. However, recent field‐ and laboratory‐scale observations have shown that in the presence of mobile colloidal particles (e.g., microbes, humic substances, clays, and metal oxides), colloids can act as pollutant carriers and thus provide a rapid transport pathway for strongly sorbing contaminants. To address this problem, we developed a one‐dimensional numerical model based on the HYDRUS‐1D software package that incorporates mechanisms associated with colloid and colloid‐facilitated solute transport in variably saturated porous media. The model accounts for transient variably saturated water flow, and for both colloid and solute movement due to advection, diffusion, and dispersion, as well as for solute movement facilitated by colloid transport. The colloid transport module additionally considers the processes of attachment/detachment to/from the solid phase and/or the air–water interface, straining, and/or size exclusion. Various blocking and depth dependent functions can be used to modify the attachment and straining coefficients. The solute transport module uses the concept of two‐site sorption to describe nonequilibrium adsorption–desorption reactions to the solid phase. The module further assumes that contaminants can be sorbed onto surfaces of both deposited and mobile colloids, fully accounting for the dynamics of colloid movement between different phases. Application of the model is demonstrated using selected experimental data from published saturated column experiments, conducted to investigate the transport of Cd in the presence of Bacillus subtilis spores in alluvial gravel aquifer media. Numerical results simulating bacteria transport, as well as the bacteria‐facilitated Cd transport, are compared with experimental results. A sensitivity analysis of the model to various parameters is also presented.

A field study was conducted to characterize the transport of rhodamine WT dye and endospores of the bacterium Bacillus subtilis through preferential flowpaths in an alluvial gravel aquifer using natural gradient tracer experiments. Preliminary tracer experiments were conducted with rhodamine WT to determine preferential flowpaths using a resin bag method, because of spatial heterogeneity at the study site. Rhodamine WT, Cl, and B. subtilis endospores were then injected 0.1 to 1.6 m below the water table, and downgradient ground water was monitored. Rhodamine WT behaved like Cl in the field and was used as a nonreactive tracer in this study. The method of time moments (MTM) and a curve fitting method (CFM) using the AT123D model were applied to estimate transport parameters of rhodamine WT and B. subtilis endospores along preferential flowpaths within the aquifer, assuming each individual flowpath to be uniform. The results from CFM fit the observed data better than did MTM. Ground water velocities through preferential flowpaths ranged from 30 to 85 m/day with a median of 63 m/day, and longitudinal dispersivities ranged from 0.71 to 5.24 m with a median of 2.70 m. These values are within the range reported in the literature for similar hydrogeological conditions. B. subtilis endospores exhibited slightly faster velocities (median retardation factor 0.86) and lower longitudinal dispersivities (0.23 to 2.4 m, median 0.79 m) compared to rhodamine WT. When multiple concentration peaks were observed in the rhodamine WT breakthrough curves (BTCs), the bacteria always arrived with the first rhodamine WT peak, regardless of whether the first peak was small or large. For those wells with a single peak for both the dye and the bacteria, the bacteria tended to coincide with the front portion of the dye BTC. This suggests that convection is relatively more important for B. subtilis endospores than for the dye, probably due to pore size exclusion of the spores. For this reason, the use of non‐reactive solute tracers to reflect the movement of microorganisms may not be appropriate. The total removal rates of B. subtilis varied from 2.4 to 9.36 day−1 (median 3.36 day−1, probably resulting from filtration, sedimentation, and irreversible adsorption, as die‐off was not observed in the field during the 45 days of the study. The longevity of B. subtilis endospores in ground water makes it a good choice to use as a bacterial tracer in the study of transport processes in the absence of die‐off.

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means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. ABBREVIATIONS: BTC, breakthrough curve; cfu, colony-forming unit, a unit indicating the number of bacteria present in a water sample; DSE, dairy-shed effl uent; PV, pore volume. ORIGINAL RESEARCHThe HYDRUS-1D mobile-immobile water model (MIM) was used to evaluate the transport of fecal coliforms, Salmonella bacteriophage, and Br in 10 soils. At a fl ux of 5 mm h −1 , a pulse of dairy shed effl uent was applied to 30 large undisturbed lysimeters, followed by water irrigation. Soil types included clayey gley soil, clay loam, silt loam, silt loam over gravels, fi ne sandy loam, dune sand soil, pumice soil, and allophanic soil. Except for dune sand, modeling results showed lower mobile water contents and dispersivities for microbes than for Br, indicating the exclusion of microbes from smaller pores. The MIM-derived removal rates were in the order: volcanic soils > greywacke-derived silt loams > granular young sandy soils, and were the most variable in clayey gley loam and silt loam over gravels. Microbial reduction was 100% in allophanic soil, 16 to 18 log m −1 in pumice soil (where the unit log is the log 10 reduction in maximum concentration compared with the original concentration), and was lowest in clayey gley soil (0.1-2 log m −1 ). For most of the other soils, the reduction was 2 to 3 log m −1 , except for 9 to 10 log m −1 for fecal coliforms in a fi ne sandy loam. The detachment rate was only 1% of the attachment rate, indicating irreversible attachment of microbes. Soil structure (macroporosity) appeared to play the most important role in the transport of microbes and Br, while soil lithology had the greatest infl uence on attenuation and mass exchange. The general pattern of predicted mobile water content agrees with the measured macroporosity, which is positively related to leaching vulnerability but negatively related to dispersivity.