Determining straining of Escherichia coli from breakthrough curves

The contribution of deeper layers in slow sand filters to pathogens removal

Slow Sand Filtration is popular in drinking water treatment for the removal of a wide range of contaminants (e.g., particles, organic matter, and microorganisms). The Schmutzdecke in slow sand filters (SSFs) is known to be essential for pathogen removal, however, this layer is also responsible for increased head loss. Since the role of deeper layers in bacteria and virus removal is poorly understood, this research investigated the removal of E.coli WR1 and PhiX 174 at different depths of a full-scale SSF. Filter material from top (0-5 cm), middle (5-20 cm) and deep (20-35 cm) layers of an established filter was used in an innovative experimental set-up to differentiate physical-chemical and biological removal processes. In the analysis, we distinguished between removal by biological activity, biofilm and just sand. In addition, we modelled processes by a one-side kinetic model. The different layers contributed substantially to overall log removal of E.coli WR1 (1.4-1.7 log₁₀) and PhiX 174 (0.4-0.6 log₁₀). For E.coli WR1, biological activity caused major removal, followed by removal within biofilm and sand, whereas, removal of PhiX 174 mainly occurred within sand, followed by biofilm and biological activity. Narrow pore radii in the top layer obtained by micro-computed tomography scanner suggested enhanced retention of bacteria due to constrained transport. The retention rates of E.coli WR1 and PhiX 174 in top layer were four and five times higher than deeper layers, respectively (k_ret 1.09 min^-1 vs 0.26 min^-1 for E.coli WR1 and k_ret 0.32 min^-1 vs of 0.06 min^-1 for PhiX 174). While this higher rate was restricted to the Schmutzdecke alone (top 5 cm), the deeper layers extend to around 1 m in full-scale filters. Therefore, the contribution of deeper layers of established SSFs to the overall log removal of bacteria and viruses is much more substantial than the Schmutzdecke.

Experiments and simulation of co-migration of copper-resistant microorganisms and copper ions in saturated porous media

Heavy metal (HV) pollutants may migrate to the groundwater environment through leaching, causing groundwater pollution. Compared with surface water pollution, groundwater pollution is complex and hidden. Existing methods for treating HV pollution in the vadose zone have had limited application owing to various problems. In recent years, microorganisms have been used in the field of pollution control and remediation owing to their outstanding adsorption and degradation properties and low cost, but their environmental safety and behavior in porous media are still poorly understood. This study aimed to investigate the migration behavior and mechanisms of copper ions in saturated porous media under the action of copper-resistant microorganisms and to establish a corresponding numerical model to simulate the results. The key parameters of adsorption and migration were determined through batch adsorption and soil column experiments. A one-dimensional soil column was used to conduct a co-migration experiment using copper-resistant microorganisms and Cu²⁺ in water-saturated quartz sand, and a co-migration mathematical model was constructed. It was found that the existence of microorganisms had an inhibitory effect on the migration of Cu²⁺ in quartz sand, and Cu²⁺ promoted the migration of microorganisms, reduced their adsorption, and increased their concentration in the column experiment effluent. The selected solute transport mathematical model had a good fitting effect on the breakthrough curves of copper ion and copper-resistant microorganisms during their co-migration. The results can provide parameters and a theoretical basis for the risk assessment and prevention of HV pollution in the saturated zone or aquifers.

Evidence for biosurfactant-induced flow in corners and bacterial spreading in unsaturated porous media

The spread of pathogenic bacteria in unsaturated porous media, where air and liquid coexist in pore spaces, is the major cause of soil contamination by pathogens, soft rot in plants, food spoilage, and many pulmonary diseases. However, visualization and fundamental understanding of bacterial transport in unsaturated porous media are currently lacking, limiting the ability to address the above contamination- and disease-related issues. Here, we demonstrate a previously unreported mechanism by which bacterial cells are transported in unsaturated porous media. We discover that surfactant-producing bacteria can generate flows along corners through surfactant production that changes the wettability of the solid surface. The corner flow velocity is on the order of several millimeters per hour, which is the same order of magnitude as bacterial swarming, one of the fastest known modes of bacterial surface translocation. We successfully predict the critical corner angle for bacterial corner flow to occur based on the biosurfactant-induced change in the contact angle of the bacterial solution on the solid surface. Furthermore, we demonstrate that bacteria can indeed spread by producing biosurfactants in a model soil, which consists of packed angular grains. In addition, we demonstrate that bacterial corner flow is controlled by quorum sensing, the cell-cell communication process that regulates biosurfactant production. Understanding this previously unappreciated bacterial transport mechanism will enable more accurate predictions of bacterial spreading in soil and other unsaturated porous media.

Flagella and Their Properties Affect the Transport and Deposition Behaviors of Escherichia coli in Quartz Sand

The effects of flagella and their properties on bacterial transport and deposition behaviors were examined by using four types of Escherichia coli (E. coli) with or without flagella, as well as with normal or sticky flagella. Packed column, quartz crystal microbalance with dissipation, visible parallel-plate flow chamber system, and visible flow chamber packed with porous media system were employed to investigate the deposition mechanisms of bacteria with different properties of flagella. We found that the presence of flagella favored E. coli deposition onto quartz sand/silica surfaces. Moreover, by changing the porous media porosity and directly observing the bacterial deposition process, local sites with high roughness, narrow flow channels, and grain-to-grain contacts were found to be the major sites for bacterial deposition. Particularly, flagella could help bacteria swim near and then deposit at these sites. In addition, we found that due to the stronger adhesive forces, sticky flagella could further enhance bacterial deposition onto quartz sand/silica surfaces. Elution experiments indicated that flagella could help bacteria attach onto sand surfaces more irreversibly. Clearly, flagella and their properties would have obvious impacts on the transport/deposition behaviors of bacteria in porous media.

Deposition and mobilization of viruses in unsaturated porous media: Roles of different interfaces and straining

The vadose zone is the first natural layer preventing groundwater pollution. Understanding virus transport and retention in the vadose zone is necessary. The effects of different interfaces and mechanisms on virus transport and retention were investigated by studying Escherichia coli phage migration in laboratory-scale columns under unsaturated conditions. The E. coli phage was used as a model virus. Colloid filtration theory, extended Derjagin-Landau-Verwey-Overbeek theory and two-site kinetic deposition model were used to calculate fitted parameters and interaction energies to assess virus retention at different interfaces. The collector diameters and the size of E. coli phages in the influent and effluent were compared to assess the effect of straining. The results indicated that the roles of solid-water interfaces (SWIs) and air-water interfaces (AWIs) in retaining E. coli phages are strongly controlled by the moisture content and hydrochemical conditions. Decreasing the moisture content and increasing the ionic strength (IS) of the suspension increased E. coli phage retention. At suspension ISs of 0.01 or 0.03 M and various moisture contents, E. coli phages were mainly retained at the SWIs rather than AWIs. When the IS was increased to 0.06 M, the viruses were strongly retained by becoming attached to both SWIs and AWIs. The role of straining in virus retention could not be ignored. Viruses were retained more at the SWIs and less straining occurred under acidic conditions than under neutral or alkaline conditions. This was mainly because of the effects of the pH and IS on surface charges and the model virus particle size. This study has important implications for modeling and predicting virus transport in soil affected by rainfall, snowmelt, and human activities (e.g., irrigation and artificial groundwater recharging).

Influence of hydraulic gradient and temperature on the migration of E. coli in saturated porous media during bank filtration: a case study at the Second Songhua River, Songyuan, Northeastern China

River bank filtration can effectively reduce the number of pathogenic microorganisms infiltrating into groundwater from surface water. Groundwater seepage velocity and temperature are considered to be important factors affecting the process, but the magnitude and mechanism of their impacts have not been clear for a long time. Based on the actual monitoring data of the Escherichia coli concentrations and soil samples of Second Songhua riverside source area, the migration of E. coli in saturated porous media under different velocities and different temperatures was studied using saturated soil column transport experiments. Concurrently, the migration characteristics of E. coli in the riverside source area were replicated by mathematical simulation. According to the field monitoring results, the concentration of E. coli decreased in the riverbank infiltration zone, and the removal rate was greater than 96%. The column experimental results showed that the lower the flow velocity was and the higher the temperature was, the greater the removal rate of E. coli was. And the flow velocity was the main factor affecting the removal of E. Coli. The mathematical simulation results showed that under the conditions of the largest hydraulic gradient (20%) and the highest concentration of E. coli (2500 MPN/100 mL) in river water, the safe exploitation distance of groundwater that did not cause a risk of E. coli pollution was more than 7 m away from the river bank. These findings are expected to provide a scientific basis for the design of water intake schemes and the optimization of mining technology.

Physical Experiment and Modeling of the Transport and Deposition of Polydisperse Particles in Stormwater: Effects of a Depth-Dependent Initial Filter Coefficient

The artificial recharge of stormwater is an effective approach for replenishing aquifer and reduce urban waterlogging, but prone to clogging by suspended particles (SP) that are highly heterogeneously sized. In this paper, the transport and deposition of SP in a sand column were investigated under a constant flow condition, for five stormwater concentrations. A depth-dependent initial filter coefficient is incorporated into the conventional filtration model. This modified model considers the heterogeneity of the particle population by lumping the capture of heterogeneous SP into a capture probability. The good agreement between the results of the modified model and the experimental results of measured outlet concentration and average specific deposit validated the modified model. The experiment data and the simulation results both indicate that the highly hyper-exponential retention profiles are caused by non-uniform deposition of heterogeneous SP; and, the conventional model was found to homogenize the spatial distribution of SP retention and overestimate retention of the porous medium. Local and overall permeability reductions were assessed by an empirical relationship and the Kozeny-Carman model, respectively. It is shown that consideration of polydisperse suspended particles is of primary importance. This study highlights the effects of polydisperse particles on SP deposition in a saturated porous medium.

Modeling Escherichia coli and Rhodococcus erythropolis transport through wettable and water repellent porous media

Water protection and bioremediation strategies in the vadose zone require understanding the factors controlling bacterial transport for different hydraulic conditions. Breakthrough experiments were made in two different flow conditions: i) an initial bacteria pulse under ponded infiltration into dry sand (-15,000 cm); ii) a second bacteria pulse into the same columns during subsequent infiltration in constant water content and steady-state flow. Escherichia coli (E. coli) and Rhodococcus erythropolis (R. erythropolis) were used to represent hydrophilic and hydrophobic bacteria, respectively. Equilibrium and attachment/detachment models were tested to fit bromide (Br^-) and bacteria transport data using HYDRUS-1D. Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended DVLO (XDLVO) interaction energy profiles were calculated to predict bacteria sorption at particles. Adsorption of bacteria at air-water interfaces was estimated by a hydrophobic force approach. Results suggested greater retention of bacteria in water repellent sand compared with wettable sand. Inverse parameter optimization suggested that physico-chemical attachment of both E. coli and R. erythropolis was thousands of times lower in wettable than repellant sand and straining was 10-fold lower in E. coli for wettable vs repellant sand compared to the exact opposite by orders of magnitude with R. erythropolis. HYDRUS did not provide a clear priority of importance of solid-water or air-water interfaces in bacteria retention. Optimized model parameters did not show a clear relation to the (X)DLVO adsorption energies. This illustrated the ambivalence of (X)DLVO to predict bacterial attachment at solid soil particles of different wetting properties. Simultaneous analysis of mass recovery, numerical modeling, and interaction energy profiles thus suggested irreversible straining due to bacteria sizing as dominant compared to attachment to liquid-solid or liquid-air interfaces. Further studies are needed to distinguish straining mechanisms (i.e. pore structure or film straining) in different hydraulic conditions.

Modeling of the transport and deposition of polydispersed particles: Effects of hydrodynamics and spatiotemporal evolution of the deposition rate

A time-distance-dependent deposition model is built to investigate the effects of hydrodynamic forces on the transport and deposition of polydispersed particles and the evolution of deposition rates with time and distance. Straining and the heterogeneity of the particle population are considered to play important roles in the decreasing distribution of deposition rates. Numerical simulations were applied in a series of sand column experiments at different fluid velocities for three different porous media. The effects of hydrodynamics forces are elaborated with the systematic variations of deposition dynamic parameters of the proposed model. With retention distributions with particle size as well as temporal and spatial evolutions of deposition rates, the transport and deposition mechanisms of polydispersed particles will be elucidated through the interplay of the variation of the particle size distribution of mobile particle populations and the geometrical change of the porous medium due to retention (straining and blocking).

Modeling of retention and re-entrainment of mono- and poly-disperse particles: Effects of hydrodynamics, particle size and interplay of different-sized particles retention

In this paper, numerical simulations of experimental data were performed with kinetic rate coefficients to characterize the retention and re-entrainment dynamics under different hydrodynamic conditions for monodisperse and polydisperse latex particles (3, 10, 16μm and the mixture). The results show that drastic increase in fluid velocity provokes hardly any remarkable decrease in retention in the presence of large energy barriers (>2000kT). Systematical increases in deposition and re-entrainment dynamic rates were observed with fluid velocity and/or particle size. Increased irreversible deposition rate indicates straining and wedging dominate deposition in this study. Excess retention of 3μm particle in the polydisperse particle suspension was observed. The origins are reckoned that deposited larger particles may hinder the re-entrainment of smaller particles near the grain-to-grain contact and can provide additional sites of attachment.

Bacterial mobilization and transport through manure enriched soils: Experiment and modeling

A precise evaluation of bacteria transport and mathematical investigations are useful for best management practices in agroecosystems. In this study, using laboratory experiments and modeling approaches, we assess the transport of bacteria released from three types of manure (cow, sheep, and poultry) to find the importance of the common manures in agricultural activities in soil and water pollution. Thirty six intact soil columns with different textures (sandy, loamy, and silty clay loam) were sampled. Fecal coliform leaching from layers of the manures on the soil surface was conducted under steady-state saturated flow conditions at 20 °C for up to four Pore Volumes (PVs). Separate leaching experiments were conducted to obtain the initial concentrations of bacteria released from the manures (Co). Influent (Co) and effluent (C) bacteria concentrations were measured by the plate-count method and the normalized concentrations (C/C0) were plotted versus PV representing the breakthrough curves (BTCs). Transport parameters were predicted using the attachment/detachment model (two-kinetic site) in HYDRUS-1D. Simulations fitted well the experimental data (R² = 0.50-0.96). The attachment, detachment, and straining coefficients of bacteria were more influenced by the soils treated with cow manure compared to the sheep and poultry manures. Influent curves of fecal coliforms from the manures (leached without soil) illustrated that the poultry manure had the highest potential to pollute the effluent water from the soils in term of concentration, but the BTCs and simulated data related to the treated soils illustrated that the physical shape of cow manure was more important to both straining and detachment of bacteria back into the soil solution. Detachment trends of bacteria were observed through loam and silty clay loam soils treated with cow manure compared to the cow manure enriched sandy soil. We conclude that management strategies must specifically minimize the effect of fecal coliform concentrations before field application, especially for the combination of poultry and cow manures, which has higher solubility and tailing behavior, respectively. Interestingly, the addition of sheep manure with all three soils had the lowest mobilization of bacteria. We also suggest studying the chemistry of soil solution affected by manures to present all relevant information which affect bacterial movement through soils during leaching.

Continuum-based models and concepts for the transport of nanoparticles in saturated porous media: A state-of-the-science review

Environmental applications of nanoparticles (NP) increasingly result in widespread NP distribution within porous media where they are subject to various concurrent transport mechanisms including irreversible deposition, attachment/detachment (equilibrium or kinetic), agglomeration, physical straining, site-blocking, ripening, and size exclusion. Fundamental research in NP transport is typically conducted at small scale, and theoretical mechanistic modeling of particle transport in porous media faces challenges when considering the simultaneous effects of transport mechanisms. Continuum modeling approaches, in contrast, are scalable across various scales ranging from column experiments to aquifer. They have also been able to successfully describe the simultaneous occurrence of various transport mechanisms of NP in porous media such as blocking/straining or agglomeration/deposition/detachment. However, the diversity of model equations developed by different authors and the lack of effective approaches for their validation present obstacles to the successful robust application of these models for describing or predicting NP transport phenomena. This review aims to describe consistently all the important NP transport mechanisms along with their representative mathematical continuum models as found in the current scientific literature. Detailed characterizations of each transport phenomenon in regards to their manifestation in the column experiment outcomes, i.e., breakthrough curve (BTC) and residual concentration profile (RCP), are presented to facilitate future interpretations of BTCs and RCPs. The review highlights two NP transport mechanisms, agglomeration and size exclusion, which are potentially of great importance in controlling the fate and transport of NP in the subsurface media yet have been widely neglected in many existing modeling studies. A critical limitation of the continuum modeling approach is the number of parameters used upon application to larger scales and when a series of transport mechanisms are involved. We investigate the use of simplifying assumptions, such as the equilibrium assumption, in modeling the attachment/detachment mechanisms within a continuum modelling framework. While acknowledging criticisms about the use of this assumption for NP deposition on a mechanistic (process) basis, we found that its use as a description of dynamic deposition behavior in a continuum model yields broadly similar results to those arising from a kinetic model. Furthermore, we show that in two dimensional (2-D) continuum models the modeling efficiency based on the Akaike information criterion (AIC) is enhanced for equilibrium vs kinetic with no significant reduction in model performance. This is because fewer parameters are needed for the equilibrium model compared to the kinetic model. Two major transport regimes are identified in the transport of NP within porous media. The first regime is characterized by higher particle-surface attachment affinity than particle-particle attachment affinity, and operative transport mechanisms of physicochemical filtration, blocking, and physical retention. The second regime is characterized by the domination of particle-particle attachment tendency over particle-surface affinity. In this regime although physicochemical filtration as well as straining may still be operative, ripening is predominant together with agglomeration and further subsequent retention. In both regimes careful assessment of NP fate and transport is necessary since certain combinations of concurrent transport phenomena leading to large migration distances are possible in either case.

Comparison of transport between two bacteria in saturated porous media with distinct pore size distribution

The transport behaviour ofEscherichia coliandKlebsiellasp. was studied under saturated flow conditions to explore the effect of pore size distribution and bacteria cell properties on microbial transport.

Experimental investigation of suspended particles transport through porous media: particle and grain size effect

Particle and grain size may influence the transportation and deposition characteristics of particles within pollutant transport and within granular filters that are typically used in wastewater treatment. We conducted two-dimensional sandbox experiments using quartz powder as the particles and quartz sand as the porous medium to study the response of transportation and deposition formation to changes in particle diameter (ds, with median diameter 18, 41, and 82 μm) and grain diameter (dp, with median diameter 0.36, 1.25, and 2.82 mm) considering a wide range of diameter ratios (ds/dp) from 0.0064 to 0.228. Particles were suspended in deionized water, and quartz sand was used as the porous medium, which was meticulously cleaned to minimize any physicochemical and impurities effects that could result in indeterminate results. After the experiments, the particle concentration of the effluent and particle mass per gram of dry sands were measured to explore changes in transportation and deposition characteristics under different conditions. In addition, a micro-analysis was conducted to better analyse the results on a mesoscopic scale. The experimental observation analyses indicate that different diameter ratios (ds/dp) may lead to different deposit formations. As ds/dp increased, the deposit formation changed from 'Random Deposition Type' to 'Gradient Deposition Type', and eventually became 'Inlet Deposition Type'.

Release of E. coli D21g with transients in water content

Transients in water content are well-known to mobilize microorganisms that are retained in the vadose zone. However, there is no consensus on the relative importance of drainage and imbibition events on microorganism release. To overcome this limitation, we have systematically studied the release of Escherichia coli D21g during cycles of drainage and imbibition under various solution chemistry and initial conditions. Results from these column studies revealed the influence of imbibition and drainage on D21g release. In particular, imbibition efficiently released cells from the air-water interface (AWI) that were initially retained under steady-state unsaturated conditions by expansion of water films and destruction of the AWI. Conversely, significant release and transport of cells during drainage only occurred below a critical water saturation (water film thickness). In this case, a fraction of the cells that were initially retained on the solid-water interface (SWI) partitioned into the mobile aqueous phase and the AWI as the receding water film thickness decreased during drainage. The efficiency of cell release from the SWI during drainage was much less than for the AWI during imbibition. Cycles of drainage and imbibition removed cells from the SWI and the AWI, respectively. However, the peak concentration and amount of cells that were released increased with the number of retained cells and the amount of drainage and imbibition, and decreased with the number of drainage and imbibition cycles. Release of cells during drainage and imbibition was found to be more pronounced in the presence of a weak secondary minimum when the ionic strength (IS) was 5 mM NaCl. Increases in the solution IS decreased the influence of water transients on release, especially during drainage. Complete recovery of the retained cells could be achieved using both IS reduction and cycles of drainage and imbibition, even when the cells were retained under favorable attachment conditions. In general, cell release was more pronounced with transients in water content than transients in IS when the IS ≥ 5 mM.

How significant to plant N nutrition is the direct consumption of soil microbes by roots?

The high degree to which plant roots compete with soil microbes for organic forms of nitrogen (N) is becoming increasingly apparent. This has culminated in the finding that plants may consume soil microbes as a source of N, but the functional significance of this process remains unknown. We used (15) N- and (14) C-labelled cultures of soil bacteria to measure rates of acquisition of microbes by sterile wheat roots and plants growing in soil. We compared these rates with acquisition of (15) N delivered as nitrate, amino acid monomer (l-alanine) and short peptide (l-tetraalanine), and the rate of decomposition of [(14) C] microbes by indigenous soil microbiota. Acquisition of microbe (15) N by both sterile roots and roots growing in soil was one to two orders of magnitude slower than acquisition of all other forms of (15) N. Decomposition of microbes was fast enough to account for all (15) N recovered, but approximately equal recovery of microbe (14) C suggests that microbes entered roots intact. Uptake of soil microbes by wheat (Triticum aestivum) roots appears to take place in soil. If wheat is typical, the importance of this process to terrestrial N cycling is probably minor in comparison with fluxes of other forms of soil inorganic and organic N.

Quantification of colloid retention and release by straining and energy minima in variably saturated porous media

The prediction of colloid transport in unsaturated porous media in the presence of large energy barrier is hampered by scant information of the proportional retention by straining and attractive interactions at surface energy minima. This study aims to fill this gap by performing saturated and unsaturated column experiments in which colloid pulses were added at various ionic strengths (ISs) from 0.1 to 50 mM. Subsequent flushing with deionized water released colloids held at the secondary minimum. Next, destruction of the column freed colloids held by straining. Colloids not recovered at the end of the experiment were quantified as retained at the primary minimum. Results showed that net colloid retention increased with IS and was independent of saturation degree under identical IS and Darcian velocity. Attachment rates were greater in unsaturated columns, despite an over 3-fold increase in pore water velocity relative to saturated columns, because additional retention at the readily available air-associated interfaces (e.g., the air-water-solid [AWS] interfaces) is highly efficient. Complementary visual data showed heavy retention at the AWS interfaces. Retention by secondary minima ranged between 8% and 46% as IS increased, and was greater for saturated conditions. Straining accounted for an average of 57% of the retained colloids with insignificant differences among the treatments. Finally, retention by primary minima ranged between 14% and 35% with increasing IS, and was greater for unsaturated conditions due to capillary pinning.

Size- and concentration-dependent deposition of fluorescent silica colloids in saturated sand columns: transport experiments and modeling

This study investigates the size and concentration effects on the transport of silica colloids in columns of sandy aquifer material. Colloid transport experiments were performed with specifically developed fluorescent labeled silica colloids in columns of a repacked natural porous medium under hydro-geochemical conditions representative of sandy aquifers. Breakthrough curves and vertical deposition profiles of colloids were measured for various colloid concentrations and sizes. The results showed that for a given colloid concentration injected, deposition increased when increasing the size of the colloids. For a given colloid size, retention was also shown to be highly concentration-dependent with a non-monotonous pattern presenting low and high concentration specificities. Deposition increases when increasing both size and injected concentration, until a threshold concentration is reached, above which retention decreases, thus increasing colloid mobility. Results observed above the threshold concentration agree with a classical blocking mechanism typical of a high concentration regime. Results observed at lower colloid concentrations were not modeled with a classical blocking model and a depth- and time-dependent model with a second order kinetic law was necessary to correctly fit the experimental data in the entire range of colloid concentrations with a single set of parameters for each colloidal size. The colloid deposition mechanisms occuring at low concentrations were investigated through a pore structure analysis carried out with Mercury Intrusion Porosimetry and image analysis. The determined pore size distribution permitted estimation of the maximal retention capacity of the natural sand as well as some low flow zones. Altogether, these results stress the key role of the pore space geometry of the sand in controlling silica colloids deposition under hydro-geochemical conditions typical of sandy aquifers. Our results also showed originally that colloid mobility in porous media is not only favored at high colloid concentrations, but also at very low concentrations, which are more likely to be observed in groundwater.

Impact of dilution on the transport of poly(acrylic acid) supported magnetite nanoparticles in porous media

This paper investigates the impact of dilution on the mobility of magnetite nanoparticles surface coated with poly(acrylic acid) (PAA). Transport experiments were conducted in a water-saturated sand-packed column for input nanoparticle solutions with total Fe concentrations ranging from 100 to 600mg/L. Particle size analysis of the synthesized nanoparticle solutions showed that PAA provides good size stability for Fe concentrations as low as about 1mg/L. Time-moment analysis of the nanoparticle breakthrough curves, on the other hand, revealed that nanoparticle mass recovery from the column decreased consistently with dilution, with greater attenuation, sharper fronts and longer tails compared to that of the tracer. Particle size analysis of the eluted solutions shows that the nanoparticle size is negatively correlated with nanoparticle concentration. Modeling results suggest that the decrease in nanoparticle mobility with input concentration can be represented using a kinetic time-dependent deposition term with finite deposition capacity and a kinetic detachment term. For field applications, the increase in particle size and detachment resulting from dilution means reduced transport efficiency of nanoparticles and reaction potential with travel distance.

Motility of Pseudomonas aeruginosa in saturated granular media as affected by chemoattractant

To examine and quantify the effects of glass beads and chemoattractant on bacterial motility in granular media, we examined the motile behavior of P. aeruginosa in a saturated granular medium and quantified the effects of glass beads and the presence of a chemoattractant. By recording individual cell trajectories in microfluidic channels under a high-speed confocal microscope, we directly measured the cell's run direction and corresponding run-length, speed and turn angle. Bacterial run speed increased in the presence of chemoattractant in both aqueous and granular media. But it decreased in glass-beads compared to in aqueous media due to the restricted pore geometry and interactions between bacteria and grain surfaces. Notably, the relatively higher frequency distribution at turn angles of 170° decreased dramatically, while the smaller peak at 70° increased and became dominant on a bimodal distribution, showing more bacteria changed directions at smaller turn angles rather than reverse their swimming directions. Additionally, the presence of glass beads also decreased the chemotactic velocity and random motility by similar proportions due to the restrictive geometry and the interactions between bacteria and glass beads surface. Our study indicates that the swimming parameters measured from aqueous media cannot be directly adopted in models for predicting bacteria travel in granular media.

Colloid straining within saturated heterogeneous porous media

The transport of 0.46 μm, 2.94 μm, 5.1 μm and 6.06 μm latex particles in heterogeneous porous media prepared from the mixing of 0.78 mm, 0.46 mm and 0.23 mm quartz sands was investigated through column transport experiments. It was observed that the 0.46 μm particles traveled conservatively within the heterogeneous porous media, suggesting that under the experimental conditions employed in this research the strong repulsive interactions between the negatively charged latex particles and the clean quartz sands led to minimal colloid immobilization due to physicochemical filtration. The immobilization of the 2.94 μm, 5.1 μm and 6.06 μm latex particles was thus attributed to colloid straining. Experimental results showed that the straining of colloidal particles within heterogeneous sand mixtures increased when the fraction of finer sands increased. The mathematical model that was developed and tested based on results obtained using uniform sands (Xu et al., 2006) was found to be able to describe colloid straining within heterogeneous porous media. Examination of the relationship between the best-fit values of the clean-bed straining rate coefficients (k(0)) and the ratio of colloid diameter (d(p)) and sand grain size (d(g)) indicated that when number-average sizes were used to represent the size of the heterogeneous porous media, there existed a consistent relationship for both uniform sands and heterogeneous sand mixtures. Similarly, the use of the number-averaged sizes for the heterogeneous porous media produced a uniform relationship between the colloid straining capacity term (λ) and the ratio of d(p)/d(g) for all the sand treatments.

The impact of biofilm growth on transport of Escherichia coli O157:H7 in sand

Understanding the transport behavior, survival, and persistence of pathogens such as Escherichia coli O157:H7 in the subsurface is essential to protection of public health. In this study, the transport of E. coli O157:H7 in a two-dimensional bench-scale sand aquifer system, hereafter referred to as the sandbox, was investigated, with a focus on the impact of biofilm development on E. coli retention and survival. Biofilm growth was initiated through flushing with unsterilized groundwater and addition of glucose, nitrate, and phosphate. Retention of E. coli from an injection test in clean sand, prior to promotion of biofilm growth, was approximately 9%. Subsequent to biofilm growth, 47% of injected E. coli cells were retained under similar flow conditions. After 10 d of no flow, sterile water was flushed through the biofouled sandbox and substantial concentrations (up to 1.5 × 10(5) cells/mL) of E. coli were measured in the effluent indicating that E. coli had survived the starvation period. Confocal laser scanning microscopy revealed that E. coli were located not only on the surface but also within the biofilm. Imposition of starvation conditions resulted in biofilm sloughing and possible mobilization of biofilm-associated E. coli.

Fecal indicator bacteria variability in samples pumped from monitoring wells

The detection of microbiological contamination in drinking water from groundwater wells is often made with a limited number of samples that are collected using traditional geochemical sampling protocols. The objective of this study is to examine the variability of fecal indicator bacteria, as observed using discrete samples, due to pumping. Two wells were instrumented as multilevel piezometers in a bedrock aquifer, and bacterial enumeration was conducted on a total of 166 samples (for total coliform, fecal coliform, Escherichia coli, and fecal streptococci) using standard membrane filtration methods. Five tests were conducted using pumping rates ranging from 0.3 to 17 L/min in a variety of purging scenarios, which included constant and variable (incremental increase and decrease) flow. The results clearly show a rapid and reproducible, 1 to 2 log-unit decrease in fecal indicator bacteria at the onset of pumping to stabilized, low-level concentrations prior to the removal of three to five well volumes. The pumping rate was not found to be correlated with the magnitude of observed bacterial counts. Based on the results, we suggest sampling protocols for fecal indicator bacteria that include multiple collections during the course of pumping, including early-time samples, and consider other techniques such as microscopic enumeration when assessing the source of bacteria from the well-aquifer system.

Involvement of cell shape and flagella in the bacterial retention during percolation of contaminated water through soil columns in tropical region

Microorganisms' retention in soil contributes to the natural purification of groundwater. Bacteria found in groundwater are generally of various shapes. The aim of this study was to assess the importance of cell shape and flagella in bacterial retention during polluted water percolation through two soil columns CA and CB, in the equatorial region in Central Africa. Percolation tests were carried out using different water loads samples which were contaminated by Escherichia coli (straight rods, peritrichous flagella), Vibrio parahaemolyticus (rods bacteria, polar flagella), and Staphylococcus saprophyticus (spherical, free-flagellum). It has been noted that showed that through soil column CA, the mean values of cells retention ratios (T(R)) varied with bacteria species considered, and from one applied water load sample to another. E. coli T(R) and that of S. saprophyticus were not significantly different (P> 0.05) for the two soil columns. V. parahaemolyticus T(R) significantly differed from that of E. coli and S. saprophyticus through soil column CA (P< 0.01) when the highest water load was applied, and through soil column CB (P< 0.05) for each of water load applied. A relative hierarchical arrangement of retained cells based on the T(R) showed that V. parahaemolyticus was less retained through the 2 soil columns. S. saprophyticus in most cases was more retained than others. The physical properties of the bacterial cell must be taken into consideration when evaluating the transfer of bacteriological pollutants towards groundwater.

Direct observations of colloid retention in granular media in the presence of energy barriers, and implications for inferred mechanisms from indirect observations

In this paper we present direct observations of retention of colloids in granular porous media over a large size range (0.21-9.0 microm) and generalize the significance of attachment in grain to grain contacts and attachment on the open surface as a function of colloid:collector ratio. We examine reversibility of attachment via these mechanisms with respect to ionic strength reduction and fluid velocity increase. We relate these direct observations to existing literature, and in some cases offer alternative interpretations of mechanisms of retention drawn from indirect observations (e.g. via column effluent and retained concentrations).

On the role of metabolic activity on the transport und deposition of Pseudomonas fluorescens in saturated porous media

A study was conducted to understand the role of cell concentration and metabolic state in the transport and deposition behaviour of Pseudomonas fluorescens with and without substrate addition. Column experiments using the short-pulse technique (pulse was equivalent to 0.028 pore volume) were performed in quartz sand operating under saturated conditions. For comparison, experiments with microspheres and inactive (killed) bacteria were also conducted. The effluent concentrations, the retained particle concentrations and the cell shape were determined by fluorescent microscopy. For the transport of metabolically-active P. fluorescens without substrate addition a bimodal breakthrough curve was observed, which could be explained by the different breakthrough behaviour of the rod-shaped and coccoidal cells of P. fluorescens. The 70:30 rod/coccoid ratio in the influent drastically changed during the transport and it was about 20:80 in the effluent and in the quartz sand packing. It was assumed that the active rod-shaped cells were subjected to shrinkage into coccoidal cells. The change from active rod-shaped cells to coccoidal cells could be explained by oxygen deficiency which occurs in column experiments under saturated conditions. Also the substrate addition led to two consecutive breakthrough peaks and to more bacteria being retained in the column. In general, the presence of substrate made the assumed stress effects more pronounced. In comparison to microspheres and inactive (killed) bacteria, the transport of metabolically-active bacteria with and without substrate addition is affected by differences in physiological state between rod-shaped and the formed stress-resistant coccoidal cells of P. fluorescens.

Migration of colloids through nonfractured clay-rich aquitards

This study examined characteristics and controls on diffusive transport of colloids through nonfractured, clay-rich glacial till. The range in molecular weight (M(w) of the colloids tested (five polymers and three natural dissolved organic carbons) was 910-15 450 Da. Hydrodynamic diameters increased from 1.45 to 6.05 nm, and aqueous diffusion coefficients decreased from 2.6 x 10(-10) to 6.3 x 10(11) m2 s(-1) with increasing M(w). All colloids were subjected to diffusion testing using undisturbed core samples placed in double reservoir diffusion cells. All colloids decreased in concentration with time in the spiked reservoirs. Concentrations in the receiving reservoirs increased for only the four smallest colloids. The lack of breakthroughs for larger colloids was attributed to straining. Transport modeling using data from colloids exhibiting breakthrough shows effective diffusion coefficients and tortuosity factors decrease from 1.5 x 10(-10) to 6.5 x 10(-11) m2 s(-1) and from 0.6 to 0.3, respectively, with increasing M(w). The effective porosities are slightly less than total porosity (0.31). Our data suggest diffusive transport through clay-rich aquitards is limited to colloids with mean diameters < 2-2.2 nm and that measuring the diameter of dissolved organic carbon from nonfractured clay-rich aquitards may be an effective method to estimate effective pore throat diameters of these media.

Microbial removal rates in subsurface media estimated from published studies of field experiments and large intact soil cores

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.

Diversity in cell properties and transport behavior among 12 different environmental Escherichia coli isolates

Escherichia coli is a commonly used indicator organism for detecting the presence of fecal-borne pathogenic microorganisms in water supplies. The importance of E. coli as an indicator organism has led to numerous studies looking at cell properties and transport behavior of this microorganism. In many of these studies, however, only a single strain of E. coli was used even though research has shown that significant genetic variability exists among different strains of E. coli. If this genetic diversity results in differences in cell properties that affect transport, different strains of E. coli may exhibit different rates of transport in the environment. Therefore, the objectives of our study were to investigate the variability in surface characteristics and transport behavior of E. coli isolates obtained from six different sources: beef cattle, dairy cattle, horse, human, poultry, and wildlife. Cell properties such as electrophoretic mobility, cell size and shape, hydrophobicity, charge density, and extracellular polymeric substance composition were measured for each isolate. In addition, the transport behavior of each isolate was assessed by measuring transport through 10-cm packed beds of clean quartz sand. Our results show a large diversity in cell properties and transport behavior for the different E. coli isolates. This diversity in transport behavior must be taken into account when making assessments of the suitability of using E. coli as an indicator organism for specific pathogenic microorganisms in groundwater environments as well as modeling the movement of E. coli in the subsurface.

Influence of organic waste type and soil structure on the bacterial filtration rates in unsaturated intact soil columns

Organic wastes are considered to be a source for the potentially pathogenic microorganisms found in surface and sub-surface water resources. Following their release from the organic waste matrix, bacteria often infiltrate into soil and may be transported to significant depths contaminating aquifers. We investigated the influence of soil texture and structure and most importantly the organic waste properties on the transport and filtration coefficients of Escherichia coli and total bacteria in undisturbed soil columns. Intact soil columns (diameter 16 cm and height 25 cm) were collected from two soils: sandy clay loam (SCL) and loamy sand (LS) in Hamadan, western Iran. The cores were amended with cow manure, poultry manure and sewage sludge at a rate of 10 Mg ha(-1) (dry basis). The amended soil cores were leached at a steady-state flux of 4.8 cm h(-1) (i.e. 0.12 of saturated hydraulic conductivity of the SCL) to a total volume of up to 4 times the pore volume of the columns. The influent (C(0)) and effluent (C) were sampled at similar time intervals during the experiments and bacterial concentrations were measured by the plate count method. Cumulative numbers of the leached bacteria, filtration coefficient (lambda(f)), and relative adsorption index (S(R)) were calculated. The preferential pathways and stable structure of the SCL facilitated the rapid transport and early appearance of the bacteria in the effluent. The LS filtered more bacteria when compared with the SCL. The effluent contamination of poultry manure-treated columns was greater than the cow manure- and sewage sludge-treated ones. The difference between cow manure and sewage sludge was negligible. The lambda(f) and S(R) values for E. coli and total bacteria were greater in the LS than in the SCL. This indicates a predominant role for the physical pore-obstruction filtration mechanisms as present in the poorly structured LS vs. the retention at adsorptive sites (chemical filtration) more likely in the better structured SCL. While the results confirmed the significant role of soil structure and preferential (macroporous) pathways, manure type was proven to have a major role in determining the maximum penetration risk of bacteria by governing filtration of bacteria. Thus while the numbers of bacteria in waste may be of significance for shallow aquifers, the type of waste may determine the risk for microbial contamination of deep aquifers.

Transport and retention of a bacteriophage and microspheres in saturated, angular porous media: effects of ionic strength and grain size

Eight saturated column experiments were conducted to examine the effects of solution chemistry and grain size on the transport of colloids through crushed silica sand. Two sizes of colloids, 0.025-microm bacteriophage (MS-2) and 1.5-microm carboxylated microspheres, were used as surrogates for the transport of pathogenic viruses and bacteria, respectively. Increasing the Ca(2+) concentration from 1 to 4.8 mM (along with background monovalent ions) resulted in complete attenuation (>6-log decrease in C/C(0)) of MS-2, but caused only a 1-log reduction (C/C(0)=0.1) in the concentration of the microspheres. Decreasing grain size from medium sand (d(50)=0.70 mm) to fine sand (d(50)=0.34 mm) resulted in substantial decreases in effluent concentrations of both the MS-2 (5-log decrease) and microspheres (>2.5-log decrease). Comparison of observed colloid retention to that predicted by a recently published correlation equation for colloid filtration revealed that the model can considerably underpredict (by 4 orders of magnitude or more) colloid retention by angular sand over distances as short as 20 cm. This indicates that state-of-the-art colloid filtration models are still limited in applicability to natural systems.

Straining of nonspherical colloids in saturated porous media

We explore the effects of colloid shape on straining kinetics by measuring the filtration of spherical and nonspherical colloids within saturated columns packed with quartz sand. Our observations demonstrate that the transport of peanut-shaped colloids matches the transport of spherical colloids with diameters equal to the minor-axis length of the peanut-shaped colloids. The straining rates of the spherical colloids vary linearly with the ratio of colloid diameter (d(p)) to sand-grain diameter (d(g)) for 0.0083 < d(p)/d(g) < 0.06. This linear relationship also quantifies the straining rates of the peanut-shaped particles provided that the particle's minor axis length is used for d(p). Results of pore-scale simulations reveal that a peanut-shaped particle adopts a preferred orientation as it approaches a pore-space constriction such that its major axis tends to align with the local flow direction. The extent of this reorientation increases with the particle's aspect ratio. Findings from this research suggest that straining is sensitive to changes in colloid shape and thatthe kinetics of this process can be approximated on the basis of measurable properties of the nonspherical colloids and porous media.

A stochastic model for colloid transport and deposition

Profiles of retained colloids in porous media have frequently been observed to be hyper-exponential or non-monotonic with transport depth under unfavorable attachment conditions, whereas filtration theory predicts an exponential profile. In this work we present a stochastic model for colloid transport and deposition that allows various hypotheses for such deviations to be tested. The model is based on the conventional advective dispersion equation that accounts for first-order kinetic deposition and release of colloids. One or two stochastic parameters can be considered in this model, including the deposition coefficient, the release coefficient, and the average pore water velocity. In the case of one stochastic parameter, the probability density function (PDF) is characterized using log-normal, bimodal log-normal, or a simple two species/region formulation. When two stochastic parameters are considered, then a joint log-normal PDF is employed. Simulation results indicated that variations in the deposition coefficient and the average pore water velocity can both produce hyper-exponential deposition profiles. Bimodal formulations for the PDF were also able to produce hyper-exponential profiles, but with much lower variances in the deposition coefficient. The shape of the deposition profile was found to be very sensitive to the correlation of deposition and release coefficients, and to the correlation of pore water velocity and deposition coefficient. Application of the developed stochastic model to a particular set of colloid transport and deposition data indicated that chemical heterogeneity of the colloid population could not fully explain the observed behavior. Alternative interpretations were therefore proposed based on variability of the pore size and the water velocity distributions.

Measuring and modelling straining of Escherichia coli in saturated porous media

Though coliform bacteria are used worldwide to indicate fecal pollution of groundwater, the parameters determining the transport of Escherichia coli in aquifers are relatively unknown. We evaluated the occurrence of both straining and attachment of E. coli ATCC25922 in columns of ultra-pure, angular, saturated quartz sand. The column experiments were conducted over a wide range of porous medium sizes, column heights, input concentrations, and pore water flow velocities. Straining and attachment were examined by modelling the breakthrough curves (with HYDRUS 1D). In addition, model output was compared with measured strained and attached bacteria via column extrusion experiments (in which sand was extruded from the column and placed in excess water) and flow reversal experiments (in which the pore water flow direction was reversed, thereby dislodging strained bacteria). Our model consisted of an attachment rate coefficient and a straining rate coefficient; both of these decreased with transport distance. The straining rate coefficient also decreased in a Langmuirian way, in response to the filling of available pore space, which in turn depended on influent bacteria concentration, quartz grain diameter, and transport distance. The maximum strained fraction was 25-30% of total bacteria mass applied to the column; the maximum attached fraction was 30-35%. The fit between modelled and measured (strained and attached) bacteria masses was acceptable, as was the sensitivity of the model output to fitted parameter values. Our results lead to a new description for the time-dependent mass balance of strained bacteria, which entails using three fitting parameters. The results also imply that column experiments in combination with retention profiles (or various column lengths) are not enough to explain the retention processes in a column. Column extrusion and flow reversal experiments provide vital additional information on the occurrence and magnitude of straining. Our straining model could be of assistance in evaluating the importance of straining and in incorporating the straining process in bacteria transport modelling.

Bacteria transport and deposition under unsaturated conditions: the role of the matrix grain size and the bacteria surface protein

Unsaturated (80% water saturated) packed column experiments were conducted to investigate the influence of grain size distribution and bacteria surface macromolecules on bacteria (Rhodococcus rhodochrous) transport and deposition mechanisms. Three sizes of silica sands were used in these transport experiments, and their median grain sizes were 607, 567, and 330 microm. The amount of retained bacteria increased with decreasing sand size, and most of the deposited bacteria were found adjacent to the column inlet. The deposition profiles were not consistent with predictions based on classical filtration theory. The experimental data could be accurately characterized using a mathematical model that accounted for first-order attachment, detachment, and time and depth-dependent straining processes. Visual observations of the bacteria deposition as well as mathematical modelling indicated that straining was the dominant mechanism of deposition in these sands (78-99.6% of the deposited bacteria), which may have been enhanced due to the tendency of this bacterium to form aggregates. An additional unsaturated experiment was conducted to better deduce the role of bacteria surface macromolecules on attachment and straining processes. In this case, the bacteria surface was treated using a proteolitic enzyme. This technique was assessed by examining the Fourier-transform infrared spectrum and hydrophobicity of untreated and enzyme treated cells. Both of these analytical procedures demonstrated that this enzymatic treatment removed the surface proteins and/or associated macromolecules. Transport and modelling studies conducted with the enzyme treated bacteria, revealed a decrease in attachment, but that straining was not significantly affected by this treatment.

Coupling of physical and chemical mechanisms of colloid straining in saturated porous media

Filtration theory does not include the potential influence of pore structure on colloid removal by straining. Conversely, previous research on straining has not considered the possible influence of chemical interactions. Experimental and theoretical studies were therefore undertaken to explore the coupling of physical and chemical mechanisms of colloid straining under unfavorable attachment conditions (pH=10). Negatively charged latex microspheres (1.1 and 3 microm) and quartz sands (360, 240, and 150 microm) were used in packed column studies that encompassed a range in suspension ionic strengths (6-106 mM) and Darcy water velocities (0.1-0.45 cm min(-1)). Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations and torque analysis suggests that attachment of colloids to the solid-water interface was not a significant mechanism of deposition for the selected experimental conditions. Effluent concentration curves and hyperexponential deposition profiles were strongly dependent on the solution chemistry, the system hydrodynamics, and the colloid and collector grain size, with greater deposition occurring for increasing ionic strength, lower flow rates, and larger ratios of the colloid to the median grain diameter. Increasing the solution ionic strength is believed to increase the force and number of colloids in the secondary minimum of the DLVO interaction energy profile. These weakly associated colloids can be funneled to small regions of the pore space formed adjacent to grain-grain junctions. For select systems, the ionic strength of the eluant solution was decreased to 6mM following the recovery of the effluent concentration curve. In this case, only a small portion of the deposited colloids was recovered in the effluent and the majority was still retained in the sand. These observations suggest that the extent of colloid removal by straining is strongly coupled to solution chemistry.

Transport of Escherichia coli bacteria through laboratory columns of glacial-outwash sediments: estimating model parameter values based on sediment characteristics

Bacterial transport through cores of intact, glacial-outwash aquifer sediment was investigated with the overall goal of better understanding bacterial transport and developing a predictive capability based on the sediment characteristics. Variability was great among the cores. Normalized maximum bacterial-effluent concentrations ranged from 5.4x10(-7) to 0.36 and effluent recovery ranged from 2.9x10(-4) to 59%. Bacterial breakthrough was generally rapid with a sharp peak occurring nearly twice as early as the bromide peak. Bacterial breakthrough exhibited a long tail of relatively constant concentration averaging three orders of magnitude less than the peak concentration for up to 32 pore volumes. The tails were consistent with non-equilibrium detachment, corroborated by the results of flow interruption experiments. Bacterial breakthrough was accurately simulated with a transport model incorporating advection, dispersion and first-order non-equilibrium attachment/detachment. Relationships among bacterial transport and sediment characteristics were explored with multiple regression analyses. These analyses indicated that for these cores and experimental conditions, easily-measurable sediment characteristics--median grain size, degree of sorting, organic-matter content and hydraulic conductivity--accounted for 66%, 61% and 89% of the core-to-core variability in the bacterial effective porosity, dispersivity and attachment-rate coefficient, respectively. In addition, the bacterial effective porosity, median grain size and organic-matter content accounted for 76% of the inter-core variability in the detachment-rate coefficient. The resulting regression equations allow prediction of bacterial transport based on sediment characteristics and are a possible alternative to using colloid-filtration theory. Colloid-filtration theory, used without the benefit of running bacterial transport experiments, did not as accurately replicate the observed variability in the attachment-rate coefficient.

Quantification of bacterial mass recovery as a function of pore-water velocity and ionic strength

Transport of bacteria in aquifer systems plays an important role in bioaugmentation, which relies upon successful bacterial delivery to a target area. In the present study, we conducted a set of laboratory column experiments under various conditions of pore-water velocity (upsilon(omega)) and ionic strength (IS) of culture medium for Pseudomonas aeruginosa, known to be a benzene-degrading bacteria, in order to investigate their relationship to mass recovery in saturated quartz sands. The column experiments revealed that both peak concentrations and mass recoveries of bacteria were lower than those of a conservative tracer KCl when deionized water was used as leaching water for all ranges of pore-water velocity (0.18-6.23 cm/min). Thus, the parameter responsible for transport of P. aeruginosa was only the deposition coefficient. Bacterial cells could not be attached to the mineral surfaces by predominance of electrostatic charge or repulsive forces over hydrophobicity or attractive forces due to the very low ionic strength ( approximately 0 mM) of deionized water. The loss of bacterial mass was attributed to the deposition in the crevice formed on the quartz surfaces, as evidenced by SEM images. For a given pore-water velocity, the ionic strength markedly influenced bacterial deposition, showing decreased peak concentrations and mass recoveries with increasing ionic strength of column leaching water. An optimum range of upsilon(omega) and IS for achieving bacterial mass recovery higher than 70% in the studied quartz sand was found such that: (i) at low IS ( approximately 0 mM), a pore-water velocity higher than 0.30 cm/min, and (ii) at pore-water velocity of 0.52 cm/min, an IS lower than 290 mM, were required, respectively.

Transport of giardia and manure suspensions in saturated porous media

Experiments were conducted to elucidate the transport behavior of cysts of Giardia and manure suspensions through several aquifer sands. Decreasing the median grain size of the sand resulted in lower peak effluent concentrations and increased deposition of the Giardia and manure particles in the sand near the column inlet. The effluent concentration curves for the manure suspensions also exhibited asymmetric shapes that tended to include larger particle sizes as the manure suspension was continuously added. Simulations of the transport of Giardia and manure particles using a simple and flexible power law model for the solid-water mass exchange term provided a satisfactory description of the effluent and spatial distribution data. The cumulative size distribution (CSD) of manure particles in the suspension initially and after passage through the packed columns was used to identify the mechanisms that were controlling the deposition of manure particles and Giardia. The CSD data indicated that manure particles were completely removed at early times by mechanical filtration and/or straining when the ratio of the particle to the median grain diameter was greater than 0.003 to 0.017. However, the CSD changed with increasing time due to deposition-induced filling of straining sites. The Giardia transport was controlled by straining. For a given sand, higher effluent concentrations of Giardia were observed in the presence than in the absence of manure suspension. The relative increase of Giardia in the effluent concentrations varied from 75 to 172%. Hence, pathogen transport studies conducted in the absence of manure suspension may underestimate transport potential in manure-contaminated environments.

Evaluation of data from the literature on the transport and survival of Escherichia coli and thermotolerant coliforms in aquifers under saturated conditions

Escherichia coli and thermotolerant coliforms are of major importance as indicators of fecal contamination of water. Due to its negative surface charge and relatively low die-off or inactivation rate coefficient, E. coli is able to travel long distances underground and is therefore also a useful indicator of fecal contamination of groundwater. In this review, the major processes known to determine the underground transport of E. coli (attachment, straining and inactivation) are evaluated. The single collector contact efficiency (SCCE), eta0, one of two parameters commonly used to assess the importance of attachment, can be quantified for E. coli using classical colloid filtration theory. The sticking efficiency, alpha, the second parameter frequently used in determining attachment, varies widely (from 0.003 to almost 1) and mainly depends on charge differences between the surface of the collector and E. coli. Straining can be quantified from geometrical considerations; it is proposed to employ a so-called straining correction parameter, alpha(str). Sticking efficiencies determined from field experiments were lower than those determined under laboratory conditions. We hypothesize that this is due to preferential flow mechanisms, E. coli population heterogeneity, and/or the presence of organic and inorganic compounds in wastewater possibly affecting bacterial attachment characteristics. Of equal importance is the inactivation or die-off of E. coli that is affected by factors like type of bacterial strain, temperature, predation, antagonism, light, soil type, pH, toxic substances, and dissolved oxygen. Modeling transport of E. coli can be separated into three steps: (1) attachment rate coefficients and straining rate coefficients can be calculated from Darcy flow velocity fields or pore water flow velocity fields, calculated SCCE fields, realistic sticking efficiency values and straining correction parameters, (2) together with the inactivation rate coefficient, total rate coefficient fields can be generated, and (3) used as input for modeling the transport of E. coli in existing contaminant transport codes. Areas of future research are manifold and include the effects of typical wastewater characteristics, including high concentrations of organic compounds, on the transport of E. coli and thermotolerant coliforms, and the upscaling of experiments to represent typical field conditions, possibly including preferential flow mechanisms and the aspect of population heterogeneity of E. coli.

Concentration dependent transport of colloids in saturated porous media

A series of column experiments was undertaken to explore the influence of colloid input concentration (2, 1, 0.5, and 0.25 times a reference concentration), colloid size (negatively charged 3.2 and 1.0 microm carboxyl latex), and sand grain size (360, 240, and 150 microm quartz sands) on transport and deposition. A similar mass of stable mono-dispersed colloids was added to each column. For a given input concentration, decreasing the sand size and increasing the colloid size resulted in increased mass retention in the sand near the column inlet and lower relative concentrations in the effluent. For a given sand and colloid, increasing the input concentration produced less deposition and higher mass recovery in the effluent, especially for coarser sands and smaller colloids. Results of a time dependent attachment (blocking) and detachment model were not consistent with this behavior because the simulations predicted much less retention near the column inlet and a decreasing number of favorable attachment sites (mass of deposited colloids) with increasing input concentration in a given system (colloid and sand). A time dependent straining model (filling of straining sites) provided a better description of the effluent and deposition data, but still could not account for the observed concentration dependent mass recovery. Alternatively, the straining model was refined to include a liberation term that assumed that straining was hindered at higher concentrations (collision frequencies) due to repulsive colloid (aqueous phase)-colloid (strained) interactions. Simulations that included straining, liberation, attachment, and detachment significantly improved the description of the experimental data.