- Top of page
- 1. Introduction
- 2. Methods
- 3. Results
- 4. Discussion
- 5. Conclusions
 Fecal bacteria are frequently found at much greater distances than would be predicted by laboratory studies, indicating that improved models that incorporate more complexity might be needed to explain the widespread contamination of many shallow aquifers. In this study, laboratory measurements of breakthrough and retained bacteria in columns of intact and repacked sediment cores from Bangladesh were fit using a two-population model with separate reversible and irreversible attachment sites that also incorporated bacterial decay rates. Separate microcosms indicated an average first-order decay rate of 0.03 log10/day for both free bacteria in the liquid phase and bacteria attached to the solid phase. Although two thirds of the column results could be well fit with a dual-deposition site, single-population model, fitting of one third of the results required a two-population model with a high irreversible attachment rate (between 5 and 60 h−1) for one population of bacteria and a much lower rate (from 5 h−1 to essentially zero) for the second. Inferred attachment rates for the reversible sites varied inversely with grain size (varying from 1 to 20 h−1 for grain sizes between 0.1 and 0.3 mm) while reversible detachment rates were found to be nearly constant (approximately 0.5 h−1). Field simulations based on the fitted two-population model parameters predict only a twofold reduction in fecal source concentration over a distance of 10 m, determined primarily by the decay rate of the bacteria. The existence of a secondary population of bacteria with a low attachment rate might help explain the observed widespread contamination of tubewell water with E. coli at the field site where the cores were collected as well as other similar sites.
- Top of page
- 1. Introduction
- 2. Methods
- 3. Results
- 4. Discussion
- 5. Conclusions
 The microbiological quality of groundwater is often better and more stable than that of surface water [Katayama, 2008]. As a result, untreated groundwater is a common source of drinking water in developing countries and also among many communities in developed countries. Growing evidence of widespread microbial contamination of groundwater, however, has prompted concern about the human health risks associated with the consumption of untreated groundwater. In the passive surveillance of 11,000 private water supplies in England, 32% of sites tested positive at least once for E. coli [Richardson et al., 2009] and 10% of 144 private water supplies surveyed in Netherlands tested positive for E. coli or intestinal enterococci [Schets et al., 2005]. In the United States, consumption of untreated groundwater water has been associated with increased risk of infection by E. coli O157:H7 [Slutsker et al., 1998], and outbreaks of this strain have been linked to contaminated groundwater [Olsen et al., 2002; Bopp et al., 2003]. In the setting that is the focus of this study, monthly monitoring of over 100 shallow (<36 m deep) tubewells in rural Bangladesh has shown that between 30 and 70% of wells are contaminated with detectable levels of E. coli, with contamination in some wells reaching levels greater than 100 colony forming units (CFU)/100 mL [van Geen et al., 2011]. Within a subset of these wells, the frequency of E. coli detection has been shown to be associated with increased likelihood of contamination by pathogenic Shigella, E. coli, and Vibrio [Ferguson et al., 2012].
 The three major processes that control microbial transport in aquifer systems are (i) the physical transport processes of advection and hydrodynamic dispersion, (ii) interactions between microbes and the aquifer's solid phase, and (iii) microbe decay [Tufenkji, 2007]. Traditional approaches to modeling microbial transport during saturated flow involve the advection-dispersion equation coupled with terms that describe attachment to and detachment from the solid phase during transport as a result of physicochemical interactions. Microbe attachment is assumed to be either irreversible, in which case microbes are permanently filtered from the mobile liquid phase, or reversible, in which case microbes can reenter the flowing liquid. Under classical colloid filtration theory (CFT), microbes are considered to irreversibly attach to the solid phase, and the rate of attachment is related to the probability of a collision with the collector surface, which is derived mechanistically from a sphere and shell model [Happel, 1958] and modified by a collision efficiency α, defined as the probability of the particle being captured [Yao et al., 1971; Rajagopalan and Tien, 1976; Logan et al., 1995]. This approach has proven successful under conditions favorable for microbial attachment; however, CFT deviates from observations in many situations of environmental relevance where an energy barrier to microbial attachment exists due to low ionic strength in the liquid phase or heterogeneity in surface charges [Johnson et al., 2007a; Tufenkji and Elimelech, 2005]. Recent evidence from micromodel studies using particle-tracking numerical models or direct visualization of microsphere surrogates show that under unfavorable electrostatic attachment conditions, particle-collector attachment mechanisms include wedging in grain-to-grain contact points [Johnson et al., 2007b; Li et al., 2006], capture by surface asperities [Yoon et al., 2006], trapping in hydrodynamic dead zones [Li et al., 2010], and weak surface attachment in a secondary energy minima [Tufenkji and Elimelech, 2005; Redman et al., 2004]. Few experiments to date have focused on determining whether these attachment mechanisms are applicable to bacterial transport under field-like conditions.
 Attachment rates for bacteria have been inferred under various conditions from aquifer-scale forced gradient and natural gradient tracer tests [Harvey et al., 1989; Bales et al., 1997; Knappett et al., 2012] or centimeter-scale column experiments [Harvey et al., 1993; Litton and Olson, 1993; Fitzpatrick and Spielman, 1973] using stained bacteria or latex microspheres with size and surface properties that are similar to bacteria. Recent reviews have focused on the apparent discrepancy between rates of bacterial transport measured in laboratory columns versus field transport experiments, generally manifested as an apparent decrease in measured attachment rates with increasing scale of the experiment [Foppen and Schijven, 2006; Dong et al., 2006; Pang et al., 2008; Scheibe et al., 2011]. One proposed explanation is that there is a subpopulation of less adhesive bacteria that are not easily detected at the scale of a typical column experiment [Bolster et al., 2000]. The transport of bacteria, and other colloidal-sized particles, with subpopulations has been modeled using a modification of CFT incorporating two independent values for the collision efficiency [Simoni et al., 1998; Foppen et al., 2007a], a bimodal distribution [Tufenkji and Elimelech, 2004], and various other probability distributions [Brown and Abramson, 2006; Abramson and Brown, 2007]. One of the major shortcomings of using CFT under unfavorable attachment conditions is the failure to account for bacterial detachment and reentrainment into the liquid phase [Johnson et al., 2007a; Tufenkji, 2007]. To account for subsequent detachment, dual-deposition kinetic models assume separate modes of bacteria-surface interaction with unique attachment and detachment rates to account for electrostatic attachment independently from other processes such as straining in dead-end pore spaces [Bradford et al., 2005; Foppen et al., 2007b], trapping at grain-to-grain contacts [Yoon et al., 2006; Basha and Culligan, 2010], or hydrodynamic retention in flow stagnation zones [Johnson et al., 2007b].
 In a dual-deposition mode kinetic model, the advection-dispersion equation, simplified to one dimension, for a saturated, homogeneous porous medium with two adsorption sites is expressed as [Schijven et al., 2002]
where c is the mass concentration of bacteria in the liquid phase, sr is the reversibly adsorbed solid-phase concentration of bacteria (in units of mass of particles per mass of solid phase), si is the irreversibly adsorbed solid-phase concentration, t is time, ρb is the bulk density of the solid phase, θ is the porosity of the medium, D is the hydrodynamic dispersion coefficient, u is the pore (or interstitial) velocity, z is distance from the inlet, μc is the decay (or die-off) rate for the liquid-phase bacteria, and μr and μi are decay rates for the solid phase reversibly and irreversibly attached bacteria, respectively.
 The rates of transfer of bacteria to the solid phase (terms 2 and 3 of equation (1)) can be expressed as
where ka is the reversible attachment rate, kd is the reversible detachment rate, and ki is the irreversible attachment rate.
 Given the multitude of properties known to influence the rates of bacterial attachment and detachment under highly uniform laboratory conditions, relatively few studies have been able to determine which of these parameters are most influential in actual field sediments. The present study applies treatments to field sediments to examine the effects of sediment layering, grain size heterogeneity, and geochemical heterogeneity on E. coli transport in saturated aquifer material. The experimental data are fit to a dual-population model with two sites that account for reversible and irreversible bacterial attachment. Parameters extracted from model fitting are used to simulate bacterial transport in an aquifer system in order to explore how dual-population transport might contribute to the observed, widespread microbial contamination of groundwater.