Original ResearchPhysicochemical Factors
Influencing the Preferential
Transport of Escherichia coliin Soils
Yusong Wang,* Scott A. Bradford, and Jiří Šimůnek

the preferential path and earlier

Laboratory and numerical studies were conducted to investigate thetransport and release of Escherichia coli D21g in preferential flow systems
with artificial macropores under different ionic strength (IS) conditions.
Macropores were created by embedding coarse sand lenses in a fine sandmatrix and altering the length, continuity, and vertical position of the lens.
The length of an artificial macropore proved to have a great impact on
the preferential transport of E. coli D21g, especially under high-IS conditions.
A discontinuous macropore (interrupted by fine sand) was found to haveless preferential transport of E. coli D21g than a continuous macropore of
the same length that was open to either the top or bottom boundary. At
low IS, more extensive transport in the preferential path and earlier arrival
time were observed for E. coli D21g than Br− as a result of size exclusion. Two
release pulses (one from the preferential path and the other from the matrix)
were observed following a reduction of the solution IS for flow systems withmacropores that were open to either the top or bottom boundary, whereas
three pulses (two from the preferential path and another from the matrix)
were observed for systems with discontinuous macropores. Numerical simulations of E. coli D21g under both constant and transient solution chemistry
conditions had very high agreement with the experiment data, except for
their capability to predict some subtle differences in transport between thevarious lens configurations.

arrival time were observed for E.

Abbreviations: BTC, breakthrough curve; DI, deionized; IS, ionic strength.

Laboratory

andnumeri-cal studies were conducted toinvestigate the transpor t andrelease of Escherichia coli D21g
in pr e f e r e nt ial f low s ys te m swith artifi cial macropores under
different ionic strength (IS) conditions. Macropore length had agreat impact on the preferential
transport of E. coli D21g, especially under high-IS conditions,but discontinuous macropores had
less preferential transport. At low
IS, more extensive transport incoli D21g than Br− as a result ofsize exclusion.

Y. Wang and J. Šimůnek, Dep. ofEnvironmental Sciences, Univ. of
California, Riverside, CA 92521; and S.
Bradford, USDA-ARS, U.S. Salinity Lab.,Riverside, CA 92507. *Corresponding
author (Ywang032@ucr.edu)
Vadose Zone J.
doi:10.2136/vzj2013.07.0120
Received 6 July 2013

© Soil Science Society of America

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The vadose zone serves as an important barrier to protect groundwater from
pathogenic microorganisms that can cause waterborne disease outbreaks (Runnells, 1976;
National Research Council, 1994; Jamieson et al., 2002). The capability of the vadose zone
to remove pathogens depends on properties of the porous media, the microbes, and thesoil solution (Harvey and Garabedian, 1991; Tan et al., 1994; Tufenkji, 2007; Ding, 2010).
An understanding and ability to simulate the influence of various factors that enhance thetransport potential of pathogens in the vadose zone is especially needed to protect water
resources from contamination. In this research, we considered three factors that are known
to enhance the transport potential of microbes in soils: (i) preferential flow; (ii) transientsin solution chemistry; and (iii) size exclusion.
Rapid water flow may occur in the vadose zone as a result of plant roots, burrowing earthworms, cracks, or natural structural heterogeneities (Wollum and Cassel, 1978; Beven andGermann, 1982; Madsen and Alexander, 1982; Unc and Goss, 2003; Cey et al., 2009).
Accurate descriptions of contaminant transport in the vadose zone is hampered by these
preferential flow pathways (Šimůnek et al., 2003) that bypass a large part of the soil matrix(Jury and Flühler, 1992). In particular, it is extremely difficult to characterize physical features (e.g., the length and configuration) of preferential flow pathways in natural systems
and the exchange of water and contaminants at the interface between the preferential
pathway and the soil matrix. Both of these factors are expected to have a critical influenceon the transport and fate of contaminants in preferential flow systems but are still poorly

Vadose Zone Journal

quantified (Harvey et al., 1993; Morley et al., 1998; Allaire-Leunget al., 2000a, 2000b; Allaire et al., 2002a, 2002b).
Many studies have investigated the transport of colloids andmicroorganisms in the field or in undisturbed soil columns withpreferential flow (Bales et al., 1989; Dean and Foran, 1992; Jarvis,
2007; Pang et al., 2008; Cey et al., 2009; Cey and Rudolph, 2009;
Guzman et al., 2009). Preferential pathways have been found to bea major contributor to the overall transport of microbes because
they are typically strongly retained in the soil matrix (Bales et al.,
1989; Abu-Ashour et al., 1994; Jiang et al., 2010). However, mostof these researches are qualitative in nature because of the difficultyin quantifying the physical and chemical complexities of the soil
matrix and macropore system. Studies using artificial macroporesprovide an opportunity to overcome many of these experimental
and modeling limitations because the macropore geometry andhydraulic properties can be well defined and controlled (Fontes
et al., 1991; Pivetz and Steenhuis, 1995; Castiglione et al., 2003;
Guzman et al., 2009; Arora et al., 2011, 2012; Wang et al., 2013).
However, systematic studies investigating the influence of thelength and configuration of artificial macropores on the preferential transport of bacteria have not yet been reported.
Solution chemistry has proven to be an important factor that
influences the transport and retention of microorganisms inporous media (Mills et al., 1994; Yee et al., 2000; Dong et al.,
2002; Chen and Walker, 2007) and in systems with preferential
flow (Fontes et al., 1991; Wang et al., 2013). The solution chemistry may change dramatically in the vadose zone during infi ltrationand drainage events as a result of differences in water quality at
the soil surface (rain, irrigation, and runoff ), evapotranspiration,and the mineral composition of the soil and groundwater. Suchtransients in solution chemistry are well known to induce therelease of colloids and microbes in homogeneous porous media(Bales et al., 1989; McDowell-Boyer, 1992; Ryan and Gschwend,
1994; Nocito-Gobel and Tobiason, 1996; Roy and Dzombak,
1996; Grolimund et al., 2001; Lenhart and Saiers, 2003; Cheng
and Saiers, 2009; Tosco et al., 2009; Bradford and Kim, 2010;
Bradford et al., 2012). Consequently, transients in solution chemistry may jeopardize the purification capability of the vadose zone
by inducing microbial release. There is presently a great need tounderstand processes that influence microbial remobilization,
especially in soils with preferential flow. Wang et al. (2013) demonstrated that transients in solution chemistry can remobilize
retained microbes and rapidly transport them in a preferential
flow system. However, numerical modeling of this microbe releaseand transport has not yet been reported.
In addition to preferential flow and transients in solution chemistry, size exclusion may also enhance the transport potential of
microbes in soils by physically restricting their transport to larger,
more conductive pore spaces (Ryan and Elimelech, 1996; Ginn,
2002). It is possible that size exclusion will be more pronounced in

Vadose Zone Journal

systems with preferential flow because of the wide difference in poresizes between the soil matrix and the high-permeability domain.
However, this issue has not yet been systematically investigated.
The objectives of this research were to: (i) investigate the influenceof the length and configuration (as shown in Fig. 1) of artificial
macropores on the preferential transport of microorganisms
under different ionic strength (IS) conditions; (ii) simulate therelease behavior of microbes with transients in solution chemistryin systems with preferential flow; and (iii) study the effect of size
exclusion on microbial transport in systems with preferential flow.
No attempt was made in this work to determine specific mechanisms of cell retention (e.g., attachment and straining) because ofcomplications arising from preferential flow.

6 Experimental Information
Porous Media and Electrolyte Solutions
Two sizes of Ottawa (quartz) sand were used in the column experiments. The median grain sizes (d50) of these sands were 120 and710 μm. To eliminate any background interference from clay particles, the sand was treated by a salt cleaning procedure describedby Bradford and Kim (2010).
Electrolyte solutions for the column experiments consisted of autoclaved, deionized (DI) (pH = 5.8) water with the IS adjusted to0, 1, and 20 mmol L−1 using NaCl and NaBr to create a range ofadhesive conditions between the bacteria and sand. The concentration of the conservative tracer Br was determined in the column
effluent using a Br-selective electrode (Thermo Scientific Orion Br
electrode Ionplus Sure-Flow).

Fig. 1. Axisymmetric representation of the four types of sand lensstructures (column center is on the left-hand side) studied in thisresearch: Type I, one lens through the whole column; Type II, onelens open to the bottom boundary; Type III, one lens open to the top
boundary; and Type IV, a discontinuous lens open to both boundaries(diagonal pattern represents coarse sand and point pattern represents
fine sand).

p. 2 of 10

Escherichia coli D21g
We selected E. coli D21g, a Gram-negative, nonmotile bacterial
strain (Walker et al., 2004), as a representative microorganism forthe column transport experiments. The culture and harvest procedures used in this study have been described by Wang et al. (2013).
In brief, E. coli D21g was cultured overnight (12–18 h) at 37°C in
Luria-Bertani broth (LB broth, Fisher Scientific) containing 0.03
mg L−1 gentamycin (Sigma), transferred onto an LB media platecontaining 0.03 mg L−1 gentamycin, and the plates were cultured
overnight (12–18h) at 37°C. The colonies were harvested into
sterile water, and then the bacterial suspension was centrifuged
and resuspended two times to remove all traces of the growth
medium. A fresh cell suspension at the desired electrolyte solution was prepared right before the start of each experiment. The
concentrations of E.coli D21g in influent, effluent, and soil solution were determined using a spectrophotometer (Unico UV-200,
United Products & Instruments) at 600 nm (Torkzaban et al.,2008) or with the spread plating method (American Public Health
Association, 1989) when necessary (e.g., at low concentrations).
The NaBr solution containing E. coli D21g at a concentrationof ?1.0 × 10 8 cells mL−1 was continuously mixed during thecolumn experiment using a magnetic stirrer. The value of theinfluent concentration (C0) was measured three times during thecourse of a transport experiment to assess the reproducibility ofthe measurements and microbial survival. The spectrophotometerreadings for E. coli D21g were within 1% of C0, and the standard
deviation of the spread plating method was 14.3% of C 0 . No
systematic decrease in C 0 was observed during the column
experiments, which indicates that little death occurred during
this interval.

Column Experiments
Column experiments were conducted in a similar way to that
described by Wang et al. (2013) with some modifications.
Preferential transport experiments were conducted in an acrylic
column that was 22 cm long and had an inside diameter of 13.2
cm. Preferential flow systems were created by packing fi ne (120μm) and coarse (710-μm) sands into the column as follows: (i)the column was fi lled with autoclaved DI water to about onethird of the column height, and a 30-cm-long plastic tube withoutside diameter of 1.14 cm was held in the center of the column;(ii) the fi ne sand was incrementally wet packed into the matrix
portion of the column (outside the plastic tube) to a height of
20 cm; (iii) excess water in the plastic tube was drained from thebottom; (iv) the tube was carefully pulled out of the column without disturbing the surrounding fi ne matrix sand and leaving a
1.14-cm-diameter hole in the center of the column; (v) the hole
was then fi lled to a height of 20 cm with various combinations ofcoarse and fine sands (Fig. 1), using a funnel, to create the desired
preferential flow lens structure; and (vi) the column was thenVadose Zone Journal

saturated with water from the bottom. Four lens configurations
were considered (Fig. 1) that are denoted as Types I (continuouscoarse sand throughout the column), II (coarse sand lens of variable length that is open at the bottom boundary), III (coarse sandlens of variable length that is open at the top boundary), and IV(coarse sand lens of variable length that is open at both boundaries
but discontinuous in the center).
A polyester membrane (Saatifil PES 18/13) with an 18-μm nominal
pore size was placed at the bottom of the column and connected
to a hanging water column (tube) to control the bottom boundarypressure at 0 cm. Solutions were delivered onto the top boundary of the heterogeneous column at a steady flow rate using a rain
simulator connected to a peristaltic pump. The water velocity was
selected to just maintain saturated conditions (several millimetersof ponding at the surface) in the column through the experiments.
The total Darcy velocity through the columns varied from 0.35 to
0.40 cm min−1 for different lengths of lens. Two pore volumes(PVs) of a selected NaCl solution were flushed through the column,
and the sand was allowed to equilibrate with this solution (Phase0) before initiating a microbial transport experiment.
Microbial transport experiments were performed in two phases,
with the same boundary conditions and flow velocity as in Phase
0. First, several PVs of microbe suspension and NaBr with theselected IS were introduced into the column at a constant rate
(Phase I). Second, NaCl solution was flushed through the column
at the same flow rate and IS as in Phase I until the effluent microbe
concentration returned to a baseline level (Phase II). Effluent samples were continuously collected during the transport experiment
at selected intervals using a fraction collector and then analyzed
for Br and microbe concentrations as described above.
After completion of Phases I and II, some of the columns at IS =20 mmol L−1 underwent an additional experimental Phase III to
examine the release of retained microbes with a reduction in solution IS. In this case, the columns were flushed with autoclaved
DI water at the same velocity as during Phases I and II until thereleased microbe concentration in the effluent returned to a baseline level. The effluent samples were collected and analyzed using
the same protocols as during Phases I and II.
The distribution of retained microbes in the heterogeneous columns
was quantified after recovery of the breakthrough curves (BTCs)(Phases I and II) for the replicate transport experiments conductedwith IS = 20 mmol L−1 solutions but without Phase III. Sand samples were taken at seven depths (spaced according to the positionof the lens) and three locations, namely: the lens, the matrix in thevicinity of the lens, and the bulk matrix. The length and positionof preferential paths were determined during extraction. The sandsamples were carefully excavated into tubes containing excess DI
water. The tubes were shaken for 15 min to liberate any reversibly

p. 3 of 10

Table 1. Model parameters from homogeneous column experiments presented in Wanget al. (2013).
MediumDispersivity

Microbe release
rate coefficient

Normalized solidphase microbe conc.

mmol L−1

cm

min−1

cm3 g−11

0.10

0.00040.01

Ionic strength

Fine sandCoarse sand

20

0.10

0.2

3.5

1

0.55

0

0

20

0.55

0

0

retained microbes, and the concentrations of E. coli D21g in theexcess solution were determined as described above.

6 Numerical Models
A two-dimensional finite element mathematic model was createdusing the COMSOL soft ware package and used to simulate thetransport, retention, and release of E. coli D21g in the heterogeneous column experiments based on the solution of the Richards
equation and the advection–dispersion equation (ADE) withterms for kinetic retention and release. The microbial transport
equations are∂( θ C )∂ ⎛⎜∂ C ⎞⎟⎟ ∂ q i C⎜⎜ θ Dij=−θψ ksw C⎟−∂t∂ x i ⎜⎝
∂ x j ⎟⎟⎠ ∂ x i[1]+ρ krs ( S − f c S3 ) H o ( S − f c S3 )∂( ρS )= θψ ksw C −ρ krs ( S − f c S3 ) H o ( S − f c S3 )∂t[2]where subscripts i and j denote coordinate directions, C [N L−3,
where N denotes number] is the microbe concentration in the
aqueous phase, S [N M−1] is the microbe concentration on thesolid phase, S3 [N M−1] is the value of S before a reduction of the
IS during Phase III, fc (dimensionless) is the fraction of microbes
on the solid phase that still remains immobilized after a reductionin IS, Dij [L2 T−1] is the hydrodynamic dispersion coefficient, qi [LT−1] is the Darcy water velocity in the ith direction, krs [T−1] is the
microbe release rate coefficient, ksw [T−1] is the microbe retention
rate coefficient, θ (dimensionless) is the soil water content, ρ [ML−3] is the soil bulk density, t [T] is time, x [L] is the spatial coordinate, and Ho is the Heaviside function that is equal to 0 when
S < fcS3 and 1 when S ≥ fcS3. The parameter ψ (dimensionless)accounts for time- and concentration-dependent blocking using aLangmuirian approach as (Adamczyk et al., 1994)
ψ = 1−S
S max

Vadose Zone Journal[3]

where S max [N M−1] is the maximum solid-phase
concentration of microbes.

In addition to Eq. [1–3], the solution IS (1:1 electrolyte) was also simulated during Phase III using an ADE
for a conservative tracer. The last term on the righthand side of Eq. [1] and [2] accounts for the releaseof E. coli D21g with a reduction in IS during Phase
III in a manner similar to Bradford et al. (2012). In
the presence of DI water, ksw was set equal to 0 basedon data presented by Wang et al. (2013) for low-IS
conditions, fc was determined from mass balance
information following the completion of Phase III, and krs wasestimated by optimization (trial and error) to the experimental
data. The value of S3 was set according to the distribution of thesolid-phase concentration of microbes at the end of Phase II. Thisapproach allowed a known amount of retained E. coli D21g to bereleased on arrival of the DI water front at a particular location.

We followed a step-by-step procedure presented by Wang et al.
(2013) to determine hydraulic properties and microbe retention
parameters for simulations in preferential flow systems during
Phases I and II. The saturated water content (θ) and bulk density(ρ) were calculated from experimental measurements to be 0.36
cm3 cm−3 and 1.7 g cm−3, respectively. The saturated hydraulicconductivities (Ks) of the matrix were determined from the breakthrough time of Br, which varied from 0.31 to 0.33 cm min−1 due
to slight differences in column packing. The saturated hydraulicconductivities of the coarse lenses were inversely estimated in
HYDRUS 2D (Šimůnek et al., 2008) using the geometry information along with the total flow rate and the hydraulic conductivityof the matrix, which varied from 10.5 to 11.1 cm min−1. The dispersivity (λ) and the retention parameters (Smax, ksw, and krs) forD21g in the coarse (lens) and fine (matrix) sands under the various
solution chemistries are presented in Table 1. These parameters
were determined from transport experiments conducted in homogeneous columns for similar sands, solution chemistries, and watervelocities (Wang et al., 2013). In this case, krs = 0 and ksw was aconstant for the given solution chemistry conditions during Phases
I and II.

6 Results and Discussion
BromideFigure 2 shows the normalized effluent concentrations (C/C0) of
Br as a function of time from columns with Type I and II lenses
when the length of the coarse sand lens ranged from 10 to 20 cm.
Two pulses of Br were observed at the outlet of all columns. The
first pulse traveled through the lens and arrived much earlier than
the second pulse from the matrix. The length of the lens had agreat impact on the arrival times of the fi rst pulse. Specifically,
the arrival time of the first pulse increased from 2 to 12 min withp. 4 of 10

Fig. 2. Observed effluent concentrations of Br− as a function of time
for columns with Type I (20 cm) and Type II (19, 18, 16, and 10 cm)
lens configurations. The bottom figure zooms on the early arrival of
Br− through the lens.

Fig. 3. Observed effluent concentrations of Br− as a function of time
for columns with the same lens length (19 cm) but with different lensconfigurations. The bottom figure zooms on the early arrival of Br−
through the lens.

a decrease in the length of the Type II lens from 20 to 10 cm, as
shown in Table 2. Type III and IV lenses also exhibited this same
trend (Table 2). As the length of the lens decreased, water andBr had to travel a longer distance through the fi ne sand, which
decreased the average velocity of both water and Br and delayed
the arrival of Br at the outlet. This decrease in velocity produceda corresponding decrease in the relative amount of Br that wastransported through the preferential pathway (the first pulse) witha decrease in the lens length. Conversely, the velocity of the matrix
was almost the same for all columns, and the arrival time of thesecond pulse was therefore similar (21.1–23.8 min).

The influence of the lens configuration (Types II, III, and IV) on
Br transport when the lens length equaled 19 cm is shown in Fig.
3. Differences in the Br BTCs for Type II (lens continuous at thecolumn bottom) and III (lens continuous at the column top) lensconfigurations were rather minor. Conversely, the Type IV (lens discontinuous in the center) lens configuration produced significantly
different BTCs from Types II and III, especially with respect to thepulse from the preferential pathway (Fig. 3). The pulse of Br fromthe preferential path had more dispersion and arrived slightly later
for Type IV heterogeneity than for the other two lens configurations. This observation indicates that the discontinuous lens createdmore disruptions to flow and transport pathways.

Escherichia coli D21g

Table 2. Breakthrough information for Br.
Type of lensconfiguration
Type I

Type II

Arrival time ofsecond pulse

Peak conc.
in first pulse(100 × C/C0)

Constant Solution
Chemistry Conditions

Length of lensArrival timeof first pulsecm——————min ———————

20

1.4

23.8

24.7

19

3.2

21.9

19.5

99.6

18

3.7

21.5

17.8

100.1

Total recovery
%
99.4

16

5.3

21.8

16.8

100.2

10

11.1

21.1

16.3

98.9

Type III

19

3.1

22.2

20.6

99.5

Type IV

19

6.6

22.5

16.6

99.6

Vadose Zone Journal

A high energy barrier limited the retention of E. coli D21g in both fine andcoarse sand when the IS equaled 1 mmol
L−1 (Wang et al., 2013). Thus, the influence of lens length and configuration wasvery similar for E. coli D21g and Br with
IS = 1 mmol L−1 (Fig. 4). Similar to Br,
the first pulse of E. coli D21g arrived laterand had less mass when the length of thep. 5 of 10

Fig. 4. Observed effluent concentrations of E. coli D21g at an ionicstrength of 1 mmol L−1 and Br as a function of time for Type III lensconfiguration with a length of 19 cm.
Fig. 5. Observed effluent concentrations of E. coli D21g at anionic strength of 20 mmol L−1 as a function of time for Type I lensconfiguration (20 cm) and Type II lens configuration (19, 18, and16 cm).

lens decreased, and was more dispersed for Type IV than II and III
lens configurations. Conversely, the arrival time for E. coli D21g in
the second pulse (from the matrix) was about 1.5 min (0.07 porevolumes) earlier than for Br in the same experiments (Tables 2 and
3; Fig. 4). This observation was ascribed to size exclusion (Fontes
et al., 1991; Ryan and Elimelech, 1996; Morley et al., 1998; Ginn,
2002), which can increase the transport velocity of particles byconstraining them to larger pore networks. It was not possible totell whether E. coli D21g also had an earlier arrival time than Br
in the first pulse (lens) because of the very rapid arrival for both
Br and E. coli D21g; however, the relative concentration of E. coli
D21g was higher than that of Br in the first pulse (Tables 2 and
3). Because Br is a conservative tracer and some E. coli D21g was
retained in the sand, this observation suggests that size exclusion
may have also enhanced the transport of E. coli D21g in the lensin comparison to Br.

D21g was able to travel through the preferential path for most of
the studied conditions, and the preferential flow pathway was the
major or even the only route for E. coli D21g transport under highIS conditions. The concentration of E. coli D21g in the effluent
decreased dramatically when the lens length decreased, and only
a trace amount of E. coli D21g was detected in the effluent when
the lens length was 16 cm (Table 3). At IS = 1 mmol L−1, the lens
length had no significant effect on the overall transport of E. coli
D21g (Table 3). Consequently, the lens length had a much greater
impact on the transport of E. coli D21g when the IS = 20 mmol
L−1 than when the IS = 1 mmol L−1 (Table 3).
Figure 6 demonstrates that the transport of E. coli D21g was
also influenced by the lens configuration (Types II and IV). In
comparison with the Type II configuration, the Type IV heterogeneity enhanced the mixing and delayed the arrival of E. coli
D21g in the first pulse when the IS = 1 mmol L−1. Similar behavior was observed for Br (Fig. 3). At a higher IS = 20 mmol L−1,
the enhanced mixing of the two flow domains in the Type IV
configuration produced greater retention of E. coli D21g in thepreferential flow path and correspondingly less recovery of E. coli
D21g in the first pulse (Table 3).

Figure 5 shows representative BTCs for E. coli D21g when the
IS = 20 mmol L−1 and the lens configuration was Type I or II. In
comparison to IS = 1 mmol L−1, the height of the energy barrier
decreased and the depth of the secondary minimum increased
when the IS = 20 mmol L−1 (Wang et al., 2013). Consequently,
the retention of E. coli D21g in the fine matrix sand became muchgreater, and no detectable amount of E. coli D21g was transported
through the 20-cm-long matrix (data not shown). However, E. coli

Table 3. Breakthrough information for E. coli D21g at 1 and 20 mmol L−1.
Type of lensconfiguration

Length of lens Ionic strength

Arrival timeof first pulse

Arrival time ofsecond pulse

Peak conc.
in first pulse(100 × C/C0) Total recoverycm

mmol L−1——————min ———————

Type II

18

1

3.6

19.9

18.7

97.4

Type III

19

1

2.2

20.7

28.4

96.4

Type IV

19

1

3.0

21.4

24.5

96.5

Type III

19

20

2.3

NA

13.8

13.9

Type IV

19

20

3.0

NA

11.2

10.0

Type II

18

20

6.5

NA

9.1

8.2

Type II

16

20

NA

NA

4.2

2.9

Type IV

16

20

NA

NA

NA

0

Vadose Zone Journal%

The spatial distribution of the retainedmass is also very important for understanding the transport and retention
behavior of E. coli D21g in systems
with preferential flow. Figure 7 shows
representative retention profiles forE. coli D21g for Types II, III, and IVlens configurations when the lens
length was 19 cm and the solution IS= 20 mmol L−1. Th ree locations wereconsidered for each case: (i) in the lensp. 6 of 10

Fig. 6. Observed effluent concentrations of E. coli D21g for Type II
and Type IV lens configurations as a function of time for (a) a lens
length of 19 cm and an ionic strength (IS) of 1 mmol L−1; (b) a lens
length of 19 cm and an IS of 20 mmol L−1; (c) a lens length of 18 cm
and an IS of 20 mmol L−1; and (d) a lens length of 16 cm and an ISof 20 mmol L−1.

(including the fi ne sand); (ii) in the matrix adjacent to the lens;and (iii) in the bulk matrix sand.
In the matrix, most retained cells were in the top few centimetersof the column, and S/C 0 rapidly decreased with an increase indistance from the column inlet. Conversely, values of S/C 0 in thecoarse-sand portions of the lenses were very low. For the Type IIlens configuration, the fi ne sand disruption in the preferential
path was located at the top. Thus, the value of S/C 0 was about
the same as in the matrix at this location (Fig. 7a). For the TypeIII lens configuration, the fine sand disruption in the preferential
path was located at the bottom, and the value of S/C 0 in thisregion was as high as that in the top matrix (Fig. 7b). For the TypeIV lens configuration, a high value of S/C 0 was located in the
middle of the preferential flow path where the fi ne-sand disruption occurred (Fig. 7c).
The greatest amount of cell retention tended to occur in thematrix adjacent to the lens. Th is location reflects the influenceof mass exchange between the lens and matrix, as well as retention in the matrix. Similar high values of S/C 0 occurred near thecolumn inlet for the three lens configurations because retention
was mainly controlled by the matrix in this location. Conversely,
distinct differences in the amount of cell retention occurred withincreasing depth for the various lens configurations because ofdifferences in the amount of mass transfer between the lens andmatrix. The higher values of S/C 0 with increasing depth in theType III and IV configurations indicates that discontinuities tolenses that are open at the column top will increase the subsequentmass transfer from the lens to the matrix and thereby enhance E.
coli D21g retention. On the other hand, cell retention adjacentto the lens in the Type II configuration was lower because the

Vadose Zone Journal

Fig. 7. Normalized solid-phase concentrations (S/C0) of E. coli D21g
as a function of depth for an ionic strength of 20 mmol L−1 (Phases
I and II), lens length of 19 cm, and lens configurations of (a) Type II,(b) Type III, and (c) Type IV for three locations: in the lens, in thematrix in the immediate vicinity of the lens, and in the matrix awayfrom the lens.

cells had to initially travel through the matrix, and many were
therefore retained in the matrix before reaching the lens.

Transients in Solution Chemistry
Figure 8 presents the release behavior of E. coli D21g during Phase
III when the influent solution IS was switched from 20 mmol L−1
to DI water. Type II, III, and IV lens configurations were considered, and the lens length was 16 cm. Table 4 gives the arrival
times and mass balance information for the release process. Release
was initiated during Phase III when the IS = 20 mmol L−1 solution was replaced by DI water, and the secondary minimum waseliminated. Similar to transport during Phases I and II, multiple
pulses of released cells were observed during Phase III because ofpreferential flow. The largest pulse of released E. coli D21g wasabout five times C0 and arrived at the outlet about 21 min after thestart of DI water flushing. This arrival corresponds with the travel
time of Br through the matrix (Fig. 2 and 3). Consequently, thishigh peak reflects cell release from the matrix, where most of thecells were retained (Fig. 7). Conversely, the smaller earlier pulses
mainly reflect mass transfer from the matrix to the lens because

p. 7 of 10

Fig. 9. Examples of observed and simulated effluent concentrations as
a function of time for Br and E. coli D21g for a Type III lens with a
19-cm length and an ionic strength of 1 mmol L−1.

Simulations with Numerical Models
Figure 9 shows examples of observed and simulated BTCs of Br
and E. coli D21g at IS = 1 mmol L−1 during Phases I and II. Figure
10 shows examples of observed and simulated BTCs of E. coli D21g
at IS = 20 mmol L−1 during Phases I and II along with cell releaseduring Phase III when the IS was reduced to DI water. The model
was able to simulate the transport of Br and E. coli D21g withvery high accuracy during Phases I and II when the IS was 1 or 20
mmol L−1. The release behavior of E. coli D21g from the matrix(the high-concentration pulse that arrived at later times) and theearlier pulse for Type II and III configurations were also well simulated by the model. Conversely, the model was not able to capture
the subtle differences in the early release behavior for Type II, III,and IV configurations. In particular, the model failed to predict
the enhanced dispersion during Phases I and II and the arrival
times of the two earlier pulses associated with mass transfer fromthe matrix to the lens in the Type IV configuration. These differences may arise because cell retention processes that occurredat textural interfaces were incompletely quantified in the model(Bradford and Bettahar, 2005).

Fig. 8. Observed effluent concentrations of E. coli D21g as a functionof time during the release process for (a) Type II lens of 16-cm length;(b) Type III lens of 16-cm length; and (c) Type IV lens of 16-cm
length. The time when the influent solution was switched from 20
mmol L−1 solution to deionized water was labeled as 0 at the start ofPhase III.

little cell retention occurred in the coarse sand lens at IS = 20mmol L−1 (Wang et al., 2013). The lens configuration (Types II, III,and IV) had a significant influence on the number and magnitudeof these earlier pulses. There was only one small pulse of released
E. coli D21g for Type II and III lens configurations at earlier times(6.9–7.2 min), whereas the Type IV configuration produced two
small pulses that arrived after 6.2 and 14.7 min. This observationsuggests that mass transfer from the matrix to the lens was more
efficient (easier) in the Type II and III configurations and less efficient in the discontinuous Type IV configuration.

6 Summary and Conclusions
The length of the lens affected the water flow and transport of Br
and E. coli D21g. In particular, a decrease in the lens length produced later arrival times and lower effluent concentrations throughTable 4. Release information for E. coli D21g.

Type of lensconfiguration

Lengthof lens

Peak conc. ofPeak conc. of
Recovery of
Arrival time of Arrival time offirst small pulse second small pulse first or first andfirst small pulse second small pulse (100 × C/C0) (100 × C/C0)
second pulsecm——————min ——————

Peak conc.of
Arrival time major pulse
Recovery ofof major pulse (100 × C/C0) major pulse

%

min

%

Type II

16

6.9

NA†

56.4

NA

17.8

23.0

437.5

78.0

Type III

16

7.2

NA

68.3

NA

22.4

23.2

548.7

69.5

Type IV

16

6.2

14.7

25.3

64.2

14.6

20.7

583.9

76.8

† NA, not applicable.

Vadose Zone Journal

p. 8 of 10

Acknowledgments
We would like to acknowledge Teresa Clapp for her help in conducting some of the transport
experiments. This research was supported by the USDA-ARS, NP 214. The USDA is an equal
opportunity provider and employer.

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Fig. 10. Examples of observed and simulated effluent concentrationsof E. coli D21g as a function of time for (a) a Type II lens of 19-cm
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