Mobilization and Transport of Naturally Occurring Enterococci in

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Mobilization and Transport of Naturally Occurring Enterococci in Beach Sands Subject to Transient Infiltration of Seawater Todd L. Russell,† Kevan M. Yamahara,† and Alexandria B. Boehm*,† †

Environmental and Water Studies, Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: This study explores the transport of enterococci (ENT) from naturally contaminated beach sands to the groundwater table via infiltrating seawater using field, laboratory, and modeling experiments. ENT were readily mobilized and transported through the unsaturated zone during infiltration events in both the field and laboratory column experiments. Detachment mechanisms were investigated using a modified version of HYDRUS-1D. Three models for detachment kinetics were tested. Detachment kinetics that are first order with respect to the rate of change in the water content and attached surface bacterial concentrations were found to provide a best fit between predicted and observed data. From these experimental and model results we conclude that detachment mechanisms associated with the rapid increases in pore water content such as air−water interface scouring and thin film expansion are likely drivers of ENT mobilization in the investigated system. These findings suggest that through-beach transport of ENT may be an important pathway through which ENT from beach sands are transported to beach groundwater where they may be discharged to coastal waters via submarine groundwater discharge.

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waters.7,20 In this pathway, the rising tide or wave uprush, the rush of water up the beach from a breaking wave, inundates FIB-laden sands and FIB are mobilized from the sands and enter the water column directly. There is another possible mechanism whereby FIB may be transported from sand to sea, ‘through-beach transport’, which has not been investigated previously. In this idealized pathway (Figure 1) FIB in

INTRODUCTION When fecal indicator bacteria (FIB) concentrations in coastal waters exceed standards, beaches are closed or posted with water quality advisories. FIB, including E. coli and enterococci (ENT), are not pathogens but can be found in high concentrations in sewage and urban runoff,1 and their concentrations have been shown to positively correlate to incidence of human illness in epidemiological studies.2−5 Across the US there were 24,091 beach closure or advisory days in 2010.6According to the National Resource Defense Council, 52% of the beach closures/advisories in the US are from unknown sources of pollution.6 These unknown sources make remediation of coastal water pollution extremely difficult, if not impossible. Recent research has identified that sources of FIB can be nonfecal in nature. These nonfecal sources may be responsible for beach closures and advisories when no clear source of pollution is present. Specifically, FIB have been found in beach sand7−12 and decaying marine and lacustrine plant material.13−18 Although beach sands have been suggested as a source of FIB, our understanding of the transport pathways and mechanisms involved is limited. Numerous studies have documented the presence of FIB in sands7−12 and have shown that they can even grow under specific conditions.8,10,19 However, only a few studies document FIB transport from sands to coastal waters.7,20 Two studies document ‘overbeach transport’ of FIB from sand to coastal © 2012 American Chemical Society

Figure 1. Conceptual model for through-beach transport. Infiltrating seawater detaches fecal indicator bacteria (FIB) from the beach sands. The FIB are transported to the groundwater table and are transported to the sea by submarine groundwater discharge (SGD).

Received: Revised: Accepted: Published: 5988

January 31, 2012 April 24, 2012 April 25, 2012 April 25, 2012 dx.doi.org/10.1021/es300408z | Environ. Sci. Technol. 2012, 46, 5988−5996

Environmental Science & Technology

Article

The pan lysimeter was rectangular and measured 45.7 cm × 30.5 cm × 15.2 cm and had a Teflon tube connected at the base to allow for sampling at the surface. Water was sampled from the lysimeter using a 60 mL syringe. The pan lysimeter was placed at a depth of 45 cm below the surface of the beach. Sand overlaying the lysimeter was not disturbed during installation; a trench was dug landward of the installation location, and then the lysimeter was inserted into the undisturbed sand at depth. A hand auger was used to install a well adjacent to the lysimeter to monitor the groundwater level. Water level relative to the surface of the beach was measured using a water level indicator (DGSI, Stone Mountain, GA). Prior to the start of the experiment, two surficial (top 5 cm) sand samples were collected from above the lysimeter. The surficial sand samples were assayed for ENT following Boehm et al.40 with slight modifications. In brief, 40 mL of sterile MilliQ water was added to the sand, the mixture was shaken for 2 min and allowed to settle 30 s, and then 10 mL of eluant was assayed for ENT using Enterolert (IDEXX, Westbrook, ME). The eluant was diluted in 90 mL of Butterfields buffer (Weber Scientific, Hamilton, NJ) and run in accordance with manufacturer directions. Dry weight of the sand was determined by baking at 110 °C for 24 h. Concentrations are expressed as most probable number (MPN)/g dry weight. The lysimeter was sampled as waves washed over the installation site near high tide. Four 50 mL samples were drawn using the syringe. Simultaneously to the collection of each lysimeter sample, a sample of seawater overlaying the lysimeter was collected. Sampling ceased when the water level in the monitoring well reached the bottom depth of the pan lysimeter. All water samples were assayed for ENT using Enterolert. Field Depth Profile of ENT. A depth profile of ENT concentrations was measured at Lovers Point to investigate whether the profile is consistent with a profile that is expected if through-beach transport is taking place. In December 2006, a core sample was taken in the middle of the beach at Lovers Point by driving a 10% HCl-acid washed sterile thin walled 3.8 cm diameter, 100 cm long polycarbonate tube capped on one end into the beach using a hand slide hammer. The core was transported to the lab on ice and processed within 6 h. In the lab, cores were sliced open using a sterile stainless steel saw into 25 sections measuring between 1 to 10 cm to preserve sections and not cross contaminate. Sand from each section was homogenized and approximately 20 g (wet weight) was placed into a sterile centrifuge tube. ENT were eluted from the sand as previously described and then enumerated using EPA method 160041 with a 48 h incubation8 due to the previously observed slow growth of ENT from beach sands. If there were too many colonies to count on a filter after incubation, 500 colony forming units (CFU) were assigned to the colony count. Moisture content of sand from each section was determined using the technique described previously. Concentrations are expressed as CFU/g dry weight. Laboratory Column Experiment. In June (‘A’) and July (‘B’) 2010, approximately 1000 g of surficial sands from Lovers Point was collected in sterile 1 L Whirl-pak bags (Nasco, Fort Atkinson, WI). Sands were composited from a 10 m2 area; only the top 3 cm of sands were collected. Seawater was also collected at Lovers Point, in knee deep water within 30 min of sand sampling, in autoclaved, triple-rinsed polypropylene bottles. Sand and seawater samples were stored on ice and transported to the laboratory. Sands and water collected during

unsaturated surface sands are mobilized and transported by the rising tide and wave uprush infiltrating through the unsaturated zone to the groundwater table. This infiltration of seawater is transient in nature as waves break on the beach intermittently. Once present in groundwater, bacteria may be transported to the coastal ocean via submarine groundwater discharge.21,22 In beach sands, FIB might be attached to the surface of the sand grain, to an air−water interface, or to an interface between air, water, and solid grain, trapped in thin films of water adjacent to the grain surface, or trapped via straining in small pores.23 Sand grains might be covered by biofilms or other organic materials that would modify bacterial surface-grain surface interactions. When seawater infiltrates through the sands, FIB may be mobilized by a variety of detachment mechanisms including shear mobilization induced by shear forces at the grain boundary, air−water interface scouring caused by propagation of the air−water interface through the pore space, thin film expansion caused by expansion of thin films as water content increases, and colloidal dispersion caused by chemical perturbations.23 Although considerable research has been devoted to understanding the fate and transport of microbes in idealized porous media,23−29 research on the mobilization and transport of microbes from natural media under transient flow, like that expected to occur during the through-beach transport pathway (Figure 1), is limited.30−36 Further, there are limited studies that examine the mobilization mechanisms and transport of indigenous microbes in beach sands inundated with seawater.7,25,37 The present study investigates ‘through-beach transport’ of ENT using field observations and coupled laboratory microcosms and mathematical modeling. In a field study, the transport of ENT from surface sands through the unsaturated zone by seawater during infiltration is investigated through deployment of a pan lysimeter. A second set of field observations investigates the ENT distribution through a vertical section of beach to determine if it matches the distribution expected in a location where through-beach transport has occurred. Laboratory column studies utilizing sand naturally contaminated with ENT investigate the mechanisms by which ENT in unsaturated sands are mobilized and transported during simulated wave uprush infiltration events. Sands with an indigenous ENT population, as opposed to seeded sands, are used to eliminate the potential for nonenvironmentally relevant attachment behaviors and biological states of the bacteria. The resulting effluent ENT concentrations are subsequently modeled using a modified version of HYDRUS-1D38,39 to gain insight into the kinetics of ENT detachment and mobilization in the sands.

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METHODS Field experiments and sand collection for laboratory experiments occurred at Lovers Point, Monterey, CA (36°37′29.88″N, 121°54′59″W). This beach was selected for the frequently high ENT concentrations found in the sand.7 The physical characteristics of the beach are described elsewhere.7 Field Observations of Through-Beach Transport. To test whether ENT in surface sands at the beach can be transported through the unsaturated zone to the groundwater table, we deployed a pan lysimeter to sample wave uprush passing through the unsaturated zone. On 3 Feb 2007 during a slack low tide, a pan lysimeter and groundwater well were installed 2.1 m seaward of the high tide line at Lovers Point. 5989

dx.doi.org/10.1021/es300408z | Environ. Sci. Technol. 2012, 46, 5988−5996


Article

Model. A modified version of the Hydrus-1D, v. 4.1439 program was used for simulating both water transport and bacterial mobilization and transport in the two replicate column experiments. We modified the program to include three bacteria detachment schemes not included in the standard release. Richard’s equation is implemented in HYDRUS-1D to model the water and hence the saturated-unsaturated conditions39

sampling event A and B were used to run two replicate column experiments. At the lab, sand samples were homogenized by hand folding for 10 min in a sterile polycarbonate beaker with a sterilized stainless steel spoon. Homogenized sand was placed into sterile Whirl-pak bags and stored at 4 °C for up to 48 h prior to the onset of the experiments. Seawater was filter sterilized using 0.2 μm pore size membranes (Nalgene, Rochester, NY), placed in autoclaved Pyrex bottles (Corning, Lowell, MA), and stored at 4 °C until use. The infiltration column consisted of a 30 cm long, 2.5 cm diameter polyvinyl chloride (PVC) column with a 100 mL, 2.5 cm diameter PVC reservoir on top, separated by a PVC union and ball valve. The bottom of the column was fitted with a 200 μm stainless steel screen to retain sand grains during infiltration events. The column and reservoir were acid washed and triple rinsed in deionized (DI) water prior to the start of each experiment. Prior to column packing, the homogenized sands and sterilized seawater were removed from the 4 °C room and allowed to warm to room temperature (21 ± 0.5 °C, approximately 4 h). Columns were packed with homogenized sand by filling in 5 cm layers using a fill and tap method. A subsample of both sand samples was dried to determine moisture content and grain size distribution (GSD) according the method ASTM C136-06.42 Each column was subjected to three successive 100 mL infiltrations. A 100 mL volume of the filter-sterilized seawater was placed in the reservoir above the column. The valve was opened, and the seawater entered the column. The second and third infiltrations occurred 20 and 40 min after the first infiltration, respectively. These infiltrations were designed to mimic transient flow of seawater at the beach during successive wave uprush events, although the time between infiltrations may be shorter than 20 min in the field. During an infiltration, seawater flowed through the packed column and exited the screened bottom where it was directed through an acid washed and triple-rinsed polypropylene funnel and deposited into a fraction collector (CF1, Spectrum Chromatography, Houston, TX). Fractions were collected in sterile polypropylene 15 mL Falcon tubes (BD, Franklin Lakes, NJ) at 2 s intervals over the first 88 s of each infiltration. Two final fractions were collected representing, respectively, effluent exiting between 89 s and 2 min and effluent exiting between 2 and 19.5 min. The valve remained open until 19.5 min to allow free air flow into the column. At 19.5 min from the start of the first and second infiltration the valve was closed, the reservoir was refilled with 100 mL of filter sterilized seawater, new tubes were placed in the fraction collector, and a new sterile funnel was installed to eliminate possible carryover between infiltrations. It should be noted that immediately after opening the valve for the second and third infiltrations, 4−5 mL of water was driven out of the pore space by the expulsion of entrapped air as the column flooded. For plotting and modeling, this volume was included in the first fraction with gravity driven infiltrating flow. Effluent was assayed for enterococci using EPA method 160041 with 48 h incubation8 within 3 h of collection. A minimum volume of 1.5 mL was required to run the assay; fractions containing less than 1.5 mL were combined with fractions that followed until the required volume of 1.5 mL was achieved. See the SI for filtration volumes and colony counting methods.

⎛ ∂h ⎞⎤ ∂θ ∂⎡ = + 1⎟⎥ ⎢K (h)⎜⎝ ⎠⎦ ∂t ∂z ⎣ ∂z

(1)

where θ is the water content [L3L−3], K is the unsaturated hydraulic conductivity [LT−1], h is the water pressure head [L], t is time [T], and z is the spatial coordinate [L]. The initial conditions were set to a pressure head, h = −100 cm. This value was selected based on a measurement of the water content in the sands which were similar for both experiments. The upper boundary condition (BC) was set to an atmospheric BC with a surface layer; this allows for ponded water on the surface and a head dependent flux (3rd type boundary condition) accurately simulating the initial condition where water is suddenly ponded on the surface when the reservoir is opened. The lower BC was set as a seepage face which allows for flow only when h > 0.39,43 The unsaturated soil hydraulic parameters, θ(h) and K(h) were estimated using the soil-hydraulic functions of van Genuchten.44 The equation utilized has six independent parameters: θr the residual soil−water content [L3L−3], θs the saturated soil−water content [L3L−3], α and n both parameters in the soil−water retention function, Ks the saturated hydraulic conductivity [LT−1], and I the tortuosity parameter in the conductivity function. To limit the possibility of nonunique solutions, the saturated water content (θs) was measured in the lab and set to the measured value and the tortuosity was set to I = 0.5. The value for the tortuosity parameter was selected based on the best estimate of the parameter for a range of soils and sands as suggested by Mualem.45 The remaining parameters were obtained using inverse parameter estimation in HYDRUS1D by minimizing the objective function39 comparing model output and the observed cumulative Darcy flux measurements for the first infiltration. These optimized parameters were then used to model the second and the third infiltrations. The effects of hysteresis were not included in the model. Bacterial detachment and transport were modeled using a modified form of the convection-dispersion equation, commonly used to model virus, colloid, and bacteria transport23,35,38,39,46 ∂θc ∂s ∂ ⎡ ∂c ⎤ ∂qc +ρ = ⎢θD ⎥ − ∂t ∂t ∂z ⎣ ∂z ⎦ ∂z

(2)

3 −3

where θ is the water content [L L ], c is the bacteria concentration in the aqueous phase [CFU L−3], s is attached concentration [CFU M−1], ρ is bulk density of the packed media [M L−3], q is the water flux velocity [L T−1], D is the dispersion coefficient for bacteria [L2 T−1], and θD = qαl where αl is the longitudinal dispersivity [L], t is time [T], and z is distance from the inlet [L]. θ and q from the hydraulic model are used as inputs to the equation. The initial conditions at each node were set to no aqueous phase bacteria and bacteria attached to surface site ‘s’ at concentrations corresponding to the concentration of enterococci per g sand prior to infiltration. It was assumed that 5990


Environmental Science & Technology so =

∑ i

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ciVi M

where kd is the first-order detachment coefficient [dimensionless]. This parametrization allowed detachment to occur when the value of ∂θ/∂t was greater than zero, indicating inundation of the node. This detachment scheme follows Cheng and Saiers36 with the following modification: positive values of ∂θ/ ∂t turned on detachment instead of a distribution of snap-on pressure head values spread over the model nodes. This detachment scheme represents the physical detachment processes associated with the propagation of the air−water interface which include air−water interface scouring and thin film expansion.36 The bacterial transport model was run with each of the three different parametrizations, and the output of the model was compared to the experimental data. The best fit parameters for bacterial detachment (kd) and when appropriate for θcrit for each detachment scheme were determined by minimizing the root-mean-square error (RMSE) between the measured and predicted log-transformed effluent ENT concentrations. When a range of kd values provided similar minimum values of RMSE, the model results with smallest value of kd from the range are presented. When the introduction of θcrit values greater than zero did not reduce the RMSE, the model results with θcrit = 0 are presented. The application of this model to our system has several limitations. The model does not account for the transport of this air driven flow that occurred at the very beginning of the second and third infiltration (described previously in column methods section). The porous media is treated as having consistent hydraulic and bacterial detachment behaviors, even though we acknowledge that the sands used in the experiment are a natural media with a distribution of grain sizes and mineral characteristics. The model does not account for hysteresis. Finally, it is acknowledged that the collection, homogenization, and repacking procedure might alter the bacterial release behavior from exact in situ conditions.

(3)

where so is the initial attached concentration, ci is the concentration of enterococci measured in each fraction i [CFU L−3], Vi is the corresponding volume of the ith fraction, and M is the dry mass of sand in the column (see Table 1 for S0). Table 1. Parameters Used in Model for Experiments A and B parameter

unit

A

B

θr θs α n ks I ρ so αl

cm3 cm−3 cm3 cm−3 cm−1 [-] cm s−1 [-] g cm−3 CFU g−1 cm

0.045 0.38 0.083 5.88 0.56 0.5 1.5 133 3

0.078 0.38 0.153 1.96 0.74 0.5 1.5 76 3

Three different detachment schemes were tested. In one parametrization, detachment was modeled as first order with respect to the attached surface concentration (detachment scheme 1)

∂s = kds (4) ∂t −1 where s is the attached surface concentration [CFU M ], and kd is a detachment constant [T−1]. This detachment scheme has typically been used by others to represent slow detachment under steady flow conditions.47−51 In the second parametrization, detachment was assumed to be first order with respect to the attached surface concentration and the fluid velocity or the fluid velocity squared (detachment schemes 2-1 and 2-2, respectively) following Saiers and Lenhart30 ∂s = kdsv m θ ≥ θcrit ∂t ∂s =0 θ < θcrit ∂t

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RESULTS AND DISCUSSION Field Through-Beach Transport. To test whether ENT in surface sands at the beach can be transported through the unsaturated zone to the groundwater table by infiltrating wave uprush, we deployed a pan lysimeter to sample seawater as it passes through the beach. As the tide rose during the experiment, the groundwater level also rose. Figure S1 shows the groundwater level relative to the depth of the lysimeter, as well as the groundwater level during the period when the lysimeter was sampled. As evidenced by the data, the lysimeter was only sampled when the groundwater level was lower than the base of the lysimeter. Therefore, water sampled from the lysimeter represents seawater that infiltrated through the unsaturated zone above the lysimeter and into the lysimeter. Two sand samples collected above the lysimeter had 2.7 and 4.7 most probable number (MPN) ENT/g dry weight. Seawater collected right before the lysimeter was sampled had 146 MPN ENT/100 mL. Figure 2 shows the concentration of ENT in water collected in the 4 pan lysimeter samples relative to the seawater. ENT in the first three lysimeter samples were between 3 and 4 times higher than ENT in the seawater. The fourth sample had lower ENT than the overlaying seawater which may reflect patchy ENT concentrations in the overlaying seawater, removal of ENT from the water phase once the sand is saturated, preferential ENT mobilization by the infiltration front, and/or depletion of sandassociated ENT after a specific volume of water has infiltrated.

(5) m‑1 −m

where kd is the detachment rate constant [T L ], v is the fluid velocity [LT−1], m is a constant set to either 1 or 2, and θcrit is a critical value of water content above which detachment occurs. This detachment scheme follows Saiers and Lenhart30 with the following modification: a single value of θcrit was implemented instead of the distribution of θcrit spread over the model nodes. This detachment scheme represents physical removal by processes proportional to the fluid velocity such as shear. In the third parametrization, detachment kinetics were modeled as first order with respect to the rate of change in water content (∂θ/∂t) and the attached surface concentration (detachment scheme 3) ∂s ∂θ ∂θ = kds >0 ∂t ∂t ∂t ∂s ∂θ =0 ≤0 ∂t ∂t

(6) 5991



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content began to raise in the capillary fringe above the groundwater table. The observed depth profile is what might be observed if ENT in the surficial sands are transported through the sand by infiltrating seawater. ENT were observed throughout the sand column, consistent with the conceptual model of through-beach transport (Figure 1). Other possible explanations for the observed ENT distribution include ENT growth within the sand or migration vertically upward from contaminated groundwater. However, given the results of the pan lysimeter experiment that showed mobilization and downward transport of ENT at this beach, it is probable that the ENT found at depth originated in surface sands. Column Experiment and Model. The results of the replicate infiltration experiments (A and B) are shown in Figures 4 and 5, respectively. Darcy flux profiles of the two trials have similar shapes, but higher velocities were observed in experiment B. The different velocities in the two experiments

Figure 2. ENT concentrations in seawater and the four 50 mL samples drawn from the lysimeter installed in the beach.

The results of the pan lysimeter experiment support the idea that through-beach transport of ENT is possible. The first three lysimeter samples had higher ENT concentrations than the seawater overlaying the location where the lysimeter was installed. This finding suggests that as seawater infiltrates the unsaturated zone, ENT are mobilized from the surface sands and enter the water phase in which they can be transported to the groundwater table. Depth Profile of ENT. A depth profile of ENT concentrations was measured at the Lovers Point field site to investigate if the profile is consistent with that expected if through-beach transport is taking place. The core collected sand throughout the unsaturated zone as evidenced by the low moisture content down to nearly the last sample (Figure 3). ENT were recovered from all samples within the depth profile and, oftentimes, exceeded our upper detection limit (black circles in Figure 3). Moisture content and ENT were not correlated (p > 0.05). ENT concentrations tended to be highest in the top 10 cm of the column and the bottom 20 cm. The bottom 20 cm represents the region where the moisture

Figure 4. Results of the column infiltration experiment A. Top panel shows Darcy flux (q) and the bottom panels shows the best model fits for detachment schemes 1−3 plotted against the observed concentrations. Fractions with concentrations below detection limit of 100 CFU/100 mL are plotted as 100 CFU/100 mL. The x-axis shows the cumulative Darcy flux (cumulative volume water/cross-sectional area of column). Vertical dotted lines indicate start of the three infiltration events.

Figure 3. Depth profile of ENT and moisture content at Lovers Point. Filled ENT circles indicate measurements that exceeded our upper limit of detection. Moisture content is percent by mass. 5992



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have very similar shapes between the two experiments. As the wetting front rapidly propagates through the column at the start each of the three infiltration events, bacterial mobilization is observed as high concentrations in the initial fractions. Thereafter, the bacterial concentrations decline. The spikes in ENT concentrations in the initial fractions of the three infiltrations suggest that detachment of ENT from these beach sands is associated with the propagation of the wetting front. The concentrations drop rapidly before the Darcy flux begins to slow. If detachment were proportional to the fluid velocity,30 we would expect to see continued elution when the velocity is high. Thus it appears that that bacterial detachment is not directly affected by water velocity. A slight increase in concentration is observed in the final fraction of each infiltration potentially caused by the propagation of the drying front.36 The hydraulic component of the model produced good fits with the cumulative water flows from the column (Figure S3). RMSEs for experiments A and B between observed and modeled cumulative Darcy flux are 0.043 and 0.041 cm, respectively. The measured and best fit parameters for the hydraulic component of the model are shown in Table 1. The bacterial transport model was implemented using the three different detachment schemes previously described. The detachment scheme that best captured the observed trends in the experiment was detachment scheme 3, which varied detachment as a function of the rate of change in water content and attached bacteria concentration. Detachment scheme 3 describes physical detachment that occurs as a result of changes in water content. This detachment scheme yielded the lowest RMSE for both experiments A and B (Table 2). Thus, the model results obtained using detachment scheme 3 are the focus of the discussion below. Figure 4 and Figure 5 show the observed and the best fit predicted bacterial concentrations for all detachment schemes for experiments A and B, respectively. The results for detachment scheme 3 are shown in the bottom panel of each figure. With detachment scheme 3, the model accurately predicts high bacterial concentrations at the beginning of each infiltration event followed by declining concentrations. The model tends to overpredict concentrations in the later fractions for each infiltration event and does not capture the slight increase in concentration observed in the final fraction. The model for A and B provided similarly strong fits with the observed data with RMSEs of 0.42 and 0.32 log ENT CFU/ mL, respectively. The values for the detachment constant (kd) that produced the lowest RMSE for the two experiments were 5.7 and 6.2 for experiment A and B, respectively. The only literature values to compare these results with are found in

Figure 5. Results of the column infiltration experiment B. Top panel shows Darcy flux (q) and the bottom panels shows the best model fits for detachment schemes 1−3 plotted against the observed concentrations. Fractions with concentrations below detection limit of 100 CFU/100 mL are plotted as 100 CFU/100 mL. The x-axis shows the cumulative Darcy flux (cumulative volume water/cross-sectional area of column). Vertical dotted lines indicate start of the three infiltration events.

likely resulted from differences in the grain size distributions and packing properties (Figure S2). In particular, the sands in A had a uniformity coefficient (Cu = d60/d10) of 1.9 and the sands in B had a Cu of 1.3. The bacterial concentrations profiles also

Table 2. Best Fit Model Parameters and RMSE for the Three Detachment Schemes Testeda

a

experiment

detachment scheme

∂s/∂t =

best fit kd [unit]

best fit θcrit [L3L−3]

RMSE [log (ENT CFU/mL)]

A A A A B B B B

1 2-1 2-2 3 1 2-1 2-2 3

kds kdsv1 kdsv2 kds(∂θ/∂t) kds kdsv1 kdsv2 kds(∂θ/∂t)

0.5 [s−1] 0.09 [cm−1] 0.0002 [s cm−2] 5.7 [-] 1.1 [s−1] 1 [cm−1] 0.0003 [s cm−2] 6.2 [-]

0 0 0 0 -

0.64 0.64 0.68 0.42 0.55 0.55 0.65 0.32

Results for experiments A and B are shown. 5993



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Cheng and Saiers.36 The authors found values of kd ranging from 0.091 to 0.90 for detachment of in situ colloids from soils subjected to intermittent wetting by freshwater. The reasonably good fit of the bacterial model with detachment scheme 3 suggests that the detachment mechanism is correlated with the rapid changes in the water content. Two detachment mechanisms that are driven by rapid changes in water content include air−water interface scouring and thin film expansion.23,30−33 It is therefore likely that one or both of these mechanisms are responsible for the observed elution behavior. Implications of Findings. The conceptual model of through beach transport (Figure 1) theorizes that ENT are transported downward from surficial sands through the unsaturated zone to the groundwater by intermittently infiltrating seawater. The pan lysimeter field data and coupled laboratory-modeling experiments confirm this pathway. Additionally, ENT were observed throughout the unsaturated zone of Lovers Point beach, a profile consistent with the conceptual model of through-beach transport. The model allowed for exploration of the mechanism whereby ENT are mobilized; results point to mobilization by the wetting front via thin-film expansion or air−water interface scouring. Future work should consider these findings on ENT transport together with ENT fate processes (i.e., growth and death) in beach sands to fully understand the potential impacts on coastal water quality, as well as human health.52 Limited work with bacterial pathogens in beach sands suggests that these may also be mobilized during infiltration events,53 so future work should investigate this possibility more fully. These experiments document that the first step of throughbeach transport is possible; ENT can be transported from surficial sands via an infiltration front with gravity to deeper sands. Future work should investigate ENT transport in the saturated region of the beach aquifer, and the possibility that they can be transported to the coastal ocean via submarine groundwater discharge54 (Figure 1). The flow of groundwater at the land-sea interface is complex and dynamic. Breaking waves and changing tides can generate flows both landward and seaward.22,55 Thus FIB mobilized from surface sands to the groundwater have the potential to be transported both toward the ocean and inland. Bacteria moving through saturated soils and beach sands have the capacity to be captured and removed by a variety of mechanisms.47,48,56 To fully understand the importance of through-beach transport, future studies will need to investigate of the entire through-beach transport process including infiltration, groundwater movement, and potential reattachment of FIB. There are several limitations to the present study. The research was carried out on beach sands from Lovers Point, so care should be taken in applying the results to other beaches. Varying beach characteristics, environmental conditions, physical and chemical makeup of the sands, and biology of ENT or other microbes may influence microbial transport. Additionally, ENT measured in the lysimeter were assayed using a different enumeration method than that used in the field depth profile and laboratory column experiments. Although the two assays should agree well,57 care should be taken in comparing the measurements directly.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 650 724-9128. Fax: 650 723-7058. E-mail: aboehm@ stanford.edu. Corresponding author address: Dept. Civil and Environmental Engineering, 473 Via Ortega, Room 189, MC 4020, Stanford, CA 94305. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Lauren Sassoubre, Mia Mattioli, Nick de Sieyes, and Keith Loague are acknowledged for their assistance with field work and modeling. Stanley Grant, Sanjay Mohanty, Keith Loague, and three anonymous reviewers provided comments that improved the manuscript. This work was supported by NSF CAREER award BES-0641406.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Some methods and Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org. 5994



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