Beach Sands along the California Coast Are Diffuse Sources of Fecal

Jun 6, 2007 - Fecal indicator bacteria (FIB) are nearly ubiquitous in California (CA) beach sands. Sands were collected from 55 beaches along the CA c...
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Environ. Sci. Technol. 2007, 41, 4515-4521

Beach Sands along the California Coast Are Diffuse Sources of Fecal Bacteria to Coastal Waters KEVAN M. YAMAHARA, BLYTHE A. LAYTON, ALYSON E. SANTORO, AND ALEXANDRIA B. BOEHM* Department of Civil and Environmental Engineering, Environmental and Water Studies, Stanford University, Stanford, California 94305-4020

Fecal indicator bacteria (FIB) are nearly ubiquitous in California (CA) beach sands. Sands were collected from 55 beaches along the CA coast. Ninety-one percent of the beaches had detectable enterococci (ENT) while 62% had detectable E. coli (EC) in their sands. The presence of a putative bacterial source (such as a river), the degree of wave shelter, and surrounding land use explained a significant (p < 0.05) fraction of the variation in both ENT and EC densities between beaches. Sand characteristics including moisture content, organic carbon, and percent fines, significantly (p < 0.05) influenced only EC densities in beach sand. We assayed 34 of 163 sand samples for salmonellae, but did not detect this bacterial pathogen. The potential for FIB to be transported from the sand to sea was investigated at a single wave-sheltered beach with high densities of ENT in beach sand: Lovers Point, CA (LP). We collected samples of exposed and submerged sands as well as water over a 24 h period in order to compare the disappearance or appearance of ENT in sand and the water column. Exposed sands had significantly higher densities of ENT than submerged sands with the highest densities located near the high tide line. Water column ENT densities began low, increased sharply during the first flood tide and slowly decreased over the remainder of the study. During the first flood tide, the number of ENT that entered the water column was nearly equivalent to the number of ENT lost from exposed sands when they were submerged by seawater. The decrease in nearshore ENT concentrations after the initial influx can be explained by ENT dieoff and dilution with clean ocean water. While some ENT in the water and sand at LP might be of human origin because they were positive for the esp gene, others lacked the esp gene and were therefore equivocal with respect to their origin. Follow-up sampling at LP revealed the presence of the human specific Bacteroides marker in water and sand.

Introduction In an effort to reduce recreational waterborne illnesses, U.S. states are required, through provisions outlined in the BEACH Act, to implement beach monitoring programs that use densities of fecal indicator bacteria (FIB) to assess risk. Similar * Corresponding author phone: (650) 724-9128; fax: (650) 7253164; e-mail: [email protected]. 10.1021/es062822n CCC: $37.00 Published on Web 06/06/2007

 2007 American Chemical Society

monitoring programs are in place around the globe. FIB are used to evaluate beach water quality because their densities in coastal waters contaminated with wastewater and urban runoff have been linked quantitatively to swimmer illness (1-3). When FIB densities exceed threshold values, beach advisories are issued warning swimmers that exposure may lead to illness. Each year, the number of beach advisories in the U.S. grows as agencies increase the number of beaches they monitor. There were 20 397 days of beach advisories in 2005 and the majority (63%) were caused by unknown, nonpoint sources of FIB (4). Inability to identify a pollution source makes remediation nearly impossible. In addition, the relationship between FIB and recreational waterborne illness has been questioned for waters contaminated with nonpoint sources of FIB (5). Efforts that elucidate “unknown” FIB sources to coastal waters and evaluate their potential to contain human pathogens are needed so that mitigation strategies can be developed and human health risks evaluated. Recent work has elucidated a potentially important nonpoint source of FIB to coastal waters: the beach itself. The presence of FIB on surficial beach sands has been documented at beaches on all U.S. coastlines (6-12), raising the possibility that at least a portion of beach advisories in the U.S. is caused by FIB from beach sand. In a limited number of studies, isolation of human pathogens from beach sand has been reported (13-16). The source of FIB and pathogens in sand remains largely unknown. It has been suggested that they emanate from animal feces, runoff, or spilled sewage; it has also been proposed that they may represent indigenous populations adapted to living in the extra-enteric environment (6, 17, 18). Environmental factors and processes that may influence the presence and density of microbial pollutants in beach sand are illustrated in Figure S1. Microbes may be deposited directly into sand by an external source such as FIB-rich bird/animal feces (9), runoff from storm drains or rivers (11, 12), sewage, or seawater. In some cases, particle associated-FIB might be deposited, or deposition of sand already contaminated with FIB may occur. Once seeded into sand, FIB may die-off or grow (8, 19). Physical removal of FIB may occur when water inundates sands causing FIB mobilization and transport. Erosion of FIB-contaminated sand might also occur, especially during storms. None of these processes is presently well-understood. In the present study, we are particularly interested in the mobilization of FIB from sand and their subsequent transport to the sea during non-storm conditions. “Over-beach” transport may occur when the rising tide or wave uprush inundates FIB-rich sands causing FIB mobilization and suspension into the water column. Alternatively, “throughbeach” transport may occur if FIB are mobilized by seawater and transported into the beach aquifer by tide- or waveforced infiltration of seawater and subsequently discharged via submarine or subaerial groundwater discharge (20, 21). Over-beach transport should result in elevated FIB during high tidal conditions, while through-beach transport should give rise to elevated FIB during low tides when groundwater discharge from beach aquifers is enhanced (21). The present study fills gaps in our understanding of microbial pollution of beach sand. First, we determine the ubiquity of the FIB in beach sands by studying the quality of sands from 55 beaches along the CA coast. Second, we examine the extent to which specific beach factors, such as degree of wave exposure, presence of a putative FIB source, VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Fifty-five beaches sampled for the CA coast sand survey are shown. Each symbol indicates the beach location. The color of the circle indicates the maximum density of ENT (left) and EC (right) measured in sand at that beach normalized to 100 g dry sand. If no detectable ENT or EC were measured an empty symbol is shown. sand composition and moisture content, and surrounding land use control the presence and densities of FIB in beach sand. Third, we test the hypothesis that FIB in beach sand can be transported from the sand to the sea by examining FIB-sand-sea interactions at a single CA beach, Lovers Point (LP). Fourth, we explored the potential for FIB in beach sand to represent a human health risk by conducting a source tracking study at LP utilizing human-specific genetic markers in enterococci and Bacteroides.

Materials and Methods California Coast Sand Survey. Sand was collected at 55 CA beaches between the Mexico and Oregon borders (Figure 1, Table S1). Beaches were chosen to represent a wide range of natural and anthropogenic conditions including sand grain size; sand organic carbon content; presence of putative FIB source such as a river, creek, or storm drain; surrounding land use; and degree of shelter from waves. At each beach, three exposed (i.e., subaerial) sand samples were collected from (1) above the high tide mark, (2) below the high tide line, and (3) near a river or storm drain, if applicable, or from a random location on the beach. Each sand sample was collected by compositing 25 mL of sand from 10 spots at the appropriate sampling locations to obtain 250 mL. Samples were homogenized by shaking, stored on ice, and returned to the lab within 48 h. Microbiological analyses for enterococci (ENT) and E. coli (EC) in sands were similar to methods described by others (12, 22) utilizing mEI (ENT) and MI (EC) agars (see Supporting Information (SI)). Ferguson et al. (11) found up to 14% of colonies cultured on mEI from sandy sediments at Huntington State Beach, CA were not ENT. Hence, the ENT colony forming units (CFUs) reported here should be viewed as presumptive ENT. Densities are reported as CFU per 100 g dry weight. Sands most enriched with FIB, as well as some sands with low FIB densities (34 samples, Table S1) were analyzed for 4516

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samonellae using a modified version of EPA Method 1682 for salmonellae enumeration in biosolids (see SI). The percent moisture content by mass (θm), percent organic carbon content by mass (Cm), and the percent fines were determined for each sand sample (see SI). Each beach was categorized based on the presence or absence of a putative FIB source (i.e., a river or storm drain), the percent land use devoted to human development (both urban and agricultural) in a 10 km radius area surrounding the beach sampling site, and the presence or absence of wave action. The presence of a putative FIB source and wave action was determined by on-site observation. Land use within a 10 km radius circle surrounding the sampling site was quantified using ARCMAP (ESRI, Redlands, CA) in conjunction with the National Land Classification Database 1992 (seamless.usgs.gov). Transport of FIB from Sand to Sea. We examined the hypotheses that FIB can be transported from the sand to the sea, and that FIB in sand emanate from a human source at asinglebeach: LoversPoint (LP), Monterey, CA (36°37′29.88′′N, 121°54′59′′W) (Figures S2 and S3). The beach is approximately 100 m long, with medium to coarse sand (φ ) 0.29, pb ) 1.5 g cm-3) and a spring tide range of 2.4 m. LP was chosen for our study because the sand harbors high densities of FIB and it is tide energy dominated in the summer (the time of our study) thus simplifying beach-sea interactions by eliminating the potential for waves to influence over-beach and through-beach transport. We chose to test our transport hypothesis using ENT because they are responsible for many of the beach advisories along the CA coast. A column experiment was conducted to determine if seawater mobilizes ENT associated with LP beach sand. Exposed sand from above the high tide line naturally contaminated with ENT at a density of 2600 CFU/100 g was collected from LP, and seawater was collected from adjacent Monterey Bay. Sand was packed into a glass chromatography column and filtered seawater was pumped through the

column at a rate of 1.5 mL min-1. No effort was made to maintain specific saturation conditions. Rather, we sought to mimic the wetting of sand by seawater as it might occur in the field. Fractions were collected and analyzed for ENT using EPA method 1600 (23). Results are displayed as concentration in fractions as a function of pore volume Vp (see SI). To assess the potential for FIB in sand to impair coastal water quality, we conducted a 24 h study at LP during a spring tide. Sampling commenced at 1400 h 8 Aug 2006. Weather conditions were clear with no fog or clouds and light winds. Every 20 min, an exposed sand sample was collected 1.5 m landward of the water level, a submerged sand sample was collected approximately 1.5 m seaward of the water level, and a water sample was collected from ankle depth along a cross-shore transect (Figure S3). Because sampling locations were relative to the water level, the precise location where samples were collected changed as the tide rose and fell. The distance from a datum located above the spring-high tide line was used to quantify the precise sampling locations. Sand samples were composited spatially during each sampling event by collecting 5 mL of the top 5 cm of sand from ten locations within 2 m alongshore. Several additional sediment and water samples were collected adjacent to a storm drain on the beach approximately 75 m from our sampling site (Figure S3). θm of the sand samples was measured in situ using a soil moisture probe (ML2X, delta-T, Cambridge, UK) (see SI). Sand samples were analyzed for ENT using the same methods described for the CA beach survey and densities were normalized to dry weight of sand. ENT in water samples were quantified using EPA method 1600 (23). ENT from 12 sand and 9 water samples (Table S2) were tested for the presence of the enterococcal surface protein (esp) gene using a method modified from Scott et al. (24) for Enterococcus faecium (see SI). The esp gene has been used as a marker for ENT of sewage origin (24-26). Since this gene is only present in about 1% of ENT in sewage (24), use of the marker may lead to false negatives. To further investigate the potential for human sewage to be present at LP we returned to the beach during dry weather in February 2007 to collect four sand samples (three from above the high tide line and one from near the storm drain) and in March 2007 to collect three water samples (two from the water flowing from the storm drain and one from the nearshore) to test for the human specific Bacteroides (HF) marker (27) (see SI). Statistical Analyses. Non-parametric methods including Spearman’s rank correlations (rs) and Kruskal Wallis tests provided by SPSS (Chicago, IL) and Matlab (Mathworks, Natwick, MA) were used to compare ENT and EC measured at various locations and times. N-way analyses of variance (ANOVAs) were used to assess the role beach and sand characteristics play in controlling the variability of FIB in beach sand along the CA coast. A sand factor was extracted using principle component analysis from Cm, θm, and percent fines data series and was necessary because these three factors were positively, significantly correlated to one another in the CA coast survey (Table S3). A high sand factor value indicated sand was high in percent fines, Cm, and θm (see SI). All results and relationships were deemed significant if p < 0.05.

Results California Coast Sand Survey. Of the 55 beaches surveyed along the CA coast, 50 (91%) had culturable ENT between 9 and 7200 colony forming units (CFU)/100 g in at least one of three samples, while 34 (62%) had culturable EC ranging between 8 and 6264 CFU/100 g (Figure 1 and Table S1). Although the range of densities observed for the indicators

is similar, ENT appears to be more prevalent than EC in CA beach sands. While a handful of presumptive positive salmonellae were obtained, none of them were confirmed as Salmonella. This result suggests that one of numerous human pathogens was not culturable from our beach sand samples. Isolation of salmonellae in UK beach sand has also been reported as unsuccessful (28) or infrequent (13). At each beach, three composite exposed sands were collected: dry sand above the high tide line (average θm of 2.4 ( 1.0%), wet sand from the swash zone (average θm of 20.7 ( 1.9 %), and sand from a location deemed by the authors to be likely to contain elevated FIB (adjacent to a river or a storm drain hereafter referred to as “target” sample, average θm of 5.1 ( 2.0 %). ENT were significantly higher in the target samples than the dry and wet samples (p < 0.05). However, EC densities in target samples were significantly higher (p < 0.05) than dry samples, but not different from wet (Figure S4). The three samples were used in aggregate to probe the relationship between FIB densities in beach sand and beach attributes. N-way ANOVAs were conducted to determine which of the independent parameters describing sand and beach attributes could explain the presence/absence and density of ENT in beach sands. The absence of wave action at a beach was the only beach attribute that explained the presence of ENT in beach sands at densities above the detection limit (adjusted r2 ) 0.555). The density of ENT in sand samples was significantly influenced by the following factors: presence of a putative FIB source, presence of wave action, land use, and the interaction between presence of a source and wave action (adjusted r2 ) 0.80). The presence of a putative FIB source, as well as lack of wave action, and a high percentage of developed land around the beach corresponded to higher ENT densities. In particular, beaches with a source that were sheltered had the highest ENT densities of various combinations of source presence and wave action. Interestingly, the sand factor did not explain a significant percent of the variability in ENT densities (even if just target samples were considered) indicating that the sand environment (as described by Cm, θm, and percent fines) is not an important controlling factor. EC presence could be predicted with a model (adjusted r2 ) 0.42) containing the following four factors: wave action, land use, the 2-way interaction between land use and wave action, and the 2-way interaction between land use and the presence of a putative FIB source. EC were more likely to be present at sheltered beaches and beaches with low levels of surrounding development. If a source was present, beaches surrounded by moderate development were more likely to have EC above our detection limit. When EC density was considered as the dependent variable, factors controlling EC presence/absence were significantly influential in the ANOVA, but so were presence of a source and the sand factor (adjusted r2 ) 0.76). The importance of the sand factor in controlling EC densities contrasts with results for ENT and suggests a moist, organic rich, fine grained sand environment is conducive to high EC densities. Transport of FIB from Sand to Sea. A series of studies at LP were conducted to determine if sand can act as a diffuse source of FIB to coastal waters. A column experiment verified that ENT can be readily mobilized from LP beach sand by seawater. When filtered seawater was applied to a column of naturally contaminated sand, the concentration of ENT decreased in effluent fractions over approximately four pore volumes, after which ENT were not detected (Figure 2). The number of ENT eluted (5113 CFU) normalized by the mass of sand in the column (184 g) indicates that 2800 CFU/100 g were mobilized and transported through the column, nearly equivalent to the density we estimated was present in the VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. ENT densities measured in fractions collected during the column experiment as a function of pore volume (defined in SI). Symbols at 0.1 CFU/mL indicate that ENT densities were beneath our lower limit of detection of 0.1 CFU/mL. sand when the experiment commenced (2600 CFU/100 g). The mobilization of sand-associated ENT in the column could have been facilitated by a propagating air-water interface, as has been observed for the mobilization of colloids in unsaturated soils subjected to transient flows (29, 30). Although the precise mechanisms whereby ENT are mobilized have yet to be determined, the experiment illustrates that at least a portion of ENT are not strongly bound to LP sand grains and may be mobilized and eluted upon exposure to seawater. We collected samples of exposed and submerged sands at LP as well as water over a 24 h period in order to compare the disappearance or appearance of ENT in sand and the water column. Overall, 144 and 72 sand and water samples were collected, respectively. There was no evidence for sand erosion or deposition during this 24 h project. ENT in LP sand exhibited spatial variability, varying over 3 orders of magnitude between below our detection limit (approximately 5 CFU/100 g) to 4452 CFU/100 g (Figure 3). Sands collected during the study had moisture contents (θm) ranging from 0.66% to fully saturated (19%), and temperatures ranging from 16.1to 28.9 °C. In aggregate, exposed sand had significantly higher ENT densities than submerged sand (Figure S5, p < 0.01). The 8 highest densities of ENT were observed in dry (average θm ) 2.5%), exposed sand near the spring-high tide line, approximately 10 m from the up-beach datum (Figure 3). ENT densities in sand were negatively correlated to moisture content (rs ) -0.515, p < 0.01), and not significantly related to sand temperature. Desiccation and heating are known to be important factors impacting the persistence of FIB and pathogens in soils, sands, and biosolids (31). Hence, the findings that ENT densities are highest in sands with low moisture content, and lack of correlation between ENT densities and temperature are surprising. However, the results are consistent with those of Oshiro and Fujioka (6) who found the highest densities of ENT in sand at Hawaiian beaches far from the water line with low moisture content. This may suggest that ENT are not sensitive to desiccation. Another possible explanation for the relationship between ENT densities and moisture content is that ENT had been deposited on the dry sand just prior to commencement of the study. We examined changes between ENT densities in sands as they were submerged and exposed during rising and falling tides (Figure S6A). During flood tides (16:00-22:00 and 5:4012:20), ENT densities were significantly lower in submerged versus exposed sands (Figure S6B and S6C, p < 0.05). This indicates that when exposed sands were inundated by the rising tide, ENT were mobilized and removed from the sand. 4518

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This is particularly important as it suggests that mobilized ENT may have entered the water column directly or been transported into the beach aquifer. During the ebb tide (22:00-5:20), there was no significant difference between ENT densities in sand when it was submerged compared to when it was exposed (Figure S6D, p ) 0.2602). While more work should be done to determine whether deposition of ENT from seawater is an important source of ENT to beach sands, our study did not verify that deposition of waterborne ENT contributed to the ENT densities in exposed sands. A stretch of submerged sands (located between 13.4 and 25.4 m, Figure S6A) was sampled twice in a row (between 20 min and 10 h apart) after it was flooded and before the tide receded. There was no significant difference between ENT measured in this section of submerged sand between sampling (Figure S6E, p ) 0.5292). Similarly, a stretch of exposed sands (between 16.2 and 34.1 m, Figure S6A) was sampled twice in succession (after 1-11 h) after it was exposed during the receding ebb tide and before it was flooded by the second flood tide. There was no significant difference in ENT densities between sampling events (Figure S6F, p ) 0.7555). Thus, we did not observe any increases in ENT in submerged or exposed sands, as would potentially be observed if ENT were growing in the sands at a sufficiently rapid rate. ENT densities in the water column at ankle depth exhibited high temporal variability (Figure 3). For example, at 19:00 the ENT density was below our detection limit (2 CFU/100 mL), and at 19:20, ENT density was 170 CFU/100 mL, above the single-sample CA Department of Health contact standard of 104 CFU/100 mL. To remove the high-frequency variability in ENT densities for simplified interpretation of longer period trends, the data were low pass filtered using a 100 min running mean. Smoothed ENT densities are characterized by a sharp increase during the first flood tide when seawater inundates exposed sand high in ENT densities (Figure 3). This finding, along with the documentation of ENT removal from exposed sand by the rising tide (Figure S6B and S6C), supports the idea that ENT from the sand were mobilized and entered the water column. We observed a small peak in ENT densities during the second flood tide near 12:00 as might be expected given that ENT densities in submerged sand were significantly lower than exposed during the second rising tide (Figure S6C). However, the spike in water ENT densities was much smaller and short-lived relative to that observed during the first high tide. The reduced impact of the second high tide on water quality may be due to the strong germicidal effect of sunlight on ENT densities in marine waters (32). We conducted an inventory analysis to determine if the spike in ENT densities in the water column observed during the first flood tide (120 CFU/ 100 mL at 21:00 h) can be explained by an influx of ENT from the sand during the first flood tide (see SI for details). We estimate that 12 × 107 CFU entered the nearshore waters to produce the observed concentration of 120 CFU/100 mL. Based on the data in Figure S6B, and making some assumptions about the volume of sand “washed” by seawater, we estimate 13 × 107 CFU were lost from the sand during the first flood tide. The number of ENT in the water is not vastly different from the amount we approximate is lost from the sand. This analysis supports the idea that ENT from the sand entered the water at LP on the flood tide giving rise to the spike in ENT at 21:00 in Figure 3. The sharp increase in ENT densities at 21:00 in nearshore LP waters was followed by a slow decrease over the remainder of the study (Figure 3). A simple box model indicates that the slow decrease can be explained by first-order decay of ENT and mixing with clean ocean water (see SI for details) as long as the residence time τ of water in the nearshore was between 18 and 26 h. Although we did not measure τ during the study,

FIGURE 3. Results from the Lovers Point study. The location of exposed (square) and submerged (circle) sand samples as a function of time during the study relative to a datum located above the spring high tide line (left axis). The sand sample locations change as the tide changes. The color of the symbol indicates the measured density of enterococci (ENT) normalized by 100 g dry sand. An open symbol indicates the density was below our detection limit. The density of ENT in the water at ankle depth, as well as the 100 min smoothed water ENT densities are shown (right axis). PDT is Pacific daylight time. we believe that 18-26 h is not unreasonable. Nearshore waters at this beach were extremely quiescent during the study. Floating wrack and foam along the beach were observed to move slowly over the 24 h study, but did not leave the study area, suggesting that the residence time of water adjacent to the beach face was on the order of a day. The source of ENT in the sand at LP has not previously been documented. A storm drain is located approximately 75 m from our study site at the south end of the cove among rocks, above the beach (Figure S3). This region does not have combined sewer-stormwater systems. Although our study was conducted during the dry season, a small volume of water was discharging from the pipe onto the sand and entering the nearshore water adjacent to the drain. The water and sand near the storm drain were assayed for ENT and found to harbor, on average, 647 CFU/100 mL (n ) 2) and 984 CFU/100 g (n ) 2), respectively. ENT grown from these 4 samples as well as ENT from 7 nearshore water and 10 sand samples collected as part of the LP study were assayed for the esp gene (Table S2). The esp gene was found in 3 of 4 water and sand samples collected at the storm drain, 3 of 10 transect sand samples and 2 of 7 nearshore water samples. These results suggest that some of the ENT at this beach, both in sand and water, may emanate from human sewage. To be certain, more work should be done to validate of usefulness of the esp gene as a marker of human ENT at CA beaches.

To further investigate the presence of human sewage at this beach, we returned at two different dry weather periods after the LP experiment (February and March 2007) to test the storm drain effluent, nearshore waters, and sand for the HF marker. We found the HF marker in two of two storm drain effluent samples, one of one nearshore water sample, and in sand samples from near the storm drain and the high tide line (Table S4, see SI). Although collected at different times than the main LP study, these data suggest that human sewage possibly impacts this beach.

Discussion FIB in Beach Sands are Widespread Along the CA Coast. Although previous studies (7, 11, 12) have documented FIB in sands at particular beaches in CA, this is the first study to document their ubiquity along the entire CA coast. This result highlights the important potential for sands to be reservoirs and perhaps sources of FIB to coastal waters. The LP study revealed that sand densities may vary over several orders of magnitude at a single beach. By collecting three composite samples during the CA survey, we attempted to smooth out potential variability and capture representative densities at each beach. Nevertheless, it should be noted that the samples we collected offer only a glimpse into the spatially and temporally dynamic densities of FIB in CA beach sand. It would be interesting to repeat this survey at a future VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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time to examine the influence of season and weather on FIB densities in beach sand. In particular, Lee et al. (12) showed ENT densities in Santa Monica Bay sand increased during storms. Specific Beach Factors Explain the Presence and Density of FIB in Beach Sands, but the Manner In Which They Influence FIB Densities Differ Between EC and ENT. ENT densities are highest at sheltered beaches with a putative FIB source, and a high degree of human development around the beach. This supports a hypothesis that ENT in beach sand along the CA coast may be anthropogenic in origin emanating from runoff or other human sources. However, more work needs to be completed to test the hypothesis because some suggest ENT are indigenous to soils and sediments (6). In contrast, EC were likely to be found at sheltered beaches, beaches with a putative FIB source, in sands of high Cm, θm, and percent fines, and at beaches with low to moderate development. These findings suggest that at some beaches (with a putative source for example) EC in sand may come from anthropogenic sources, but at others, in particular those with low levels of surrounding development, EC may be seeded from other natural sources or be a member of the indigenous microbial community (18). The finding that moist, carbon rich, fine sands are likely to harbor elevated EC is particularly interesting, as these are the conditions that have been shown to be most favorable for EC regrowth in sediments (8, 12). We Found ENT in at Least One Sand Sample at Nearly Every Beach (50/55), but EC at Significantly Fewer (34/55) (p < 0.01). Assuming that potential sources of FIB to sands (Figure S1) are rich in both ENT and EC, this result indicates differential survival, growth, or removal of the two indicators. Future work should examine the survival of these bacteria in beach sands with variable characteristics. FIB-Contaminated Beach Sands Can Act as Diffuse FIB Sources. At LP, ENT from sand were mobilized by the rising tide and transported into the water column where densities exceeded the CA singe-sample standard. This mode of transport of FIB from the sand to the sea is referred to as over-beach transport, and a signature of this transport is rising ENT densities with the rising tide. Shibata et al. (33) found the highest FIB densities occurred with high tide at a tide-energy dominated Florida beach with contaminated sands, suggesting over-beach transport may be occurring there as well. More work should be done to examine the importance of sand as a source of ENT to waters at other beaches. We Determined That at Least a Portion of ENT are Not Strongly Attached to Sand Grains and Can be Readily Mobilized Upon Inundation by Seawater. To fully understand the dynamics of FIB and pathogens in beach sand, studies need to be conducted to examine the association between target organisms and various sizes and types of beach sand grains, the potential for organisms to be present in thin films on grain surfaces, and the ability of seawater and freshwater to mobilize them via thin film expansion release, air-water interface scouring, and shear mobilization (34). Our column studies (present here and in Boehm et al. (35)) have only examined mobilization and transport of ENT in beach sand. Gram-negative bacteria have different surfaces than Gram-positive bacteria, so attachment, mobilization and transport of EC may be different from ENT. It is Yet to be Determined if Exposure to Sands Contaminated with FIB Represent a Health Risk. Salmonellae were not detected from CA-wide survey sand samples using culture methods. Microbial source tracking at LP illustrated that human sewage might be present in beach sands based on the presence of HF and esp gene markers. 4520

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Acknowledgments We acknowledge the Boehm Lab, Keeney Willis, Steve Weisberg, Matt Charette, and anonymous reviewers who assisted with the work and/or provided suggestions for improving the manuscript. George Somero provided space at Hopkins Marine Station. This work was supported by NOAA Oceans and Human Health grant NA04OAR4600195 and NSF CAREER award to A.B.B., the UPS Foundation, and an NSF graduate fellowship to B.A.L.

Supporting Information Available Further details on the methods and results, Figures S1-S6, and Tables S1-S4. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Kay, D.; Fleisher, J. M.; Salmon, R. L.; Jones, F.; Wyer, M. D.; Godfree, A. F.; Zelenauch, J.; Shore, R. Predicting likelihood of gastroenteritis from sea bathing- Results from randomised exposure. Lancet 1994, 344, 905-909. (2) Haile, R.; Witte, J.; Gold, M.; Cressey, R.; McGee, C.; Millikan, R.; Glasser, A.; Harawa abd N.; Ervin, C.; Harmon, P.; Harper, J.; Dermand, J.; Alamille, J.; Barrett, K.; Nides, M.; Wang, C. The health effects of swimming in ocean water contaminated by storm drain runoff. Epidemiology 1999, 10, 355-363. (3) Wade, T. J.; Pai, N.; Eisenberg, J. N. S.; Colford, J. M., Jr. Do U.S. Environmental Protection Agency water quality guidelines for recreational waters prevent gastrointestinal illness? A systematic review and meta-analysis. Environ. Health Perspect. 2003, 111, 1102-1109. (4) National Resources Defense Council. Testing the waters, 2006. http://www.nrdc.org/water/oceans/ttw/titinx.asp. (5) Colford, J. M., Jr.; Wade, T. J.; Schiff, K. C.; Wright, C. C.; Griffith, J. G.; Sandhu, S. K.; Burns, S.; Hayes, J.; Sobsey, M.; Lovelace, G.; Weisberg, S. B. Water quality indicators and the risk of illness at non-point source beaches in Mission Bay, California. Epidemiology 2007, 18, 27-35. (6) Oshiro, R.; Fujioka, R. Sand, soil, and pigeon droppingssSources of indicator bacteria in the waters of Hanauma Bay, Oahu, Hawaii. Water Sci. Technol. 1995, 31, 251-254. (7) Boehm, A. B.; Fuhrman, J. A.; Mrse, R. D.; Grant, S. B. A tiered approach for the identification of a human fecal pollution source at a recreational beach: Case study at Avalon Bay, Catalina Island, California. Environ. Sci. Technol. 2003, 37, 673-680. (8) Desmarais, T. R.; Solo-Gabriele, H. M.; Palmer, C. J. Influence of soil on fecal indicator organisms in a tidally influenced subtropical environment. Appl. Environ. Microbiol. 2002, 68, 1165-1172. (9) Whitman, R. L.; Nevers, M. B. Foreshore sand as a source of Escherichia coli in nearshore water of a Lake Michigan beach. Appl. Environ. Microbiol. 2003, 69, 5555-5562. (10) Alm, E. W.; Burke, J.; Spain, A. Fecal indicator bacteria are abundant in wet sand at freshwater beaches. Water Res. 2003, 37, 3978-3982. (11) Ferguson, D. M.; Moore, D. F.; Getrich, M. A.; Zhowandai, M. H. Enumeration and speciation of enterococci found in marine and intertidal sediments and coastal water in southern California. J. Appl. Microbiol. 2005, 99, 598-608. (12) Lee, C. M.; Lin, T.; Lin, C.-C.; Kohbodi, G. A.; Bhatt, A.; Lee, R.; Jay, J. A. Persistence of fecal indicator bacteria in Santa Monica Bay beach sediments. Water Res. 2006, 40, 2593-2602. (13) Bolton, F.; Surman, S. B.; Martin, K.; Wareing, D. R.; Humphrey, T. J. Presence of Campylobacter and Salmonella in sand from bathing beaches. Epidemiol. Infect. 1999, 122, 7-13. (14) Obiri-Danso, K.; Jones, K. Intertidal sediments as reservoirs for hippurate negative campylobacters, salmonellae and faecal indicators in three EU recognized bathing waters in North West England. Water Res. 2000, 34, 519-527. (15) Elmanama, A. A.; Fahd, M. I.; Afifi, S.; Abdallah, S.; Bahr, S. Microbiological beach sand quality in Gaza Strip in comparison to seawater quality. Environ. Res. 2004, 99, 1-10. (16) Pianetti, A.; Bruscolini, F.; Sabatini, L.; Colantoni, P. Microbial characteristics of marine sediments in bathing area along Pesaro-Gabicce coast (Italy): A preliminary study. J. Appl. Microbiol. 2004, 97, 682-689. (17) Ott, E.-M.; Mller, T.; Mller, M.; Franz, C. M. A. P.; Ulrich, A.; Gabel, M.; Seyfarth, W. Population dynamics and antagonistic

(18)

(19)

(20) (21) (22)

(23)

(24)

(25)

(26)

potential of enterococci colonizing the phyllosphere of grasses. J. Appl. Microbiol. 2001, 91, 54-66. Byappanahalli, M. N.; Whitman, R. L.; Shively, D. A.; Sadowsky, M. J.; Ishii, S. Population structure, persistence, and seasonality of autochthonous Escherichia coli in temperate, coastal forest soil from a great lakes watershed. Environ. Microbiol. 2006, 8, 504-513. Anderson, M. L.; Whitlock, J. E.; Harwood, V. J. Persistence and differential survival of fecal indicator bacteria in subtropical waters and sediments. Appl. Environ. Microbiol. 2005, 71, 30413048. Longuet-Higgins, F. R. S. Wave set-up, percolation, and undertow in the surf zone. Proc. R Soc. London Ser. A 1983, 390, 283-291. Nielson P. Tidal dynamics of the water table in beaches. Water Res. Resour. 1990, 26, 2127-2134. Whitman, R. L.; Shively, D. A.; Pawlik, H.; Nevers, M. B.; Byappanahalli, M. N. Occurrence of Escherichia coli and enterococci in Cladophora (Chlorophyta) in nearshore water and beach sand of Lake Michigan. Appl. Environ. Microbiol. 2003, 69, 4714-4719. U.S. Environmental Protection Agency. Method 1600: Membrane Filter Test Method For Enterococci in Water; Technical 1 Report; Office of Water and Hazardous Materials, U.S. Environmental Protection Agency: Washington, DC, 1997. Scott, T. M.; Jenkins, T. M.; Lukasik, J.; Rose, J. B. Potential use of a host associated molecular marker in Enterococcus faecium as an index of human fecal pollution. Environ. Sci. Technol. 2005, 39, 283-287. McDonald, J. L.; Hartel, P. G.; Gentit, L. C.; Belcher, C. N.; Gates, K. W.; Rodgers, K.; Fisher, J. A.; Smith, K. A.; Payne, K. A. Identifying sources of fecal contamination inexpensively with targeted sampling and bacterial source tracking. J. Environ. Qual. 2006, 35, 889-897. McDonald, J. L.; Hartel, P. G.; Gentit, L. C.; Belcher, C. N.; Gates, K. W.; Rodgers, K.; Fisher, J. A.; Smith, K. A.; Payne, K. A. Detection of human-derived fecal pollution in environmental waters by use of a PCR-based human polyomavirus assay. Appl. Environ. Microbiol. 2006, 72, 7567-7574.

(27) Santoro, A. E.; Boehm, A. B. Frequent occurrence of the humanspecific Bacteroides fecal marker at an open coast marine beach: Relationship to waves, tides, and traditional indicators. Environ. Microbiol. 2007, (Online Early Articles). doi: 10.1111/ j.1462-2920.2007.01319.x. (28) Obiri-Danso, K.; Jones, K. Distribution and seasonality of microbial indicators and thermophilic campylobacters in two freshwater bathing sites on the River Lune in northwest England. J. Appl. Microbiol. 1999, 87, 822-832. (29) El-Farhan, Y. H.; DeNovio, N. M.; Herman, J. S.; Hornberger, G. M. Mobilization and transport of soil particles during infiltration experiments in and agricultural field, Shenandoah Valley, Virginia. Environ. Sci. Technol. 2000, 34, 3555-3559. (30) Auset, M.; Keller, A. A. Intermittent 1 filtration of bacteria and colloids in porous media. Water Resour. Res. 2005, 41, doi: 10.1029/2004WR003611. (31) Zaleski, K. J.; Josephson, K. L.; Gerba, C. P.; Pepper, I. L. Survival, growth, and regrowth of enteric indicator and pathogenic bacteria in biosolids, compost, soil, and land applied biosolids. J. Residuals Sci. Technol. 2005, 2, 49-63. (32) Boehm, A. B.; Grant, S. B.; Kim, J. H.; McGee, C. D.; Mowbray, S.; Clark, C. D.; Foley, D.; Wellmann, D. Decadal and shorter period variability of surf zone water quality at Huntington Beach, CA. Environ. Sci. Technol. 2002, 36, 3885-3892. (33) Shibata, T.; Solo-Gabriele, H. M.; Fleming, L. E.; Elmir, S. Monitoring marine water recreational water quality using mutiple microbial indicators in an urban tropical watershed. Water Res. 2004, 38, 3119-3131. (34) DeNovio, N. M.; Saiers, J. E.; Ryan, J. N. Colloid movement in unsaturated porous media: Recent advances and future directions. Vadose Zone J. 2004, 3, 338-351. (35) Boehm, A. B.; Shellenbarger, G. G.; Paytan, A. Groundwater discharge: A potential association with fecal indicator bacteria in the surf zone. Environ. Sci. Technol. 2004, 38, 3558-3566.

Received for review November 29, 2006. Revised manuscript received April 16, 2007. Accepted April 30, 2007. ES062822N

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