CHAPTER 3. SURFZONE BACTERIA PATTERNS

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Chapter 1. Introduction and Background

Chapter 2. Methodology

Chapter 3. Surfzone Bacteria Patterns

Chapter 4. Subtidal Circulation Pathways

Chapter 5. Newport Canyon Transport Pathway

Chapter 6. Sea Breeze

Chapter 7. Tidal Transport Pathways

Chapter 8. Sediment Resuspension and Transport near the OCSD Outfall

Chapter 9. Nearshore Circulation and Transport Pathways

Chapter 10. Spatial and Temporal Patterns of Plume Tracers

Chapter 11. Temporal and Spatial Patterns for Surfzone Bacteria before and after Disinfection of the OCSD Outfall

Acknowledgements

The total and fecal coliform and enterococci concentrations measured in the surfzone between July 1, 1998, and December 31, 2001, provided by OCSD, were analyzed with respect to their temporal and spatial variability. The methods used to collect and process these samples are described in Chapter 2. Station locations are shown in Figure 3-1.

15,909 samples from the numbered surfzone stations from 39,000 feet south of the Santa Ana River to 39,000 feet north of the Santa Ana River (including those sampled sporadically as well as those sampled regularly, but excluding Talbert Marsh and Santa Ana River) were analyzed for the three bacterial indicators. The distribution of samples by concentration is shown in Figures 3-2, 3-3, and 3-4 for total coliform, fecal coliform, and enterococci, respectively. Given what we know about the confidence limits on the bacterial concentration measurements, we attempted to group the measured concentration ranges into those which seemed very unlikely to have exceeded the AB411 single-sample standards (SS) (green), those which may or may not have actually exceeded AB411 SS (blue), and those which almost certainly did exceed SS (red). Total coliform is a little harder to interpret than the other two indicators, since there are two criteria, one involving a ratio to fecal coliform. Noble et al. (2003) only gives confidence limits for the total coliform = 10,000 standard. The blue bars make an attempt to include the total coliform = 1,000 standard, with the assumption that the ratio test is met. The majority of samples have bacterial concentrations less than, or equal to, the minimum detection limit (these percentages are listed in Figures 3-2, 3-3, and 3-4).

3.1.1. Relationship to Spring-Neap Tidal Cycle

Previous work (MEC, 2000; Grant et al., 2000) suggested that there was a relationship between the phase of the moon and the surfzone bacterial concentrations. Figure 3-5 shows measured Los Angeles sea level, “spring tides”, and new and full moons for the HB PIII period. Averaged over 1998-2001, the spring-tide high water occurs 0.6 days after a new or full moon.

As described in Section 2.1.2, three types of surfzone bacteria event days were defined based on exceedance of AB411 standards (Table 1-1). Type 1 is based on total and fecal coliform in the stations 3N-12N region, type 2 on enterococci exceedances at multiple stations, and type 3 picks up any enterococci exceedances in the stations 3N-12N region which occur on days not already categorized as type 1 or 2 (Table 3-1).

These events are calculated using the daily subsampled data set (if multiple samples were available for a station for a given Pacific Standard Time (PST) day, the sample closest in time to that of the average sampling time for that station, calculated over May 1–October 31, 2001, was used) to minimize the prejudicial effect of the round-the-clock hourly sampling done for six 2-day periods only in 2001. Out of the 1280 days, of which 692 were sampled, type 1 events (red) occur on 148 days, type 2 events (blue) occur on 67 days, and type 3 events (orange) occur on 75 days (Figure 3-6). Without subsampling to no more than one sample per day per station, an additional 6 days qualifies as a type 1 event and an additional 10 days as a type 2 event. While throughout the 3.5-year record additional surfzone samples were occasionally taken in between those regularly collected at 3000-foot intervals, note that in 1999 this was done more often, including in the stations 3N to 12N area. These contributed to a larger number of bacterial events, as defined here, relative to what would have been seen with only the regularly sampled stations.

If we consider individual samples, however, as opposed to days on which the previously defined events occur, we find that enterococci exceeds AB411 standards in many more samples than total and fecal coliform, as has been previously noted by Grant et al. (2000), and others. Out of a total of 14,866 surfzone samples (taken from the regularly sampled numbered stations, so that those sampled only sporadically or for short periods, as well as Talbert Marsh and Santa Ana River, are not included) taken during the 3.5-year period, 834 had enterococci concentrations greater than the AB411 SS, whereas only 292 had total or fecal coliform concentrations exceeding AB411 SS. 198 samples exceeded AB411 standards for both enterococci and coliform bacteria.

Further analysis was performed to quantify the relationship between tidal height (or range) and the occurrence of bacterial contamination in the surfzone. Each day in the 3.5-year record was classified by its proximity in time to the day on which spring tide, as defined in Section 2.5.1, occurred. The results reported below are for the bacterial events calculated using the full data set and AB411 SS. Results from calculations carried out using the data set subsampled to no more than daily are not significantly different. Figure 3-7 shows the percentage of days on which a type 1, type 2, or any type 1, 2, or 3, event occurs. There is a higher likelihood of a type 1 or 2 bacterial event occurring the day after the spring-tide high water than on any other day in the fortnightly cycle. Of all the days in a fortnightly cycle, the day that spring tide occurs has the greatest chance (60%) of experiencing an AB411 exceedance in the stations 3N-12N region. If we now take all of the bacterial events and classify what day they fall on relative to the spring tide, we find that about 50% of them occur within ± 2 days of the spring tide (Figure 3-8).

Finally, we also looked at the relationship between the height of the higher high water (HHW) each day and the probability of a bacterial event occurring. In this portion of the analysis we considered a simple logistic regression model. In this model, each day has a probability of having an event. Denote the probability of an event on day i by pi. Then the logistic or “logit” model says:

where tidei is the height of the higher high tide on day i. We fit three separate models of this sort (one for each of event type 1 and 2, plus one in which the response was type 2 or type 3). In each case we found no added predictive power was gained by including a term quadratic in tide height, nor, not surprisingly, a term marking nearness to spring tide. Thus each model is of the form seen in Figure 3-9. In each case the tide height term is “statistically significant,” indicating that the size of the effect we see in the model is such that it is very unlikely that an effect of that size would have arisen in the sample if in fact there were no such relationship in the population.

It is interesting to note that, in this area, the phases of the diurnal and semidiurnal tidal constituents are such that in recent years (1998-2001) the largest spring tides have fallen in the summer (May-August) and winter (December-February). It also turns out that the larger of the two ebb tides (HHW to lower low water) during the summer spring tides falls at night, with the higher high tide at 0800–1000 PST. Given that the beat period resulting from the fortnightly cycle produced by the semidiurnal constituents M2 and S2 (14.77 days) and fortnightly cycle produced by the diurnal constituents O1 and K1 (13.66 days) is very nearly half a year (186.62 days), the timing of the largest spring tides relative to the annual cycle changes very little year to year.

3.2. HB PIII Temporal and Spatial Patterns: June–October 2001

The percentage of samples possibly exceeding the AB411 SS (blue plus red bars of Figures 3-2, 3-3, and 3-4) is not significantly different for the HB PIII period as opposed to the 1998-2001 period, except that there is a slightly higher percentage of enterococci with values >35 (22% vs. 17%) for the summer 2001 period, due to the nighttime sampling. Figures 3-10, 3-11, and 3-12 are contour plots of the logarithm of total coliform, fecal coliform, and enterococci concentrations, respectively, for the surfzone stations sampled during HB PIII. These are made using the daily subsampled data set. While the fortnightly pattern discussed earlier is evident in all three bacterial indicators, it is clearest in total coliform. Also, while all three indicator species show higher concentrations preferentially in a band between stations 3N and 12N, this is particularly true for fecal coliform. Occasionally, relatively high levels of enterococci (>300 MPN/100 mL) are found almost simultaneously all the way from stations 9S to 15N. Total coliform rarely exceeds 300 MPN/100 mL south of the Santa Ana River, but on one occasion (in mid-August) total coliform exceeds 200 MPN/100 mL from stations 15S to 12N.

The temporal relationship between various physical processes and surfzone bacterial concentrations high enough to trigger beach closures in accordance with AB411 standards was explored using bacterial “events” as defined in section 2.1.2. During the HB PIII study, almost every surfzone sample that exceeded an AB411 SS, fell on a day characterized as a type 1, 2, or 3 event, as seen in Figure 3-13. Hourly sea level, as measured at Los Angeles, is included in this figure to show graphically the preference for bacterial events occurring close to the time of spring tide, as discussed above.

In addition to calculating the events in terms of exceeding the AB411 SS, we also defined a set of events based on single samples exceeding the prescribed monthly geometric mean (MM) standards. We calculated events using the daily subsampled data set for both the single sample and monthly mean limits (Chapter 2). Figures 3-14 and 3-15 show these four sets of bacterial events. Using the lower monthly mean standard increases the number of days on which there are bacterial events, and using the data set subsampled to daily decreases the number of days on which there are events. Referring to Figure 3-14, it can be seen that if all the events (type 1 and/or 2 and 3) are considered together (i.e., all the colored dots), only one event (type 1 on ~July 11), calculated from the full data set using the monthly mean standard (row 3), occurs which is not contiguous with either an event, or an unsampled day next to an event, calculated using the SS with the full data set (row 1). If events are considered separately by type (Figure 3-15), two additional type 1 events defined by monthly mean appear that are not contiguous with SS type 1 events. They are, however, contiguous with type 3 events, and type 3 events occur by definition only on days when there is no type 1 or type 2 event, so a given day can swap between being a type 1 or 3, or a type 2 or 3, for the four-event data sets considered here. Figure 3-15 also illustrates that the large-scale enterococci events (type 2) are significantly reduced (there are fewer brown dots than red dots) when the daily subsampled, rather than the full, data set is used with the SS. Taken together, these figures reveal that, while the number of contiguous days during which bacterial concentrations exceed AB411 standards expands or contracts depending on whether the SS or monthly mean standard is applied and on whether all samples or the daily subsampled data is used, the timing of the events is essentially unchanged. In other words, if the Huntington Beach closures were based solely on single daytime samples, using either the AB411 SS or monthly mean standard, the length of the closures would vary, but the number of closures would not.

3.2.1. Temporal Relationship between High Bacterial Concentrations and Physical Phenomena

In an effort to relate the occurrence of bacterial concentrations exceeding AB411 standards to physical phenomena hypothesized to facilitate these events, the time series of bacterial events calculated with the SS and all data from June 14-October 9, 2001 (when the majority of the moorings were in the water) were used. In addition to the bacterial events, Figure 3-16 shows the days on which cold water was observed to be unusually close to shore, days on which the mid-shelf stratification was unusually weak, as well as the height of HHW and a characterization of the surface wave field for every day.

Cold water near shore was determined by two methods. In the first method, the temperature of the sewage outfall plume was determined by looking at the T/S distribution from the moorings in two-week increments from June 17 through October 15, 2001. The maximum temperature of the plume T/S anomaly was picked as an upper limit of the temperature of the plume, and this value was compared with the temperature time series at the nearshore moorings (HB01 and HB03). If the 10-m temperature at mooring HB01 was less than, or within 0.25ºC of, the warmest temperature in the sewage outfall plume, it was designated as a cold-water nearshore event. The result is the same whether mooring HB03 or HB01 is used. In the second method, the temperatures measured by the moorings along the main line were used to determine when the 12ºC isotherm was inshore of the 30-m isobath and the water at HB03 was cooler than 13ºC. The July 23-26 occurrence of cold water near shore, judged by both of these criteria, coincides with the largest cold-water sloshing event of the HB PIII project as identified by the 14ºC isotherm reaching 5 m, or higher, at the HBN2 site on the 11-m isobath.

Unusually weak stratification over the mid-shelf was also identified by two methods. One criteria specified that the water column at HB03 be well-mixed from top to bottom. The other criteria identified days with reduced stratification by specifying that the temperature difference, at any time during the day, between thermistors at two out of three moorings (listed below) be less than the mean temperature difference minus two standard deviations of the temperature difference. The calculation was carried out using the hourly low-pass-filtered (3-hour cut-off) data. The moorings, thermistors, and temperature differences used are as follows:

HB01: surface to 10 m, DT < 0.41°C

HB03: 5 to 15 m, DT < 0.70°C

HB05: 5 to 20 m, DT < 1.06°C

The hourly surface-wave directional spectra were averaged into daily spectra. These were examined by eye and characterized as consisting of southerly swell, westerly wind waves, southerly wind waves, a combination of the above, or no waves. An example of each is shown in Figures 3-17a, 3-17b. 55% of days with a bacterial event (of any of the previously defined types) coincided with southerly swell, while 45% of days with a bacterial event occurred on days with either no waves to speak of, or westerly wind waves alone. These are nearly identical to the percentages of all days in the study period with (54%) and without (46%) southerly swell, thus indicating that bacterial events did not occur preferentially during southerly swell. Considering each of the three event types separately, type 1 (58%) and type 3 (50%) events occur in concert with southerly swell in close to the same proportions as southerly swell occurs in general (54%), while type 2 bacterial events have a slight bias towards southerly swell, with 75% of them occurring on southerly swell days.

Other than large tidal ranges (i.e., spring tides), the only other physical variable pictured in Figure 3-16 that co-occurs with bacterial events more often than not is the occurrence of reduced stratification in the nearshore zone. Four out of five of those occurrences (each set of contiguous days is considered as an occurrence) coincide with type 1 events. However, during these events the plume is below the warm unstratified water, and is maintained offshore. Evidence for the internal tide swash into the nearshore zone, hypothesized to facilitate transport of sewage plume bacteria to shore, is greatest during July 23-26. The only exceedance of the AB411 SS in the Huntington Beach area on these days was a single sample taken at 3N on July 23 with a measured enterococci concentration of 110 MPN/100 mL. Even if you consider the lower monthly mean standard, the only additional exceedances are an enterococci value of 60 at station 6N and fecal coliform equal to 220 at station 12N on July 23. The timing of the cold swashes and the surfzone sampling at stations 3N–12N (Table 3-2) was such that the samples could have captured high bacterial concentrations if they were brought in to shore together with the cold water.

3.2.2. Hourly Round-the-clock Sampling during Cruise Periods/Spring Tides

During the six cruise periods, scheduled to coincide with the occurrence of spring tides, samples were taken for bacteriological analysis in the surfzone at stations 15S to 21N every hour for 48 hours (with the exception of the first cruise in May when the hourly sampling was done for only 36 hours, with one several-hour break).

Consistent with the results of Boehm et al. (2002) for data from May 2000, contour plots of these data show a strong day-night cycle in all three fecal indicator bacteria, with the highest values occurring at night. Figures 3-18, 3-19, and 3-20 show this for the cruise in early July. While there are similarities in the spatial patterns for all three indicators, there are also differences. The higher concentrations tend to be found north of the Santa Ana River, but when there are elevated levels south of the Santa Ana River, they occur to some degree in all three indicators (e.g., 2000-2300 PDT July 19 and 2100-2200 PDT July 20) (Figures 3-21, 3-22, and 3-23). The highest values of total coliform tend to occur at station 0 (next to the mouth of the Santa Ana River), and there is some suggestion of upcoast (northwestward propagation) from there, with a speed of about 30 cm/s (Figures 3-18 and 3-21). Fecal coliform values generally peak at station 6N (Figures 3-19, 3-22, and 3-24), while enterococci values tend to be high not only at station 6N, but also at the southern end of the range (Figures 3-20, 3-23, and 3-25) and occasionally at the northern end (Figure 3-26). Enterococci have a minimum from stations 6S to 0. These patterns also hold for the daily subsampled data shown in Figures 3-10, 3-11, and 3-12.

The onset of elevated total coliform values at station 0 occurs between about 1200 and 0300 (Figures 3-18, 3-21, and 3-27), shortly prior to the nighttime low tide (0300-0600) (Table 3-3). Since the time of low tide varies so little among the six intensive sampling periods, it is impossible to say based on this alone whether the timing of the elevated bacteria levels is related to the phase of the semidiurnal tidal cycle, the diurnal light cycle, or both. Bacterial concentrations are not generally elevated near the daytime low tide, the higher low tide of the two, except as noted below.

On the few occasions when there are high values of coliform in the middle of the day (e.g., ~1330 PDT on June 19 and 20 and ~1500 PDT on July 6) (Figures 3-27, 3-28, and 3-18, respectively), enterococci values are only slightly elevated (Figure 3-25), if at all, while at night all three indicators show notably higher concentrations. Note that these midday increases in bacteria level have their maxima at station 0 for all three indicators, and occur close to the time of the higher low tide (1354 PDT on June 19, 1318 PDT on June 20, and 1542 PDT on July 6).

When the timing of samples exceeding the AB411 SS is examined in these hourly data, they also occur predominantly at night, particularly for enterococci which has the lowest standard. Boehm et al. (2002) noted that enterococci falls below detection limits earlier in the day than total and fecal coliform. Enterococci exceedances, wherever they occur, do so overwhelmingly between sunset and sunrise (Figures 3-29, 3-30, and 3-31). As previously stated, however, this time period also coincides closely with the time between HHW and lower low water. Note, however, that the onset of enterococci exceedances precedes both the high tide (the start of the ebb tidal flow) and the sudden drop in near-bottom nearshore temperature (e.g., nights of July 19 and 20) (Figure 3-31). This could indicate that the day-night cycle in enterococci is more strongly influenced by sunlight-induced die-off than by tidal influence on either a landward or seaward source. The coliform exceedances, however, do not show this strong relationship to the day-night cycle, nor does such a pattern appear when a cut-off even lower than the AB411 SS is used for visualization (Figures 3-32, 3-33, and 3-34). The predominance of high coliform values near the time of low tide is evident, however.

The surfzone bacteria data for stations 3N-12N for the whole HB PIII period show no significant relationship with sample temperature. The relationship with time of day is due solely to the sampling strategy in this mostly once per day data set. There is a weak inverse correspondence between bottom pressure (equivalent to sea level for this purpose) and total and fecal coliform. When the hourly data from the six intensive sampling periods is examined together, however, the relationships between bacteria and sea-level height, time of day, and phase of semidiurnal tide that were pointed out for the individual high-resolution sampling periods becomes more obvious.

Figure 3-35 shows the concentration of all three fecal indicator bacteria at all stations, 15S to 21N, for all hourly data from the six cruise periods plotted as a function of time of day. Again we see that enterococci values rebound as soon as the sun sets, while coliform values don't rise until later in the evening, and don't reach values as high as seen in the early morning. When the same surfzone bacterial data is plotted versus sea level (Figure 3-36), we see that coliform values are highest (with a few exceptions) when sea level is low, whereas enterococci have no obvious relationship to sea level. The total coliform peak that is seen 5-10 hours after HHW at station 0 (Figure 3-37) also occurs north of the Santa Ana River (represented by station 6N here), while the weaker peak 15-20 hours after HHW only appears at station 0, perhaps indicating die-off or dilution to the background levels before the source waters are advected past station 6N. Fecal coliform shows a pronounced peak at station 6N, but not at station 0, 6-10 hours after HHW. The enterococci pattern is quite different from total and fecal coliform.

These results suggest that coliform levels are controlled more by phase of the tide, and enterococci by time of day. However, due to the fact that all the hourly sampling in this study was separated by about two weeks so that the larger ebb always fell at night, we can not definitively separate the two effects. Also, without reliable information as to the die-off rate of the different bacterial indicators under the prevailing temperature and salinity conditions, we cannot say how that influences the temporal patterns.

Taken together, however, these results suggest that tidal flow out of the Santa Ana River may be a source of high bacteria concentration, particularly for total coliform. There is also some indication that there may be a local source of bacterial contamination, particularly high in fecal coliform, near station 6N. Enterococci appear to have multiple sources, and exhibit a particularly pronounced day-night cycle. Boehm et al. (2002) suggested that the most likely answer to the question "Why is the surf zone so rapidly re-supplied with indicator bacteria after the sun goes down?" is that there is a continuous supply of indicator bacteria to the surfzone. These data would support that suggestion for enterococci. Grant et al. (2001), also using May 2000 data, identified the Talbert Marsh as a net source of enterococci and characterized the temporal variability of enterococci concentration in terms of flood vs. ebb tides, but did not look at the influence of time of day.

3.3. References

Boehm, A.B. et al., 2002. Decadal and shorter period variability of surf zone water quality at Huntington Beach, California. Environmental Science and Technology, v. 36, p. 3885-3892.

Grant, S.B. et al., 2001. Generation of Enterococci bacteria in a coastal saltwater marsh and its impact on surf zone water quality. Environmental Science and Technology, v. 35, p. 2407-2416.

Grant, S.B. et al., 2000. Huntington Beach Water Quality Investigation Phase II: An analysis of ocean, surf zone, watershed, sediment and groundwater data collected from June 1998 through September 2000. Final Report.

MEC, 2000. Huntington Beach closure: relationships between high counts of bacteria on Huntington Beaches and potential sources. Final Report.

Noble, R.T. et al., 2003. Comparison of beach bacterial water quality indicator measurement methods. Environmental Monitoring and Assessment, v. 81, p. 301-312.