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Major Findings 5.1

Noble, Xu, Rosenfeld, Largier, Hamilton, Jones, and Robertson, 2003, Huntington Beach Shoreline Contamination Investigation, Phase III: U.S. Geological Survey Open-file Report 03-62, version 1.0

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1. Background
2. Hypotheses
3. Objectives and Methods
4. Measurement Program
5. Major Findings:
5.1. Surfzone Bacterial Contamination Patterns
5.2. Outfall Plume Tracking
5.3. Coastal Transport Processes
6. Transport Processes
7. Conclusions
8. References
9. Acknowledgements


5. Major Findings

5.1. Surfzone Bacterial Contamination Patterns

5.1.1. Multi-year patterns
The total and fecal coliform and Enterococci concentrations measured in the surfzone between 1 July 1998 and 31 Dec. 2001 were analyzed with respect to their temporal and spatial variability. The majority of samples have bacterial concentrations less than, or equal to, the minimum detection limit (58% of samples for total coliform, 72% for fecal coliform, and 65% for Enterococci). Single sample bacteria concentrations were compared to both the single sample (SS) and the geometric mean monthly (MM) AB411 standards:

   Total coliform > 10,000 mpn / 100 ml (SS); 1,000 mpn / 100 ml (MM)
   Total coliform > 1000 mpn / 100 ml and Total coliform / fecal coliform < 10 (SS)
   Fecal coliform > 400 mpn / 100 ml (SS); 200 mpn / 100 ml (MM)
   Enterococci > 104 mpn / 100 ml (SS); 35 mpn / 100 ml (MM)

There are numerous exceedances of the AB411 single sample standards (3% of the samples for total coliform, 2% for fecal coliform and 6% for Enterococci). Here, the total coliform exceedances are conservatively defined as having values greater than 1000 mpn/100ml. The majority of those exceedances are for Enterococci, as was found in previous studies (Grant et al. 2000). The percentage of samples exceeding the AB411 standards is not significantly different for the HB Phase III period when compared to the 1998-2001 period. Because of nighttime sampling, there is a slightly higher percentage of the single sample Enterococci concentrations with values greater than the allowable geometric monthly mean level in the summer of 2001 (22% vs. 17%).

The incidence of high concentrations of bacteria confirm a previously identified relationship with the lunar cycle (MEC, 2000; Grant et al., 2000), such that higher bacteria concentrations are associated with maximum tidal ranges (i.e. spring tides). The larger the tidal range (or equivalently, the lower the daily lower low water, or the higher the daily higher high water), the higher the probability of a contamination event. It is interesting to note that in southern California, the timing of the largest spring tides relative to the annual cycle changes very little year to year. The phases of the diurnal and semidiurnal tidal constituents are such that the largest spring tides fall in the summer (May-August) and winter (December-February). The larger of the two ebb tides (higher high water to lower low water) falls at night during the summer spring tides, with the higher high tide at 8-10 PM local time.

5.1.2. Summer 2001 patterns
Surfzone bacteria were collected once per day, five days per week, during the HB Phase III study period. Increased levels of all three fecal indicator bacteria are found more frequently in the surfzone north of the Santa Ana River than south of it (Figure 3.  Log10 of total coliform, fecal coliform, and Enterococci concentration (mpn/100ml) are plotted versus time and distance alongshore.  Figure 3 - 1.51MB PDF file ). Excessive levels of total and fecal coliform are generally confined to the 0-15N and 3N-9N regions, respectively. High concentrations of Enterococci are found throughout the range on a number of occasions.

These distinct spatial patterns for coliform and Enterococci bacteria could be classified into three types of events. In section six of this report, we will determine whether specific coastal-ocean processes are related to the presence or absence of these event categories.
  Type 1: Total or fecal coliform exceeded AB411 single sample standards at one or more of the stations from, 3N to 12N during a 24-h period starting at 0000 PST.
  Type 2: Enterococci exceeded AB411 standards at three or more of the numbered surfzone stations from 39S to 39N, including at least one station from 3N to 12N during the same 24-h period.
  Type 3: Enterococci exceeded AB411 at any station from 3N to 12N on any day on which there is not a type 1 or type 2 event. This type was added to ensure that all days on which any single sample standard was exceeded in the Huntington Beach area was counted as an event.
Almost every surfzone sample between 39S and 39N that exceeded an AB411 single sample standard during the HB Phase III study fell on a day characterized as a type 1, 2, or 3 event (Figure 4. Days on which bacterial events occurred.  Sea level measured at Los Angeles is shown at the bottom of the figure. Figure 4 - 203KB PDF file ). The sea level data confirm that contamination events tend to occur during spring tides.

In addition to the daily samples, additional bacteria samples were taken in the surfzone at stations 15S to 21N every hour for 48 hours during spring tides while the six hydrographic cruises were being conducted. The only exception is that during the first cruise in May, hourly sampling was done for only 36 hours, with one several-hour break. All three fecal indicator bacteria show a day-night cycle, with the highest values occurring at night. This pattern is most pronounced for Enterococci, which rebounds to high nighttime values almost as soon as the sun goes down (Figure 5. The ratio of Enterococci concentrations to the AB411 single sample standard at stations 15S to 21N for the 2nd intensive sampling period are plotted together with sampling times at station 0 (pink dots), Los Angeles sea level (red line), and temperature 0.5m above bottom at HB01 (blue line). Only samples exceeding 104 mpn/100ml are shown here. A local time reference is used. Figure 5 - 87KB PDF file ). Enterococci exceedances, wherever they occur, do so overwhelmingly between sunset and sunrise.

Even though total coliform has a day-night cycle, it shows a stronger relationship to the semidiurnal tidal cycle. The highest values are found preferentially near low tide at station 0 (next to the mouth of the Santa Ana River) and nearby stations to the north of it. Data also suggest that there is upcoast (northwestward propagation) from station 0, with a speed of about 30 cm/s (0.7 mph) (Figure 6.  The log10 of total coliform concentration (mpn/100 ml) is plotted versus local time and distance alongshore. Black x's indicate the time and location of each sample. Figure 6 - 59KB PDF file ).

These results suggest that total and fecal coliform levels are controlled principally by the phase of the tide. Enterococci seem to be controlled more by the day-night cycle. However, we cannot definitively separate the two effects because the hourly sampling programs were separated by about two weeks. Hence, night samples always are taken on the larger ebb tide. Also, without reliable information as to the die-off rate of the different bacterial indicators under the prevailing light, temperature, and salinity conditions, we cannot say how the temporal patterns are influenced by the bacterial life span.

These results suggest that tidal flow out of the Santa Ana River may be a source of high bacteria concentration, potentially impacting the beach to 15N particularly for total coliform. There is some indication that there may also be a local source of bacterial contamination, particularly high in fecal coliform, near 6N.

Enterococci, which have caused many of the beach postings, appear to have multiple sources. Also, it is interesting to note that the concentrations of Enterococcus observed in the surf zone are often higher than the concentrations observed in the effluent plume. This enhanced concentration suggests that the source of the bacteria may not be the effluent plume. The bacterial ratios in the effluent plume suggest that if high Enterococcus counts were due to the plume then they should be associated with high total and fecal coliform counts. This association is often missing in the surfzone (Figure 3.  Log10 of total coliform, fecal coliform, and Enterococci concentration (mpn/100ml) are plotted versus time and distance alongshore.  Figure 3 - 1.51MB PDF file ).

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U.S. Department of the Interior, U.S. Geological Survey, Western Region Coastal and Marine Geology
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Maintained by: Michael Diggles
Created: January 23, 2003
Last modified: December 9, 2005 (mfd)