CHAPTER 10. SPATIAL AND TEMPORAL PATTERNS OF PLUME TRACES

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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

In the summer of 2001 hydrographic surveys were conducted six times during spring tides to map the distributions of the water-column tracers of the plume (Figure 10-1). Prior research indicated that the periods of beach contamination were most common at spring tides. During each survey, the plume tracers were mapped for a continuous 48-hour period. The region between 400 m from the beach and 3.3 km offshore was mapped at 4-hour intervals with CTD profiles, resulting in 12 realizations. The region between 3.3 km and offshore was mapped with a towed undulating vehicle (TUV) each 8 hours, producing up to 6 three-dimensional maps of the region. Beach sites were sampled at hourly intervals.

Four types of flow conditions were observed during the six mapping surveys (Table 10-1):

1. Upcoast flow at plume depth,

2. Downcoast flow throughout water column,

3. Downcoast flow in upper layer and upcoast flow in lower layer, and

4. Downcoast flow in upper layer and sheared flow in lower layer.

The plume was defined by low-salinity anomalies, high fecal bacteria measurements, and elevated ammonium counts in the following manner:

1. The salinity anomaly was at least 0.005 psu less than the predicted ambient salinity. The predicted ambient salinity was calculated from the measured temperature using a set of linear equations to define the ambient temperature-salinity (T-S) relationship. The ambient T-S was determined by examining the T-S relationship for observations that were outside of the influence of the effluent plume–either offshore or upcurrent from the outfall. For the salinity anomaly, we primarily considered observations where the temperature was less than 14°C as being part of the effluent plume. Normally, there was a distinct minimum in the absolute value of the salinity anomaly above 14°C, and often a secondary feature near-surface where it is believed that runoff contributes to the low salinity signal.

2. Fecal indicator bacteria abundance was elevated, particularly for water temperatures less than 14°C, and where the salinity anomaly was at least 0.005 psu less than the predicted ambient salinity. Ammonium was used as a secondary indicator. If elevated bacteria concentrations were found, and there was no significant salinity anomaly or increase in ammonium above background, then we did not consider that bacteria to be due to the outfall.

10.2. Where Was the Plume Observed?

Corresponding to the list of flow regimes above, several examples from these periods demonstrate different characteristic distributions associated with each type of regime. In all of the cases the general pattern of the plume distribution conformed to the direction of the measured flow in that the plume extended along the coast in the direction of the measured currents (Table 10-1). The depth of the plume was generally in agreement with plume modeling results using the RSB multiport diffuser model (Roberts et al., 1989a; 1989b; 1989c).

10.2.1. Upcoast Flow

The only case when the currents appeared to be flowing upcoast was during the first survey (May 21-22, 2001). The flow direction was inferred from the plume because there was no current data available at this time. The plume remained offshore when the flow at plume depth was upcoast presumably because the flow followed bottom topography that veers away from the coast (Figure 10-2). Three indicators of the presence of the plume (salinity anomaly, ammonium concentration, and fecal coliform abundance) all showed that the plume was submerged and offshore, tending to follow the topography. Shoreward transport of the plume was not likely during upcoast flow because the isopycnals tend to tilt downward toward the coast (Chapter 4), providing a density boundary between the offshore plume and the nearshore surface layer.

10.2.2. Downcoast Flow

When the currents below 30 m were flowing downcoast, two types of plume distribution were observed: 1) the plume separated from the shelf break at the offshore end of Newport Canyon and advected downcoast; and 2) a portion of the plume followed the shelf break and entered the canyon. During the fourth cycle (from 1201 to 1706 PDT on June 20, 2001), the plume remained at the offshore end of the survey line and appeared to separate from the topography at the mouth of Newport Canyon (Figure 10-3). In contrast, during Survey 6 (September 15-17, 2001), the effluent plume was found in the canyon and near to shore at the beginning of the survey (Figure 10-4). By the end of the cruise, the plume was no longer present within the canyon, and it was more offshore, similar to the observations from Survey 2 in June.

10.2.3. Sheared Flow

During Survey 4 (July 19-21, 2001), the mean alongshore currents were vertically sheared between 24- and 44-m water depth during the 48-hour period of the survey. The depth of the shear varied with the tide phase, but in general the current was downcoast above 24 m. Below 24 m, however, the flow rotated clockwise with increasing depth until it became upcoast at 44 m. In this situation the flow affecting the plume was mostly upcoast (Figure 10-5). Although the bulk of the plume appeared to advect upcoast from the outfall, the complex vertical current structure also caused parts of the plume to move downcoast from the outfall. The downcoast portion of the effluent plume appeared to penetrate into Newport Canyon, but bacteria concentrations were low.

10.2.4. Weak Flow

An example from the third cruise (July 5-7, 2001) shows the distribution of the salinity anomaly (Figure 10-6). Like the other plume observations during the summer, the plume was submerged and remained below depths of 25 m in the offshore region. In this example, when the flow below 30 m was weak, the plume is present both upcoast and downcoast from the outfall diffuser. In the most downcoast section, it appears that the plume extended shoreward into Newport Canyon and may have shallowed some but still remained below 20-m water depth.

Although the plume shallowed nearer to shore, it generally remained below depths of about 15 m. The plume was observed at a distance of 2 km from shore on several occasions, but at 0.8 km from shore maxiumum total coliform and E. coli concentrations were usually low compared with offshore, nearshore, and beach concentrations (Figure 10-3 and Table 10-2). Occasionally, the bacteria concentrations within 2 km of shore were correlated with the cross-shelf currents.

10.3. Cross-shelf Transport Due to Currents

An important objective of this study was to determine whether cross-shelf currents transported the plume to the beach. Boehm et al. (2002) have speculated that cross-shelf transport due to internal tides is a likely mechanism for transporting bacteria from the OCSD outfall to the shore. Because of the intensive grid sampling in the region between 0.4 and 3.3 km offshore, it is possible to characterize the cross-shelf transport associated with the plume and cross-shelf currents. During the third cruise between July 5 and 7, 2001, the alongshore currents were relatively weak below 30-m water depth (Table 10-1). During this period there was a good relationship between the cross-shore velocities and the distribution of plume. As shown in Figure 10-7, bacteria concentrations increased on the alongshelf Lines 3 and 4 when the currents were shoreward. This close relationship between the abundance along these two lines and the cross-shelf currents was only observed during the third survey cruise when the currents at depth were relatively weak. During other cruises this relationship was not readily apparent.

Because cross-shelf currents can transport the effluent plume shoreward, we examined the cross-shelf distribution of bacteria for all of the cruises. Summaries of maximum concentrations for each alongshelf hydrographic line for each cruise are given in Table 10-2. During each survey, Line 2, 0.8 km offshore from the beach, always had lower maximum concentrations of bacteria than were observed at the beach and along Line 1. It was usually lower than Line 3 (with the exception of cruise 4), and always lower than Line 4. Even though exceedances of AB411 standards for total coliform and E. coli may occur offshore in the water column and along Line 1 and at the beach, exceedances did not occur along Line 2, 0.8 km from shore, during any of the five cruises from June through September. The conclusion from this is that bacteria from the plume may extend shoreward as close as 2 km from the shore, but the bacteria do not appear to cross into the region within about 1 km of the shore. Higher bacteria concentrations 0.4 km (400 m) from shore are likely to come from transport from the beach via rip currents and other cross-shore surfzone exchange processes.

10.4. Bacterial Relationships within the Plume

Fecal indicator bacteria in the effluent plume were well correlated with each other (Figure 10-8). Enterococci are often stated to remain viable for longer time periods (lower T90) than either total or fecal coliforms. However, in the submerged plume measured offshore, the ratio of total coliform:fecal coliform:enterococci is typically about 25:5:1 and is relatively conservative with distance away from the outfall discharge. This suggests that die-off rates in the plume may be relatively uniform. The relationships appear to become less clear as concentrations of a given indicator fall below 100 MPN/100 mL. If there is any difference in relative die-off of bacteria within the plume, enterococci appear to decrease more rapidly compared to total coliform bacteria (Figure 10-5) when the enterococci concentration falls below 100 MPN/100 mL and total coliform bacteria fall below 1000 MPN/100 mL. Whether this is real, or an artifact of the error at lower concentrations, is uncertain.

In the earlier Plume Tracking SPS, high bacteria concentrations were observed at least 12.5 km downcoast from the outfall. Transit times from the outfall were estimated to be on the order of 2 days. Therefore, the die-off rates of the bacteria may be low when the plume is submerged well beneath the surface where it is below the euphotic zone and in relatively cool water.

10.5. Variability on the Beach

We examined the variability of bacteria on the beach during the six intensive sampling periods. Three trends, or modes of variability, in the beach bacteria distribution were observed. The first mode was diurnal variability of all three indicator bacteria. This was a spatially ubiquitous pattern that occurred during each of the studies. The second major pattern was during low-low tide periods in the daily cycle, which typically occurred during the night for each of our studies. Total coliform would increase first in the vicinity of the mouths of the Santa Ana River and Talbert Marsh and then propagate upcoast from the Santa Ana River and Talbert Marsh region at rates that were on the order of 0.25-0.3 cm/s (0.48-0.57 knots). These rates are consistent with the wave-induced surfzone transport that would result from southerly waves with a wave height of about 0.75 m. The third mode of variability was that single sites would have high bacterial concentrations. These were sometimes repeatable with the tidal cycle and sometimes not. In one case, it was found that there was a leaking bathroom at station 6N that accounted for high fecal-indicator bacteria at the beach (Orange County Sanitation District, 1999).

The variability observed on the beach suggests that alternative sources of bacteria can account for many of the exceedances on the beach. Because many of the beach postings and closures have resulted from enterococci, it suggests that the source of the bacteria may not be the effluent plume, especially since the concentrations of enterococci observed in the surfzone are often higher than the concentrations observed in the effluent plume. Based on the relationships observed in the effluent plume, high enterococci counts should also be associated with high total and fecal coliform counts.

10.6. Other Low-Salinity Sources

In addition to the outfall plume observed beneath the surface and offshore, a nearshore, surface, low-salinity feature was often present in the region during our hydrographic surveys. This feature was present in at least four of the six hydrographic surveys. When present this low-salinity layer could extend alongshore across the entire sampling region and extended offshore for a distance of 2-3 km. This low-salinity layer is much warmer than the plume and represents runoff from the Los Angeles and San Gabriel Rivers that has been transported downcoast by near-surface currents.

An important question with regard to this plume is whether there is evidence of elevated bacterial concentrations within the river plume that could contribute to the high bacterial concentration on the beach. For example, during the third intensive bacterial sampling event (July 5-7, 2001) the runoff plume was very evident in the nearshore region as indicated by the three-dimensional map of the salinity anomaly (Figure 10-6). In an examination of observations within the river plume one instance of total coliform abundance greater than 24000 MPN/100 mL was observed within the water that was identified as river plume (Figure 10-9).

10.7. Summary and Conclusions

The plume was sampled under a range of conditions throughout the summer of 2001. The six survey periods provided a range of current conditions over the shelf off Huntington Beach, California. Four distinct types of flow regimes were observed during the study:

1) Upcoast flow,

2) Downcoast flow,

3) Sheared flow, and

4) Weak flow.

In each case the distribution of the plume correlated with the currents in the vicinity of the effluent plume. The most complex case was that of sheared flow when the evidence of the plume was found both upcoast and downcoast from the outfall diffuser.

When the currents are downcoast, the plume was observed in Newport Canyon. But when present in the canyon, effluent indicators were not observed above a depth of 25 m. Indication of the plume in the canyon was also observed when sheared flow resulted in a portion of the plume advecting downcoast. However, the presence of the plume in the canyon may be transient, as observed during Survey 6, when the plume was observed in the canyon at the beginning of the study, but was offshore from the canyon by the end of the 48-hour period.

Cross-shelf transport of the plume was observed. Elevated bacterial concentrations were found as close as 2.2 km from shore. This is consistent with the observations of Boehm et al. (2002). The cross-shelf oscillations coincided with internal tide variations in the cross-shelf currents (Figure 10-7); however, the elevated bacteria concentrations did not penetrate inshore of 1 km in any of the surveys. Thus, while internal tides certainly can move the plume cross-shelf, as Boehm et al. (2002) hypothesized, it does not appear that the transport routinely brings bacteria near the surfzone.

Many of the bacterial postings and closures that occur on Huntington Beach are due to exceedances of enterococci. While elevated levels of enterococci exist within the plume, it appears that they may dissipate away from the core of the plume more rapidly than the coliform bacteria. This is contrary to what is often expected. Thus, the many beach closures due to enterococci suggest that a source of bacteria other than the outfall was polluting the beach. This is particularly so when high enterococci counts are not correlated with similarly elevated levels of coliform indicator bacteria.

There is considerable variability in the location of the plume with varying flow regimes over the San Pedro shelf. However, within the context of this study, there was no direct evidence of the plume being transported sufficiently close to shore to contribute to the surfzone contamination that is frequently observed along Huntington Beach.

For an expanded summary of the entire hydrographic plume tracer study see Jones (2004).

10.8. References

Boehm, A.B., B.F. Sanders, and C.D. Winant, 2002. Cross-shelf transport at Huntington Beach. Implications for the fate of sewage discharged through an offshore ocean outfall. Environmental Science and Technology, v. 36, p. 1899-1906.

Jones, B., 2004, Huntington Beach Shoreline Contamination Investigation, Phase III: Hydrographic and plume tracer data report. In preparation.

Orange County Sanitation District, 1999. Huntington Beach Closure Investigation, Phase I: Final Report. Fountain Valley, CA.

Roberts, P.J.W., W.H. Snyder, and D.J. Baumgartner, 1989a. Ocean outfalls: 1. Submerged wastefield formation. Journal of Hydraulic Engineering-Asce, v. 115, p. 1-25.

—. 1989b. Ocean outfalls: 2. Spatial evolution of submerged wastefield. Journal of Hydraulic Engineering-Asce, v. 115, p. 26-48.

—. 1989c. Ocean outfalls: 3. Effect of diffuser design on submerged wastefield. Journal of Hydraulic Engineering-Asce, v. 115, p. 49-70.