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Major 2:22 PM 1/28/2003Findings 5.2

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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Abstract
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

Contacts

5. Major Findings (Continued)


5.2. Outfall Plume Tracking

5.2.1. Spatial Patterns of the plume
During the summer of 2001, the distributions of water properties that act as tracers of the plume were mapped during 6 spring tides between May and September 2001 (Figure 1a. Map of the region, the mooring sites and surfzone sampling stations. Sediment transport instruments are deployed at HB03, HB05, HB07, and HB11. The meteorological buoy is at HB07. Inset shows instrumentations of a typical mooring. Figure 1a - 2.41MB PDF file ). Periods of maximum tidal range (spring tides) were selected as beach contamination is most common during spring tides. The plume tracers were mapped repeatedly for 48 hours during each period. The region between 400 m and 3.3 km (0.25 to 2 miles) offshore of the beach was mapped at 4-hour intervals with CTD profiles, resulting in 12 realizations during each period. The region between 3.3 km (2 miles) and beyond the shelf break was mapped with a towed undulating vehicle every 8 hours, which produced up to 6 three-dimensional maps of the offshore region during each period.

The OCSD outfall plume was defined by a combination of a low-salinity anomaly and elevated fecal indicator bacteria. In waters less than 14ÁC, salinities more than 0.005 below what would be expected from temperature are taken as an indication of plume presence (Figure 7. T-S plot for all of the TUVtransects for Cruise 5, August 19-21, 2001. In this case, upcurrent was upcoast from the outfall. The equations describe the lines in T-S space and are used to calculate the salinity anomaly. In this example, anomalously low salinities below 14.5°C are due to the effluent plume and above 17°C result from the influence of surface runoff into the coastal ocean. Figure 7 - 231KB PDF file ). Within this temperature region, bacteria abundances greater than the detection limits were considered to be due to the outfall plume.

Both measurements and subsequent predictions from a model that used the continuously measured density field from the mooring near the outfall site (HB07) during the summer of 2001 showed that the outfall plume remained submerged below the thermocline over the 4 months of the program. The average depth of the top of the plume was 31m (102 ft) beneath the surface. An earlier measurement period that preceded the Phase III, during late August and early September 1999, the plume never rose to less that 14 m (46 ft) even when the stratification was weak.

In most cases the plume remained offshore and extended along the coast in the direction of the measured currents. For example, during the third cruise (July 5-7), the plume was submerged and remained below depths of 25 m (82 ft) in the offshore region. Because the flow below 30 m (98 ft) was weak and variable, the plume was advected both up and downcoast from the outfall diffuser (Figure 8. Spatial distribution of the salinity anomaly showing the distribution of effluent plume during a period of weak alongshore flow during July 5-7, 2001. The effluent plume is clearly seen offshore and beneath 30 meters depth, centered on the outfall which extends from the beach to about 60 meters depth. The diffuser is the portion of the line underneath the offshore end of Tow 8. Another region of low salinity water is found at the surface nearshore. This water is runoff probably from Los Angeles and San Gabriel Rivers. Figure 8 - 2.52MB PDF file ). It appeared that the plume extended shoreward into Newport Canyon and may have shoaled somewhat but still remained below 20 m (66 ft) water depth.

An additional low salinity feature was often present at the sea surface in the nearshore region of our study area (Figure 8. Spatial distribution of the salinity anomaly showing the distribution of effluent plume during a period of weak alongshore flow during July 5-7, 2001. The effluent plume is clearly seen offshore and beneath 30 meters depth, centered on the outfall which extends from the beach to about 60 meters depth. The diffuser is the portion of the line underneath the offshore end of Tow 8. Another region of low salinity water is found at the surface nearshore. This water is runoff probably from Los Angeles and San Gabriel Rivers. Figure 8 - 252MB PDF file ). This feature was found 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 (1.2-1.9 miles). This low salinity layer is much warmer than the plume and possibly represents runoff from the Los Angeles and San Gabriel Rivers that has been transported downcoast by near-surface currents.

5.2.2. Bacteria Concentrations
An important objective of this study was to determine whether cross-shelf currents transported the outfall plume to the beach. Cross-shelf currents can move the plume shoreward, especially during the time when the alongshore currents below 30 m (98 ft) depth are relatively weak, as was observed during the third cruise (Figure 9. Concentration of E. Coli plotted as lines parallel with the shore. The top panel shows the distribution of coliforms on the beach during the four hours of this sampling cycle. The vertical axis in this panel is time and the horizontal axis is distance alongshore relative to the Santa Ana River. The remaining panels show the concentration of E. Coli in the water column offshore from the beach. In these panels the vertical axis is depth and the horizontal axis is the same as in the top panel. Figure 9 - 604KB PDF file ). During this survey, the lowest concentrations of bacteria were found on Line 2, which is 0.8 km (0.5 miles) from the beach. Higher bacteria concentrations were observed inshore along Line 1 (0.4 km from shore (0.25 miles)) and in the surfzone. Higher concentrations were also observed offshore, along Line 3 (2 km from shore (1.2 miles)) and Line 4. This pattern was typical of the entire set of cruises. The lowest concentrations of bacteria were always found along Line 2, even though there may have been exceedances of AB411 standards offshore and inshore of this line. The inference from this is that bacteria from the plume may extend shoreward as close as 2 km (1.2 miles) from the beach, but the bacteria do not appear to cross into the region within 1 km (0.6 miles) of the beach. The higher bacteria concentrations found 0.4 km (0.25 miles) from shore along Line 1 are likely associated with the high concentrations observed on the beach.

Fecal indicator bacteria in the submerged effluent plume were well correlated with each other. The ratio of total coliform: fecal coliform: Enterococcus is typically about 25:5:1 in the main body of the plume. This ratio is relatively constant with distance away from the outfall discharge. This suggests that die-off rates for the 3 bacterial species in the plume may be relatively uniform in this environment. In earlier plume tracking surveys (MEC, 2001), high bacteria concentrations were observed at least 12.5 km (7.8 miles) downcoast from the outfall. Transit times from the outfall were estimated to be on the order of 2 days. Both these results suggest that the die-off rates of the bacteria may be low in the center of the plume, where the waters are dark and cold.

The ratios among the 3 fecal indicator bacteria become more variable at lower bacterial concentrations. Enterococcus appears to decrease more rapidly compared to total coliform bacteria when the Enterococcus concentrations falls below 100 MPN/100 ml and total coliform bacteria fall below 1000 MPN/100 ml. Whether this is real or an artifact of increased variability at lower concentrations is uncertain.

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URL of this page: http://pubs.usgs.gov/of/2003/of03-62/objectives.html
Maintained by: Michael Diggles
Created: January 23, 2003
Last modified: March 10, 2005 (mfd)