Hypotheses

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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2. Hypotheses

Due to strong stratification, field observations have shown and plume models predict that OCSD‰s effluent plume remains trapped below the thermocline in the summer season (MEC final report, 2001). Hence, the primary hypothesis in this program is that coastal ocean processes that transport water and suspended material below the thermocline have the greatest potential to transport the OCSD plume into the nearshore region (water depths of 10-15 m (33-49 ft) during the summer months. If the plume was carried into the nearshore region, it was possible that local beaches were at risk of significant bacterial contamination.

Internal tides
Internal tides are the most likely coastal-ocean process that could transport the submerged wastewater plume into the nearshore regions. Once nearshore, other possible pathways could bring the water from the nearshore region into the surfzone. These nearshore pathways include:
  a. If the effluent plume is transported into the nearshore region, it could enter the proximity of the AES Corporation power plant cooling water intake and discharge pipes and be entrained by the intake and/or discharge jet. Through either pathway, the effluent would then contaminate the thermal plume, which could easily be moved onshore through buoyant spreading, wind forcing or other processes.
  b. Shoaling, or run-up, of the internal tides could transport sub-thermocline water potentially contaminated with effluent into the surf zone and onto the beach ( Figure 2 - 114KB PDF file).

Other subsurface transport mechanisms
Subsurface cross-shelf transport by other coastal ocean processes could move the submerged plume to the nearshore region. Possible transport mechanisms include:
  a. Upwelling of sub-thermocline water driven by alongshore, low-frequency winds
  b. Forced swash of sub-thermocline waters driven by diurnal winds.
  c. Transport via Newport Canyon.
The effluent plume may be transported into Newport Canyon by downcoast currents, then upwelled out of the canyon. Upcoast flow in the nearshore region may transport this water toward Huntington Beach.
  d. Vertical mixing
The subsurface effluent plume could be mixed into the near-surface layers of the coastal ocean by winds, breaking internal waves or other ocean-mixing processes. The contaminated water may then move more easily ashore as it is no longer trapped beneath the thermocline.
  e. A combination of specific subtidal transport events
For example, if subtidal flows near the outfall are weak or stalled, the effluent would not be carried as quickly as it is normally carried from the region. Hence, a larger volume of effluent is potentially available to be transported toward the beach.

Sediment transport processes
Particulate mater discharged by the OCSD outfall may settle out near the outfall and may contain high levels of bacteria. Surface or internal waves could resuspend this bottom sediment and the particle-bound bacteria. Cross-shelf transport processes may then bring this material toward the shore. The research program was designed to monitor this process, although the TAC ad-hoc committee thought that sediment-transport processes were not the most likely mechanism for transporting effluent bacteria to shore because fine material normally moves from shallow to deeper water.

Buoyant particles
It is possible that buoyant particles (e.g., oil and grease) discharged at the outfall could reach the surface even when the water column is stratified; bacteria may adhere to these particles. The TAC ad-hoc committee discussed transport processes in the sea-surface microlayer, but did not think these processes would account for either the high values or the spatial and temporal patterns of bacterial contamination of the beach. Monitoring data from OCSD have shown poor association (r2=0.08) between grease, oil and fecal coliform bacteria in the offshore region (SAIC and MEC, 1991). Shoreline microbiology sampling have shown that the presence of grease particles on the beach is rare. Therefore, this element was not included as design criteria for either the hydrographic or moored sampling programs.

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