Part 7: Cold Water and Bacterial Transport

Objective: identify water masses that may contain bacteria from the sewage outfall. Determine whether the presence of these water masses at the beach actually causes bacterial contamination at the beach. Determine whether the sewage-type water at the beach is connected with sewage-type water at the outfall.

What is sewage-type water?

Treated sewage from the OCSD outfall is released on the bottom of the ocean at a depth of 60m. The stuff that comes out of the pipe goes through a diffuser to mix it with the water around it. The mixture of water and treated sewage then spreads and rises, looking somewhat like a plume of smoke. You may remember the crabs in "Finding Nemo" browsing on thin streams of stuff rising from a sewage pipe? The picture is accurate, if not the analogy to "manna from heaven".

The water released from the sewage outfall, as well as containing partially treated sewage, is slightly cooler, fresher and less dense than the water into which it is released. It rises a bit above the pipe, but cannot reach the surface. The ocean is vertically stratified, with warm, less dense water at the surface, and cool, denser water at the bottom. The sewage plume cannot rise above the level where it is the same density as the ocean water.

Now have a look at the first figure on this page. The x-axis is time, spanning July through September, 2001. The y-axis is depth. Red indicates warm water (22oC is a comfortable swimming temperature), and blue indicates cold water. An isotherm is a level where a given temperature of water is found. Can you identify the 14oC isotherm?. Notice that this isotherm rises and falls, and is lowest in early September, when the surface water is warmest. Now look at the jagged white line that marks the upper edge of the sewage plume -- does it look like there is ever sewage in water warmer than 14oC?

Plume stays below
14 degree isotherm
Cold water touches coast
due to winds and internal tides
Cold water appears
after contaminations
Cold fresh water appears
near outfall and at beach
but not in between

When do we see sewage-type water?

If the sewage effluent stays in water colder than 14oC, our next question should be whether that cold water ever reaches the beach. The second figure on this page shows temperatures at several locations on the coast, during a two-week period in summer of 2000. Each line is a different location, and the height of the line shows the water temperature. Note that at some stations, the temperature stays cool for the entire period; these are the offshore stations in deep water. At the coastal stations, the water temperature varies dramatically, from about 21oC down to about 13oC. If you lay a straight edge horizontally across the plot at 14oC on the y-axis, you will see that even the warmest regions experience cool water for a few hours at a time.

The third figure on this page shows the times of bacterial contamination events and also the times when cool water hit the beach. The x-axis is time, covering the same period as the bacterial concentration plot we discussed in the previous section. The vertical bars mark times when the beach was closed due to bacterial contamination, either from coliforms or enterococcus. The blue undulating line shows the amplitude of the tides (not the tides themselves, just the way the size of the tides varies!). Do the bacterial contamination events occur during large or small tides?

The row of blue diamonds on the third figure shows times when cool water (less than 14oC) was observed at the beach. Did this occur during beach contamination events, before the contamination or after the contamination?

Where do we see sewage-type water?

To understand the fourth figure, imagine you are in a plane looking diagonally down through the water. The beach runs along the upper left, and the sewage outfall is near the bottom center of the plot. The water that comes out of the sewage outfall is both cooler and fresher than the ocean above the outfall. Bright red spots show cool, fresh, sewage-type water. On the particular day when these data were collected, sewage-type water was seen right near the outfall, and also near the beach. BUT it was not seen anywhere in between. The scientists carefully reviewed hundreds of plots like this, but could not find any times when the two red blobs of sewage-type water were connected.

The Scientists Concluded that the sewage from the outfall plume was very unlikely to travel to the beach. First, the sewage plume stays in water cooler than 14oC, and somewhat fresher than the ambient ocean water. Water of this temperature was seen on the beach several times, but the cool events occured after bacterial contamination events. They refer to this as a "temporal disconnect". Second, cool, fresh sewage-type water was observed near the beach and the outfall, but never in between. The scientists called this a "spatial disconnect".

Assignment 7:

  1. Is water from the sewage outfall warm or cool?
  2. Is water of the right temperature ever seen at the beach?
  3. Is the sewage-type water observed before or after bacterial contamination events?
  4. When the sewage-type water is observed at the beach, is that water connected with water at the outfall?
  5. Do you believe the scientists conclusion?

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