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