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Pycnocline or thermocline

Density and thermal stratification: The intense heating caused by sunlight results in the formation of a light, warmer layer of water floating on top of a much larger mass of cold, dense water. Where these two bodies of dense water meet there is a zone of rapid change in the water temperature, called the thermocline or picnocline. Such a thermocline is a permanent feature of both tropical and temperature waters. When the surface water of temperature seas is warmed in the summer a further seasonal thermocline can develop much closer to the surface. This is much stronger feature than the permanent thermocline in temperature waters. This thermocline plays an important role in determining seasonal changes in marine primary production in temperature seas. Thermal stratification represents a physical barrier to the mixing of the water column. There is little or no exchange of nutrient through a thermocline.

The stratification phenomena is the existence of two homogeneous water layers and separated by a thin thermocline layer. In such a case, we can say that the water is stratified. There is no exchange of pollutants through this picnocline layer.

Example:

Calculation in both XY-plane (parallel to water surface) and XZ-plane (perpendicular to water surface). The fucshia square represents a point source (position of the outfall pipe). The red colour represents high pollutant concentrations. The effluent flow rate is 0,15 m^3/s, the current velocity is  0,04 m/s and the current angle is 45 (NE). The Stratified model has been used in the calculation. The  fucshia square represents the vertical discharge source (outfall locations).  We have a three-dimensional image of the pollutant plume with these two computer screens.

BOD pollution map produced by continuous discharge in this region. The stratified model has been used. Top: the orange horizontal line indicates us the position of the XZ-Plane. Bottom: the orange horizontal line indicates us the position of the XY-Plane and the blue horizontal line indicates us the position of the water surface plane.  The fucshia square represents a point source (position of the outfall pipe). The red colour represents high pollutant concentrations. The violet horizontal line (bottom) indicates us the position of the pycnocline (or thermocline) layer plane. We can see that there is no exchange of pollutants through the pycnocline layer.