Destratification

Stratification or layering of the water column is a progressive disruption to a healthy water body. Deoxygenated water is denser than oxygenated water, so it sinks to the bottom and stays there. Cold water is denser than warmer water, so it sinks to the bottom and stays there.

So colder, less oxygenated water accumulates in the benthic (bottom) zone. Over time more and more layers develop as the level of cold deoxygenated water increases in the water column. Because these layers are heavier and denser they become “locked in” by gravity.

Stratification can, therefore, be quantified by measuring the temperature and dissolved oxygen levels of the water at different depths.

The data in the graph shows that the temperature difference between the surface and the bottom of a 30 feet deep lake is 22°F at the start (76°F at top, 54°F at bottom).

After 6 weeks the temperature difference has halved to 11°F (73°F at top, 62°F at bottom).

After 10 weeks the temperature difference between top and bottom is 2°F (65°F at top, 63°F at bottom). This shows that the water column is destratified.

The data in the graph shows that the Dissolved Oxygen at the surface is 5.15mg/l but only 0.4mg/l at the bottom when remediation began.

After 6 weeks Dissolved Oxygen levels have increased over 500% to 2.2mg/l at the bottom.

After 10 weeks Dissolved Oxygen levels have increased to 5.9mg/l at the bottom. This is greater than the 5.15mg/l at the surface initially. 5mg/l is the recognized minimum for aerobic conditions, so reoxygenation of the whole water column to an aerobic condition has been achieved.