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Measuring Dissolved Oxygen tells us a lot about Water Quality

Written by Janice M on . Posted in Water & Environmental

We can’t see the oxygen dissolved in water that is essential for aquatic life. It is not the bubbles in water, nor is it the O part of the water molecule H20. Rather, it is a measure of the amount of free oxygen molecules in water.

Too Little Oxygen:

Dissolved oxygen (DO) is a vital indicator of an aquatic ecosystem because it is essential to life – for respiration and other chemical reactions. Low DO levels can affect the creatures that live in a body of water. In fact, an entire aquatic system can "die" if there is not enough DO in the water – this process is called eutrophication.

A low DO level in drinking water makes water taste "flat" – shake up a drink of water that has been standing and the taste will improve.

Too Much Oxygen:

Too much oxygen can also be dangerous. Oxygen saturation (over 100%) can be harmful to aquatic life.

Drinking water and industrial water with a high DO level will cause increased corrosion in the water pipes. For this reason, industrial operators try and keep the DO levels in process water low.

Factors that influence Dissolved Oxygen Levels:

  • Aquatic life – organisms living in water use up some of the DO for life. Bacteria also use oxygen when they decompose organic materials, such as animal waste
  • Temperature – cold water holds more DO than warm water. A measurement taken early in the morning when the temperature is lower may be as much as 1 mg / l higher than a temperature taken at noon
  • Photosynthesis – counteracts the temperature a little as sunlight results in more photosynthesis, which produces oxygen
  • Turbulence – more turbulence creates increased opportunities for oxygen to enter the water
  • Altitude – there is less oxygen in the atmosphere at higher altitude and less DO
  • Minerals / salinity – distilled water can absorb the most oxygen. The more minerals or salt in the water, the less oxygen the water can hold
  • Fertilisers / algal blooms – fertilisers lead to an over-production of algae, which limits the sunlight and plant growth in lower levels of water. Dying plants add to the decay, a lower level of photosynthesis and lower oxygen levels
  • Waste material – human and agricultural waste in water can also use up the oxygen in water to support the increasing amounts of bacteria

Example:

A man-made dam may experience less turbulence than a river. Add a hot summer temperature to the dam and warm, stagnant water, which may also be at a high altitude with less oxygen in the atmosphere, then throw in some dead leaves, fertiliser run-off from the nearby farms and sewage …
These factors can lead to a serious situation with many dead fish and eutrophication, such as the Hartebeespoort Dam in South Africa.

Dissolved Oxygen Measurements:

Dissolved Oxygen is most often measured in-the-field. Various options are available:

Dissolved Oxygen Levels / Standards:

Aquatic Life:

0 – 2 mg / l: DO levels too low to support aquatic life
2 – 4 mg / l: very few fish and aquatic insects can survive
4 – 7 mg / l: good for many aquatic animals, low for cold water fish
7 – 11 mg / l: ideal for most stream fish

A saturation greater than 100% may result in increased algae which can lead to eutrophication.

Effluent / Waste water*:

At least 75% saturation for discharge of effluent water.

* DWA – Requirements for the Purification of waste water or effluent, Act No. 991 – 18 May 1984.

 

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