Water Quality Indicators: Dissolved Oxygen

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Factors that influence Dissolved Oxygen

Factors affecting DO

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How to use this page: This is an indicator page. Examine this page for detail on the indicator and use that information to establish a metric for the indication of water quality. This is the final step in examining a sustainable community for its environmental attributes, water sub-attributes, and finally a water indicator. After completing this page, please go back and review other indicators and see discern metrics and weights for the AHD process.

 

Why is Dissolved Oxygen an indicator of Water Quality?


                   Biologically speaking, the level of oxygen is a much more important measure of water quality than fecal coliforms. Dissolved oxygen is absolutely essential for the survival of all aquatic organisms ( not only fish but also invertebrates suach as crabs, clams, zooplankton, etc). Moreover, oxygen affects a vast number of other water indicators, not only biochemical but esthetic ones like the odor, clarity and taste. Consequently, oxygen is perhaps the most well-established indicator of water quality.


How does Dissolved Oxygen affect Water Quality?


Adequate dissolved oxygen is necessary for good water quality. Oxygen is a necessary element to all forms of life. Natural stream purification processes require adequate oxygen levels in order to provide for aerobic life forms. As dissolved oxygen levels in water drop below 5.0 mg/l, aquatic life is put under stress. The lower the concentration, the greater the stress. Oxygen levels that remain below 1-2 mg/l for a few hours can result in large fish kills. Total dissolved gas concentrations in water should not exceed 110 percent. Concentrations above this level can be harmful to aquatic life. Fish in waters containing excessive dissolved gases may suffer from "gas bubble disease"; however, this is a very rare occurrence. The bubbles or emboli block the flow of blood through blood vessels causing death. External bubbles (emphysema) can also occur and be seen on fins, on skin and on other tissue. Aquatic invertebrates are also affected by gas bubble disease but at levels higher than those lethal to fish. 


What affects Dissolved Oxygen and consequently Water Quality?


The main factor contributing to changes in dissolved oxygen levels is the build-up of organic wastes. Decay of organic wastes consumes oxygen and is often concentrated in summer, when aquatic animals require more oxygen to support higher metabolisms. Accurate data on concentrations of dissolved oxygen (DO) in water are essential for documenting changes to the environment caused by natural phenomena and human activities. Sources of DO in water include atmospheric re-aeration and photosynthetic activities of aquatic plants. Many chemical and biological reactions in ground water and surface water depend directly or indirectly on the amount of oxygen present.
Some pollutants, such as acid mine drainage, produce direct chemical demands on oxygen in the water. Dissolved oxygen is consumed in the oxidation-reduction reactions of introduced chemical compounds, such as nitrate (NO31-) and ammonia (NH41+), sulfate (SO42-) and sulfite (SO32-), and ferrous (Fe2+) and ferric (Fe3+) ions. For a more detailed explanation of factors that influence Dissolved Oxygen please click below:

Factors affecting DO


What level of Dissolved Oxygen is preferable for a sustainable community?


 High DO levels in a community water supply are good because it makes drinking water taste better. However, high DO levels speed up corrosion in water pipes. For this reason, industries use water with the least possible amount of dissolved oxygen. Water used in very low pressure boilers have no more than 2.0 ppm of DO, but most boiler plant operators try to keep oxygen levels to 0.007 ppm or less.
                  Low dissolved oxygen levels indicate an excessive demand on the oxygen of the system. The build up of organic material from human activities is one source of oxygen depletion. Microorganisms in the stream consume oxygen as they decompose sewage, urban and agricultural runoff, and discharge from food-processing plants, meat-packaging plants, and dairies.

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Where to go now that you have reviewed an indicator:

Now that you have reviewed an indicator you should create a metric (see Step 4 of the Analytical Hierarchy Processes) that indicates the importance of this indicator in your decision process. Keep this step in mind as you go through one of the other indicators below. Once you have chosen a metric for each of your indicators, you should decide how they collectively measure the sustainability of water by weighting each indicator (see Step 5 of the Analytical Hierarchy Process). These will be applied in an algorithm (see Step 6 of the Analytical Hierarchy Process) to give you the final measurement for Water and Sustainability.

 

 

Author: Shawn Dayson Shifflett