Dissolved Oxygen

Once absorbed, oxygen is either incorporated throughout the water body via internal currents or is lost from the system. Flowing water is more likely to have high dissolved oxygen levels than is stagnant water because of the water movement at the air-water interface. In flowing water, oxygen-rich water at the surface is constantly being replaced by water containing less oxygen as a result of turbulence, creating a greater potential for exchange of oxygen across the air-water interface. Because stagnant water undergoes less internal mixing, the upper layer of oxygen-rich water tends to stay at the surface, resulting in lower dissolved oxygen levels throughout the water column. Oxygen losses readily occur when water temperatures rise, when plants and animals respire, and when microbes aerobically decompose organic matter.

Dissolved oxygen may play a large role in the survival of biota in temperate lakes and reservoirs during the summer months, due to a phenomenon called stratification. Seasonal stratification occurs as a result of water's temperature-dependent density. As water temperatures increase, the density decreases. Thus, the sun-warmed water will remain at the surface of the water body (forming the epilimnion), while the more dense, cooler water sinks to the bottom (hypolimnion). The layer of rapid temperature change separating the two layers is called the thermocline (Smith, 1990).

At the beginning of the summer, the hypolimnion will contain more dissolved oxygen because colder water holds more oxygen than warmer water. However, as time progresses, an increased number of dead organisms from the epilimnion sink to the hypolimnion and are broken down by microorganisms. Continued microbial decomposition eventually results in an oxygen-deficient hypolimnion. If the lake is in a eutrophic state, this process may be accelerated and the dissolved oxygen in the lake could be depleted before the summer's end.

Microbes play a key role in the loss of oxygen from surface waters. Microbes use oxygen as energy to break down long-chained organic molecules into simpler, more stable end-products such as carbon dioxide, water, phosphate and nitrate (Dunne et al., 1978). As the organic molecules are broken down by microbes, oxygen is removed from the system and must be replaced by exchange at the air-water interface.

Each step above results in consumption of dissolved oxygen. If high levels of organic matter are present in a water, microbes may use all available oxygen.

Numerical Categories:

Criteria to maintain designated use:
Designated Use Lowest acceptable DO levels (mg/l)*

Environmental Effects: The introduction of excess organic matter may result in a depletion of oxygen from an aquatic system. Prolonged exposure to low dissolved oxygen levels (<5 - 6 mg/l ) may not directly kill an organism, but will increase its susceptibility to other environmental stresses. Exposure to < 30% saturation (<2 mg/l oxygen) for one to four days may kill most of the biota in a system. If oxygen-requiring organisms perish, the remaining organisms will be air-breathing insects and anaerobic (not requiring oxygen) bacteria (Gower, 1980).

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Recreation: If all oxygen is depleted, aerobic (oxygen-consuming) decomposition ceases and further organic breakdown is accomplished anaerobically. Anaerobic microbes obtain energy from oxygen bound to other molecules such as sulfate compounds. Thus, anoxic conditions result in the mobilization of many otherwise insoluble compounds. The breakdown of sulfate compounds will often impart a "rotten-egg" smell to the water, affecting its aesthetic value and preventing recreational use.

Sources: Low dissolved oxygen levels may occur during warm, stagnant conditions that prevent mixing. (For more information, see Temperature section.) In addition, high natural organic levels will often cause a depletion of dissolved oxygen. (For more information, see Organic Matter section.)

Mode of Transport: Not applicable.

(APHA, 1992)

1. Iodometric Method: Most reliable method. Requires the addition of divalent manganese solution, followed by a strong alkali, in a stoppered glass bottle. The dissolved oxygen oxidizes to manganous hydroxide precipitate. With the addition of iodide, the oxidized manganese reverts back to divalent state, releasing iodide equivalent to original dissolved oxygen content. Iodide is titrated with thiosulfate.
   • Interferences: Oxidizing agents may release iodines from iodides (false positive). Reducing agents may reduce iodine to iodide (false negative). Air entrapped in the sample bottle will also interfere.
   
   
   
2. Azide Method: This method is suitable for samples containing more than 50 ug/l nitrite and not more than 1 mg/l of ferrous iron.
   • Interferences: Oxidizing and reducing agents may provide false positives or negatives.
   
   
   
3. Permanganate Modification: This method is suitable for samples containing ferrous iron.
   • Interferences: High concentrations of ferric iron will interfere. Can be overcome by the addition of 1 ml KF.
   
   
   
4. Alum Flocculation Modification:
   • Interferences: High suspended solids may consume iodide in acid solution. This interference is removable by alum flocculation.
   
   
   
5. Membrane Electrode Method: This method is ideal for field testing. Not applicable for industrial and domestic wastewater. Polarographic or galvanic oxygen-sensitive membrane electrodes are composed of two metal electrodes in contact with a supporting electrolyte that is separated from the test solution by a selective membrane.
   
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   • Interferences: The presence of hydrogen sulfide gas will desensitize the electrode cells.
   
   
   

NCSU Water Quality Group

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