Dissolved oxygen analysis measures the amount of gaseous oxygen (O2) dissolved in an aqueous solution. Dissolved oxygen is one of the most important parameters in aquatic systems. This gas is an absolute requirement for the metabolism of aerobic organisms and also influences inorganic chemical reactions. Therefore, knowledge of the solubility and dynamics of oxygen distribution is essential to interpreting both biological and chemical processes within water bodies. Oxygen gets into water by diffusion from the surrounding air, by aeration (rapid movement) and as a waste product of photosynthesis. The amount of dissolved oxygen gas is highly dependent on temperature. Atmospheric pressure also has an effect on dissolved oxygen. The amount of oxygen (or any gas) that can dissolve in pure water (saturation point) is inversely proportional to the temperature of water. The warmer the water, the less dissolved oxygen. Please see Figure 2 in the discussion of temperature.
Methodology: When performing the dissolved oxygen test, only grab samples should be used and the analysis should be performed immediately. Therefore, this is a field test that should be performed on site.
Most of the sampling teams will use a modified Winkler method for determining dissolved oxygen. This is a multi-step chemical method which involves adding a chemical which reacts with the oxygen or "fixes" it. Other steps include addition of reagents which develop color. Then the amount of that compound is determined by addition (drop by drop) of a second chemical solution of known concentration until a color change occurs. The amount of chemical used in the last step is used to calculate the amount of dissolved oxygen.
If the instrument is available, the YSI oxygen probe may be used to analyze dissolved oxygen. The temperature of the water and the atmospheric pressure must to be known in order to calculate ppm (parts per million) of dissolved oxygen. The oxygen probe contains a solution of potassium chloride (KCl) which will absorb oxygen. As more oxygen is diffused into the solution, more current will flow through the cell. Lower oxygen pressure (less diffusion) will mean less current.
Table IV relates the solubility of oxygen to temperature in fresh water.
Solubility Of Oxygen in Fresh Water (100% Saturation)
| Temperature oC | Dissolved Oxygen
Dissolved Oxygen | 1
| 14.2
| 24
| 8.5 | 2
| 13.9
| 25
| 8.4 | 3
| 13.5
| 26
| 8.2 | 4
| 13.2
| 27
| 8.1 | 5
| 12.8
| 28
| 7.9 | 6
| 12.5
| 29
| 7.8 | 7
| 12.2
| 30
| 7.7 | 8
| 11.9
| 31
| 7.5 | 9
| 11.6
| 32
| 7.4 | 10
| 11.3
| 33
| 7.3 | 11
| 11.1
| 34
| 7.2 | 12
| 10.8
| 35
| 7.1 | 13
| 10.6
| 36
| 7.0 | 14
| 10.4
| 37
| 6.8 | 15
| 10.2
| 38
| 6.7 | 16
| 9.9
| 39
| 6.6 | 17
| 9.7
| 40
| 6.5 | 18
| 9.5
| 41
| 6.4 | 19
| 9.3
| 42
| 6.3 | 20
| 9.2
| 43
| 6.2 | 21
| 9.0
| 44
| 6.1 | 22
| 8.8
| 45
| 6.0 | |
Source: Derived from "Standard Methods for the Examination of Water and Wastewater"
Environmental Impact: In a nutrient-rich water body the dissolved oxygen is quite high in the surface water due to increased photosynthesis by the large quantities of algae. However, dissolved oxygen tends to be depleted in deeper waters because photosynthesis is reduced due to poor light penetration and due to the fact that dead phytoplankton (algae) falls toward the bottom using up the oxygen as it decomposes. In a nutrient-poor water body there is usually less difference in dissolved oxygen from surface to bottom. This difference between surface and bottom waters is exaggerated in the summer in reservoirs, stream-pools, and embayments when thermal layering occurs which prevents mixing. The surface may become supersaturated with oxygen (>100%) and the bottom anoxic (virtually no oxygen). Shallower reservoirs and actively flowing shallow streams generally are kept mixed due to wind action in the shallow reservoirs and physical turbulence created by rocks in the stream beds.
Adequate dissolved oxygen is needed and necessary for good water quality. Oxygen is a necessary element to all forms of life. Adequate oxygen levels are necessary to provide for aerobic life forms which carry on natural stream purification processes. 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 oxygen 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. Aquatic invertebrates are also affected by gas bubble disease but at levels higher than those lethal to fish.
Criteria: Kentucky Water Quality criteria for aquatic life require that the average dissolved oxygen remain above 5.0 mg/L and that the instantaneous minimum not fall below 4.0 mg/L.