3.2 Dissolved oxygen
Organic and inorganic nutrients are the basic food supply essential for maintaining the plants and animals in natural watercourses. Equally essential to aquatic life is a supply of oxygen, needed for respiration. Oxygen dissolved in the water is also needed in the biodegradation of organic matter by aerobic (oxygen-consuming) bacteria. A measure of this oxygen demand can be obtained experimentally and is defined as the biochemical oxygen demand (BOD). The BOD is a measure of the polluting capacity of an effluent due to the oxygen taken up by micro-organisms as they decompose the organic matter it contains.
Oxygen dissolved in natural waters arises from two main sources – the atmosphere and the process of photosynthesis. Atmospheric air containing 21% oxygen by volume can dissolve in water up to a limit. Green plants in the presence of sunlight generate oxygen. These two sources replenish the oxygen used up in aerobic processes by aquatic organisms. The solubility of oxygen in water depends on the temperature, pressure, and the amount of dissolved solids present.
Table 2 shows the solubility of oxygen from air at atmospheric pressure in pure water at various temperatures. This is calculated using the expression Cs = 14.65 – 0.41022T + 0.00791T2 − 0.00007774T3 where Cs is the solubility of oxygen in water at 1 atmosphere pressure, and T is the temperature in degrees Celsius. As can be seen, the solubility decreases with an increase in water temperature.
The solubility Cs is expressed in grams per cubic metre. (This is the same as mg l−1, mg/l or ppm, parts per million. You may like to verify this for yourself.) Cs is the maximum amount of oxygen in grams which can be held in one cubic metre of solution – called the saturation concentration. A value of 5 g m−3 of dissolved oxygen is considered to be the minimum required to support a balanced population of desirable aquatic flora and fauna. When you consider from Table 2 that the saturation concentration of dissolved oxygen at 15°C is only 10.01 g m−3, it is evident that oxygen concentrations need not fall very much before the balance of aquatic life is threatened.
Decreasing the atmospheric pressure above the water decreases the saturation concentration of dissolved oxygen. Therefore, streams at high altitudes are unable to dissolve as much oxygen as those at the same temperature nearer sea level.
Increasing the concentration of dissolved salts also lessens the saturation concentration of dissolved oxygen in water, and the correction which must be subtracted for each gram of total salts per 1000 g of water is also shown in Table 2. It is for this reason that the amount of oxygen needed to saturate sea water is less than that required to saturate fresh water at the same temperature and pressure.
Table 2 Saturation concentration of oxygen in water at different temperatures
|Temperature (°C)||Solubility of oxygen in water Cs (g m−3) in equilibrium with air at 1 atmosphere (1.013×105 N m−2 or 101.3 kPa)||Correction to be subtracted for each degree of salinity (expressed as g total salts per 1000 g water)|
A sample of sea water from the Arabian Gulf, at 30°C, has a total salt content of 44 g per 1000 g water. If the sample is found to be 25% saturated with oxygen, what is the oxygen content in g m−3?
At 30°C, saturated pure water contains (from Table 2) 7.36 g m−3 of oxygen.
Since the sample is highly saline sea water, a correction has to be applied.
Thus, if the sea water were saturated at 30°C, it would have
But since it is only 25% saturated, the oxygen present
The rate at which oxygen dissolves in water is dependent on several factors. One of these, the oxygen deficit (D), is the difference between the saturation concentration of oxygen (Cs) and the concentration of oxygen actually present (C) i.e. D = Cs − C. The oxygen deficit is the driving force for the replenishment of oxygen used up in polluted water. The greater the oxygen deficit is, the greater the transfer rate of oxygen into the water. Other factors important in the dissolution of oxygen in water include the turbulence of the water, its ratio of surface area to volume, the presence of animals and plants in the water, and any chemicals present. These will be discussed later.
A river at a certain location has a dissolved oxygen content of 8.1 g m−3. Using the data given in Table 2, calculate the oxygen deficit, if the river water has a temperature of 10°C.
Figure 8 illustrates how the oxygen concentration varies between the water surface and the interior of a water body when oxygen is consumed in the water. The resultant oxygen deficit causes oxygen to be transferred from the surface into the water body. As mentioned earlier, the greater the deficit, the greater the rate of oxygen transfer into the water.
The rate of oxygen transfer into a water body also depends on the turbulence of the water, since this helps transport oxygen from the surface layers to the main body of the water. Rapidly flowing turbulent streams are therefore able to take up oxygen more rapidly than smoothly flowing, slow ones.
Another factor governing the transfer of oxygen into a watercourse is the ratio of surface area to volume. A large surface area permits a greater diffusion of oxygen into the water. Hence shallow, wide rivers are reoxygenated more rapidly than deep, narrow ones. Agitation increases the ratio of surface area to volume, as, for example, when water flows over dams and weirs, and when waves are produced by strong winds. A further advantage of agitation is the entrainment of air bubbles as air is drawn into the water body.
The amount of oxygen in a water body at any given time depends not only on the characteristics mentioned above but also on biological and other factors. Almost all aquatic animals and plants use oxygen in carrying out their metabolic processes and so are constantly tending to increase the oxygen deficit. If organic pollutants are present, the oxygen deficit is increased further as biodegradation takes place. At the same time as oxygen is being consumed, oxygen replenishment via photosynthesis and natural aeration takes place.
Figure 9 shows graphically the processes of oxygen demand and replenishment. Curve (a) shows the oxygen demand of a polluted water sample. Curve (b) shows the reaeration process observed when oxygen is forced to dissolve in the water due to the oxygen deficit created by the biodegradation taking place. The net result of the oxygen demand and replenishment processes is illustrated by curve (c), which is called the dissolved oxygen sag curve. This is in effect the difference between the demand and replenishment curves.
There are diurnal and seasonal differences in oxygen concentration. Figure 10 illustrates the diurnal variation that may occur. This variation is related to plant growth, light intensity and temperature. The amount of dissolved oxygen rises to a maximum during the day because of photosynthesis occurring in daylight. It decreases through the night because none is produced by photosynthesis, but respiration (using up oxygen) continues as it does during the daylight hours. This extreme diurnal variation occurs mainly between April and October because the lower temperatures during the rest of the year tend to slow down or inhibit the metabolic processes and plants become dormant.
Figure 11 illustrates seasonal changes in dissolved oxygen. An increase occurs in the summer months because of longer days (more daylight) and therefore increased photosynthetic activity.
In some circumstances, oxygen supersaturation can occur, i.e. more oxygen is dissolved in the water than the saturation concentration allows. This occurs because plants produce pure oxygen (whereas air contains 21%). Therefore, when photosynthesis is responsible for the oxygenation of the water, rather than atmospheric aeration, up to five times the saturation concentration is theoretically possible at the same temperature and pressure. In practice, 500% is never attained, but up to 200% has been recorded, in a shallow river with profuse plant growth, on bright sunny days.
Which of the following events would not affect the rate of transfer of oxygen from the atmosphere to a body of water?
A Doubling the oxygen deficit.
B Large amounts of salts being discharged into the water.
C A slight breeze blowing over the water.
D Water flowing over a weir.
E Raising the temperature of the water by 10°C.
C. Only if the wind is strong enough to cause turbulence will it result in an increase in the rate of oxygen transfer across the water-air interface; otherwise it will not significantly affect the rate of transfer. The atmosphere contains approximately 21% oxygen by volume so there is always plenty of oxygen in the air immediately above the interface, even when the air is not moving.
When is the level of dissolved oxygen in a river likely to be at its highest and at its lowest?
In warm weather when aquatic plants are growing rapidly, their photosynthetic processes usually result in a diurnal variation in the concentration of dissolved oxygen; it is high during the day and low at night. During summer days, the water may become supersaturated with dissolved oxygen, especially during the afternoon. After sunset, the oxygen-producing phase of photosynthesis ceases and the concentration of dissolved oxygen declines, generally reaching its lowest levels just before dawn. Variations of up to 10 g m−3 have been recorded in 24 hours.
The oxygen demand as a result of bacterial decay of leaves and other organic matter that have fallen into the water means that levels of dissolved oxygen will also be very low in the autumn.