Environmental Engineering Reference
In-Depth Information
If there was no DO production or consumption in a stream or river, it would be expected that the
DO concentration would be that in equilibrium with the atmosphere, the saturation concentration. If
the water concentration was greater than the saturation concentration, then the DO net transfer would
be out of the water, while if the water concentration was lower, the net transfer would be into the water
until equilibrium was once again achieved, a process called reaeration. The rate of the transfer, or the
reaeration rate, depends on the streamlow, wind, and other factors that will be discussed later.
When an organic material is introduced and heterotrophic organisms are available to consume it,
oxygen is consumed (deoxygenation). In a stream or river, the material would also move downstream.
Assuming uniform low conditions, the distance downstream could be estimated as a product of the
average velocity and travel time. If the rate of deoxygenation is greater than the rate of reaeration, the
DO concentration would decline. The decline would continue as the oxygen-consuming materials
move downstream with the low until the rate of deoxygenation equals the rate of reaeration. This
point would be the critical concentration point, where the DO concentration is lowest. Often, the goal
of a wasteload allocation (or TMDL determination) would be to ensure that the critical concentration
is not less than the water quality standard, often with some margin of safety included. The critical
point is not static and could move further upstream or downstream as lows vary.
Beyond the point of the critical concentration, the DO concentration would increase until at some
distance downstream equilibrium conditions with the atmosphere are achieved. However, other pro-
cesses may occur that impact the DO proile, such as other oxygen demands (e.g., sediment demands)
and plant productivity and respiration. Productivity and respiration may also result in diel variations,
so that, for example, in some highly productive systems, oxygen concentrations may exceed saturation
during the day and drop to their lowest levels, following nighttime respiration, in the early morning
hours. Each of these processes will be discussed in greater detail in the following sections.
5.5.1 S
aturatIon
Air contains about 21% oxygen. However, the solubility of oxygen in water is much less. The water
concentration will tend toward an equilibrium condition so that there is no gradient in the partial
pressure of oxygen in the atmosphere and the partial pressure of air in water. The DO saturation
concentration is that which is in equilibrium with atmospheric concentrations. The saturation con-
centration decreases with increasing temperature and dissolved solids concentrations, as illustrated
in Figure 5.21. As a result, for example, waters in warm saline environments would be expected to
have lower DO concentrations than cold freshwater mountain streams. Also, since the partial pres-
sure of oxygen in the atmosphere decreases with altitude, there is a corresponding decrease in the
saturation concentration, as illustrated in Figure 5.22.
T
(°F)
40
50
60
70
80
90
100
16
Freshwater (
S
= 0 ppt)
12
8
4
Saltwater (
S
= 35 ppt)
0
0
10
20
T
(°C)
30
40
FIGURE 5.21
Variations in dissolved oxygen saturation concentrations as a function of temperature.
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