silicon for the diatoms has allowed the blue-greens to proliferate resulting in less
competition to use the large amounts of nitrogen and phosphorus contributed from
the Mississippi River.
11 Thermal Impacts on the Aquatic Environment
With increasing human activities, and the accompanying increasing demand for
energy, more and more waste heat is being produced. Where possible, means are
being considered for the utilization of this waste heat. In other instances, the heat is
merely wasted to the environment. The two main categories of the environment
receiving this waste heat are the atmosphere and the water. Discharge of heat into the
atmosphere may even impact lakes or streams by changing precipitation patterns. If
dry cooling towers are used, the atmosphere is heated and the relative humidity
thereby decreases. On the other hand, if wet cooling towers are used, significant
amounts of water vapor are passed into the atmosphere, thereby increasing the
relative humidity and increasing the possibility of precipitation downwind. In
terms of discharge into bodies of water, this can directly increase the temperature
of the body of water, resulting in increased evaporation and loss of water in the
surface lakes and streams.
Excess heat in the aquatic environment may be considered to be another form of
pollution: a resource out of place. Every human action produces heat in some form or
another. It is the main source of energy and energy itself is usually expended in the
form of heat: friction produces heat. The problem is compounded by the fact that
industries are expanding and concentrating. When there were many small mills
spread around the country, there was no serious problem. However, because of the
economy of scale, there are now fewer but much larger mills located at a few choice
locations. Thus, the same amount of heat may be produced, but it is discharged into a
much more limited local area where it has a greater impact. One of the greatest
sources of heat is from electrical power generation. A major problem in a thermal
power generation system is the efficiency of utilizing the heat. For fossil fuels, the
efficiency is only about 40%, whereas for a nuclear fuel the efficiency is only 33%.
Thus, over half of the heat that is generated to produce the electricity must be
discharged in some satisfactory manner. Much effort has been expended in finding
uses for this waste heat. However, particularly with nuclear power reactors, the heat
is frequently produced in a location remote from where it can be readily used. The
losses in transmitting such heat long distances preclude its use at distant locations.
Thus large amounts of waste heat frequently reach the aquatic environment.
There are many problems that occur as the temperature of a lake or stream is
increased. Some of these are obvious, but some represent problems that are difficult
to assess. Probably the simplest problem that can be seen is the reduction in the
saturation value of dissolved oxygen in a stream at an elevated temperature. At
20
C, approximately 9.2 mg/L of oxygen is the saturation value. At 30
C this is
7.6 mg/L; at 40
C it is 6.6 mg/L. Thus it may be readily seen that there is less
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
283
competition to use the large amounts of nitrogen and phosphorus contributed from
the Mississippi River.
11 Thermal Impacts on the Aquatic Environment
With increasing human activities, and the accompanying increasing demand for
energy, more and more waste heat is being produced. Where possible, means are
being considered for the utilization of this waste heat. In other instances, the heat is
merely wasted to the environment. The two main categories of the environment
receiving this waste heat are the atmosphere and the water. Discharge of heat into the
atmosphere may even impact lakes or streams by changing precipitation patterns. If
dry cooling towers are used, the atmosphere is heated and the relative humidity
thereby decreases. On the other hand, if wet cooling towers are used, significant
amounts of water vapor are passed into the atmosphere, thereby increasing the
relative humidity and increasing the possibility of precipitation downwind. In
terms of discharge into bodies of water, this can directly increase the temperature
of the body of water, resulting in increased evaporation and loss of water in the
surface lakes and streams.
Excess heat in the aquatic environment may be considered to be another form of
pollution: a resource out of place. Every human action produces heat in some form or
another. It is the main source of energy and energy itself is usually expended in the
form of heat: friction produces heat. The problem is compounded by the fact that
industries are expanding and concentrating. When there were many small mills
spread around the country, there was no serious problem. However, because of the
economy of scale, there are now fewer but much larger mills located at a few choice
locations. Thus, the same amount of heat may be produced, but it is discharged into a
much more limited local area where it has a greater impact. One of the greatest
sources of heat is from electrical power generation. A major problem in a thermal
power generation system is the efficiency of utilizing the heat. For fossil fuels, the
efficiency is only about 40%, whereas for a nuclear fuel the efficiency is only 33%.
Thus, over half of the heat that is generated to produce the electricity must be
discharged in some satisfactory manner. Much effort has been expended in finding
uses for this waste heat. However, particularly with nuclear power reactors, the heat
is frequently produced in a location remote from where it can be readily used. The
losses in transmitting such heat long distances preclude its use at distant locations.
Thus large amounts of waste heat frequently reach the aquatic environment.
There are many problems that occur as the temperature of a lake or stream is
increased. Some of these are obvious, but some represent problems that are difficult
to assess. Probably the simplest problem that can be seen is the reduction in the
saturation value of dissolved oxygen in a stream at an elevated temperature. At
20
C, approximately 9.2 mg/L of oxygen is the saturation value. At 30
C this is
7.6 mg/L; at 40
C it is 6.6 mg/L. Thus it may be readily seen that there is less
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
283