Thermal Pollution of Water Courses
L. W. CADWALLADER, Vice President
Potomac Electric Power Company,
Washington, D. C.
Rivers and lakes have a natural temperature pattern which, in the absence of
industry and population, varies directly with atmospheric and weather conditions.
In the past, many rivers and lakes in this country have frozen over in the wintertime and reached a natural summertime temperature of perhaps 85 F. This is to
say that in most instances nature alone produces severe temperature variations between summer and winter water temperatures.
The biological significance of temperature has been recognized by students
of the natural sciences as one of the most important characteristics of the aquatic
environment. One of the most significant forces in bringing about evolution
through the geological ages has been variation in temperature. Geographical
areas ofthe earth are characterized primarily by their temperature characteristics.
Thus, we recognize the Arctic and Antarctic zones, the temperature zones, and
the tropical zone. Even within a specific zone, there are seasonal temperature
variations which greatly influence the aquatic environment and the type of aquatic
community which may be found in the various bodies of water. On a smaller
scale, we recognize changes in day and night temperatures.
Because these various temperature cycles are so basic, the physiological
changes and overall behavior of aquatic organisms, as influenced by temperature,
have been studied for many years. Aside from the recognized importance of
temperature on the aquatic organisms, the ease of measuring this environmental
characteristic has influenced the amount of research work performed. It would
not be surprising if a search revealed that temperature effects have been studied
as much as any other single environmental factor.
Although temperature effects have been intensely studied, specific information needed to evaluate the effects of heated discharge on receiving waters is
meager. This is because most of the studies were performed in laboratories, using
a single type of organism or a group of organisms with a limited diversity of the
species. In a natural aquatic environment there exists a great diversity of aquatic
life. This is not simply an uncoordinated mass of living things. Instead, the
aquatic life is organized into an intricately balanced system, often referred to as
the biodynamic cycle. Within this system are found a host of organisms, ranging
from bacteria and algae through the higher forms, such as well known fish and
shellfish.
This biodynamic cycle is a very competitive one, which might be thought of
as a food chain between the lower and higher forms of aquatic life. Actually,
this food chain does not consist of a single series of links, but rather as a series of
chains that are sometimes interlinked. Thus, conditions which break one series
of links do not necessarily destroy the overall cycle. When conditions become
severe, as in the case of extreme pollution, the cycle may become broken and
the higher forms of aquatic life eliminated. In studying the effects of changing
the aquatic environment, we must be concerned with the whole pattern of life,
rather than just one group. Because the largest part of the research background
has not been oriented to show the effects of temperature on species competing
under natural conditions, we lack much of the information needed to evaluate the
effects of heated discharges.