STREAM POLLUTION 
403 
Arsenic may be cited as an example of a substance which can be accumulated by the 
fish via the gastrointestinal tract after long exposure to low concentrations. 
To summarize, on the basis of locus of injury to the fish, stream pollutants may 
be classified as — 
(1) Those substances injurious to the gills and other external structures of the 
fish without any marked absorption beyond the gills. Death of the fish exposed to 
these pollutants results from anoxemia, through respiratory and circulatory failure, 
and through interference with the excretory functions of the gills. 
(2) Those substances killing the fish by specific toxic action after absorption 
through the gills, the lining of the mouth, and other external structures. 
(3) Those substances killing the fish by specific toxic action after absorption 
from the gastrointestinal tract, which region they have reached in water swallowed 
by the fish. 
LETHALITY OF SPECIFIC SUBSTANCES OCCURRING IN 
STREAM POLLUTANTS 
GENERAL CONSIDERATIONS 
In this section, data from the literature and from experimental assays made dur- 
ing the present studies are presented covering the lethality of various substances 
which are the active harmful agents in different types of stream pollutants. These 
biochemical and biophysical findings in general may be applied to pollution condi- 
tions ranging from acute to mild; although in the evaluation of all individual cases 
of pollution, caution must be exercised, as biological data of any sort are rarely suit- 
able for rigid application. One particular component of a given waste may consti- 
tute the major pollution hazard of that waste; in fact, the dangerous qualities of many 
wastes center largely around one substance in each effluent rather than the effluent 
mixture as a whole. In such cases, lethality tables are particularly helpful, but the 
synergistic and antagonistic actions of other substances in the stream water as well 
as in the waste itself must be borne in mind always. For example, the lethality of 
copper sulphate for fishes under certain conditions is decreased by the presence of 
calcium chloride and increased by sulphuric acid, and both of these compounds occur 
free or combined in certain industrial wastes. Again the toxic actions of small 
amounts of chlorine and of ammonia on aquatic life singly are different from the ac- 
tions of the same quantities of these substances when both are present, since together 
they may form either ammonium chloride or chloramine, depending upon conditions; 
and the toxicity of the resultant mixture may vary accordingly. 
From the data presented in the following tables, however, the harmless, injurious, 
and lethal ranges of various substances may be computed readily with reasonable 
accuracy for particular pollution cases if due regard be given to the chemical and 
physical conditions of the water of the stream involved. Tests should be made, of 
course, to detect the presence of unusual and unexpected substances in the local com- 
plex which might alter the action of the pollutant. 
In addition to the appraisal of the pollutant on the basis of the lethal action of its 
major component or components, when the pollution is acute, mild, or intermittent, 
