number of substances will be beneficial. In this 

 way, many water courses have a capacity to ab- 

 sorb materials to advantage. But the addition of 

 what may be considered beneficial substances must 

 be controlled so that they will not exceed favorable 

 limits. 



The osmotic concentration of the body 'fluids of 

 a fresh water animal is generally the maximum 

 concentration of dissolved material that the ani- 

 mal will tolerate. In some animals, notably some of 

 the fresh water moUusks, the body fluids have an 

 osmotic concentration as low as 50 milliosmoles 

 (the equivalent of about 0-025 molar or 1,500 

 mg/1 sodium chloride). If the dissolved materials 

 are relatively innocuous, having only an osmotic 

 effect, it is judged that the total dissolved materials 

 in a water course may be increased to a certain 

 extent but they should not exceed 50 milliosmoles 

 if the fauna is to be maintained. 



Many species of diatoms are very sensitive to 

 changes in chloride and other salt concentrations. 

 Some species, such as those in mountain streams 

 and in black water streams of the coastal plains, 

 can live only in waters with extremely low concen- 

 trations of salts. The addition of salts to such 

 streams will eliminate many desirable species of 

 diatoms and permit undesirable species to flourish. 

 Such changes may reduce the desirable food 

 sources and bring about nuisance problems as 

 well. It is believed that the total dissolved mate- 

 rial in a water course should not be increased by 

 more than one-third of that which is characteristic 

 of the natural conditions of such a water course. 



The toxicity of substances added to natural 

 waters often depends on the substances already 

 present in the receiving waters. With synergism, 

 the toxicity increases, and with antagonism it de- 

 creases. Again the reaction of the toxic substances 

 may produce, in some cases, new products of 

 greater toxicity, and in others, products of lesser 

 toxicity. 



In view of the many factors that become in- 

 volved in the disposal of soluble materials in na- 

 tural waters, it is evident that no simple answer is 

 available. Therefore, bioassays should be used to 

 determine the amounts of the materials that may 

 be tolerated without reducing the productivity of 

 the water course in question. 



Recommendation: Dissolved materials are of two 

 types: those that are toxic at very low concentrations 

 and those, such as the salts of the earth metals, that 

 are required in certain concentrations for a productive 

 water and become harmful only at high concentrations 

 by exerting an osmotic effect. If the dissolved materials 

 are relatively innocuous, i.e., their harmful effect is an 

 osmotic one at high concentrations, it is judged that the 

 total dissolved materials of this type may be increased 



to a certain extent but they should not exceed 50 mil- 

 liosmoles in waters where diversified animal popula- 

 tions are to be protected. Further, to maintain local 

 conditions, total dissolved materials should not be in- 

 creased by more than one-third of the concentration 

 that is characteristic of the natural condition of the 

 water. When dissolved materials are being increased, 

 bioassays and field studies should be used to determine 

 how much of the materials may be tolerated without 

 reducing the productivity of the desired organisms. 



Acidity alkalinity, and pH 



Acidity and alkalinity are reciprocal terms. 

 Acidity is produced by substances that yield hydro- 

 gen ions on hydrolysis and alkalinity is produced 

 by substances that yield hydroxyl ions. Other defi- 

 nitions state that a substance is acid if it will neu- 

 tralize hydroxyl ions and a substance is alkaline if 

 it will neutralize hydrogen ions. The terms "total 

 acidity" and "total alkalinity" are often used to 

 express the buffering capacity of a solution. Acidity 

 in natural waters is caused by carbon dioxide, min- 

 eral acids, weakly dissociated acids, and the salts 

 of strong acids and weak bases. Alkalinity is 

 caused by strong bases and the salts of strong 

 alkalies and weak acids. 



An index of the hydrogen ion activity is pH. 

 Even though pH determinations are used as an 

 indication of acidity and/or alkalinity, pH is not a 

 measure of either. As pointed out in the first sen- 

 tence in the previous paragraph, acidity and al- 

 kalinity are reciprocal terms. Indeed, a water may 

 have both an acidity and alkalinity at the same 

 time. Total acidity, by definition, is the amount of 

 standard alkali required to bring a sample to pH 

 8.3. Total alkalinity, similarly, is the amount of 

 standard acid required to bring a sample to pH 

 4.5. Both are expressed in equivalents of CaCOg. 

 Under these circumstances, there is a relation- 

 ship between pH, acidity, and alkalinity since, by 

 definition (see Standard Methods for the Exami- 

 nation of Water and Wastewater, 12th edition. 

 1965), any water with a pH of 4.5 or lower has no 

 measurable alkalinity and a water with a pH of 

 8.3 or higher has no measurable acidity. 



In natural waters, where the pH is in the vicinity 

 of 8-3, acidity is not a factor of concern. In most 

 productive, fresh, natural waters, the pH falls in 

 the range between 6.5 and 8.5 (except when in- 

 creased by photosynthetic activity). Some aquatic 

 organisms have been found to live at pH 2 and 

 lower and others at pH 10 and higher; however, 

 such organisms are relatively few. Some natural 

 waters with a pH of 4 support fish and other or- 

 ganisms. In these cases the acidity is due primarily 

 to carbon dioxide and humic acids and the water 



40 



