has little buffering capacity (low total alkalinity). 

 Other natural waters with a pH of 9.5 also support 

 fish, but in such situations the waters are not 

 regarded as highly productive. 



Acids that dissociate to a high degree do not 

 appear to be toxic at pH values above 6.0. They 

 are toxic if added in sufficient quantities to reduce 

 the pH to less than 6.0 Acids that dissociate to a 

 low degree are often toxic at pH values consider- 

 ably above 6.0. In the latter condition, toxicity is 

 due either to the anion or to the compound itself; 

 e.g., hydrogen cyanide (HCN), hydrogen sulfide 

 (H2S), and hypochlorous (HCIO) and tannic 

 acids. 



Alkalies that dissociate to a high degree do not 

 appear to be toxic at pH values below 9.0. Alka- 

 line compounds that dissociate to a low degree are 

 often toxic at pH values less than 9.0 and their 

 toxicity is due either to the cation or to the undis- 

 sociated molecule. Ammonium hydroxide is an 

 example. Temporarily high pH levels often are 

 produced in highly productive waters through pho- 

 tosynthetic activity of the aquatic plants by con- 

 verting the carbonate to the hydroxide, which re- 

 sults in an increased pH. Because these high pH 

 levels prevail for only a few hours, they do not 

 produce the harmful effects of continuous high 

 levels due to the presence of strong alkalies. 



Addition of either acids or alkalies to waters 

 may be harmful not only in producing adverse acid 

 or alkaline conditions, but also by increasing the 

 toxicity of various components in the waters. The 

 addition of strong acids may cause the formation 

 of carbonic acid (free CO2) in quantities that are 

 adverse to the well-being of the organisms present. 

 A reduction of about 1.5 pH units can cause a 

 thousand-fold increase in the acute toxicity of a 

 metallo-cyanide complex. The addition of strong 

 alkalies may cause the formation of undissociated 

 NH4OH or un-ionized NH3 in quantities that may 

 be toxic. The availability of many nutrient sub- 

 stances varies with the acidity and alkalinity. At 

 higher pH values, iron tends to become unavail- 

 able to some plants. 



The nonlethal limits of pH are narrower for 

 some fish food organisms than they are for fish. 

 For example, Daphnia magna does not survive 

 experimentally in water having a pH below 6.0. 



The major buffering system in natural waters is 

 the carbonate system. This system not only neutra- 

 lizes acids and bases so as to reduce the fluctua- 

 tions in pH, but also forms an indispensable reser- 

 voir of carbon for photosynthesis, because there is 

 a decided limit on the rate at which carbon dioxide 

 can be obtained from the atmosphere to replace 



that in the water which becomes fixed by the 

 plants. Thus the productivities of waters are 

 closely correlated with the carbonate buffering 

 systems. The addition of mineral acids preempts 

 the carbonate buffering capacity and the original 

 biological productivity is reduced in proportion to 

 the degree that such capacity is exhausted. It is as 

 necessary, therefore, to maintain the minimum es- 

 sential buffering capacity as it is to confine the pH 

 of the water within tolerable limits. 



Recommendation: (1) In view of the above con- 

 siderations and their importance for the production and 

 well-being of aquatic organisms, no highly dissociated 

 materials should be added in quantities sufficient to 

 lower the pH below 6.0 or to raise the pH above 9.0. 



(2) To protect the carbonate system and thus the 

 productivity of the water, acid should not be added 

 sufficient to lower the total alkalinity below 20 mg/1 

 expressed as CaCOs. 



(3) The addition of weakly dissociated acids and 

 alkalies should be regulated in terms of their own 

 toxicities as established by bioassay procedures. 



Hardness 



Hardness was originally considered as the 

 capacity of water to precipitate or neutralize soap. 

 In natural waters, hardness is chiefly attributable 

 to calcium and magnesium ions. Other ions, such 

 as strontium, barium, aluminum, manganese, iron, 

 copper, zinc, and lead also are responsible for 

 hardness, but since they are present in relatively 

 minor concentrations, their role usually can be 

 ignored. Hardness, like acidity and alkalinity, is 

 expressed in terms of CaCOa but the hardness of 

 a water is not necessarily equal to either the acidity 

 or alkalinity. Hardness in natural waters is gener- 

 ally correlated with dissolved solids but there are 

 exceptions. 



Generally, the biological productivity of a water 

 is directly correlated with its hardness, but hard- 

 ness per se has no biological significance because 

 productivity depends on the specific combination 

 of elements present. Calcium and magnesium con- 

 tribute to hardness and to productivity. Most other 

 elements that contribute to hardness reduce biolog- 

 ical productivity and are toxic when they produce 

 a substantial measure of hardness. Because hard- 

 ness of itself has no biological significance, and 

 because some elements which contribute to hard- 

 ness may enhance biological productivity (while 

 other contributing elements are toxic), it is rec- 

 ommended that the term hardness be avoided in 

 dealing with water quality requirements for aquatic 

 life. 



41 



