As has already been noted, it is expedient to provide requirements for 

 the first of the two water uses for fresh water and only for the second for 

 sea water because in these cases these are the highest requirements for 

 water quality. We will examine the level of permissible pollution taking 

 into account these requirements and then the suitability or degree of pollu- 

 tion can be ascertained by comparing the existing water quality to the per- 

 missible concentrations. 



Quality of water is shaped by aquatic organisms on the basis of hydro- 

 chemical and hydrological regimes. Toxicants in the water change the 

 hydrochemical composition of the surface water and have a definite effect, 

 depending on concentrations, on the processes determining its quality. 



Pollutants discharged into surface waters gradually degrade or are 

 transformed to less active states. The degree and rate of breakdown de- 

 pend primarily on the nature of the pollutant, organisms involved in decom- 

 position, time and physical and chemical factors (pH, o , salinity and 

 hardness ) . 



Each of the factors enumerated can accelerate or retard decomposition. 

 Natural organic substances are fairly easily decomposed by bacteria, proto- 

 zoans, fungi and other aquatic organisms. Organochlorine pesticides and 

 detergents created by man and also heavy metals and long-lived radioactive 

 isotopes retain their toxicity for a long time and enter into the food 

 chain. Saprophytic and nitrifying bacteria from the Nitrosomonas and 

 Nitrobacter groups grow and multiply poorly in the presence of toxicants; 

 as a result, the process of recovery of the water is retarded. Figure 1 

 shows data on the effect of triethyl stannic chloride on biological oxida- 

 tion and nitrification. Delay in decomposition of organic substances due 

 to the effect of toxicants results in an increase (accumulation) of pollu- 

 tants. If the toxicants enter waterways (rivers, canals, etc.) in signifi- 

 cant concentrations (see Figure 1), then the water is polluted at great 

 distances from the emission source. The nature of the pollutant and, pri- 

 marily, its capability to be broken down by microorganisms will play a de- 

 cisive role in the degree of pollution. 



The specific composition and number of aquatic organisms play an impor- 

 tant role in removing pollution of water. But they themselves are subject 

 to the effect of toxic agents and therefore their biological activity and 

 number depend on the quality and quantity of the toxicants. All of the 

 vital processes of aquatic organisms and, consequently, the rate of detoxi- 

 fication of the water medium, depend on time. In the final analysis, 

 aquatic organisms break down all toxicants or remove their toxicity, but in 

 what time period? For us it is important now that these processes occur 

 rapidly and completely but we can have little effect on the rate. 



The physical and chemical medium has an essential meaning both for 

 vital activity of aquatic organisms that detoxify, and for the mode of de- 

 composition of toxicants (oxidation, ionization, hydrolysis, etc.). 



