Since organophosphorus pesticides are easily dissolved in water, heavy 

 rains contribute to their intensive runoff from agricultural fields to re- 

 servoirs. For example, after a rainfall of 2.1 vm, the phosalon content of 

 the water body located near an orchard treated with this chemical exceeded 

 the permissible concentrations by 7 to 9.6 times, and after a rainfall of 

 21.1 nm a 12-fold excess was reported (Ivantshenko 1978). 



Decomposition of organophosphorus compounds in water compared with or- 

 ganochlorine compounds occurs very rapidly (Table 2). The time of degrada- 



TABLE 2. PERSISTENCE OF SELECTED ORGANOPHOSPHORUS PESTICIDES IN WATER 



tion depends on the concentration of hydrogen ions, and temperature 

 (Melnykov, et al_. 1977); and it is dependent upon the number of bacteria de- 

 composing these compounds (Ulyanova, et al_. 1979). 



Both the intensity and duration of effects upon water bodies are pri- 

 marily determined by the length of time that pesticides stay in the soil of 

 catchment areas. Depending on the type of soil, humidity, and pH, pesti- 

 cides may be retained for extended periods of time and with surface water 

 flows, enter reservoirs (Table 3). 



Organophosphorus pesticides in concentrations most commonly found in 

 water bodies, show a high toxicity for aquatic animals, especially for 

 planktonic invertebrates and aquatic insects (Table 4). In 48-hour expo- 

 sures to 0.001 mg/l solutions of malathion, Simocephalus vetulus became less 

 mobile and died after being placed in freshwater for recovery. The 48-hour 

 LC50 for the eggs of carp is approximately 0.01 mg/£, but for their larvae 

 the value is ten times as high (Prokopenko, et aj_. 1976). Eight day larval 

 forms of freshwater invertebrates demonstrate depression changes after three 



85 



