Only in isolated cases has evidence been presented 

 that selenium occurs in water in sufficient amounts 

 to produce selenosis in man or animals. Moreover, 

 water containing high concentrations of selenium 

 is generally unpalatable to livestock {143). Un- 

 like certain of the other elements considered, 

 selenium poses an additional problem in that it 

 is readily transmitted through the mammary gland 

 to the milk (747). 



Selenium is an essential trace mineral and of 

 special concern in the small safety range between 

 requirement (1 to 2 mg/kg of feed) and toxicity 

 (5mg/kg). 



Sodium: Various salts of sodium occur in con- 

 siderable concentrations in the earth's crust and 

 these may be leached into surface waters. Also, in 

 some areas there is considerable production of 

 sodium salts from deep wells of the petroleum 

 industry. High concentrations of various sodium 

 salts in water are deleterious to both plants and 

 animals. Waters containing 2,700 mg/1 of Na 

 (as NaCl) were toxic to chicks {168) and a 

 threshold limit of 2,000 mg/1 of sodium for live- 

 stock has tentatively been suggested {160). There 

 are considerable differences in the sensitivities of 

 different species of livestock to sodium concen- 

 trations in water. 



Sulfate: A threshold limit of 1,000 mg/1 for 

 sulfates in drinking water has been suggested 

 by Stander {160). There are reports that levels 

 of 2,104 mg/1 of sulfate caused progressive 

 weakening and death in cattle (2 ) and 2,500 mg/1 

 of sulfate caused diarrhea in dogs {19). 



Vanadium: It is questionable that significant 

 levels of vanadium will occur in surface waters. 

 Little data .are available on toxic effects of vana- 

 dium in water per se; however, increased mortality 

 on a seleniferous ration has been attributed to 

 addition of 5 mg/1 of vanadium {115). 



Zinc: There are very many opportunities for 

 contamination of water by zinc, both from natural 

 sources and from its many industrial uses. Animals 

 appear to tolerate significant amounts of zinc. 

 Rats fed water containing 50 mg/1 of zinc show 

 no harmful effects {3, 199). 



Organic Wastes and Algae: A vast number of 

 organic compounds too numerous to list here can 

 find their way into soil and surface waters as con- 

 taminants. Since the most numerous and perhaps 

 the most important of these will be discussed in the 

 section on herbicides and pesticides below, they 

 will not be further described here. Attention is di- 

 rected, however, to contamination of waters used 



for livestock by organic matter, particularly algal 

 growths. 



It is difficult to generalize on effects of algae 

 because they differ markedly. Some types of green 

 algae serve as food for certain aquatic species and 

 their harvest for use as livestock feeds has been 

 suggested. Other types of algae, notably the blue- 

 green type, are patently toxic and can cause death 

 both of aquatic species and of livestock. Probably 

 the first report of livestock poisoning by "water 

 bloom" was recorded in Austraha in 1878 {59) 

 and similar descriptions have appeared since. In 

 late July 1946, numerous deaths occurred among 

 animals drinking algae-contaminated water from 

 upper Des Lacs Lake in North Dakota {25). 

 Canadian studies have implicated Aphanizome- 

 non, Anabaena, and Anacystis blooms in such 

 situations. The first-named genus was much more 

 plentiful than the other two and it was apparently 

 the major factor in toxicity. Animals were reported 

 to have died shortly after drinking water from a 

 lake containing these plants and a suspension of 

 the algae killed laboratory mice and rats within 

 20 hours. 



A freshwater dinoflagellate, Gymnodinium was 

 apparently responsible for mass death of plankton- 

 feeding shad {126). Fish poisoned by phytoplank- 

 ton and consumed by birds have been reported to 

 cause their death {189), presumably a similar 

 fate could befall animals and man. 



Pesticides and Herbicide Residues: Pesticide 

 and herbicide residues have been a cause of con- 

 cern to livestock owners from the time the agri- 

 culturalist first used these materials to protect 

 crops or livestock from pests or disease. The 

 cheapest diluent and spreading agent is water and 

 even relatively insoluble compounds are formu- 

 lated so they may be dispersed in water. Leftover 

 formulations in open containers may be consumed 

 by thirsty livestock, or may enter the water supply 

 through improper dispositions. To a lesser degree, 

 a water supply may be accidently contaminated 

 with these compounds, leading to poisoning. In 

 the presence of microorganisms, silt, or other 

 colloidal or suspended matter in water, many com- 

 pounds accumulate in the nonaqueous substances. 

 These, rather than the water itself, when assimi- 

 lated, provide the poisoning effects which assume 

 increasing importance in water supplies today. 

 To date, however, no reported example has been 

 found of toxicity in livestock due to pesticides or 

 herbicide contaminants of water supplies in 

 general. 



Pesticides and herbicides, along with other com- 

 pounds which have dangerous properties when out 



137 



