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I V ** 



dietary intake levels of >100 ppm lead as toxic to cattle. Soukup 

 (1972) found a maximum lead value of 0.044 mg/L in Helena Valley 

 water, well below the permissible criteria of 0.1 mg/L. The 

 possibility of high levels of lead in forage and soil, suggests 

 that the drinking water criteria of 0.05 ppm lead may be most ap- 

 propriate for the Helena Valley. 



The most appropriated hazard level for cadmium concentrations 

 in livestock water of the Helena Valley will depend on cadmium 

 levels found in forage and soils under background conditions. The 

 0.5 ppm criteria reported by the NRC (1974) may be the most 

 applicable. Chaney (1984) and NRC (1980) have given a value of 

 0.5 mg/kg cadmium in forage as the chronic toxicosis tolerance 

 level. However data discussed by Hansen and Chaney (1984) showed 

 that the 0.5 mg/kg cadmium value was based upon conservative 

 estimates for cadmium accumulation in animal livers. They felt 

 that when the Cd:Zn ratio is <1.0%, cadmium in feed may reach 5 

 ppm with little accumulation in liver and kidney tissues of 

 animals. However, the drinking water standard and the FWPCA 

 livestock criteria of 0.01 mg/L may be insufficient to prevent 

 cadmium toxicosis under conditions of heavy contamination. 



Zinc tolerence is high in animals and dietary intake exceed- 

 ing 2000 ppm may be required to produce zinc toxicosis (Puis 

 1981). The 1972 study of the Helena Valley indicated a maximum 

 forage content of 232.0 ppm (dry wt . ) zinc (Hindawi and Neely 

 1972). Soils sampled in the same study contained a maximum of 

 5200 ppm zinc and the mean for sites 0.67 to 10 miles from the 

 smelter was found to be 79 ppm (Miesch and Huffman 1972) . It is 

 apparent that the recommend zinc limit of 25 mg/L for livestock 

 water will provide a sufficient margin of safety except in areas 

 with very high soil contamination. 



No data were found that would document the heavy metal 

 content of snowmelt runoff and its consumption by livestock. 



4.2 Water Quality Levels for Irrigation 



Water quality criteria for irrigation must take into consid- 

 eration the nature of the specific water constituent, soil charac- 



136 



