Acute and chronic toxicity of hydrogen cyanide to seven fish species and two invertebrates was determined in dynamic flow-through bioassays. The 96-hr LC50 concentrations of HCN for juvenile fish ranged from 57 micrograms/l for rainbow trout at 10C to 191 micrograms/l for field stock fathead minnows at 15C. The fry and juvenile fish were similar in their sensitivity to HCN with eggs the most tolerant stage. The 96-hr LC50 concentrations for invertebrates ranged from 176 micrograms/l for Gammarus to 2328 micrograms/l for Asellus at 18C. Long-term tests conducted with fathead minnows demonstrated that the concentrations of HCN having no adverse effect on egg production was between 12.9 and 19.6 micrograms/l. Chronic tests with brook trout demonstrated that on the basis of spawning success the maximum acceptable toxicant concentration (MATC) was between 5.7 and 11.2 micrograms/l HCN. Long-term tests with bluegills showed that no spawning occurred at HCN concentrations as low as 5.2 micrograms/l. Chronic experiments with invertebrates demonstrated that the highest concentration of HCN having no adverse effect was between 29 and 40 micrograms/l for Asellus and between 16 and 21 micrograms/l for Gammarus. Experiments conducted with intermittent and sublethal diurnal exposure regimes demonstrated that adverse effects of early growth of the fathead minnow are lessened as the exposure period is reduced. Using the toxic unit approach, it also was demonstrated that the Zn-HCN and ammonia-HCN mixtures were more acutely toxic and the Cr-HCN mixture less toxic than what would be predicted from simply additive interaction. This report covers a period from November 1, 1973 to September 30, 1978.

Grant no. Study was conducted in cooperation with University of Minnesota Agricultural Experiment Station, St. Paul, Minn. Jan. 1979. Includes bibliographical references (pages 110-113).