Spectrographic analysis of 969 river samples by the FW-PCA from 1963 to 1965 showed zinc in 80% of all the samples. Concentrations (Pickering 1968) varied from 0.003 to 1.080 mg/l, with 0.136 mg/l reported as the mean concentration in drinking water from 37 U.S. locations (Kehoe, Cholak and Largent 1944). In greater than trace concentrations, zinc is harmful to aquatic organisms. The toxic concentration varies with pH and hardness. The 96hr median tolerance limit (TLm) for fathead minnows is 4.7 mg/l zinc concentration with 8 pH and 50 mg/l hardness, and it is 35.5 mg/l with 6 pH and 200 mg/l hardness (Mount 1966). Long-term tests on minnows in water with 200 mg/l hardness show egg production reduced 50% with the zinc concentration at 0.009 times the 96-hr TLm value (Brungs 1969), an amount lower than that found in some natural streams. Soluble zinc concentrations can be reduced by ion exchange and precipitation processes.
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