Elemental nickel seldom occurs in nature, but nickel compounds are found in many ores and minerals.
As a pure metal it is not a problem in water pollution because it is not affected by or soluble in water. Many nickel salts, however, are highly soluble in water. Nickel toxicities occur in nature in conjunction with high levels of chromium in soils developed serpentine rocks.
Poisoning of human beings by nickel or nickel salts is almost unknown, therefore, data on the toxicity of nickel to man is rare.
Nickel appears to be less toxic to fish and river crabs than copper, zinc, brass and iron.
For stickle backs, however, the lethal limit has been reported as 0.8 mg/L of nickel.
Nickel combines readily with cyanide to form a nickel cyanide complex that is relatively stable. It can be present in water at concentrations greater than 100 mg/L as cyanide without harm to fish life if the water is moderately alkaline.
In acidic waters, however, the complex breaks down and releases HCN.