Mercury, silver, arsenic, cadmium, chromium, copper, lead, nickel and zinc are heavy metal compounds present in our waters. They are toxic to man and aquatic organisms in varying degrees. They can be serious pollutants because these stable compounds can have persistent toxic effects for many years following their deposit.
The heavy metal compounds, chromium, cadmium, mercury and lead have no known biological function in animal life and can act synergistically with other substances to increase toxicity. Some toxic effects are cumulative and are harmful to the degree that the dosage and resultant concentrations approach lethal thresholds.
The fishery industry has sustained economic losses in recent years when unacceptable levels of mercury or other heavy metals were discovered in fish from contaminated waters.
Each of the heavy metals is discussed briefly below showing specific levels that should not be exceeded.
Arsenic is found to a small extent in nature in elemental form. It occurs mostly in the form of arsenites, of true metals or as pyrites. Arsenic is cumulative in the tissues of many organisms and eventually exerts its effects even though the natural level is low.
Sources of arsenic are from mining debris and wastes and certain insecticides and herbicides.
U.S. Public Health Service drinking water standards recommend an upper limit of 0.05 mg/L of arsenic. Aquatic organisms seem to be able to tolerate levels up to 2 mg/L. with respect to lower forms of aquatic life, arsenic concentrations of 3 to 14 mg/L have not harmed may fly nymphs and 10 to 20 mg/L have been harmless toward dragon and damsel flies.
Concentrations of 2 to 4 mg/L of arsenic are reported not to interfere with self purification of streams. Bacteria grow even in the presence of 10,000 mg/L of potassium arsenate and algae are not killed at 1,000 mg/L of arsenate.
Cadmium in its elemental form is insoluble in water. It occurs largely as the sulfide which is often an impurity in zinc ores. Cadmium is more likely to be precipitated at high pH values. The toxicity of cadmium towards fish and other aquatic organisms indicates that the lethal concentration to fish varies from about 0.01 to 10 mg/L depending on the test animal, the type of water, temperature and time of exposure.
Concentrations of 0.03 mg/L of cadmium were not harmful to 1 and 2 year old tench, carp, rainbow trout and char, nor to the crustacea, worms and insect larvae on which they fed.
Cadmium acts synergistically with other substances to increase toxicity. Cadmium concentrations of 0.03 mg/L in combination with 0.15 mg/L of zinc from galvanized screens caused mortality of salmon fry.
Cadmium can form a complex with cyanide in plating baths, but in dilute solutions the complex is almost completely disassociated and highly toxic. Synergism of the toxic cadmium and cyanide ions liberated in the disassociation is indicated.