Very few natural waters contain detectable concentrations of mercury.
However, the element may be introduced into the water through disposal of mining and metallurgical or other industrial waste. Mercuric salts occur in nature chiefly as the mercury sulfide (HgS) known as cinebar, but numerous synthetic, organic and inorganic salts of mercury are commonly used. These include mercuric chloride, mercuric cyanide, mercuric nitrate, mercuroorganic compounds and metalic mercury, all of which are toxic in varying concentrations to aquatic life.
Methyl and ethyl mercuric salts are also extremely toxic and hazardous to living organisms. Therefore, mercury and mercuric salts are considered to be highly toxic to humans.
They are readily absorbed by way of the gastro-intestinal tract and fatal doses for man vary from 3 to 30 grams.
For fresh water fish, concentrations of 0.004 to 0.02 mg/L of mercury have been reported as harmful.
Fish can accumulate mercury in their tissues directly from the surrounding water or through ingestion of contaminated food supplies.
Mercury concentrations in living tissue should not exceed 0.5 mg/L.
Drinking water standards are set at 0.005 mg/L for the maximum limit tolerable.
The minimum lethal concentration of mercury salts has been reported for phytoplankton to range from 0.9 to 60 mg/L of mercury.
The toxic effects of mercuric salts are accentuated by the presence of traceamounts of copper.