Toxic Elements Often Found in Water

Heavy metal ions are perhaps the most common of all water pollutants. The heavy metals include such frequendy encountered elements as lead and mercury, as well as many less common ones like cadmium, chromium, nickel, and copper. These metals can, at times, be acutely toxic, causing immediate symptoms, but often they are chronically toxic in very small quantities. Chronic toxicity is characterized by nagging symptoms that lessen normal body functions. Inadequate disposal of wastes from mining or industrial activities causes these metals to find their way into water supplies. In addition, some farming activities and the disposal of household wastes can contribute to the presence of heavy metals in our water supplies. 

The heavy metals are considered heavy because they have high atomic weights. 

Mercury is a fairly common heavy metal. Its elemental form at room temperature is that of a liquid that is volatile with the characteristic shininess of a metal. Mercury atoms are readily oxidized to Hg [mercury(I) ion] and Hg + [mercury (11) ion]. Both of these ionic forms are toxic, and their effects can be cumulative (i.e., repeated exposures will increase the toxic effects because the body does not easily rid itself of the element). 

Mercury is widely used in industry and can still occasionally be found in homes in mercury thermometers. The disposal of used fluorescent lamps represents a major source of mercury in water. Fluorescent lamps contain small amounts (about 60 mg per lamp) of mercury, which vaporizes inside the lamp and helps carry the electric current between the electrodes. Discarded lamps of this type are ultimately crushed when they are disposed of in landfills. This releases the mercury to become oxidized and then to dissolve in any water that comes in contact with the waste. 

Once released into streams, lakes, or oceans, metallic mercury sinks to the bottom, where it is converted by microorganisms to methymercury (CHgHg). In this stable and persistent form, it accumulates in the flesh of large marine creatures (e.g., tuna, swordfish, marine mammals). Once there, it becomes a hazard to the health of humans who consume these creatures, and numerous health advisories have been issued about consuming mercury-contaminated seafood. 

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