Heavy Metals (Pb,
Hg, Cd, Se)
The most common metal pollution in freshwater
comes from mining companies. When the pH in
water falls, metal solubility increases, and the
metal particles become more mobile. Metals can
become “locked up” in bottom sediments, where
they remain for many years. Metals are non-
biodegradable and can cause decreased
reproductive rates. Increased selenium
concentrations have been shown to interfere with
increased birth defects.Nitrate (NO 3 – ) Most excessive amounts of nitrates come from
human-based activities, such as runoff from
fertilized land, animal wastes from feedlots, and
treated municipal waste effluent. It has been
implicated as the primary cause of the dead
zones in the Gulf of Mexico, the Chesapeake
Bay, and the Long Island Sound. Nitrates also
get reduced to nitrites, which can be harmful to
humans and fish.
Nitrite (NO 2 – ) Nitrites occur in water as an intermediate
product in the biological breakdown of organic
nitrogen being produced either through the
oxidation of ammonia or the reduction of nitrate.
The presence of large quantities of nitrites is
indicative of wastewater pollution.
pH
Many aquatic life-forms are very sensitive to the
pH levels of the water. Changes in water pH can
result in increased mortality of eggs and
juveniles, decalcification of bone, and
physiological stress. Pollution tends to make
water acidic and increases the solubility of heavy
metals. Most bodies of water have the highest
biological diversity when the pH is near 7. Most
natural waters will have pH values from 5.0 to
8.5. Fresh rainwater may have a pH of 5.5 to 6.0
(due to carbon dioxide dissolving in the water,
making a weak carbonic acid solution). The