Toxic metals can be present in industrial, municipal, and urban runoff, and by definition, are harmful to humans and aquatic biota. Increased urbanization and industrialization have increased the levels of trace metals, especially heavy metals, in water ways. There are over 50 elements that can be classified as heavy metals, but only 17 that are considered to be both very toxic and relatively accessible. Mercury, lead, arsenic, cadmium, selenium, copper, zinc, nickel, and chromium, however, should be given particular attention in terms of water pollution and runoff/discharge effects. Toxicity levels depend on the type of metal, it's biological role, and the type of organisms that are exposed to it.
Availability of Heavy Metals
Toxic metals are often added to the streams as salts (sulfides, phosphates and carbonates), are very insoluble in hard waters and usually travel with sediment. The transformation into readily accessible materials (for organisms) is a complex process and depends on many factors such as pH, sediment presence, and hardness. The availability of these metals is determined by precipitation-dissolution reactions which are strongly affected by pH. Therefore, at a lower pH, metallic ions (heavy metals in general) are more available and more reactive. Many of these metals then undergo methylation, as a result of bioaccumulation where bacteria absorb these elements and convert them from a metallic state into a toxic organometallic state. By becoming incorporated with an organic component, these metals become readily available to the first trophic level of the food chain and eventually lead to biological magnification throughout the system.
Testing Information
Since even low concentrations of heavy metals can cause serious harm to aquatic ecosystems, very sensitive and precise instruments are required to appropriately measure these substances in water samples.
Technically speaking, a measure of the availability of heavy metals to the first trophic level of organisms can made by taking the fraction extracted by a hydroxylamine hydrochloride reagent or by a chelating cation exchange resin.
