![]() pH is a measure of the concentration of hydrogen (H+) ions dissolved in water. PH tends to be a master variable in this whole process. The aquatic environment is more susceptible to the harmful effects of heavy metal pollution because aquatic organisms are in close and prolonged contact with the soluble metals. As a result, many heavy metals tend to be sequestered at the bottom of water bodies. Streambed sediments exhibit the same binding characteristics found in the normal soil environment. Metal behavior in the aquatic (streams, lakes and rivers) environment is surprisingly similar to that outside a water body. By contrast, soil bound metals tend to stay in place. The soluble form of metals is thought to be more dangerous because it easily is transported and more readily available to plants and animals. The negative charges of these various soil particles tend to attract and bind the metal cations and prevent them from becoming soluble and dissolved in water. Soil organic matter tends to have a variety of charged sites on their surfaces, some positive and some negative. Most clay minerals have a net negative charge. Soil particles and loose dust also carry charges. Zinc and copper, for instance, both carry a 2+ charge. ![]() Most heavy metals are cations, meaning they carry a positive charge. ![]() These processes are governed by the chemical nature of metals, soil and sediment particles, and the pH of the surrounding environment. Whether in the soil or aquatic environment, metals can be transported by several processes. In either case, the metals enter the soil or are channeled into a storm drain. During precipitation, the bound metals will either become soluble (dissolved) or be swept off the roadway with the dust. On the road surface, most heavy metals become bound to the surfaces of road dust or other particulates. Motor oil also tends to accumulate metals as it comes into contact with surrounding parts as the engine runs, so oil leaks become another pathway by which metals enter the environment. Brakes release copper, while tire wear releases zinc. Smaller amounts of many other metals, such as nickel and cadmium, are also found in road runoff and exhaust.Ībout half of the zinc and copper contribution to the environment from urbanization is from automobiles. Lead concentrations, however, consistently have been decreasing since leaded gasoline was discontinued. Zinc, copper, and lead are three of the most common heavy metals released from road travel, accounting for at least 90 of the total metals in road runoff. However, roadways and automobiles now are considered to be one of the largest sources of heavy metals. To many people, heavy metal pollution is a problem associated with areas of intensive industry. ![]()
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