To many people, heavy metal pollution is a problem associated with areas of intensive industry. However, roadways and automobiles now are considered to be one of the largest sources of heavy metals. 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. Lead concentrations, however, consistently have been decreasing since leaded gasoline was discontinued. Smaller amounts of many other metals, such as nickel and cadmium, are also found in road runoff and exhaust.
About half of the zinc and copper contribution to the environment from urbanization is from automobiles. Brakes release copper, while tire wear releases zinc. 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.
On the road surface, most heavy metals become bound to the surfaces of road dust or other particulates. During precipitation, the bound metals will either become soluble (dissolved) or be swept off the roadway with the dust. In either case, the metals enter the soil or are channeled into a storm drain. Whether in the soil or aquatic environment, metals can be transported by several processes. These processes are governed by the chemical nature of metals, soil and sediment particles, and the pH of the surrounding environment.
Most heavy metals are cations, meaning they carry a positive charge. Zinc and copper, for instance, both carry a 2+ charge. Soil particles and loose dust also carry charges. Most clay minerals have a net negative charge. Soil organic matter tends to have a variety of charged sites on their surfaces, some positive and some negative. 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. The soluble form of metals is thought to be more dangerous because it easily is transported and more readily available to plants and animals. By contrast, soil bound metals tend to stay in place.
Metal behavior in the aquatic (streams, lakes and rivers) environment is surprisingly similar to that outside a water body. Streambed sediments exhibit the same binding characteristics found in the normal soil environment. As a result, many heavy metals tend to be sequestered at the bottom of water bodies. Some of these metals will dissolve. 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.
pH tends to be a master variable in this whole process. pH is a measure of the concentration of hydrogen (H+) ions dissolved in water. H+ is the ion that causes acidity; however, it is also a cation. As a cation it is attracted to the negative charges of the soil and sediment particles. In acid conditions, there are enough H+ ions in to occupy many of the negatively charged surfaces of clay and organic matter. Little room is left to bind metals, and as a result, more metals remain in the soluble phase. In areas still suffering from acid rain, the effects of pH are even more pronounced. The increase in acidity causes heavy metals to become more soluble.
Common Metals in Road Runoff
- Lead: leaded gasoline, tire wear, lubricating oil and grease, bearing wear
- Zinc: tire wear, motor oil, grease, brake emissions, corrosion of galvanized parts
- Iron: auto body rust, engine parts
- Copper: bearing wear, engine parts, brake emissions
- Cadmium: tire wear, fuel burning, batteries
- Chromium: air conditioning coolants, engine parts, brake emissions
- Nickel: diesel fuel and gasoline, lubricating oil, brake emissions
- Aluminum: auto body corrosion
What Can I Do?
- Learn More. Heavy metal pollution from road runoff is dangerous for aquatic wildlife, but other types of non-point source pollution like excess nutrients and sediment erosion are just as harmful to habitat.
- Wash Your Car the Right Way. Make sure that the soap and chemicals from washing your car aren't going straight into local streams.
- Drive Less. Exercise your commuting options by carpooling, using public transportation or bicycling.
- Volunteer! Lead a storm drain marking project to educate the community about what we all can do to keep harmful pollutants out of our waterways.