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(Conservation Currents, Northern Virginia
Soil and Water Conservation District, March 2005)
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.
The effects of pH are even more pronounced in the Washington,
DC metropolitan area because of the problem of acid rain. Acid
rainfall can cause a large increase in acidity and a corresponding
increase in the amount of heavy metals becoming soluble.
- 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
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