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Metals
are found naturally occurring in surface waters in minute quantities as a
result of chemical weathering and soil leaching. However, concentrations greater than those
occurring naturally can be toxic to human and aquatic organisms. Elevated levels are often indicative of
industrial pollution, wastewater discharge, and urban runoff, especially from
areas with high concentrations of automobiles.
Airborne contaminants from coal-fired power plants may also contribute
metals to the atmosphere which are then carried to land by precipitation and
dry fallout. Because metals sorb readily
to many sediment types, they may easily enter streams in areas with high
sediment runoff. Another source of heavy
metals can be runoff from agricultural fields using sewage sludge as
fertilizer, which sometimes is permitted to contain up to 1500 mg metal/1 kg
fertilizer. Copper: The standard of 7.0 ug/l has been
established to protect aquatic life. In most areas, ambient levels are usually
below 1.0 ug/l. Wear of brake linings
has been shown to contribute concentrations of copper, lead, and zinc. Copper has a relatively high content in brake
linings. Copper is also present in
leaded, unleaded, and diesel fuel emissions. Lead: A
standard of 25.0 ug/l has been established to protect aquatic life, while the
normal ambient level is usually below 1.0 ug/l.
Lead may be present in industrial wastewater and was once common in road
runoff from the use of leaded gasoline.
Roadside soils still generally contain high lead levels, resulting in
elevated stream concentrations if these soils are subject to erosion. Zinc: The surface water standard is 50.0 ug/l. Typical ambient levels of zinc are approximately 5.0 ug/l. Zinc is a major metal component of tire rubber, brake linings, and galvanized crash barriers. Studies have been conducted linking this to zinc contamination from urban runoff. Because zinc is a by-product of the auto tire vulcanization process as well as the galvanization of iron. Zinc is a byproduct of the auto tire vulcanization process.
This
graph shows median conductivity levels at each site compared to the average
median for the region. All of the sites
monitored exhibit very low conductivity levels.
Even the highest conductivity measured at any of the sites is well below
the average median for the region.
Highest levels occur at the sites that also show the greatest problems
from sedimentation, Mill Creek, Pine Creek, and Cedar Creek.
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