Aluminum: Abundant in rocks and clays, and can be mobilized from soils by natural weathering. Sources also include effluents from industries that use aluminum in their processing or use alum as a flocculent. Low concentrations
are not a concern, but toxicity of aluminum increases if the pH of the water is less than 6.
Arsenic: Released naturally into the aquatic environment as a result of weathering of arsenic-containing rocks, from industrial and municipal discharges, and from combustion of fossil fuels.
Barium: A common element in the earth's crust, although only trace levels are normally found in natural surface waters.
Boron: Released naturally into the aquatic environment from weathering of rocks and soil, and is present in municipal sewage.
Cadmium: Present in trace concentrations in fresh water as a result of natural weathering processes. Concentrations above about 0.01 mg/L can usually be attributed to human activities such as mining, agriculture and the
burning of fossil fuels. Concentrations above this level may be toxic to aquatic life.
Chromium: Released into rivers by weathering of rocks and soil, and in industrial discharges such as from metal plating plants. Chromium is toxic to certain aquatic invertebrates at low concentrations, but fish are less
Cobalt: Released into the environment from weathering of cobalt-rich ores and from anthropogenic sources such as emission from coal burning industries.
Copper: Sources to aquatic environments include the weathering of copper minerals and native copper, and numerous sources from human activities. Background concentrations in surface waters are usually below 0.02 mg/L.
Higher levels are generally related to human activities.
Iron: Released naturally into the aquatic environment by weathering of sulphide ores and rocks and leaching from sandstones. Human sources include the burning of coal, acid mine drainage, mineral processing, sewage and
Lead: Toxic to fish and other organisms, particularly in soft water. Although there are natural sources of lead, human sources often supply a greater quantity to surface waters. Sources include weathering of sulphide
ores, urban runoff, atmospheric deposition and industrial and municipal discharges.
Manganese: An essential trace element for living things. Soils, sediments and rocks are significant natural sources of manganese, whereas industrial discharges are the primary source from human activities.
Mercury: Deposits occur in all types of rocks, and human sources of mercury to the aquatic environment include industrial and municipal discharges, atmospheric deposition or industrial emissions and leaching from landfill
sites. Mercury is of particular concern because of its toxicity to aquatic organisms and its adverse effects on human health.
Nickel: Enters the aquatic environment through the weathering of rocks and as a result of human activities, primarily the burning of fossil fuels and from smelting and electroplating industries.
Selenium: Enters surface waters as a result of weathering and erosion of soils and discharge from copper and lead refineries and municipal wastewater.
Vanadium: Released to the aquatic environment primarily by surface erosion. The major human-related sources are from atmospheric deposition as a result of emissions associated with oil, gas and steel production.
Zinc: Enters the aquatic environment primarily as a result of geochemical weathering and industrial and municipal discharges. Zinc is an essential element for living organisms.