Geoscience Reference
In-Depth Information
anions are the predominant form of dissolved chromium in waters that are alkaline
and mildly oxidizing. While hexavalent chromium contamination is generally
associated with industrial activity, it can occur naturally.
Chromium and certain chromium compounds are classified as substances
known to be carcinogenic. EPA classifies chromium as a ''de minimis'' carcino-
gen, meaning that the minimum amount of the chemical set by the US Occupa-
tional Safety and Health Administration (OSHA) is considered to be carcinogenic.
Chromium compounds vary greatly in their toxic and carcinogenic effects. Tri-
valent chromium compounds are considerably less toxic than the hexavalent
compounds and are neither irritating nor corrosive.
Breathing high levels of chromium (VI) can cause irritation to the nose,
including nosebleeds, ulcers, and holes in the nasal septum. Ingesting large
amounts of chromium (VI) can cause stomach upset and ulcers, convulsions,
kidney and liver damage, and even death. Skin contact with certain chromium (VI)
compounds can cause skin ulcers. Some people are extremely sensitive to chro-
mium (VI) or chromium (III). Allergic reactions consisting of severe redness and
swelling of the skin have been noted. Several studies have shown that chromium
(VI) compounds can increase the risk of lung cancer. Animal studies also have
shown an increased risk of cancer. The World Health Organization determined that
chromium (VI) is a human carcinogen. The US Department of Health and Human
Services (DHHS) determined that certain chromium (VI) compounds are known to
cause cancer in humans.
3.5.4 Lead
Lead is a bluish-white lustrous metal. It is very soft, highly malleable, ductile, and
a relatively poor conductor of electricity. It is very resistant to corrosion but
tarnishes on exposure to air. It is one of the oldest metals used by humans and has
been used widely since 5000 BC; lead was known to the ancient Egyptians and
Babylonians. The Romans used lead for pipes and in solder. The element has four
naturally occurring stable isotopes, three of which result from the decay of natu-
rally occurring radioactive elements (thorium and uranium). Although lead is
seldom found uncombined in nature, its compounds are widely distributed
throughout the world, principally in the ores galena (a lead sulfide ore), cerussite
(PbCO
3
), and anglesite (PbSO
4
).
To date, the single most important commercial use of lead is in the manufacture of
lead-acid storage batteries. However, for most of the twentieth century, the most
important environmental source of Pb was gasoline combustion. It is also used in
alloys, such as fusible metals, antifriction metals, and solder. Lead foil is made with
lead alloys. Lead is used for covering cables and as a lining for laboratory sinks,
tanks, and the ''chambers'' in the lead-chamber process for the manufacture of
sulfuric acid. It is used extensively in plumbing. Because it has excellent vibration-
dampening characteristics, lead is often used to support heavy machinery.
