Science of Water : Concepts and Applications

204 The Science of Water: Concepts and Applications
example of a nonpoint source (NPS) of pollution is rainwater carrying topsoil and chemical contaminants
into a river or stream. Some of the major sources of nonpoint pollution include water
runoff from farming, urban areas, forestry, and construction activities. The word runoff signals
a NPS that originated on land. Runoff may carry a variety of toxic substances and nutrients, as
well as bacteria and viruses with it. NPSs now comprise the largest source of water pollution,
contributing approximately 65% of the contamination in quality-impaired streams and lakes.
RADIONUCLIDES
When radioactive elements decay, they emit alpha, beta, or gamma radiations caused by transformation
of the nuclei to lower energy states. In drinking water, radioactivity can be from natural
or artifi cial radionuclides (the radioactive metals and minerals that cause contamination).
These radioactive substances in water are of two types: radioactive minerals and radioactive gas. The
USEPA reports that some 50 million Americans face increased cancer risk because of radioactive
contamination of their drinking water.
Because of their occurrence in drinking water and their effects on human health, the natural
radionuclides of chief concern are radium-226, radium-228, radon-222, and uranium. The source of
some of these naturally occurring radioactive minerals is typically associated with certain regions of
the country where mining is active or was active in the past. Mining activities expose rock strata, most
of which contains some amount of radioactive ore. Uranium mining, for example, produces runoff.
Another source of natural radioactive contamination occurs when underground streams fl ow through
various rockbed and geologic formations containing radioactive materials. Other natural occurring
sources where radioactive minerals may enter water supplies are smelters and coal-fi red electrical generating
plants. Sources of man-made radioactive minerals in water are nuclear power plants, nuclear
weapons facilities, radioactive materials disposal sites, and mooring sites for nuclear-powered ships.
Hospitals also contribute radioactive pollution when they dump low-level radioactive wastes into sewers.
Some of these radioactive wastes eventually fi nd their way into water supply systems.
While radioactive minerals such as uranium and radium in water may present a health hazard in
these particular areas, a far more dangerous threat exists in the form of radon. Radon is a colorless,
odorless gas created by (or as a by-product of) the natural decay of minerals in the soil. Normally
present in all water in minute amounts, radon is especially concentrated in water that has passed
through rock strata of granite, uranium, or shale.
Radon enters homes from the soil beneath the house, through cracks in the foundation, through
crawl spaces and unfi nished basements, and in tainted water, and is considered the second leading
cause of lung cancer in the United States (about 20,000 cases each year), after cigarette smoking.
Contrary to popular belief, radon is not a threat from surface water (lake, river, or aboveground
reservoir), because radon dissipates rapidly when water is exposed to air. Even if the water source
is groundwater, radon is still not a threat if the water is exposed to air (aerated) or if it is processed
through an open tank during treatment. Studies show that where high concentrations of radon occur
within the air in a house, most of the radon comes through the foundation and from the water;
however, radon in the tap water, showers, baths, and cooking (with hot water) will cause high concentrations
of radon in the air.
Note, however, that radon is a threat from groundwater taken directly from an underground
source—either a private well or from a public water supply and where treatment of the water does
not include exposure to air. Because radon in water evaporates quickly into air, the primary danger
is from inhaling it from the air in a house, not from drinking it.
THE CHEMICAL COCKTAIL
Previously, we referred to a glass of water fi lled to the brim from the household tap. When we hold
such a full glass (though we usually do not hold it; instead, we fi ll the glass and drink the water,