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What contaminants may be found in drinking water?
There is no such thing as naturally pure
water. In nature, all water contains some impurities. As water flows
in streams, sits in lakes, and filters through layers of soil and rock
in the ground, it dissolves or absorbs the substances that it touches.
Some of these substances are harmless. In fact, some people prefer mineral
water precisely because minerals give it an appealing taste. However,
at certain levels minerals, just like man-made chemicals, are considered
contaminants that can make water unpalatable or even unsafe.
Some contaminants come from erosion of
natural rock formations. Other contaminants are substances discharged
from factories, applied to farmlands, or used by consumers in their homes
and yards. Sources of contaminants might be in your neighborhood or might
be many miles away. Your local water quality report tells which contaminants
are in your drinking water, the levels at which they were found, and the
actual or likely source of each contaminant.
Some ground water systems have established
wellhead protection programs to prevent substances from contaminating
their wells. Similarly, some surface water systems protect the watershed
around their reservoir to prevent contamination. Right now, states and
water suppliers are working systematically to assess every source of drinking
water and to identify potential sources of contaminants. This process
will help communities to protect their drinking water supplies from contamination,
and a summary of the results will be in future water quality reports.
For more information
- Read a list of the
drinking water contaminants that EPA regulates , including their sources
in drinking water and their potential health effects.
- How does arsenic get in drinking water? How do people use vinyl chloride? a set of fact sheets provides information
on each contaminant that EPA regulates, including its tradename(s),
areas in which it is commonly found, its possible health effects, etc.
- Non-point source pollution, such as runoff from farmlands and urban
stormwater, is one of the greatest threats to water quality today. To
learn more about this threat, see EPA's Non-point
source pollution site.
- To learn about the source water assessment process in your state, visit
EPA's local drinking
water information web site, which will help you find the state's web
site and source water protection coordinator.
- To find information about locations where there are substances present
that may contaminate sources of drinking water (for example, Superfund
sites), visit EPA's Envirofacts web site, where you can search by your location, zip code, etc. You can
also use the
Enviromapper function to plot the information that you find on a map.
Where does drinking water come from?
A clean, constant supply of drinking water is essential to every community. People in large cities frequently drink water that comes from surface water sources, such as lakes, rivers, and reservoirs. Sometimes these sources are close to the community. Other times, drinking water suppliers get their water from sources many miles away. In either case, when you think about where your drinking water comes from, it's important to consider not just the part of the river or lake that you can see, but the entire watershed. The watershed is the land area over which water flows into the river, lake, or reservoir.
In rural areas, people are more likely to drink ground water that was pumped from a well. These wells tap into aquifers--the natural reservoirs under the earth's surface--that may be only a few miles wide, or may span the borders of many states. As with surface water, it is important to remember that activities many miles away from you may affect the quality of ground water.
Your annual drinking water quality report will tell you where your water supplier gets your water.
For more information:
How is drinking water treated?
When a water supplier takes untreated water from a river or reservoir, the water often contains dirt and tiny pieces of leaves and other organic matter, as well as trace amounts of certain contaminants. When it gets to the treatment plant, water suppliers often add chemicals called coagulants to the water. These act on the water as it flows very slowly through tanks so that the dirt and other contaminants form clumps that settle to the bottom. Usually, this water then flows through a filter for removal of the smallest contaminants like viruses and Giardia.
Ground water is naturally filtered as it passes through layers of the earth into underground reservoirs known as aquifers. Water that suppliers pump from wells generally contains less organic material than surface water and may not need to go through any or all of the treatments described in the previous paragraph. The quality of the water will depend on local conditions.
The most common drinking water treatment, considered by many to be one of the most important scientific advances of the 20th century, is disinfection. Most water suppliers add chlorine or another disinfectant to kill bacteria and other germs.
Water suppliers use other treatments as needed, according to the quality of their source water. For example, systems whose water is contaminated with organic chemicals can treat their water with activated carbon, which adsorbs or attracts the chemicals dissolved in the water.
For more information:
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