Where Does My
Drinking Water Come From?
Drinking water comes from surface water and
ground water. Large-scale water supply systems tend to rely on surface water
resources, and smaller water systems tend to use ground water. Including the
approximately 23 million Americans who use ground water as a private drinking
water source, slightly more than half of the population receives its drinking
water from ground water sources.
Surface water includes rivers, lakes, and reservoirs. Ground water is pumped
from wells that are drilled into aquifers. Aquifers are geologic formations that
contain water. The quantity of water in an aquifer and the water produced by a
well depend on the nature of the rock, sand, or soil in the aquifer where the
well withdraws water. Drinking water wells may be shallow (50 feet or less) or
deep (more than 1,000 feet). Your water utility or your public works department
can tell you the source of your public drinking water supply.
What is a Public Water System?
The Safe Drinking Water Act (SDWA)
defines a public water system as one that serves piped water to at least 25
persons or 15 service connections for at least 60 days per year. Such systems
may be owned by homeowner associations, investor-owned water companies, local
governments, and others. Water that does not come from a public water supply,
and which serves one or only a few homes, is called a private supply.
Community water systems are public systems that serve people year-round in
their homes. EPA also regulates other kinds of public water systems--such as
those at schools, factories, campgrounds, or restaurants--that have their own
water supply. The data shown in this report cover only community water systems
because they are the source of most drinking water.
How Does Water Get To My
Faucet?
In a typical community water supply
system, water is transported under pressure through a distribution network of
buried pipes. Smaller pipes, called house service lines, are attached to the
main water lines to bring water from the distribution network to your house.
In many community water supply systems, water pressure is provided by pumping
water up into storage tanks that store water at higher elevations than the
houses they serve. The force of gravity then "pushes" the water into
your home when you open your tap. Houses on a private supply usually get their
water from a private well. A pump brings the water out of the ground and into
a small tank within the home, where the water is stored under pressure.
How Do Public Water Suppliers
Treat My Water To Make It Safe?
Water suppliers use a variety of
treatment processes to remove contaminants from drinking water. These
individual processes may be arranged in a "treatment train" to
remove undesirable contaminants from the water. The most commonly used
processes include filtration, flocculation and sedimentation, and disinfection.
Some treatment trains also include ion exchange and adsorption. A typical
water treatment plant would have only the combination of processes needed to
treat the contaminants in the source water used by the facility. If you want
to know what types of treatment are used for your water supply, contact your
local water supplier or public works department.
Flocculation/Sedimentation
Flocculation refers to water treatment
processes that combine small particles into larger particles, which settle out
of the water as sediment. Alum and iron salts or synthetic organic polymers
(alone, or in combination with metal salts) are generally used to promote
coagulation. Settling or sedimentation is simply a gravity process that
removes flocculated particles from the water.
Filtration
Many water treatment facilities use
filtration to remove remaining particles from the water supply. Those
particles include clays and silts, natural organic matter, precipitants from
other treatment processes in the facility, iron and manganese, and
microorganisms. Filtration clarifies water and enhances the effectiveness of
disinfection.
Ion Exchange
Ion exchange processes are used to
remove inorganic constituents if they cannot be removed adequately by
filtration or sedimentation. Ion exchange can be used to treat hard water. It
can also be used to remove arsenic, chromium, excess fluoride, nitrates,
radium, and uranium.
Adsorption
Organic contaminants, color, and taste-
and odor-causing compounds can stick to the surface of granular or powdered
activated carbon (GAC or PAC). GAC is generally more effective than PAC in
removing these contaminants. Adsorption is not commonly used in public water
supplies.
Disinfection (chlorination, ozonation)
Water is often disinfected before it
enters the distribution system to ensure that dangerous microbes are killed.
Chlorine, chloramines, or chlorine dioxide most often are used because they
are very effective disinfectants, and residual concentrations can be
maintained to guard against biological contamination in the water distribution
system. Ozone is a powerful disinfectant, but it is not effective in
controlling biological contaminants in the distribution pipes.
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EPA is
in the process of developing regulations limiting the amount of
disinfection by-products (DBPs). DBPs are contaminants that form when
disinfectants react with organic matter that is in treated drinking
water. Long-term exposure to some DBPs may increase the risk of cancer
or other adverse health effects.
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How Much Does
It Cost To Treat And Deliver My Drinking Water?
On a per gallon basis, water is cheap.
On average, water costs are slightly more than $2 per 1,000 gallons, although
the costs tend to be lower for large water systems. Treatment accounts for
about 15 percent of that cost. Other costs are for equipment (such as the
treatment plant and distribution system) and labor for operation and
maintenance of the system.
U.S. Environmental Protection Agency
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