16 ChemMatters | OCTOBER/NOVEMBER 2017
U T T
can cause cancer.
tors are substances
that interfere with
hormone functions in our
bodies and can affect growth,
metabolism, and reproduction. The sources
of these contaminants are typically associ-
ated with human actions. After pesticides
are applied to lawns and farm fields, they
are leached by groundwater or flow off from
the land in water streams. Carcinogens and
endocrine disruptors enter the water system
through a variety of sources, including sew-
age. Examples of these chemicals include
thiabenazadole, 1,2-dichloropropane, and
Depending on the specific chemicals
found—for example, a particular pesticide—
the water-treatment facility will consider different treatment options. But these options are
typically the same types of treatments used
to remove disinfection by-products and heavy
metals. They can use specialized filters, activated carbon, coagulation, and precipitation
(see Filtration, step 4, in the illustration on the
Keeping the water safe
In Western countries, most of the time
getting a safe glass of water is as simple as
turning on the faucet. We don’t think about the
distance water has to travel from its source
to the tap and all the potential contaminants
that it can pick up along the way. Thanks to
chemistry and the various techniques used in
a water-treatment plant, these contaminants
are removed before the water leaves the plant.
Then, the water has to be carefully controlled
again so that it does not pick up any additional
contaminants between the treatment plant and
your glass. It is a complex journey!
How Are Contaminants Regulated
and Their Levels Determined?
THE U.S. ENVIRONMENTAL PRO-
TECTION AGENCY (EPA) SETS THE
OVERALL STANDARDS FOR DRINK-
ING WATER. The EPA must follow a
specific process, as outlined in the Safe
Drinking Water Act, to determine
which potential pollutants need to be regulated and what the regulatory limits
are. Currently, more than 90 contaminants are regulated. These
contaminants were selected based on the potential adverse health effects on
people. Regulations are set based on scientific data that the EPA has reviewed
to determine the maximum contaminant level goal (MCLG). This is
the maximum level of contaminants present in drinking water
that will not adversely affect
the health of people, with a
margin of safety.
Many of the regulated limits are
given in milligrams per liter (mg/L), or
parts per million, and some may even
be reported in parts per billion (ppb).
The limit on arsenic (As) is 0.010 mg/L
or 10 ppb. It is important to note that
parts per billion is based on mass and
not moles. Ten ppb of As is about 2
atoms of As per billion molecules of water.
treatment procedure must be determined on a
If heavy metals enter the drinking water
by leaching from the lining of pipes in the
distribution system, special filtration systems
are required to remove the heavy metals at
the tap. This is a problem that cities with older
water systems face because pipes used to
be made of lead or lead solder. If necessary,
water-purification plants can add compounds
called corrosion inhibitors to water. Corrosion
inhibitors create a protective layer on the pipe
and prevent heavy metals from leaching. This
protective layer can build up naturally in cities
that have hard water.
and endocrine disruptors
Potentially harmful contaminants need to be
managed in drinking water. These include pesticides, carcinogens, and endocrine disruptors. Chemical pesticides are used by farmers
Ground Water and Drinking
Water. U.S. Environmental
Geological Survey, Dec 9,
html [accessed Aug 2017].
Making Water Safe in an
Emergency. U.S. Centers for
Disease Control and Prevention,
Jan 23, 2016: http://www.cdc.gov/
water-safe.html [accessed Aug 2017].
Dennis, B. Schools Around the Country Find Lead
in Water, with No Easy Answers. The Washington
Post, July 4, 2016: https://www.washingtonpost.
2b336e293a3c_story.html [accessed Aug 2017].
Frankie Wood-Black is a science writer who
lives in Tonkawa, Okla. This is her first article in