11.3
Testing Water Quality
Water quality is an important indicator of the health of a water
system. How do we know if the water quality is good or poor?
Turbidity
turbid: not clear; containing tiny
specks of sediment or other solid
matter
Sediment (bits of solid matter) makes water turbid (cloudy). Clear
water is usually of better quality than turbid water. You can see the
turbidity of water simply by looking at it. Environmental scientists
measure turbidity by slowly lowering a two-coloured disk into the
water until it is no longer visible, and then recording this depth
(Figure 1).
Temperature
Figure 1 Scientists lower a disk
called a “Secchi disk” into a lake to
measure the water’s turbidity.
Warm water contains less dissolved oxygen than cold water. If the
water does not contain enough oxygen, aquatic organisms will die.
Increasing the temperature of water by artificial means is called
thermal pollution. Nuclear power plants and some industries use
large volumes of water for cooling machinery. This warmed water is
then returned to rivers and lakes (Figure 2).
thermal pollution: artificially
raising the temperature (for example,
of water)
Figure 2 Nuclear power plants use lake water as a coolant.
Deforestation along a waterway reduces shade along the shoreline,
which also causes thermal pollution. Thermal pollution reduces the
quality of the water.
Acidity
Figure 3 An indicator scale can be
used to discover whether a sample is
acidic or basic.
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Sci8_UnitD_Chap11.indd 296
Water becomes acidic if it contains certain dissolved chemicals, such
as sulfuric acid or nitric acid. We can measure the acidity of water
with an indicator that turns different colours in acidic and basic
solutions (Figure 3). Water is “acidic” if its pH is below 7 and “basic” if
its pH is above 7. Good-quality fresh water has a pH between 6.5 and
8.5. Acid rain, industrial pollution, and chemical spills can affect the
acidity of water. Most fish cannot reproduce in acidic water.
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Salinity
LINKING TO LITERACY
How can you test for salt in water without tasting it? A conductivity
apparatus is used to determine whether a solution can carry an
electrical current. Salt water is a good conductor of electricity.
Therefore, testing a sample of water for conductivity will tell you
whether the solution contains salt (Figure 4).
Concept Map
To help you learn about water
quality, draw a concept map.
Start with the words “water
quality,” then draw a line and
write “turbidity.” Repeat this for
each of the factors that affect
water quality. Around each
factor, make notes using lines
and boxes. Write connecting
words on the lines between
boxes. Concept maps are a
useful tool for boosting your
learning.
Figure 4 The dissolved salt in the salt solution completes the circuit and lights the bulb.
The fresh water contains no salt. It is not conductive, so its bulb does not light up.
Other Dissolved Substances
Dissolved substances (solutes) in water include:
• chlorine—added to kill unwanted organisms in drinking water
• nitrates—from fertilizer runoff from fields, golf courses, and gardens
• iron and copper—dissolved as water flows over certain rocks
• calcium and magnesium—also dissolved from rocks, making the
water hard
We can use chemically treated paper test strips to show whether a
chemical is present in the water. Sometimes, we also need to know the
quantities of solutes that are present. In the proper amounts, some of
these solutes can be beneficial. For example, chlorine is used in the
water treatment process to kill harmful bacteria. However, too much
chlorine can be toxic to animals. Other solutes, such as DDT, are toxic
even in tiny amounts. What are the safe concentrations, and how
much is too much?
To learn about maximum
acceptable concentrations of
solutes in water,
Go to Nelson Science
Unit Task
If you choose to develop a technology to solve a water issue for the
Unit Task, which of the water quality tests discussed in this section might be useful?
CHECK YOUR LEARNING
C
1. List at least three factors mentioned in this section that
decrease water quality.
3. What is thermal pollution? How do nuclear power plants
contribute to thermal pollution?
2. How can you test a water source to see whether it is acidic
or not?
4. How could each of the following substances enter a
freshwater source?
(a) chlorine
(c) calcium
(b) nitrates
(d) acid
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