Water Quality
Standards Covered: 8.E.1.3
We rely on water to live. Safe drinking water is of the utmost importance. Sometimes
chemicals in the water can make it dangerous to drink. Even though the water appears clear, it
can still be harmful. Odorless, tasteless and colorless contaminants can all foul water sources.
Toxins from living things, chemicals from bedrock or runoff from waste treatment plants can
create unsafe conditions. Arsenic is one example. It easily dissolves in water and can poison
people. Other heavy metals like copper and lead also cause harm.
Scientists monitor water to learn about its conditions and to determine its safety for drinking.
Water quality is a measure of the physical, chemical and biological factors that affect a body of
water. Natural and artificial forces often change these factors. Monitoring water quality starts
with a sample of water. Scientists or workers test the water to see what the conditions are like
at any given time. Scientists use field and laboratory equipment to analyze the water.
Many measurements must be taken over a long period of time. This tells scientists what the
normal conditions are for that water system. Knowing what is normal makes it easier to detect
any changes that may occur. There are many physical and chemical factors scientists look at to
determine water quality. Water quality often determines the number and types of organisms
that can live in a water system.
Physical Factors
Water temperature is a measure of how cold or warm the water is. It tells us what kinds of
plants and animals are able to live in the water. Temperature also affects the current in bodies
of water. The temperature remains nearly constant for small streams and creeks. In deeper
bodies of water, the temperature varies with size and depth. Water temperature also changes
with the seasons. In summer, the Sun warms surface water, while deeper water stays relatively
cool. In winter, surface water cools down due to the cooler temperatures of the surrounding
air. The amount of shade surrounding the water also impacts its temperature. Water in the
shade in generally much cooler than water in the sunlight. The weather can make a difference
too. Severe storms with hail or cold rain enter a water system and lower its temperature. Tides
cause water temperatures to fluctuate as the tides move in and out. This happens especially in
estuaries and sounds.
It is important to monitor the temperature because it can affect other conditions in the water.
It changes the ability of water to dissolve gases. Generally, cooler water contains more
dissolved oxygen than warmer water. The temperature also determines the types of organisms
that live in the water. Certain organisms are adapted to specific temperature ranges. In
extreme temperatures, some animals can no longer breed and some are not able to survive.
Scientists measure turbidity as another factor of water quality. Turbidity is a measure of how
cloudy or clear the water is. It is determined by the amount of suspended particles in water.
These particles prevent light from being able to pass through the water. Suspended particles
can be things like soil particles, phytoplankton (single-celled algae) and microbes. All of these
contribute to turbidity. If they increase in the water, less light can pass through. This raises the
turbidity of the water. If water has high turbidity, it appears cloudy or opaque. If water has a
low turbidity, the water appears clearer.
Turbidity affects the water quality in several ways. Turbid water tends to have a higher
temperature. This is because suspended particles can absorb heat well. Water with a lot of
particles in it decreases the amount of light reaching plants. This decreases the rate of
photosynthesis. Organisms that depend on photosynthesis for food and oxygen are affected.
Sometimes particles suspended in water may endanger organisms directly. For instance, some
organisms have trouble finding food in turbid water. Sediments may also clog fish gills and kill
the fish.
Turbidity is caused by natural factors like erosion and algae growth. Tides can also change the
turbidity of water. Finally, weather also changes turbidity. Hurricanes, high winds and storms
all stir up sediments in water and increase turbidity. Man-made causes of turbidity include
runoff from construction sites and agriculture. Waste discharge and boat propellers also make
water more turbid.
Stream flow is the volume of water that moves over a certain point in a fixed period of time. It
is usually expressed in cubic feet per second or meters per second. It is measure by calculating
the distance water travels over time. Stream flow affects the quality of water, as well as the
organisms and their habitats. Some organisms are adapted to fast flowing streams. Others
need slow, clam pools of water. Stream flow is controlled by the amount of water moving from
a watershed into the stream channel. It changes with the weather and the seasons. In
summer, evaporation rates are higher, and there is a lot of vegetation around a stream. These
remove water from a stream and from the ground, reducing stream flow. Water used for
agricultural purposes like irrigation can also reduce the flow. Dams built for electric power
generation can cause sudden changes in stream flow. A dam may block the flow of the water
and release it later in a surge. Small, slower streams are most affect by pollution because they
cannot dilute pollutants as easily as fast streams. The flow also determines the amount of
dissolved oxygen in a stream. Generally faster-moving water has a higher level of oxygen than
slower-moving water. This is because swift currents flow over rocks and other land surfaces
making them better able to dissolve atmospheric oxygen.
Physical Factors
Recall that water is great solvent. It has the ability to dissolve gases from the atmosphere.
Oxygen in water is dissolved oxygen. It is important to note that this oxygen is different from
oxygen in water molecules. Dissolved oxygen is oxygen gas. Organisms need oxygen to respire
and breathe. Healthy water has enough dissolved oxygen to sustain aquatic life. Dissolved
oxygen is one indicator of polluted water. A high amount of dissolved oxygen usually means
the water is healthy. A low amount means the water is unhealthy. Oxygen enters water in 2
main ways. The first is from oxygen in the air. As surface water moves, it interacts and mixes
with air. This allows oxygen from the air to enter the water. Wind, waterfalls, rapids and waves
all help integrate oxygen into water. The second pathway is through aquatic plants. They add
oxygen to water through photosynthesis. Photosynthesis is the process plants use to trap solar
energy. One by-product of this chemical reaction is oxygen.
Dissolved oxygen is often measure in parts per million (ppm). This means for every one million
molecules of water, some of them are oxygen molecules. Dissolved oxygen ranges from 0-18
ppm. Water needs at least 5-6 ppm to be considered healthy. At this level, water can support a
variety of life. Really low or really high levels are dangerous for aquatic life. Hypoxia occurs
when the water contains low levels of oxygen, between 2-4 ppm. When water has little or no
oxygen, 0-1 ppm, anoxia occurs. These conditions are often causes by pollution and lead to
stress and even death in aquatic organisms. Dissolved oxygen levels do not exceed 14 ppm very
often. If they do, a disease can occur that forms oxygen bubbles inside an animal’s blood
vessels. This disease can be deadly.
Dissolved oxygen levels naturally change throughout the day. Levels are usually at their lowest
in the morning and increase during the day. This is because plants have stopped the production
of oxygen over night, while both plants and animals still use it to respire. Organisms like
bacteria and fungi use oxygen when breaking down dead and decaying organic matter (plant
and animal material). Levels also change with the weather. For instance, high winds increase
the amount of dissolved oxygen in water. Water temperature also affects oxygen levels. Cold
weather generally holds more oxygen than warm water.
The pH of water is a measure of how acidic or basic (alkaline) a substance is. It is ranked on a
scale from 0 (acidic) to 14 (alkaline). Acidity increases as the pH get lower. Salt water is slightly
more basic than fresh water. A slight change in pH can have major impacts on aquatic life. This
is because the pH scale is on a logarithmic scale. A drop in the pH by 1.0 unit produces a 10fold increase in acidity. So, a water sample with a pH of 5.0 is 10 times more acidic than one
with a pH of 6.0. And, a water sample with a pH of 4.0 is 100 times more acidic than one with a
pH of 6.0.
Healthy water has a pH between 6.5 and 8.0. This water can support a variety of organisms. If
the pH is higher or lower, the number of organisms is typically reduced. Acidic water is stressful
on organisms. Acidity can affect the ability of fight to hatch eggs. It can also prevent some
animals from growing hard shells that they need to survive. Water that is very acidic (below 4)
or very basic (above 10) can be deadly. The pH of water can be measured by a pH meter or by
an indicator. Nitrate is an important nutrient for aquatic plants. It is a form of nitrogen.
Nitrate that enters the water from land is dissolved more easily than other nutrients like
phosphorus. This makes it more useful in determining water quality. Only small amounts of
nitrate occur in water naturally it comes from decomposing organic matter and waste. Most of
the nitrate in water comes from artificial sources. Septic systems, wastewater and runoff from
fertilized lawns all add nitrate to water. Nitrate is an essential nutrient for organisms. But, too
much can adversely affect organisms and lower water quality. An excessive amount of nitrate
and phosphorus is called eutrophication. It may lead to an increase in plant growth, especially
of algae. A sudden population growth in algae is called an algal bloom. Algal blooms cloud
water and deplete the water of oxygen (hypoxia) that animals need to survive.
Biological Factors
You can tell how healthy water is by the number and variety of organisms living in it. Their
survival tells us a lot about the water quality. Bio-indicators are organisms we use to monitor
the health of an ecosystem. Physical or chemical changes that occur in the organisms are a sign
that a problem may exist. Changes in their population or physical deformities are signs that the
water is becoming unhealthy. Particularly chemical pollutants and wastes products can be
monitored in this way. Toxins can be measured in the tissue of animals as well. A change in an
animal’s behavior may also suggest that a problem exists.
Pollution affects some organisms more than others. Some animals cannot survive in polluted
waters. They are called intolerant. Animals that can survive with difficulty in polluted
conditions are facultative. And animals that do very well in polluted conditions are tolerant.
Macroinvertebrates are organisms that do not have a backbone. They are small in size but can
still be seen with the eye. They are often used as bio-indicators in water systems, because they
do not move around much, are easy to see and have long life cycles. A healthy water system
will have a wide variety of macroinvertebrates. Examples of intolerant kinds are mayflies,
stoneflies, and caddisfly larvae. Those that are facultative are dragonflies and damselfly larvae.
And, tolerant ones include rat-tail maggots and midge larva, among others. Scientists can
sample the number and type of organisms living in a watershed to determine its overall health.
If they discover a wide diversity of species, including intolerant species, living in an ecosystem,
it is safe to say the system is healthy. However, if they find only a few select species, mostly
tolerant ones, they can say the system is unhealthy.
Microorganisms are organisms that are too small to be seen with the naked eye. An example is
phytoplankton. They are also useful for measuring water quality because they are abundant in
the water. This makes them easy to sample. They produce special proteins called stress
proteins when contaminants enter the water. If these stress proteins are detected, it is a good
sign that the water quality is being threatened.