The U.S. Geological Survey has been measuring water for decades. Millions
of measurements and analyses have been made. Some measurements are taken
almost every time water is sampled and investigated, no matter where
in the U.S. the water is being studied. Even these simple measurements
can sometimes reveal something important about the water and the
environment around it.
The results of a single measurement of a water's properties are actually less
important than looking at how the properties vary over time. For example,
if you take the pH of the creek behind your school and find that it is
5.5, you might say "Wow, this water is acidic!" But, a pH of 5.5
might be "normal" for that creek. It is similar to how my normal body temperature (when I'm not sick) is about 97.5 degrees, but my third-grader's normal temperature is "really normal" -- right on the 98.6 mark. As with our temperatures, if the pH of your creek begins to change, then
you might suspect that something is going on somewhere that is affecting
the water, and possibly, the water quality. So, often, the
changes in water measurements are more
important than the actual measured values.
pH is only one measurement of a water body's health; there are
others, too.
Choose from this list to find out what they are and
how they can reveal something about water.
Temperature
pH
Specific conductance
Turbidity
Dissolved oxygen
Hardness
Suspended sediment
Water temperature is not only important to swimmers and fisherman, but also
to industries and even fish and algae. A lot of water is used for cooling purposes in power plants that generate electricity. They need cool water to start with, and they generally release warmer water back to the environment. The temperature of the released water can affect downstream habitats. Temperature also can affect the ability of water to hold oxygen as well as the ability of organisms to resist certain pollutants.
pH is a measure of how acidic/basic water is. The range goes from 0
- 14, with 7 being neutral. pHs of less than 7 indicate acidity, whereas
a pH of greater than 7 indicates a base. pH is really a measure of
the relative amount of free hydrogen and hydroxyl ions in the water. Water
that has more free hydrogen ions is acidic, whereas water that has more
free hydroxyl ions is basic. Since pH can be affected
by chemicals in the water, pH is an important indicator of
water that is changing chemically. pH is reported in "logarithmic units,"
like the Richter scale, which measures earthquakes. Each number
represents a 10-fold change in the acidity/basicness of the water. Water
with a pH of 5 is ten times more acidic than water having a pH of six.
Pollution can change a water's pH, which in turn can harm animals and plants
living in the water. For instance, water coming out of an abandoned coal
mine can have a pH of 2, which is very acidic and would definitely affect
any fish crazy enough to try to live in it! By using the logarithm
scale, this mine-drainage water would be 100,000 times more acidic than
neutral water -- so stay out of abandoned mines.
Diagram about pH
Picture of a pH meter
Specific conductance
is a measure of the ability of water
to conduct an electrical current. It is highly dependent on the amount
of dissolved solids (such as salt) in the water. Pure water, such
as distilled water, will have a very low specific conductance, and sea water
will have a high specific conductance. Rainwater often dissolves
airborne gasses and airborne dust while it is in the
air, and thus often has a higher specific conductance than distilled water.
Specific conductance is an important water-quality measurement because it
gives a good idea of the amount of dissolved material in the water.
High specific conductance indicates high dissolved-solids concentration; dissolved solids can affect the suitability of water for domestic, industrial, and agricultural uses. At higher levels, drinking water may have an unpleasant taste or odor or may even cause gastrointestinal distress. Additionally, high dissolved-solids concentration can cause deterioration of plumbing fixtures and appliances. Relatively expensive water-treatment processes, such as reverse osmosis, are needed to remove excessive dissolved solids from water.
Agriculture also can be adversely affected by high-specific-conductance water, as crops cannot survive if the water they use is too saline, for instance. Agriculture can also be the cause of increases in the specific conductance of local waters. When water is used for irrigation, part of the water evaporates or is consumed by plants, concentrating the original amount of dissolved solids in less water; thus, the dissolved-solids concentration and the specific conductance in the remaining water is increased. The remaining higher specific-conductance water reenters the river as irrigation-return flow. In a USGS study in Colorado, USA, specific conductance was found to vary during the year as a result of the temporal variability of streamflow. As this chart shows, specific conductance generally was lowest in the Arkansas RIver near Avondale, Colorado, in May to August, when streamflow generally was largest, and increased with decreasing streamflow in the fall, winter, and spring.
Often in school, students do an experiment where they connect a battery to a light bulb and run two wires from the battery into a beaker of water. When the wires are put into a beaker of distilled water, the light will not light. But, the bulb does light up when the beaker contains salt water (saline). In the saline water, the salt has dissolved, releasing free electrons, and the water will conduct an electrical current.
Turbidity is the amount of particulate matter that is suspended in water. Turbidity measures the scattering effect that suspended solids have on light: the higher the intensity of scattered light, the higher the turbidity. Material that causes water to be turbid include:
- clay
- silt
- finely divided organic and inorganic matter
- soluble colored organic compounds
- plankton
- microscopic organisms
Turbidity makes the water cloudy or opaque. The picture to the left shows highly turbid water from a tributary (where construction was probably taking place) flowing into the less turbid water of the Chattahoochee River in Georgia. Turbidity is measured by shining a light through the water and is reported in nephelometric turbidity units (NTU). During periods of low flow (base flow), many rivers are a clear green color, and turbidities are low, usually less than 10 NTU. During a rainstorm, particles from the surrounding land are washed into the river making the water a muddy brown color, indicating water that has higher turbidity values. Also, during high flows, water velocities are faster and water volumes are higher, which can more easily stir up and suspend material from the stream bed, causing higher turbidities.
![](pictures/TurbidityMeter.jpg) Turbidity can be measured in the laboratory and also on-site in the river. A handheld turbidity meter (left-side picture) measures turbidity of a water sample. The meter is calibrated using standard samples from the meter manufacturer. The picture with the three glass vials shows turbidity standards of 5, 50, and 500 NTUs. Once the meter is calibrated to correctly read these standards, the turbidity of a water sample can be taken.
![Picture of a turbidity sonde.](pictures/TurbiditySond.jpg) State-of-the-art turbidity meters (left-side picture) are beginning to be installed in rivers to provide an instantaneous turbidity reading. The right-side picture shows a closeup of the meter. The large tube is the turbidity sensor; it reads turbidity in the river by shining a light into the water and reading how much light is reflected back to the sensor. The smaller tube contains a conductivity sensor to measure electrical conductance of the water, which is strongly influenced by dissolved solids (the two holes) and a temperature gauge (the metal rod).
You can't tell by looking at water that there is oxygen in it (unless you remember that chemical makeup of a water molecule is hydrogen and oxygen). But, if you look at a closed bottle of a soft drink, you don't see the carbon dioxide dissolved in that - until you shake it up and open the top. The oxygen dissolved in lakes, rivers, and oceans is crucial for the organisms and creatures living in it. As the amount of dissolved oxygen drops below normal levels in water bodies, the water quality is harmed and creatures begin to die off. Indeed, a water body can "die", a process called eutrophication.
Although water molecules contain an oxygen atom, this oxygen is not
what is needed by aquatic organisms living in our natural waters. A
small amount of oxygen, up to about ten molecules of oxygen per
million of water, is actually dissolved in water. This dissolved
oxygen is breathed by fish and zooplankton and is needed by them to
survive.
Rapidly moving water, such as in a mountain stream or large river,
tends to contain a lot of dissolved oxygen, while stagnant water
contains little. Bacteria in water can consume oxygen as organic matter decays. Thus, excess organic material in
our lakes and rivers can cause an oxygen-deficient situation
to occur. Aquatic
life can have a hard time in stagnant water that has a lot of rotting,
organic material in it, especially in summer, when dissolved-oxygen
levels are at a seasonal low.
The amount of dissolved calcium and magnesium in water determines
its "hardness." Water hardness varies throughout the United
States. If you live in an area where the water is "soft,"
then you may never have even heard of water hardness.
But, if you live in Florida, New Mexico,
Arizona, Utah, Wyoming, Nebraska, South Dakota, Iowa, Wisconsin, or Indiana,
where the water is relatively hard, you may notice that
it is difficult to get a lather up when washing your hands or clothes. And,
industries in your area might have to spend money to soften their
water, as hard water can damage equipment. Hard water can even shorten the
life of fabrics and clothes! Does this mean that students who
live in areas with hard water keep up with the latest fashions since
their clothes wear out faster?
![Map of U.S. showing water hardness, by watershed.](graphics/HardnessMap.gif)
More information: Hard water and water softening - Stephen Lower
Suspended sediment is the amount of soil moving along in
a stream. It is highly dependent on the speed of
the water flow, as fast-flowing water can pick up and suspend more
soil than calm water. During storms, soil is washed from the
stream banks into the stream. The amount that washes into a stream
depends on the type of land in the river's watershed and the vegetation surrounding the river.
If land is disturbed along a stream and protection measures are not taken,
then excess sediment can harm the water quality of a stream.
You've probably seen those short, plastic fences that builders put up
on the edges of the property they are developing. These silt fences
are supposed to
trap sediment during a rainstorm and keep it from washing into a
stream, as excess sediment can harm the creeks,
rivers, lakes, and reservoirs.
Sediment coming into a reservoir is always a concern; once it enters
it cannot get out - most of it will settle to the bottom. Reservoirs can
"silt in" if too much sediment enters them. The volume of
the reservoir is reduced, resulting in less area for boating, fishing,
and recreation, as well as reducing the power-generation capability
of the power plant in the dam.
Do you want to test your local water quality?
Water test kits are available from World Water Monitoring Day (WWMD). Teachers and water-science enthusiasts: Do you want to be able to perform basic water-quality tests on local waters? WWMD offers inexpensive test kits so you can perform you own tests for temperature, pH, turbidity, and dissolved oxygen.
World Water Monitoring Day is an international education and outreach program that builds public awareness and involvement in protecting water resources around the world.
Information on this page is from A Primer on Water Quality, by Swanson, H.A., and Baldwin, H.L., U.S. Geological
Survey, 1965
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