Introduction to Turbidity
What is it?
Think of turbidity as the opposite of clarity. It is a measure of how cloudy a water body is. Most people have seen how rivers turn brown after a heavy rain. Soil particles carried by runoff cause this to happen.
Why do high levels of turbidity matter?
High amounts of soil in the water will block sunlight from reaching the bottom of a river or a lake in shallow water. When the water is turbid, floating particles absorb heat from the sun and cause the water temperature to rise. Higher temperatures cause oxygen levels in the water to fall, limiting the ability of fish and insects to survive there.
Another effect is that the floating particles may clog fish gills. When these particles sink, they can smother and kill fish and aquatic insect eggs that lay on the bottom. Turbidity can also limit plant growth. This happens when sunlight cannot reach the plants’ leaves.
The combination of warmer water, less light and oxygen depletion makes it impossible for some forms of aquatic life to survive.
How do turbidity levels rise?
Higher turbidity can be caused by human activity like cutting trees and removing vegetation next to a body of water. Trees provide shade to keep the water cooler, and trees and other plants help block mud and soil from washing into the water. When roads and parking lots are constructed without the proper silt fencing, more soil and mud are likely to reach the water.
Further Information on Turbidity
The American Public Health Association (APHA) defines turbidity as “the optical property of a water sample that causes light to be scattered and absorbed rather than transmitted in straight lines through the sample.” In simple terms, turbidity answers the question, “How cloudy is the water?”
Light’s ability to pass through water depends on how much suspended material is present. Turbidity may be caused when light is blocked by large amounts of silt, microorganisms, plant fibers, sawdust, wood ashes, chemicals and coal dust. Any substance that makes water cloudy will cause turbidity. The most frequent causes of turbidity in lakes and rivers are plankton and soil erosion from logging, mining, and dredging operations.
Measuring Turbidity
The most accurate way to determine water’s turbidity is with an electronic turbidimeter . The turbidimeter has a light source and a photoelectric cell that accurately measures the light scattered by suspended particles in a water sample. The results are reported in units called Nephelometric Turbidity Units or NTUs.
You also can measure turbidity by filtering a water sample and comparing the filter’s color (how light or dark it is) to a standard turbidity color chart. You’ll need the following equipment to do this: filter apparatus (Gelman or other manufacturer), some white membrane filters and a standard color chart to compare your findings. Your teacher will show you how to operate the filter equipment and will provide a color chart.
The procedure for using the Gelman filter apparatus to determine the turbidity of a water sample is as follows:
- Place a white gridded filter on the filter apparatus. You may handle the filter with your fingers; it’s not necessary to use a sterilized tweezers.
- Use a plastic cup or bucket to take a water sample from the river, lake or stream. Be sure you scoop only the water, not the sediment on the bottom.
- Pour 100 milliliters (mL) of your water sample into the top of the filter apparatus. The unit is graduated in milliliters. Just fill it to the 100-mL mark.
- Filter the sample. You may need to use a hand-operated vacuum pump to pull your sample through the filter.
- Remove the filter from the machine and let it dry.
- Estimate the turbidity of your sample by comparing its color to the color chart furnished by your teacher.
- Refer to the information below for a discussion of what these values mean.
|
Table 9. Turbidity level of water for industrial use |
|
|---|---|
|
Industrial Use |
Maximum Turbidity Units |
| Beverages |
1-2 |
|
Food products |
10 |
|
Water used in boilers |
1-20 (varies with type of boiler) |
|
Making high grade paper |
5-25 |
|
Making rayon |
1 |
|
Making cotton |
25 |
|
Baking |
10 |
|
Water used for cooling |
50 |
|
Ice making |
0.5 (same as drinking water) |
|
Tanning leather |
20 |
| References 2 | |
Drinking Water Standards
The APHA specifies drinking water turbidity shall not exceed 0.5 NTUs. However, some scientists think this standard is too generous. They want to see the value reduced to 0.1 NTUs.
Turbidity Effects on Fish and Aquatic Life
Turbidity effects fish and aquatic life by:
Interference with sunlight penetration. Water plants need light for photosynthesis. If suspended particles block out light, photosynthesis—and the production of oxygen for fish and aquatic life—will be reduced. If light levels get too low, photosynthesis may stop altogether and algae will die. It’s important to realize conditions that reduce photosynthesis in plant result in lower oxygen concentrations and large carbon dioxide concentrations. Respiration is the opposite of photosynthesis. (See Carbon Dioxide.)
Large amounts of suspended matter may clog the gills of fish and shellfish and kill them directly.
Suspended particles may provide a place for harmful microorganisms to lodge. Some suspended particles may provide a breeding ground for bacteria.
Fish can’t see very well in turbid water and so may have difficulty finding food. On the other hand, turbid water may make it easier for fish to hide from predators.
The table below shows the amount of plankton per acre which may be expected in ponds of different turbidities.
| Table 10. Plankton density as a function of water turbidity | |||
|---|---|---|---|
| Factor measured |
Clear ponds |
Intermediate ponds |
Muddy ponds |
| Average turbidity units: |
less than 25 |
25-100 |
over 100 |
| Amount of fish in pounds per acre: |
162 |
94 |
29 |
| Comparative amount of plankton caught in nets |
12.8 |
1.6 |
1 |
Note that the average amount of plankton in pristine (clear) water is almost 13 times that found in turbid (muddy) water. Turbidity in pristine water apparantly comes from the healthy plankton population itself, an excellent food source for many fish.
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