Water Clarity

We can use water clarity as one way to describe a river or lake. Actually, we use the concept of "turbidity" to describe water: turbidity is a measure of the amount of particulates suspended in water (“particulates” are things like sand, silt and algal cells). The more suspended particulates there are in the water, the cloudier it is, the higher the turbidity level—and the lower the clarity or the water. You can think of turbid water as the opposite of clear water.

If the water is turbid there will be very little light reaching the bottom of the river. This means that there won’t be enough light for plants to grow and there won’t be aquatic plants in the river or algae growing attached to the bottom.

The Kaw is a naturally turbid river and native fish and invertebrates are adapted to low visibility—that’s why catfish and sturgeon have small eyes and highly developed chemical senses (they can “taste” with their whiskers when searching for food). A lot of this natural turbidity is from coarse particulates like sand. In contrast, bank erosion, stormwater run-off and dredging can add too much fine silt to the water. This is considered a form of pollution because the finer particulates can be harmful to aquatic life. Fine silt clogs the gills of fish, mussels, and other animals that live in the river, and it smothers bottom organisms like mussels. It also clogs up the spaces between pebbles and sand particles on the bottom, making it difficult for bottom insects and other invertebrates to live in their underground burrows. Chemical pollutants and nutrients are attached to silt and wash in with it, causing even worse water quality problems.

In addition to silt, high turbidity can also be caused by algal blooms. Algal blooms can cause problems when the algae dies and decays, reducing oxygen levels in the water and causing fish kills.

Materials

  • Bucket or jar for collecting water sample
  • Turbidity tube (you can use a secchi disk in deeper water)
  • Field notebook and pencil

Methods

Turbidity Tube:

1. Collect a sample of water to test. You should test it immediately, don't let the particulates settle out.

2.Close the drain tube on the bottom of the column by squeezing the white plastic clamp. Fill the Plexiglass column with your water sample.

3. Make sure you are in a bright area with good lighting.

4. Look down through the tube. You shouldn’t be able to see all the way to the bottom (if you can the turbidity is too low to measure using this method and you should record 60+cm). While looking down through the opening of the tube, partially open the clamp on the drain tube.

5. Slowly let the water out, controlling the rate by squeezing the clamp.

6. As soon as you begin to see the black-and-white pattern on the bottom tighten the clamp on the drain tube.

7. Record the water level using the measuring scale on the side of the tube. The greater the depth of the water column, the clearer the water, and therefore the lower the turbidity (think of turbidity as being the opposite of clarity).

 

Secchi Disk:

1. Find a safe place in the shade to lower your secchi disk (this can be done from things like a boat, a bridge, a pier). You do this in the shade so that the reflection off the surface of the water doesn't interfere with your ability to see the secchi disk.

2. Lower the secchi disk to the surface of the water and measure how far it is to the surface--you can use an object like the deck of the boat or the top of the bridge as a stable point to measure from. Write down this distance.

3. Slowly lower the secchi disk until you can no longer see it. Pull it up a short distance and lower back down to make sure you have the right depth. Record the depth.

4. Slowly bring the secchi disk back up until you can see it again. Record this depth.

5. The average between the two depths is the value we are looking for:

add together (depth at which the disk disappeared) + (depth at which the disk reappeared) and divide this number by two to find the average.







Record your observations on the table below.

 

 

Turbidity

 Rating

1- Poor

2- Fair

3- Good

4- Best

 Description

Turbidity reading is less than 10cm

Turbidity reading is 10 to 30cm

Turbidity reading is 30 to 60 cm

Turbidity reading is 60 cm or higher

 Comments

 

 

 

 

 

 

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Laura Calwell,
Jul 11, 2011, 12:25 PM