Sedimentation Demonstration
Levels 1&2

Objectives:

The students will understand that soils are made up of different grain sizes and that this affects how the soils settle out of water. By extension, sedimentation in waterways will be illustrated.

Instructor Background:

Soil is made up of particles of many sizes. Scientists classify or name the soil according to its size. Soil scientists classify soil particles, smallest to largest, as clay, silt, sand, and gravel. Depending on its size, the soil will settle out of water or interact with chemicals in the environment differently.

Silt and clay are said to be fine-grained soils. Clay particles are smaller than 0.002 millimeters in diameter. Silt particles are larger than clay, and are between 0.002 and 0.05 millimeters in diameter. Some clay particles are so small that ordinary microscopes cannot see them. When silt and clay are dry and compact, there is very little space between the particles and water cannot pass easily through them. But, once silt and clay particles are mixed with water, they take a long time to settle out.

Individual silt and clay particles have a slight electrical charge. This causes them to act like little magnets, attracting and reacting with each other and with other chemicals in the environment. This is a very important property of fine-grained soils and is especially relevant in sediment contamination processes.

Sand and gravel are said to be coarse-grained soils. Sand is from 0.05 to 2.0 millimeters in diameter. Gravel is greater than 2.0 millimeters. Even when packed together, water can pass easily through sand and gravel. These particles are heavy in comparison to silt and clay particles and settle out of water quickly. Individual sand and gravel particles do not possess an electrical charge and do not tend to react with other chemicals in the environment. They are said to be chemically inert.

When soil particles are washed into streams and rivers, where and when they settle out will depend on the size of the particle. Water flowing faster than about 2 feet per second will keep most sand and finer particles in suspension. As the water slows down, the soil will settle out, largest particles first. Water must be moving very slowly for the smallest particles to settle out.

Materials:

1. Six quart jars (empty mayonnaise jars or canning jars will do).
2. Soil samples of clay, silt, and sand (from your Natural Resource Conservation Service agent, County Extension Agent, garden center, decorative rock retailer) bags of sand, dolomite clay and topsoil (for silt) are available in stores… do NOT use potting soil; there is too much floating material in it.
3. Paper plates.
4. Water.
5. Alum (available in grocery stores, usually where they have spices and canning supplies).
6. Shovel.
7. Paper sacks.

Procedure:

1. At least 2 days before presenting the activity, fill one jar about 1/3 full with a mixture of sand, silt, and clay in about equal parts. Add enough water to fill the jar. Shake the mixture thoroughly and set aside to let the soil settle out.
2. Two days later, display on paper plates some sand, silt, and clay. Have the students pick up and let each kind of soil sift through their fingers. Feel the texture and particle size.
3. Show the students the settled-out jar of soil samples. Explain what you did 2 days ago to make this sample. Have them look closely at the three layers. Have them name the layers.
4. Split the students into several groups. Have each group collect a sample of soil from a different location (schoolyard playground, garden area, roadside). They can gather the soil during school time or bring it from home. Put about 2 cups of material in the paper sack and bring it back to the classroom.
5. Spread the samples out on separate paper plates. Crush any lumps and remove rocks or trash.
6. For each soil type, fill a quart jar one-quarter full with soil, label, and add water until the jar is three-quarters full. Close the lid and shake hard. Place on a counter where it will be undisturbed.
   
   Students will see that the mixture separates into layers. The coarse-grained sediments (sand and gravel) will fall to the bottom first, the particles of silt and clay next. Material that stays floating is organic matter. Organic material is made up of dead leaves, twigs, stems, and decomposing parts of animals and plants. Most of the heavier sand will settle out in about 1 minute. Silt particles will settle out in about 5 minutes. The fine clays may take 48 hours or more. If the soil does not seem to be settling, open the jar, add one tablespoon of alum, and gently stir to avoid disturbing already-settled sediment. Put the lid back on and let settle. (Alum is aluminum potassium sulfate, AlK(SO4)2·12H2O. In water the aluminum and potassium have the effect of neutralizing the slight negative charge of the silt and clay particles, causing them to clump together and fall out of solution more quickly.)
7. After 2 days, have the students compare their soil settle to the one you did 4 days ago. Look closely at the layers with the magnifying glass.

Discussion Questions for the Sedimentation Demonstration:

1. When does soil become sediment?

    
    

2. Which particles settled first?

    
    

3. Are there distinct divisions in layers?

    
    

4. Why do clay and silt take so long to settle out?

    
    

5. Using a magnifying lens, which layer was the coarsest?

    
    

6. Which was the most compact?

    
    

7. Which had the most organic matter?

    
    

8. Why did adding the alum affect the settlement?

    
    

9. Which particles did the alum affect?

    
    

Soils are composed of particles that vary in size. The combination of the various-sized particles present creates the "texture" of the soil. Common textural classifications include gravel, sand, silt and clay; gravel being the coarsest and clay the finest.

One of the simplest ways to classify soil is by texture. If you are identifying particles in the field, you can use the following guidelines to classify soil textures.

Particles from 1 to 3 inches in diameter are considered gravel. Larger pieces are either cobbles, boulders, or rocks. Sand ranges in size from about 0.25 inch in diameter to grains that can hardly be seen. The finer particles--clays and silts--are difficult to see, but you can feel them with your thumb and forefinger.

Sand is broken down into several subdivisions. Most classification systems include coarse, medium, and fine grains. Because of the small size of its particles, fine sand is easily transported by water.

Soil particles smaller than sand are generally called "fines." These include both silt- and clay-size particles. Silt is coarser than clay. Dry, pulverized silt and clay have a powdery look. In the field you can test for fines (silt and clay) by rubbing the soil between your fingers. If it feels smooth, only fines are present. If it feels gritty, there are particles larger than silt and clay present. This method will enable you to make a rough estimate of the amount of fines in the soil.

When it is moist, clay has a plastic or doughy consistency. Silt may also have some plasticity but not as much as clay. To distinguish between silt and clay in the field, slowly moisten a sample of clay or silt soil and try to roll it into a pencil-sized ribbon between your hands. If you can roll the sample into a ribbon 6 inches in length without breaking it, you can assume the soil has a high clay content.