BACKGROUND:

Ground water is water stored under the surface of the Earth. Convey to the students that ground water sometimes takes hundreds of years to accumulate. A good example of this occurs in the Sahara Desert in Africa, where ground water has accumulated for thousands of years and is still being used for drinking and irrigation. Ground water is stored in the ground below us, where different types of rocks can act as reservoirs. Sand or sandstone is the best material for a reservoir and forms extensive aquifers underground, but not all sands are created equal, as students will find out in lab.

Filtering water through different porous substances cleans water naturally in the ground. Substances like charcoal and diatomite (white powder that people put in pools), help to filter small particles that may be suspended in water.

The ground on which we live, is made up of many layers of different types of rocks. Water can move into the tiny pores within the rocks or soil. As they move, the different rocks can trap the particles in the pores. Some rocks have more pores than other rocks and can act as a reservoir of water or an aquifer. Some municipal water supplies depend on ground water for a source of drinking water. Filtering techniques are used in municipal water supplies. Water from a reservoir is cleaned through a combination of filtering and chemical processes.

PROCEDURE:

1. At the beginning of this lab, review the main components of the water cycle.  This lab shows students how ground water moves through rocks that have a high porosity.  
     
2. Students will discover that large grains allow water to move freely and also have large pore spaces (can hold more water).  You may want to use the diagram on the right to help you illustrate this principle. Follow the directions on the student lab sheet.  Do not use plastic beakers or graduated cylinders because the sand will scratch the plastic.
     
3. Have the students measure and compare their results in order to draw conclusions. Sand size and sorting of the sand particles do have a lot to do with how water moves. You may want to introduce the term Porosity (meaning the pore space available for liquid) and Permeability (how freely the liquid moves through the sand). In the diagram to the right, you can notice that water can only flow through open space. The best aquifers have large porosities and permeability.  In poorly sorted sands, there is usually more pore space available. 
     
4. This is just an introduction to these concepts. You can reuse the sand even if it is wet.  However you must drain the water and if you are doing more than one lab in a row, you may want to save dry sand for students to do the sorting of the sand.  If the sand grains are large  it tends to have more porosity and permeability.  The smaller the grains of sand the less pore space allowed for the movement of water.  The small amount of sand used in this experiment may cause differences in results.  The objective is just to recognize  water movement. 
     
5. In exercise 3, students will be testing how to filter water.   The tube is just a small water bottle, that is filled with   different layers of material that will help clean the water.  You may want students to prepare this at home and then experiment in class.  You will require a lot of different types of soil and sand if you are doing this in class.   Use different sand, rock, charcoal, or similar items that may help clean the water.  Get a plastic water bottle (small) and make 5-7 nail size holes on the bottom of the bottle. (You may want parents to help with this at home.)  
     
6. Have the students add one type of material one at a time so that layers are formed. For example first add one inch of sand then   add one inch of charcoal. A funnel, if you have one, can make this step easier, otherwise just have them use their hands as if they were in a  sand box.  Make sure the materials are dry when you are making the layers.
     
7. Use the worksheet to record the type of materials used to fill the bottle.  Fill in the type of material that you used for each layer and assign it a color in the legend  area. Using the colors you have indicated in the legend, have the student color the different layers in the bottle shape to match what their bottle actually looks like.  Each student will demonstrate if their filter can clean dirty water in class. You may want to test out the filter, by adding dirty water and see how well your filtering tube can clean dirty water.

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