Ground water is water found beneath the surface of the earth. It may be flowing in between the particles of soil or through cracks in rocks. Most ground water comes from precipitation that has soaked into the ground through the surface soil. All ground water eventually reappears as surface water either in the form of springs, or by feeding directly into streams or lakes or the ocean.
Many regions of the country rely on ground water for their water supply. Wells get their water by being drilled into the ground water supply. Ground water supplies water to nearly half the households in our country and is the primary source of irrigation water for agriculture in the United States. This dependence on ground water is causing problems in some sections of the country where the demand for water is exceeding the supply of ground water. In some areas in Texas and Oklahoma, so much ground water has been removed from the system that the landscape has actually sunk and changed shape. In other areas, ground water is threatened when the surface is covered so that the ground water supply cannot be recharged. On Long Island, where ground water is there only source of fresh water, the governments have had to create ground water recharge basins that are designed to direct rainwater into the ground water because so much of the ground of Long Island is covered with houses and asphalt that the natural recharging process cannot keep up with the demand for water.
Most people picture well water or spring water as being exceptionally pure and good to drink because it has been filtered and protected by the earth and is far away from any sources of contamination. Unfortunately, we have shown that humans can pollute even waters in the depths of the earth and that ground water is quite vulnerable to contamination. Soil is simply not a good enough filter to protect ground water from some of the contaminants that we have dumped on the ground over the years. There are many activities that can lead to ground water contamination, including: Industrial and agricultural waste disposal, poorly designed landfills, leaking underground storage tanks, failing septic systems, improperly applied agricultural fertilizers and pesticides, and the list goes on. Once these substances are released on the ground, they move down through the soil and into the ground water. Some of these chemicals, such as phosphates or the bacteria from septic systems, are filtered by the soil before they reach ground water unless the ground water is very close to the surface. Other chemicals, more soluble in water, are not likely to be absorbed by soil particles and are more likely to move to contaminate the ground water. The type of soil and quantity of chemical involved also affect the possibility of ground water contamination.
Once ground water is contaminated it takes a very long time, if it is even possible, for it to purify itself. In surface waters, there are a variety of chemical and biological reactions that help to speed the purification of contaminated water. These reactions include evaporation to the atmosphere, biological uptake, and breakdown by sunlight or microorganisms. Since none of these processes are available to ground water, purification is almost impossible. Slow movement of ground water and the colder temperatures within the earth also slow down the purification process. Even when a contamination source is very small, such as a leak from a home heating oil tank, the expense of cleaning up the ground water can be astronomical.
The best way to ensure a constant supply of clean ground water is to protect the water now rather than trying to clean it up later. Connecticut has state laws created under and “Aquifer Protection Act”, that are designed to keep activities using potentially contaminating substances out of areas where ground water is used or could be used as a source for drinking water.
Making a Ground water Model
*large glass or clear plastic container
*mixture of sand and gravel
Pour the sand and gravel mixture into the container, making a layer several inches thick that slopes toward one side of the container. Add water until you can see the level of the water within the soil in the container. Explain that the visible line of water is the “water table” or the upper limit of the ground water. Below the water the pores between the grains of sand and gravel are filled with water and this water is the ground water.
Continue adding water to the container until there is standing water visible over the lowest side of the sand and gravel mixture and the water table is visible in the mixture on the other side of the container.
Ask the group to compare the level of the water on both sides of the container
. Explain that swamps, streams and ponds are actually areas where the water table is higher than the surface of the ground because of uneven topography.
Ask the group what would happen if more water was added to the container
. (“pond” would get larger as water table got higher)
or if a well was placed in the dry land area
(“pond” will get smaller if too much water is removed and the water table is lowered). Emphasize the tie between precipitation, ground water and surface water.
Soil As A Filter
for each group you will need:
*3 jars, 1 with a lid
*2 coffee filters
*student worksheets (included at the end of this unit)
*large container for waste water
Divide the groups into groups of four or five. Have the groups set up their soil filters as described on their worksheet and place the funnels over two of the jars. The experimental solutions are mixed in the third jar as described on the worksheet. The soil should be a more effective filter than the sand because soil has smaller particles, making it easier for substances to absorb into soil. The vegetable oil should be filtered out by the soil, but the sugar and food coloring should pass through both filters.