Objective
Students will explore the behavior of groundwater through two simple experiments. Student will observe percolation of water though soil and dissolution of soluble "pollutants" and the effects of density differences in keeping freshwater aquifers separate from saltwater aquifers underneath them.

Materials
test tubes (2 per group)
salt
sand
tap water
eye dropper
food coloring (yellow and blue)
sugar free "Kool-Aid" (dark-colored flavor)

Procedure

Percolation and Dissolution

1. This experiment simulates the action of rain water and sandy soil, and the dissolution of salt or another leachate such as fertilizer, herbicide or an insecticide. Fill a test tube with 2-3" of sand.

2. Add a few ml of water from a dropper (do not saturate the sand completely). Observe as the water flows through and around the sand particles. This is percolation.

3. Add a layer of dark-colored, "Kool-Aid" crystals to the surface of the sand. The crystals represent a soluble pollutant. Slowly add more water from the dropper. Observe the pollutant as it dissolves and flows (leaches) in solution through the sand. How might this same process affect a pond near a farm where fertilizer or pesticides have been used?

Density in Relation to Groundwater

1. Prepare a concentrated solution of table salt in water and color it light yellow with food coloring. Prepare a sample of tap water colored dark blue. Fill a test tube about full with the yellow salt water. Using a dropper, slowly add the blue tap water by allowing it to gently flow down the side of a tipped test tube.

2. Observe that the blue freshwater (lower density) floats upon and is kept separate from the yellow saltwater (higher density). A narrow band of green can be observed between the blue and yellow layers. This indicates that some restricted mixing occurs. This is true in an aquifer where the soil prevents any turbulence. In underground areas near the ocean, the salt groundwater forms the floor of a freshwater aquifer.

3. Fill a second test tube about full with light blue tap water. Using the same procedure introduce yellow saltwater to the test tube. Notice that these waters mix immediately as the more dense saltwater flows into the less dense freshwater, resulting in a green mixture.