National Ground Water Association

Page Content By Kimberly Mullen, CPG

Objective

Students will be able to define terms pertaining to groundwater such as permeability, porosity, unconfined aquifer, confined aquifer, drawdown, cone of depression, recharge rate, Darcy’s law, and artesian well. Students will be able to illustrate environmental problems facing groundwater, (such as chemical contamination, point source and nonpoint source contamination, sediment control, and overuse).

Introduction

Looking at satellite photographs of the planet Earth can illustrate the fact that the majority of the Earth’s surface is covered with water. Earth is known as the “Blue Planet.” Seventy-one percent of the Earth’s surface is covered with water. There also is water beneath the surface of the Earth. Yet, with all of the water present on Earth, water is still a finite source, cycling from one form to another. This cycle, known as the hydrologic cycle, is an important concept to help understand the water found on Earth. In addition to understanding the hydrologic cycle, you must understand the different places that water can be found—primarily above the ground (as surface water) and below the ground (as groundwater). Today, we will be starting to understand water below the ground.

General definitions and discussion points

“What is groundwater?”

Groundwater is defined as water that is found beneath the water table under Earth’s surface.

“Why is groundwater important?”

Groundwater, makes up about 98 percent of all the usable fresh water on the planet, and it is about 60 times as plentiful as fresh water found in lakes and streams. Because groundwater is not visible (in most cases), it is often overlooked when considering all of the water on Earth, and yet, water beneath the land surface is a valuable resource. Protecting it from contamination and carefully managing its use will ensure its future as an important part of ecosystems and human activity.

How does groundwater move through rock and/or soil?

Water in the ground travels through pores in soil and rock, in fractures, and through weathered areas of bedrock. 

Other important definitions

1. The amount of pore space present in rock and soil is known as porosity.
2. The ability of fluids to travel through the rock or soil is known as permeability.
3. The permeability and porosity measurements in rock and/or soil can determine the amount of water that can flow through that particular medium. A “high” permeability and porosity value means that the water can travel very quickly.
4. Groundwater can be found in aquifers. An aquifer is a body of water-saturated sediment or rock in which water can move readily.
5. There are two main types of aquifers: unconfined and confined.
6. An unconfined aquifer is an aquifer that is exposed to the surface of the land. Because this aquifer is in contact with land, it is impacted by meteoric water and any kind of surface contamination. There is not an impermeable layer to protect this aquifer.
7. A confined aquifer is an aquifer that has a confining layer that separates it from the land surface. This aquifer is filled with pressurized water (due to the confining layer).
8. If the water is pressurized at a high enough value, when a well is drilled into the confining aquifer, water rises above the land surface. This is known as a flowing artesian water well.
9. The pressure of the water is called the hydraulic head. Groundwater movement or velocity is measured in feet (or meters) per second.
10. Darcy’s law is one method used to compute this value (because groundwater cannot be viewed completely, like surface water, volume cannot be accurately measured). Darcy’s law uses both the hydraulic head value, the hydraulic gradient, and the permeability of the material that the aquifer is traveling. The actual computation is velocity = permeability x hydraulic gradient (hydraulic head/distance).
11. In some areas, the bedrock has low permeability and porosity levels, yet groundwater can still travel in the aquifers. Groundwater can travel through fractures in the rock or through areas that are weathered. Limestone, for example, weathers in solution, creating underground cavities and cavern systems. At the land surface, these areas are known as“karst”.
12. The voids in the rock, created as limestone goes into solution, can cause collapses at land surface. These collapses are known as sinkholes. Sinkholes often are a direct conduit to the groundwater and are areas where contamination can easily infiltrate the aquifers. On topographic maps, sinkholes appear relatively circular with hacher marks (indicating a depression); they may or may not be filled with water (depending on the groundwater levels). These areas also can have land subsidence as mass wasting occurs in areas with a sudden change in slope and contact with water.
13. Porosity and permeability of the sediment, soil, and bedrock in the area also affects therecharge rate of the groundwater. This means that in some areas, the groundwater can be pumped out faster than it can replenish itself. This creates a number of problems.
14. One of these problems is called “drawdown.” This is a lowering of the aquifer near a pumping well. This can occur in areas where the well is pumping faster than the groundwater aquifer is recharged. This creates voids in the bedrock and can lead to additional land subsidence or sinkholes (as there is no longer water present and the void cannot hold the weight of the material above and collapses).

Demonstration

This uses a large Plexiglass groundwater simulator.

Instructions:

1. Make sure that the groundwater simulator and all other materials were set up prior to class beginning. Simulator should be in the very front of the class for all students to see.
2. Stand behind the simulator. Show students each feature of the simulator. Emphasize the new vocabulary words that were just introduced earlier in the lesson. Be sure that students are recording their observations in their lab books, along with the definitions of the new words.
3. Students will be evaluated by turning in their observations and definition at the end of the class. Review the standard format for lab observations.
4. Ask for at least two to three student volunteers. One will be the groundwater recharge person and the other will be the “polluter”. The third can be the one that empties the buckets and fills bottles as needed.
5. Have the recharge student begin to fill the reservoir with plain tap water. As this is slowly saturating the sand, gravel and clay, ask students what they see. Water begins to fill the simulator and enter the wells. Finally, water enters the river and pond.
6. Point to the clay layer (it's black). Clay is the confining layer in the simulator. Explain that this could be bedrock. Point to the confining aquifer. Ask students to list the wells (they are numbered) that are drilled into the confined aquifer, and in the unconfined. What are the benefits?
7. Ask for another volunteer. This will be the well pump student. Ask the student to pump one of the wells in the confined aquifer. What happens to the water levels in all of the wells when this is done vigorously? Review drawdown and cone of depression and how this can work with contamination.
8. Now, ask the "polluter" to fill up the UST (underground storage tank) with red dye. This will represent gasoline (high BTEX and with MTBE!). Ask them to fill the landfill with blue or green dye. This represents the leachate. Both leak!
9. Have student record their observations. The chemicals are slowly leaking into the unconfined aquifers and river and pond.
10. Now, have the pumper pump the well hard again. Have students pay attention to what happens (the contamination is going towards that well and polluting the other wells in its path). Explain how industries can easily do this.
11. Eventually the recharge reservoir also becomes contaminated.
12. Students should observe that the confining aquifer was the last to be polluted.

Summary

Students will typically have many different questions concerning the terms and the scenario. Take time and really go over it with them. By watching the aquifers and seeing what happens with the simulator, students can understand and apply these difficult concepts to the real world. Students can also mark the different features on the simulator using a grease pencil.