For the most part, water is like energy: it is neither created nor destroyed. With the exception of a few chemical reactions that create and use water, most water cycles around the earth simply change the form of the water. For example, one water droplet in New York can condense, become part of a cloud, and then fall from the sky as rain or snow in Chicago, IL. From Chicago it might travel through waterways down the entire length of the Mississippi River to the Gulf of Mexico and end up in the ocean for years. One day, it might evaporate and travel in the air; then, the cycle starts over again. Another water droplet could be sprayed from a sprinkler onto corn crops in Iowa, taken up and used by a corn plant, and transpired into the air again. If the same water droplet was not taken up by the corn, but instead filtered through the ground, it could become part of the groundwater and travel through the ground for thousands, even millions of years. Each water droplet has its own story, but the entire process (called the water cycle) is summarized as the following components: evaporation, transpiration, condensation and precipitation (see Figure 1).

Figure 1. The water cycle. click for copyright

A lot of water travels over the surface of the Earth, but we tend to forget about what we cannot see under the ground — the groundwater. Groundwater makes up about 90% of the useable fresh water on the Earth. It is a very important resource as well as a very important part of the water cycle. Groundwater interacts with surface water. Water can flow from a lake or stream into the ground, travel for miles underground, and then flow back into another lake or stream or be pumped out through a well. Lakes or streams that lose water to the groundwater are called losing streams or lakes. And appropriately, lakes or streams that gain water from groundwater are called gaining streams or lakes.

Soil is everything that collects on the surface of the Earth that is loose or unconsolidated. It is made up of little grains that can come from the weathering of rocks or minerals or the biodegradation of organic matter that has died, such as sea algae or tree and plant leaves. In addition, all soils have a little bit of water. There are many different kinds of soils. Each soil has a different composition of rock/mineral grains, organic matter and water. Some common types of soil are sand, gravel, silt and clay.

Each of the soils mentioned above is different because they have different grain sizes, as illustrated in Figure 2. Sand has the largest grains and clay has the smallest grains. Small grains can be packed very tightly, whereas large grains cannot be packed as tightly. For example, if you were to fill your classroom with balls, could you fit more bowling balls in the classroom or more golf balls in the classroom? You could fit more golf balls, because they are smaller and can be more tightly packed together. The spaces in between the grains are called pores. Soils with larger grains have larger pores and vice versa. The measure of how much pore space a specific soil has is called porosity. The mathematical definition is:

Figure 2. Pore spaces. click for copyright

The larger the pores, the more room there is for water to flow through the soil. Therefore, water flows through soils with high porosity more easily. Permeability measures how easily water flows through soil. Soils with high porosity have high permeability, and soils with low porosity have low permeability. Porosity and permeability are used most often to determine how quickly and easily groundwater flows.

Although it is slow, water normally flows through the ground. How much water flows through the ground, or the groundwater flow rate (Q), is determined by Darcy's Law. Darcy's Law can be written as follows.

The groundwater flow rate is controlled by hydraulic conductivity (k) and the hydraulic gradient (I). Hydraulic conductivity is a direct measure of the permeability and porosity of the soil (i.e., if the soil has a high porosity, then it has a high permeability and, thus, a high hydraulic conductivity and flow rate). The hydraulic gradient is dependent on the change of elevation. Consider a mountain with one side that is very steep and one side that is gently sloping; would water flow faster down the steep side or the gently sloping side? (Answer: It would flow down the steep side faster.) Groundwater flows just like surface water — it generally flows from an area of high elevation to a lower elevation. The steeper the change in elevation, the quicker the water flows. Hydraulic gradient measures the change in groundwater elevation (i.e., the steepness of the slope).

An aquifer is a permeable layer of rock or soil that is saturated with water. Technically, an aquifer must be able to yield useable amounts of water when pumped. Most aquifers are soils such as sand or gravel that become filled with water. The level that the water raises to is called the water table. There are two different types of aquifers: confined and unconfined. Confined aquifers are "pressurized" by impermeable layers of rock or soil called confining layers above and below the aquifer. Because water cannot flow through a confining layer, the aquifer acts like a pipe: the water is under pressure and flows faster. In an unconfined aquifer, the water is not confined by anything and the water level — or water table — can move up and down. Below the water table, all the pores in the soil are filled with water, or are saturated. This is called the saturated zone. Above the water table, there is soil that is not saturated with water. This is called the unsaturated zone or vadose zone. See Figure 3 for a visual illustration of the layers of an aquifer.

Figure 3. Layers of an aquifer. click for copyright

When it rains, water is filtered through the unsaturated zone and ends up in the groundwater. Once it is in the groundwater, it can travel far distances under the ground, until it reaches a well. There, it remains until someone pumps it for drinking water or irrigation use. Unfortunately, this is how aquifers and groundwater become polluted. For example, if someone spills gasoline on the road, it will eventually end up in the groundwater. On average, groundwater moves very slowly, around 0.01 to 0.000001 cm/s (that is on the order of feet per day). Because groundwater moves so slowly, pollutants create plumes, or areas of contaminated groundwater. Imagine what would happen if you poured a lot of food coloring into a lake: would it all stay right where you poured it? No, it would spread out and eventually become diluted in the lake. Pollutants like gasoline are similar when they seep into groundwater: when spilled, they can contaminate the groundwater, then be pumped up from wells, and wind up being used as drinking water (see Figure 4).

Figure 4. Pollution migrating under the ground. click for copyright

Discussion: Ask students the following questions and encourage them to come up with several answers. Tell them that there is no right answer. Explain to the students that they can talk freely, but have to talk one at a time. If they cannot talk one at a time, then have them raise their hands to answer. Keep a list of their answers on the board.

• Where do you think we get our drinking water? (Possible answers: reservoirs, lakes, rivers, streams, wells and groundwater.)

Question and Answer: Ask the students the following questions.

• What is groundwater? (Answer: water under the ground)
• How could groundwater become polluted? (Possible answer: If there are pollutants on the ground, then they can seep through the ground into the groundwater.)
• How do we use groundwater? (Answer: Groundwater is a major source of drinking water.)
• How fast does groundwater move? (Answer: slowly)
• What different things pollute the groundwater? (Possible answers: chemicals such as chlorinated solvents; e.g., TCE, PCE, and PCP, fertilizers such as nitrate and pesticides such as DDT.)
• What is an aquifer? (Answer: water stored in permeable rock and soil that produces a useable amount of water when pumped.)
• What do engineers have to do with groundwater? (Possible answers: Environmental engineers are challenged to clean the groundwater and restore it to its natural or a usable state; they work to keep the groundwater free of harmful chemicals that could potentially get into a drinking water supply and make people sick.)

Letter to the Mayor: A textile industry used to own a plant on a hill outside of Happy Town. The plant is known to have dumped various chlorinated chemicals into an unlined lagoon next to the plant from when they opened in 1955 until they went bankrupt in 1980. The town does not consider this lagoon to be a risk, but GR Water Engineering Firm, who employs the students, did some tests and found out that the local water table is really shallow. The groundwater is flowing from the lagoon toward the town well. Have students write a persuasive letter to the Mayor of Happy Town explaining the danger of their town's groundwater supply becoming polluted.

Their letters should include some description of how groundwater flows and how long it will take for water supply to be polluted. Give the students a list of the following facts.

1. The groundwater is flowing from east to west.
2. The lagoon is directly east of the town's well.
3. The groundwater is flowing 0.5 miles/year.
4. The lagoon is approximately 25 miles from the town well.

(Note: The pollution would reach the town in 50 years at this rate. It would start entering the town's water supply in approximately 2005 and will continue to do so for 25 years from that time.)