An Underground River

Figure 1

Engineering Connection

Groundwater is an important source of drinking water in the world, and one that is at growing risk of becoming polluted by fertilizers, pesticides and industrial pollutants such as dry cleaning chemicals and dyes. Environmental engineers are challenged to clean the groundwater and restore it to a natural or usable state so that it remains free of harmful chemicals that could contaminate the drinking water supply and make people sick.

Figure 1:

Aquifer
Aquitard
Unsaturated Zone
Water Table
Confined Aquifer
Unconfined Aquifer
Deep Well
Sort Well
Artesian Well

Introduction/Motivation

Is there actually a river under the ground? In some sense yes, but it does not look like a river that we see outside. Water under the ground saturates the soil or dirt below the surface. It flows like a river, but very, very slowly. This slow moving water under the ground is called the groundwater.

All of the ground is made up of individual particles of soil — some tiny and some large. The spaces between particles are called pores. The measure of how much volume of pore space exists in specific soil is called porosity. The larger the pores, the more room 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. Sometimes the water stores up in a large underground area, such as a lake; this is called an aquifer. Aquifers are often used as a water resource for drinking water.

Groundwater is a very important source of drinking water in the US and other parts of the world. It is also at risk of becoming polluted by a range of chemicals, including pesticides, fertilizers, dyes, pharmaceuticals (medicines), dry cleaning chemicals, spilled gasoline and paint, and other chemicals that are released by human activity. After the 1940s, production of these types of chemicals drastically increased, and they were used for many industrial and agricultural processes.

Unfortunately, many people did not know how bad the chemicals were and disposed of them irresponsibly: dumping chemical waste straight into rivers, streams, lagoons or anywhere convenient. The result of carelessly disposing of chemicals is a very toxic, very dangerous pollution that ultimately seeps through the ground into the groundwater. 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.

Water Cycle

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 2).

Figure 2: The Water Cycle

Groundwater

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, while lakes or streams that gain water from groundwater are called gaining streams or lakes.

Porosity and Permeability

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. Many different kinds of soils exist and each 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 these soil types is different due to 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 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:

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. Refer to the associated activity How Full Is Full? for students to learn about porosity and permeability and relate these concepts to groundwater flow.

Groundwater Flow

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

v = kl

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 (that is, 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 (that is, the steepness of the slope).

What is an Aquifer?

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. Two different types of aquifers exist: 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 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.

Groundwater Pollution

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 such as gasoline behave in a similar way when they seep into groundwater: when spilled, they can contaminate the groundwater, then be pumped up from wells, and end up in your drinking water (see Figure 5).

Figure 5

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