Groundwater Contamination

The science background section provides teachers with more in-depth information on the phenomena groundwater contamination students explore in this unit. Below is an excerpt from the science background section on groundwater contamination.

Accessing Water

The study of geology that focuses on the distribution and movement of groundwater in the soil and rocks of Earth’s crust is called hydrogeology. Scientists are particularly interested in this field because of the important role that groundwater plays in human development. Given that only 2.5 percent of all water on Earth is fresh water, and 70 percent of all fresh water is frozen in ice caps and glaciers, much of the population growth and economic development around the world has been possible because people have had access to groundwater.

About 1.5 billion people today depend on groundwater for their drinking water. In the United States, 50 percent of people in the United States use groundwater for drinking water, agriculture, and industry, especially in the western United States.

People drill wells into the ground to access groundwater. This water can then be brought to the land surface by a pump. However, the level of the water table can change over time. The water table is the highest point in an aquifer from which water can be obtained. Natural factors that influence the water table level include changes in the water cycle patterns and changes to the geology of the region. For example, in times of drought, the water table drops because the amount of precipitation seeping into the ground declines. However, people often turn to aquifers during times of drought to make up for the loss of available surface water, decreasing water levels even more.

Healthy Aquifers

When people withdraw excessive amounts of groundwater, they upset the natural balance of the water cycle. Earth is constantly changing as matter cycles and energy flows through it. In the same way that the interacting parts of an ecosystem act as checks on one another, so do the various parts of Earth’s systems interact to maintain balance.

Aquifers remain stable when the amount of water being added to the aquifer from precipitation is roughly the same as the amount of water leaving the aquifer over time. Aquifer recharge occurs when water seeps into the ground and replenishes the aquifer, such as when precipitation soaks into the ground. Aquifer discharge occurs when water leaves the aquifer, such as when it seeps into a spring or when people use wells. When people pump too much groundwater, they deplete the aquifer, causing wells to run dry. If the rate of discharge from an aquifer is greater than the rate of recharge, the aquifer will become depleted. Some aquifers take hundreds or thousands of years to recharge.

Regions around the world are already experiencing groundwater depletion. For example, the largest aquifer in North America is called the Ogallala Aquifer, and it lies underneath seven western states: Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. It is used for human drinking water, as well as for industrial and agricultural use. The majority of the water that fills the Ogallala Aquifer has been there for millions of years. However, the Ogallala Aquifer is currently being depleted faster than it can be replenished. This worries many people because it is the single most important source of fresh water in this region.

Groundwater pumping can also harm ecosystems. Because groundwater is so connected to surface water, the removal of groundwater impacts the quality of surface water. Aquifers discharge groundwater to the surface, adding water to many streams and rivers. When people pump too much water from an aquifer, there is less water that is discharged to streams. This harms the aquatic habitats of many species, including fish and amphibians that live in streams.

Keeping aquifers healthy is also important to scientists because groundwater is some of the cleanest water on Earth. After studying the structure of aquifers, scientists realized that the particles of rock that make up aquifers act as a natural filter as water moves through the layers of materials. Filtration is the process of separating solid matter from a fluid by having the fluid pass through the pores of another substance, called a filter. Because they act as filters, aquifers provide an ecosystem service—the positive benefits that an ecosystem provides to people. As water moves from Earth’s surface underground, it is filtered, becoming purer. By the time water has moved through the aquifer, many pollutants have been removed.

The water quality in deeper aquifers is often better because more contaminants are filtered out. This is because aquifers are natural filters that trap sediment and other particles such as bacteria. Aquifers act like coffee filters. In the same way that coffee filters trap coffee grounds but allow the coffee to flow through, the pore spaces in an aquifer's rock or sediment prevent sediments and other larger particles from flowing through while allowing the movement of water.

However, different aquifers are made up of different kinds of rocks, which have different pore sizes. For example, some aquifers are igneous or metamorphic rock, which are generally impermeable. They hold water in cracks or fractures, and water does not filter through them as easily. As a result, they are less able to filter out contaminants.

Studying Earth’s Systems

Because of the critical role that groundwater has in supporting life, scientists are trying to better understand groundwater on Earth. Groundwater is difficult to study because it is underground. It is also unevenly distributed around the planet. As a result, scientists map the geology and the underground structures of different areas to understand how groundwater moves through different materials, how quickly it is recharged in various locations, and how it interacts with other Earth systems. To do this, scientists use deep wells up to 2,000 feet underground to monitor the groundwater and to observe what happens when additional groundwater is removed.