An aquifer is defined as “a geologic unit which can store and supply significant quantities of water,” but that may not mean a lot to you unless you are a hydrogeologist. A simpler definition is anyplace underground which holds enough water to be useful. This article will provide a quick breakdown of what is and what is not an aquifer, give examples of different types of aquifers, and point you to some resources where you can learn more about the important natural resource called underground water.
Let's start by breaking down the hydrogeologist’s definition of an aquifer. A “geologic unit” is the ground upon which you are standing and everything underneath. It can be made of solid rocks, loose particles like sand or clays, or anything in between; even dirt is a geologic unit.
The second important part of the definition above is the term “significant” quantities of water. Significant to whom? Significant to people, of course! To be called an aquifer, the underground materials must provide enough water to be of use to people.
Water is stored within the small spaces between particles of sand and other sediments or within rocks (these spaces are called “pores”), as well as within the cracks or fractures in solid rocks. In an aquifer there are many pores or fractures connected together such that a lot of water is stored there. When people dig or drill a well into an aquifer, water flows into the hole and is easily pumped out for use.
In some cases, like in clays, water may be stored in the material, but the material is so tight or the pores are not connected such that it is impossible to pump the water out. This is called an aquiclude.
Aquifers are an important part of the hydrologic cycle. Fresh water evaporates from oceans, forms clouds which then produce rain. When rain falls onto the ground it either runs off into rivers or streams and finally to lakes or back to the oceans. But a lot of water also seeps into the ground, where it is then called groundwater.
Rivers, lakes, oceans and any other water bodies that we can see (that are above ground) are not aquifers. They hold surface water. Aquifers hold groundwater, and are always underground. Eventually, water may reach the end of an aquifer, for example on the side of a hill, where it flows out into a spring, at which point it is again called surface water.
Just like surface water, groundwater flows and moves around in the pores and fractures of the earth, but it does so very slowly. Imagine some raindrops that fall on a mountain, some of which run off into a stream rushing down the side of the mountain and some of which seep into the ground and begin flowing in the aquifer. The water in the stream may reach the ocean at the bottom of the mountain in a few days or weeks. The water in the aquifer, however, may take years or even tens of thousands of years to reach the ocean!
A common misconception about aquifers is that they are underground lakes, like was recently shown in the James Bond movie “Casio Royal.” Aquifers are only underground lakes in one very, particular case (which does not occur under a sandy desert as shown in the movie!). Underground lakes only occur in areas with “karst geology,” or where the rocks are made of limestone, like in Florida or Japan. In limestone, huge networks of caves are carved out of solid rock through natural chemical reactions and it is here, and only here, where you might find an underground lake. A hydro-geologist would call this a “karst aquifer,” not an underground lake.
Aquifers are classified as “unconfined” or “confined.” An unconfined aquifer is generally within the top layers of the earth and, specifically, the top of the water in the aquifer is exposed to air within the ground. The top level of the aquifer is free (or unconfined) to move up and down according to rainfall, evaporation or withdrawals by people.
A confined aquifer is overlain with an aquiclude, and is thus the top of the aquifer is not exposed to air and doesn't move, but instead the water pressure in the aquifer changes according to water coming in or out of the aquifer.
A special kind of confined aquifer is an “artisan” aquifer, where no pump is needed to extract the water. The water pressure in an artisan aquifer is very high and so when a well is dug, water rises above the surface of the earth without a pump. A famous example of this phenomenon is the fountains in Dijon, France where Henry Darcy developed some of the basic scientific theories about how water flows underground.
All aquifers are very a very important resource for our water supply. 20% of the freshwater on Earth is groundwater and as such must be protected for future generations. It may seem difficult to pollute groundwater because it is underground, but unfortunately, this is not true.
Just like water seeps into the ground, so can any other liquid and any solid waste can be dissolved into rain and also seep into the ground. When you pour out a cup of soda onto the ground, you are polluting the groundwater with sugar, acid and other harmful chemicals. If a huge manufacturing company dumps its waste into unlined ponds, these seep into the ground and pollute aquifers and make people very sick. The movie “Erin Brokovich” was based on a famous case such as this.
Now you know the basics about aquifers. They are rocks and sediments that hold water underground which can be exploited by people and are an important natural resource. To learn more or to look at some interesting pictures and diagrams, there are several sites on the internet which can help, like the US Geological Survey’s education pages; start with a basic lesson in aquifers and then explore more about water resources on the water education page. Whatever your interest level, remember that all natural resources, including groundwater and aquifers, are important to everyday life, even if you can't see them!
Sources: Freeze, Allen R. and John A. Cherry. Groundwater. Prentice Hall, New York, 1979.