What is an aquifer?

You may have heard someone say that their well sits over an underground river or lake, but rivers and lakes create the impression of a water surface with air above, when in fact groundwater exists in the tiny spaces between grains of sand, silt, and clay, or in the cracks common in bedrock.

Most wells — whether they're used by community water systems, industries, or private well owners — draw their water from an aquifer. An aquifer is a geologic unit, usually either sand and gravel or crack and crevice bedrock, that can provide usable quantities of water to a well. That means that the aquifer has to be porous enough to hold enough water for a well to pump and that the water has to be able to flow through the aquifer easily enough to supply the well as it is pumped. This is permeability. The geologic units that slow or prevent the movement of water are called confining layers, or aquitards. There can be multiple aquifers and aquitards at a single location, as shown here.

Porosity is a measure of the relative volume of pore space per unit volume of geologic unit. So an aquifer with the porosity of 25% means that 25% of the total space it occupies is void space, full of water. Porosity can vary. For example, a sand and gravel aquifer can have a porosity of 20 to 30%. Even clay, which is an aquitard, can have high porosity upwards of 50%.

Clay is an aquitard because it is hard for water to move through it, which we will discuss in a minute. Bedrock can be highly fractioned and creviced so that overall it has enough void space to be an aquifer, or it can be almost solid throughout with a porosity of only a few percent. If your well is in a geologic unit with low porosity, you might have limited water availability. In many bedrock aquifers, if a well doesn't go through multiple or large fractures or crevices, water availability can be limited to just a few gallons a minute or less.

As we said earlier, aquifers don't just have to have enough water — The water also has to flow through them easily to be able to replenish a pumping well. This is defined as an aquifer's permeability.

In Figure 1, we see the typical permeability for the major types of aquifer materials. Large grained, unconsolidated deposits — like sands and gravels — have high permeability, meaning that water can easily flow between the grains to a well. Some sand and gravel aquifers can allow water to move through them at a rate of as much as 3,000 feet a day. Water doesn't move through all aquifers that fast, but that's a potential upper limit for most aquifers. But as the particles get smaller, it gets harder and harder for water to move between them, lowering the permeability.

Really small particles, like clays and silts, may have high porosity up to 50% water by volume. But because the particles are so small, it's hard for the water to move around the particles to a well. Geologic units made up of clay and silt are generally considered to be aquitards, which means that they don't supply a sufficient quantity of water to a well.

Permeability is important because a well generally draws water from an area of the aquifer around the well. If water can't get to the well easily, it will limit how much water can be pumped, and pumping it too high a rate will draw down the water in a well to the pump intake, interrupting the water supply.

In bedrock aquifers, most of the water is stored in fractures and crevices. To be an aquifer, the bedrock has to have enough fracturing and crevices, as well as connections between those openings, to supply sufficient water to a well. The rock itself isn't very porous and doesn't hold much water. One exception to this is sandstone, which not only has fractures and crevices, but can also hold water in the rock itself, or not completely cemented.

The aquifer you get your water from, and how that aquifer is recharged, will tell you a lot about your water supply. Like other natural resources, the groundwater available to you is dependent on your local conditions, mainly the geology of your location. In some cases, you might have options where multiple aquifers are available, but for many, there may only be one choice.

For some well owners, there is no real aquifer available for water supply at the location they want to put in a well. There are no sand and gravel aquifers, nor are there dependable bedrock aquifers. It could be that the ground water and deeper geologic units aren't potable. In other words, the water is unfit to drink. An example of this would be a bedrock aquifer that contains salt water. In these cases, the only groundwater option might be a shallow dug or bored well. These wells are meant to be storage reservoirs, collecting water from near-surface infiltration and/or thin lenses of sand in areas that otherwise have mostly clay or silt-based geology.

The Private Well Class is a collaboration between the Rural Community Assistance Partnership and the University of Illinois through the Illinois State Water Survey and the Illinois Water Resource Center, and funded by the United States Environmental Protection Agency. The views expressed here solely from the class authors and not endorsed or reviewed by US EPA for more information on private wells, sign up for our free 10-week email course at www.privatewellclass.org.