Bannock County Groundwater

Aquifer Basics

What is an aquifer and what is groundwater?

An aquifer is any water-saturated rock or sediment body that contains sufficient quantities of water, for example, enough to supply water to a city or a single residence. This water, called groundwater, occupies the pore spaces, cracks, and spaces between soil and rock particles. In most cases, aquifers are not underground rivers.

Water movement in aquifers is highly dependent on the permeability of the aquifer material or how well the pore spaces or cracks are interconnected. In some permeable aquifers, groundwater may move several feet in a day; in other places, it moves only a few inches in a century. Groundwater moves very slowly through relatively impermeable materials such as clay and shale.

Water in an aquifer can be discharged into springs, streams, or withdrawn from the ground by wells. This discharged water must be replaced by new water to replenish or recharge the aquifer. Thus, every aquifer has a recharge zone or zones.

The boundaries of an aquifer are sometimes difficult to define and commonly merge with the boundaries of other aquifers, forming an aquifer system.  If the top of an aquifer is the water table, then the aquifer is called an unconfined aquifer.  If an aquifer has at least one layer of clay or low-permeability sediment at its top, it is called a confined aquifer. Water in a confined aquifer, if tapped by a well or spring, will rise above the top of the aquifer and may flow from the well or spring onto the land surface under artesian pressure. In this case, the aquifer is called an artesian aquifer.

Pumping can affect the level of the water table

If an aquifer is shallow enough and permeable enough to allow water to move through it at a rapid-enough rate, then people can drill wells into it and withdraw water. The level of the water table can naturally change over time due to changes in weather cycles and precipitation patterns, streamflow and geologic changes, and even human-induced changes, such as an increase in impervious surfaces on the landscape and over pumping.

The pumping of wells can have a great deal of influence on water levels below ground, especially in the vicinity of the well, as this diagram shows. If water is withdrawn from the ground at a faster rate that it is replenished, either by infiltration from the surface of from streams, then the water table can become lower, resulting in a “cone of depression” around the well. Depending on geologic and hydrologic conditions of the aquifer, the impact on the level of the water table can be short-lived or last for decades, and it can fall a small amount or many hundreds of feet. Excessive pumping can lower the water table so much that the wells no longer supply water—they can “go dry.”

What is the hydrologic cycle?

The hydrologic cycle is a description of the generic reservoirs of water on earth including moisture in the atmosphere, water in plants and animals, lakes, rivers, wetlands, polar ice caps and glaciers, groundwater, oceans and seas and how water moves between these reservoirs in various forms (gas, liquid, solid).  It is an interconnected cycle – water falls as precipitation, is used by plants and animals, infiltrates into the ground, and is evaporated and transpired back into the atmosphere. All the water that has ever existed on earth moves through this continuous cycle.

Because of these interconnections, human effects on the hydrologic cycle can cause problems.  For example, groundwater extraction from shallow aquifers can alter stream flows.  And conversely, diversion of water from streams and rivers for irrigation can have a major impact on local groundwater tables, causing water levels to decline.

Some information on this page is from “Ground Water and the Rural Homeowner, Pamphlet”, U.S. Geological Survey, by Roger M. Waller, 1982.

Protect Our Aquifer…..We Drink It!


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