What is groundwater?
Groundwater comes from rain and snowmelt that seeps into the ground. Gravity pulls the water down through the spaces between particles of soil or through cracks in rocks. Eventually the water reaches a depth where all openings in soil or rock are filled with water; this is called the saturated zone. The water in the saturate zone is called groundwater.
The top of the saturated zone is called the water table. The water table rises and falls according to the season and the amount of rain and snowmelt that has occurred during a particular year. In Oregon it is higher in the winter and spring, and at its lowest around October.
Note that bedrock below the saturated zone prevents the water from penetrating more deeply.
Water moves through spaces in soil and rock
Different kinds of soil and rock vary in the size of the spaces for water to move through. It is easier for water to move through bigger spaces. Gravel has very large spaces so water moves through it very fast. On the other hand, the spaces in clay are so small that almost no water moves through.
Some layers of rock are so solid that they don't let water move through; this is called an impermeable layer. Others are very crumbly or have lots of big cracks. If the cracks are connected to each other, then water can move thorough the rock.
Permeability decreases moving left to right due to soil and rock type.
A saturated soil or rock layer with spaces that allow water to move through it is called an aquifer. Aquifers may be separate by layers of rock or clay that do not allow water to move through it.
In Oregon there are three main types of aquifers:
Many layers of basalt from ancient lava flows cover much of eastern Oregon and parts of the northern Willamette Valley. Between the layers of basalt there is soil and gravel that was on the surface at the time of each flow. Cracks in the basalt and the layers between flows form aquifers.
In the river valleys there are layers of sand and gravel forming aquifers, sometimes separated by layers of clay.
Bedrock with connected cracks forms aquifers in many parts of the state, especially in the hills above river valleys.
Groundwater below a layer of solid rock or clay is said to be in a confined aquifer. The rock or clay is called a confining layer. A well that goes through a confining layer is known as an artesian well.
The groundwater in confined aquifers is usually under pressure. This pressure causes water in an artesian well to rise above the aquifer level. If the pressure causes the water to rise above ground level, the well overflows and is called a flowing artesian well.
Unconfined (water table) aquifers
Aquifers that are not below a confining layer are called unconfined aquifers. Because the top of these aquifers is the water table, they also are called water table aquifers. In a water table aquifer, the water level in a well is the same as the water table level outside the well.
This section, including most illustrations, was adapted with permission from "What is Groundwater?" by Lyle Raymond, Jr. (© Cornell Cooperative Extension, Cornell University, July 1988).