Although groundwater can be found beneath all land surfaces in Washington, not all groundwater can be drawn into wells. An aquifer is a geologic formation that is capable of providing a usable amount of water to a well or spring. The amount of water a well will yield depends on the porosity and permeability of the aquifer and the well's construction. The amount of empty space in a material, or its porosity, determines how much water it will hold...the more pores, the more water.

Porosity

Porosity is expressed as a percentage of the total volume of a material. For example, the porosity of a certain sand might be 30%; that is, 30% of the total volume of the sand is pore space and 70% is solid material.

Bedrock is pretty solid. So it's porosity is determined by the amount of fractures in the rock. So, highly fractured bedrock holds more water than less fractured bedrock.

Bedrock is the rock that lies beneath all soil and loose rocks on the surface of the earth. It is the earth's crust. If a well intersects bedrock fractures that are filled with water, bedrock can serve as an aquifer.

Generally, fractures within the first 200 to 300 feet of the surface will supply enough water for private, home use. Some highly fractured zones known as faults can yield many thousands of gallons per day, and may be developed for municipal or industrial use.

Most of the mountainous areas of Washington have highly fractured bedrock. In eastern Washington, basaltic bedrock areas are generally well fractured.

Permeability

The ease or difficulty with which water flows through a material is controlled by the material's permeability. A material that is very permeable allows water to pass through it easily. Permeability is important because it determines how quick ground water can be drawn into a pumping well. In bedrock, permeability depends on how well the fractures in the rock are connect with each other.

Well-connected fractures will allow water to move easily; poorly-connected fractures will not. In a sand and gravel pit, permeability depends on the size of the pore spaces between the grains of material. Water moves quickly through large pore spaces and slowly through small spaces.

Porosity and permeability are related, but are not the same. A material can be very porous and hold a large volume of water, but not be very permeable. For example, clay may be twice as porous as sand, but a pumping well will not be able to pull the water from the pores between the clay particles fast enough to supply the well with water. Very small pore spaces resist flow reducing permeability.

Porosity determines the capacity of a material to hold water.

Permeability determines its ability to yield (provide) water.

Geologists call gravel, soil, rock and other loose material that lie on top of the bedrock "Surficial deposits" or "overburden". (The rest of us call it soil.) Most surficial deposits consist of a range of material types and sizes, such as, clay or sand. So, the ease which we can retrieve water also depends on the surficial deposits.