Aquifers

Although groundwater can be found beneath most land surfaces in Massachusetts, not all groundwater can be drawn into wells. To yield significant quantities of water, wells must be located in aquifers. An aquifer is a geologic formation that is capable of yielding a significant amount of water to a well or spring. All the spaces and cracks, or pores, between particles of rock and other material in an aquifer are saturated with water. Water can move through the pores toward a spring or other discharge area or toward a pumping well.

More pores, more water: Porosity
The porosity of a material determines how much water it will hold--the more pores, the more water. Porosity is expressed as a percentage of the total volume of a material. For example, the porosity of a certain sand might be 30 percent; that is, 30 percent of the total volume of the sand is pore space and 70 percent is solid material. That means that 30 percent can be filled with water, or more than 2 gallons of water per cubic foot!

Water moves through pores: Permeability
The ability of a material to transmit water is called permeability. It is important to understand this concept because permeability determines whether groundwater can actually be drawn into a pumping well. In consolidated rock, such as granite, permeability depends on how well the fractures in the rock are interconnected. In an unconsolidated material, such as sand and gravel, permeability depends on the size of the pore spaces between the grains of material.

Porosity and permeability are different
Porosity and permeability are related, but they are not the same thing. 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 clay particles fast enough to supply the well. Very small pore spaces create a resistance to flow that reduces permeability. Porosity determines the capacity of the material to hold water. Permeability determines its ability to yield water.

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