DEFINITION OF CONE OF DEPRESSION

A type of depression in the water table

formed by overpumping from a well. Wells are normally

drilled to penetrate well below the water table to allow for

seasonal rises and falls of the water table related to changes

in the balance between recharge and discharge from the

aquifer. If a water well is used to pump water out of the

aquifer faster than it can be recharged, then the water table

level around the well will be drawn down to a lower level,

approaching that of the intake pipe on the well. If the aquifer

is characterized by uniform material in terms of porosity and

permeability, then this area of drawdown will form a cone in

which the water table is depressed around the well. This in

turn creates a steeper groundwater slope and steeper

hydraulic gradient toward the well, causing the groundwater

to flow faster toward the well intake pipe.

The size of a cone of depression around a well depends

on the permeability of the ground, and the amount and rate

that groundwater is withdrawn. Faster and larger amounts of

withdrawal create larger cones of depression, and the cones

tend to have steeper sides in areas with lower permeability.

The formation of cones of depression is a result of the overdraft

of the aquifer, defined as when more water is taken out

of the aquifer than is replaced by recharge. Regional drawdown

and overdraft can cause the water table to be lowered

significantly on a regional scale, causing wells to go dry. In

coastal areas drawdown of the aquifer can lead to seawater

intrusion in which the seawater gradually moves into the

aquifer, replacing the freshwater with unusable salty water.

See also AQUIFER; GROUNDWATER.

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