How a Well Affects an Aquifer

Cone of Depression

As the well pumps, the ground water elevation around the well drops, typically in the shape of an inverted cone. This drawdown cone is also referred to as the cone of depression and it is the area where the ground water elevation or water table is depressed due to pumping, see Figure 15. In an unconsolidated, porous aquifer setting, the cone of depression forms around the well head in an ever expanding circle as more water is pumped from the aquifer. In a consolidated aquifer setting, the cone of depression will follow the subterranean fracture systems and may take an unpredictable shape as the cone expands outwards to pull more water into the well. In an artesian system, the cone may extend for hundreds of feet.

Any water and contaminants (if present) within the cone of depression around the well is eventually captured and drawn into the well and your water supply system. If the cone extends out and beneath a river or stream, the well will begin pumping river water that has been pulled through the river bed, through the aquifer, into the well. If the cone extends out and beneath a contaminant source, such as a leaky gas station storage tank or a land fill, the well will begin to draw the contaminants into the well. If the cone intercepts a neighboring cone of depression from a nearby well, the rate at which the ground water elevation drops will rapidly increase, and both wells may run dry faster.

The rate at which the ground water elevation rebounds to its original or near original level after the pumping stops is important to the overall operation of the well. If water level recovery is slow, excessive use—over a weekend, for example—may temporarily cause the well to go dry. Pumping from the well should be managed to reduce this type of cyclic dewatering.