A cone of depression forms in the water table or potentiometric surface of an aquifer when a well is pumped. This depression results from the water level or pressure dropping around the wellbore, creating a hydraulic gradient that causes groundwater to flow toward the well. The cone’s size and shape are dynamic, constantly changing as long as the well is pumping. Determining when this growth will cease is a primary interest for water managers.
The Dynamic Process of Cone Enlargement
When a pump is activated, the cone of depression expands outward and downward as the pressure reduction propagates through the aquifer. Initially, the withdrawn water is sourced entirely from the aquifer’s internal storage (water held within the rock or sediment pores). This early stage is a transient process where water levels continue to decline over time away from the well.
The lateral growth of the cone is the movement of a pressure wave, which slows down as it moves farther from the well. As long as the rate of extraction exceeds the rate at which new water flows into the depressed area, the cone must widen and deepen. This continuous expansion increases the hydraulic gradient over a larger area to meet the pumping demand.
Aquifer Properties That Govern the Spread
The physical characteristics of the aquifer material control the rate and shape of the cone’s propagation. Transmissivity (T) describes the aquifer’s ability to transmit water through its saturated thickness. High transmissivity allows water to flow easily, resulting in a cone that spreads widely but remains shallow.
Conversely, low transmissivity resists water flow, forcing the cone to become narrow and deeper around the well to create a steep gradient sufficient to supply the pump. The other important characteristic is the storage coefficient (S) or specific yield (Sy), which quantifies the volume of water released from storage for a given drop in water level. Aquifers with lower storage coefficients, particularly confined aquifers, see the cone expanding more rapidly and extensively because less water is released from storage per unit of pressure drop.
Achieving Equilibrium: When Drawdown Ceases
The enlargement of the cone of depression stops when the system reaches a steady state, or equilibrium. This point is reached when the volume of water pumped is precisely balanced by the amount of water flowing into the cone’s area of influence from an external source. Until this balance is achieved, the cone must continue to expand by drawing water out of the aquifer’s storage.
The long-term stabilization relies on a process called capture, where the pumping well intercepts water previously discharging naturally elsewhere. This captured water might have been flowing toward a distant stream, lake, or wetland, or lost to evaporation by plants.
Another mechanism for stabilization occurs when the cone expands to intersect a constant head boundary, such as a large river or lake. When the cone reaches a surface water body, the difference in head can induce flow from the river or lake directly into the aquifer, known as induced recharge. In both scenarios, the external source provides the necessary water to match the pumping rate, halting the continued decline of the water level. In large, confined aquifers lacking these boundaries, the cone may continue to expand for a very long time, drawing water from storage for generations.
Practical Consequences of Extensive Drawdown
When a cone of depression expands widely, it has significant implications for the surrounding environment and other water users. The most immediate consequence is well interference, where the cone from one pumping well overlaps with and lowers the water level in a neighboring well. This combined effect accelerates water level decline, potentially causing shallower wells to run dry.
The capture of water intended for natural discharge can lead to stream or wetland depletion, reducing the base flow that sustains surface water ecosystems. In coastal areas, reduced groundwater pressure allows saltwater to move inland into the aquifer, contaminating the freshwater supply (saltwater intrusion).
In aquifers composed of fine-grained sediments, the loss of water pressure can cause the sediment structure to collapse. This results in land subsidence, or the sinking of the ground surface, which can damage infrastructure.