The cone of depression is a hydrogeological feature that represents the localized lowering of the groundwater level caused by the extraction of water from an aquifer. This phenomenon occurs when a well actively pumps water, creating a temporary depression in the surrounding water table or potentiometric surface. Understanding this depression is fundamental to managing groundwater resources, as it directly impacts the availability of water for the well causing the effect and for neighboring wells. The effect is not permanent and begins to dissipate once the pumping stops, allowing the water level to recover.
How Pumping Creates the Cone
The formation of the cone of depression begins the moment a well starts extracting water from an aquifer at a rate that exceeds the local rate of water flow toward the well. Before pumping, the groundwater is at its static water level, which is relatively flat across a small area. As water is removed, the water level immediately inside the well casing drops, a process known as drawdown. This initial drop establishes a hydraulic gradient, meaning a slope of water pressure, between the higher water level in the aquifer and the lower level inside the well.
Water naturally flows from areas of high pressure to areas of low pressure, so the established gradient forces the surrounding groundwater to move radially toward the well screen. Since the pumping rate is greater than the rate at which the aquifer materials can transmit water to the well, the water level in the aquifer begins to drop in a curved pattern. The physical properties of the aquifer material, such as its hydraulic conductivity, play a large role in how quickly this depression forms and expands. Aquifers with high permeability allow water to flow more easily, which can limit the depth of the cone but allow it to spread over a wider area.
The cone continues to deepen and expand outward until a state of dynamic equilibrium is reached, where the rate of water flowing into the cone from the surrounding aquifer equals the constant rate of water being pumped out. In unconfined aquifers, the cone represents an actual physical depression in the water table itself. Conversely, in confined aquifers, where the water is trapped under pressure between impermeable layers, the cone is represented by a lowering of the potentiometric surface.
Describing the Shape and Extent
The shape of the depression is three-dimensional and generally resembles an inverted cone, which is why it is given its name. The maximum vertical distance the water level has dropped from its original static level occurs closest to the pumping well; this specific measurement is called the drawdown. As the distance from the well increases, the amount of drawdown progressively decreases, causing the characteristic sloping sides of the cone.
The horizontal boundary of the cone of depression is defined by the radius of influence, which is the radial distance from the pumping well to the point where the water level remains unaffected by the pumping. Beyond this radius, the water table or potentiometric surface is still at its original elevation. The slope and extent of the cone are dynamic and change based on the pumping rate and the duration of pumping. A higher pumping rate or a longer period of continuous pumping will result in a deeper cone with a larger radius of influence.
The geological characteristics of the aquifer also dictate the cone’s geometry. A less transmissive aquifer, one that resists water flow, will lead to a deep, steep-sided cone with a smaller radius. In contrast, a highly transmissive aquifer will produce a shallow cone that spreads out over a larger area. When pumping ceases, the cone of depression gradually begins to fill in as the surrounding groundwater flows back into the area, and the water level recovers toward the original static level.
The Practical Effects of a Cone of Depression
The most immediate practical consequence of a cone of depression is well interference, which occurs when the cones of depression from two or more nearby wells overlap. This overlapping causes an additive effect, resulting in a greater total drawdown in each well than would occur from a single well pumping alone. If the combined drawdown is significant, a neighboring well may experience a severe drop in its water level, potentially causing it to run dry or requiring the well owner to install a deeper pump.
In areas near surface water bodies, an expanding cone of depression can intersect a river, stream, or wetland, leading to an effect known as induced recharge or streamflow reduction. Once the groundwater level drops below the level of the surface water body, the cone begins to draw water from the river or lake into the aquifer to supply the pumping well. This diversion of flow can lower stream levels, deplete wetlands, and negatively impact local aquatic ecosystems that depend on consistent water levels.
Long-term, deep cones of depression in coastal regions can trigger saltwater intrusion. The lowered freshwater pressure allows denser saltwater to migrate inland and contaminate the aquifer. Furthermore, excessive and prolonged drawdown in unconfined aquifers can reduce the pressure that supports the overlying soil structure, potentially leading to land subsidence.
Conversely, the cone of depression can be strategically utilized in environmental remediation. This involves creating a capture zone around a contaminated site to prevent the spread of a pollutant plume.