Groundwater drawdown defines the vertical distance the water level drops within an aquifer due to water removal. This lowering affects the water table in unconfined aquifers or the potentiometric surface in confined aquifers. Drawdown is always measured as the difference between the original, non-pumping water level, known as the static water level, and the water level observed while pumping is occurring. Understanding this concept is fundamental to sustainable water resource management because it indicates the aquifer’s ability to supply water without being depleted over time.
How Pumping Creates Drawdown
When a well begins to extract water, the surrounding water cannot instantly flow in to replace the removed volume. This imbalance forces the water level inside the well to drop immediately, creating a pressure gradient in the aquifer material. Water then flows radially toward the well to compensate for the extraction. This movement causes the water level in the immediate vicinity of the well to decline, forming the characteristic cone of depression.
The cone of depression is a funnel-shaped decline in the water surface, with the deepest point directly at the pumping well. This geometry represents the change in hydraulic head that drives groundwater flow toward the point of extraction. The size and depth of this cone are directly related to the rate and duration of pumping. A higher pumping rate or longer pumping period will result in a wider and deeper cone of depression, extending the area of influence further into the aquifer.
The cone represents a dynamic equilibrium between the water being pumped out and the water flowing into the area. Once pumping ceases, the pressure gradient is removed, and the water levels gradually begin to recover toward their original static level. This recovery process demonstrates the aquifer’s capacity to return to a balanced state when the stress of extraction is lifted. The extent and speed of this recovery are also important indicators of the aquifer’s physical properties.
Aquifer Properties That Influence Drawdown
Drawdown magnitude depends heavily on the physical characteristics of the aquifer. Two intrinsic properties govern how the aquifer material responds to pumping: transmissivity and storativity. Transmissivity describes how easily water can flow horizontally through the saturated thickness of the aquifer. A formation with high transmissivity, such as coarse gravel, allows water to move quickly toward the well, resulting in a wide but relatively shallow cone of depression.
Conversely, a low-transmissivity material, like fine sand or fractured rock, restricts the flow of water, meaning the aquifer cannot supply the well as easily. This resistance to flow causes a steep, deep, and more localized cone of depression near the well bore. The second defining property is storativity, which measures the volume of water an aquifer releases from storage per unit decline in water level. In unconfined aquifers, this is often called the specific yield, representing the water released by gravity drainage of the pore spaces.
Confined aquifers, where the water is under pressure, have a much lower storativity because water is released only through the compression of the aquifer material and the expansion of the water itself. This difference explains why pumping in a confined aquifer can produce widespread drawdown across a large area, even with a relatively small water-level change. An unconfined aquifer can release a larger volume of water for a similar drop in the water table.
Monitoring Groundwater Levels
Tracking groundwater levels assesses the health and sustainability of an aquifer system. Monitoring is typically done using observation wells, which are non-pumping boreholes installed to measure the water level response to pumping or natural fluctuations. Measurements are taken manually using electric water-level sounders or continuously using pressure transducers. These submerged sensors record water pressure and convert it to a water level reading over time.
The most precise way to understand an aquifer’s potential for drawdown is through a controlled pumping test. During this test, water is removed from a pumping well at a constant rate for a set period, and the resulting drawdown is carefully measured in surrounding observation wells. Hydrogeologists analyze the time-drawdown data to calculate the aquifer’s specific hydraulic properties, such as its transmissivity and storativity. This allows for the prediction of future drawdown under various pumping scenarios, which is important for managing water allocations.
Effects of Significant Drawdown on the Environment and Wells
Excessive or sustained drawdown creates significant consequences for nearby wells and the environment. One common issue is well interference, which occurs when the cones of depression from two or more active wells overlap. This overlapping effect increases the total drawdown at each well, potentially reducing the water yield or causing a neighbor’s well to fail if the water level drops below the pump intake.
Well failure occurs when the water table falls below the bottom of the well screen or the pump intake, causing the well to run dry. Beyond the local impact on wells, deep or prolonged drawdown can disrupt the connection between groundwater and surface water bodies, such as rivers and wetlands. This results in reduced baseflow to streams, diminishing surface water supplies and negatively affecting ecosystems that depend on a shallow water table.
In certain geological settings, particularly those with interbedded layers of clay, excessive groundwater extraction can lead to land subsidence. As the water is removed from the compressible clay layers, the sediment compacts, resulting in a permanent sinking of the ground surface. Along coastlines, significant drawdown can also induce saltwater intrusion. Saltwater intrusion occurs when reduced freshwater pressure allows denser saltwater from the ocean to move inland and upward, contaminating the freshwater supply wells.