Well drawdown is a fundamental measurement in hydrogeology that describes the drop in water level within a well when the pump is actively running. This vertical distance, typically measured in feet, reflects the stress placed on the underground water source, or aquifer, by the act of pumping. Understanding and tracking this measurement is a direct way to assess a water well’s health, efficiency, and the long-term sustainability of the local groundwater supply.
The Mechanics of Drawdown
The calculation of drawdown requires two distinct measurements: the static water level (SWL) and the pumping water level (PWL). The static water level is the resting, natural level of water in the well when the pump has been off long enough for the water to stabilize and fully recover. The pumping water level, also called the dynamic water level, is the stabilized water level observed while the pump is operating at a constant discharge rate.
Drawdown is simply the difference between the static water level and the pumping water level, representing the amount the water surface has lowered due to extraction. This lowering of the water surface in the aquifer immediately surrounding the well forms a physical phenomenon known as the cone of depression. The cone of depression is a conical-shaped dip in the water table that is deepest at the well bore and becomes shallower as the distance from the well increases.
The cone forms because a gradient, or slope, must exist to force water from the surrounding aquifer toward the well screen to replace the extracted volume. The size and depth of the cone expand until the rate of water flowing into the well equals the rate at which the pump is removing it. In unconfined aquifers, this is a physical depression of the water table, while in confined aquifers, it represents a reduction in the pressure head.
Factors Influencing Drawdown
The extent of well drawdown depends on both the characteristics of the aquifer and the operational parameters of the well itself. The rate at which water is pumped has a direct relationship to drawdown; a higher pumping rate demands quicker replenishment, resulting in a deeper cone of depression. If the pump removes water faster than the aquifer can transmit it, the drawdown increases significantly, and the well may temporarily run dry.
Aquifer properties, such as permeability, are major determinants of drawdown. Permeability measures how easily water flows through the aquifer’s rock or sediment material. Aquifers with low permeability exhibit greater drawdown because the material resists the rapid movement of water toward the well. Conversely, highly permeable aquifers can transmit large volumes of water with only a small drop in the water level, leading to lower drawdown.
The design and condition of the well structure also influence drawdown. A poorly developed well, where fine sediment remains around the well screen, can become clogged over time, reducing efficiency. This clogging, or incrustation from mineral deposits like iron, restricts the flow of water into the well. This forces the pump to pull harder, creating excessive drawdown to achieve the required flow rate.
Effects on Well Performance
Excessive drawdown reduces the well’s overall capability and increases costs. One immediate effect is a substantial increase in energy consumption and corresponding electricity costs. Since the pump must lift water from the pumping water level to the surface, greater drawdown means the pump is lifting water a greater vertical distance, requiring more work.
High drawdown can also lead to a reduced well yield, which is the volume of water the well can produce over time. If the pumping water level drops too low, the pump may begin to draw in air, a condition known as “pumping off,” which can temporarily interrupt the water supply and damage the pump. Sustained, excessive drawdown can also shorten the lifespan of the well and pump components.
The continuous stress of high drawdown can lead to increased sedimentation, pulling fine particles from the aquifer into the well bore. If the water level drops low enough to expose the well screen, it introduces more oxygen into the aquifer. This can enhance the growth of bacteria that cause well plugging and further reduce flow, accelerating wear and tear on the pump and the well structure.
Managing Excessive Drawdown
Managing high drawdown involves preventative measures, operational adjustments, and regular maintenance. The most direct adjustment is reducing the well’s pumping rate. Lowering the speed or discharge volume reduces stress on the aquifer, allowing the water level to stabilize at a higher, more sustainable pumping level. This is often achieved by installing a correctly sized or variable speed pump that is flow-controlled.
Periodic maintenance, known as well rehabilitation, is an effective method for correcting drawdown issues caused by well blockage. Techniques such as chemical treatment (like acidizing) or physical methods (such as jetting) can dissolve mineral incrustation and remove sediment from the well screen and surrounding formation. Restoring the well’s original efficiency reduces the resistance to flow, which lowers the necessary drawdown.
Ensuring the pump is correctly selected and placed is an important preventative step. The pump intake should be set deep enough to accommodate the expected pumping water level, plus a safety margin, even during periods of maximum anticipated drawdown. Regularly monitoring both the static and pumping water levels over time helps track trends and provides an early warning system for a declining water supply or reduced well performance.