A water well accesses an aquifer, an underground layer of permeable rock or sediment that stores and transmits water. This supply is relied upon for drinking, agriculture, and industry worldwide. Overpumping occurs when water is withdrawn from a well faster than natural processes, such as rain and snowmelt, can replenish the aquifer. This unsustainable extraction creates a deficit, leading to a cascade of physical, chemical, and environmental consequences that extend far beyond the well itself.
The Well Runs Dry
The most immediate consequence of overpumping is drawdown, the lowering of the water level inside the well during pumping. As water is continuously pulled out, a temporary, inverted cone-shaped depression forms in the water table around the well, called the cone of depression. If the pumping rate is too high, this cone deepens, causing the water level to drop below the pump intake. This results in the well running dry and the pump losing suction.
Repeatedly pumping a well dry severely stresses mechanical components, often leading to premature pump failure. The pump must work harder and longer to lift water from a greater depth, which increases energy consumption and electricity costs. Homeowners may face the expense of drilling the well deeper to chase the falling water level. Alternatively, they may install a large storage tank to allow the well to recover between short pumping cycles. In the long term, overpumping permanently decreases the well’s yield and lifespan.
Changes in Water Chemistry
Overpumping can drastically degrade the quality of the remaining groundwater, potentially making it unusable or unsafe. In coastal areas or near deep layers of naturally saline water, excessive pumping causes saltwater intrusion. By lowering the freshwater pressure in the aquifer, the denser saltwater is pulled into the well, contaminating the fresh supply.
The reduced volume of fresh water also concentrates natural contaminants present in the aquifer materials. For instance, high concentrations of naturally occurring minerals like arsenic or dissolved solids can be drawn into the well as the water table drops. Rapid pumping can also increase the water’s turbidity, or cloudiness, by pulling fine clay and silt particles into the well from the surrounding rock formation.
The introduction of more oxygen into the aquifer due to aggressive drawdown can enhance the growth of certain bacteria, leading to biofouling. This process creates a gelatinous film inside the well and the aquifer’s pores, which restricts water flow and decreases the well’s efficiency. Mineral incrustation, the precipitation of dissolved minerals onto the well screen, is another form of plugging caused by pressure and temperature changes from heavy pumping.
Land Subsidence and Ground Movement
The removal of groundwater can cause a profound geological consequence known as land subsidence. Groundwater occupies the tiny pore spaces between soil and sediment grains, and this water pressure helps support the weight of the overlying earth. When this pressure is removed through overpumping, especially in aquifers with compressible clay and silt layers, the weight of the land above causes the aquifer material to compress.
This compression causes the ground surface to sink, which can be a gradual process or lead to earth fissures and sinkholes. Subsidence is largely irreversible; the land will not rebound even if the aquifer is refilled. This permanent compaction destroys the aquifer’s ability to store water in the future. The sinking ground can cause extensive damage to infrastructure, including roads, pipelines, canals, and building foundations.
Impacts on Local Ecosystems
Overpumping an aquifer impacts the wider environment because groundwater and surface water systems are interconnected. Groundwater provides the baseflow that sustains perennial streams, rivers, springs, and wetlands, especially during dry seasons. When the water table is lowered, the natural discharge of groundwater that feeds these surface features is reduced or eliminated.
This reduction in flow can cause streams to dry up completely, leading to a loss of habitat for fish and other aquatic organisms. Wetlands, which depend on a shallow water table, can disappear, resulting in the death of vegetation and increased susceptibility to fire. The ecological health of a river system can be severely threatened when groundwater extraction causes water levels to drop significantly during the dry season.