Groundwater serves as a hidden reservoir, holding vast amounts of water within underground layers of rock and sediment called aquifers. This water is a source for drinking, agriculture, and industry worldwide. Groundwater overdraft arises when water is pumped from these aquifers at a rate faster than it can be replenished by natural processes like rainfall. This imbalance leads to a sustained decline in water levels with significant long-term effects.
Depletion of Water Supply
The most immediate result of prolonged groundwater overdraft is the lowering of the water table, as pumping outpaces recharge. For communities who depend on wells, this can mean their water source disappears as the water table falls below their existing equipment. The only solution is often to drill deeper, more expensive wells to chase the receding water.
This depletion forces an increase in energy consumption. Pumping water from greater depths requires more power, leading to higher operational costs for municipalities, agricultural operations, and individual homeowners. In dry years, reliance on groundwater increases as surface water supplies become scarce, accelerating the rate of depletion and the associated costs.
The consequences of this depletion are visible in many regions. For instance, in California’s Central Valley, decades of overdraft for agricultural use have led to significant declines in groundwater storage. Between 2011 and 2014, the water loss in this area was equivalent to about 12 million acre-feet annually.
Land Subsidence
A dramatic consequence of groundwater overdraft is land subsidence, the gradual sinking of the ground surface. Aquifers are composed of materials like gravel, sand, and clay, with water filling the pore spaces between these particles. The pressure of the water helps support the overlying land, and when it is pumped out, the pores can collapse, causing the land to sink permanently.
This process is similar to squeezing a wet sponge. As aquifers are dewatered, the geological formations compact, which reduces the aquifer’s total storage capacity. This means that even if water levels were to rise again, the aquifer would not hold as much water as it once did, and the damage is often irreversible.
Subsidence can cause significant damage to infrastructure, including buildings, roads, railways, and underground utilities. In coastal regions, the sinking of land increases the risk of flooding from high tides and storm surges. One of the most extreme examples occurred in California’s San Joaquin Valley, where the ground level dropped by over nine meters between 1925 and 1975.
Water Quality Degradation
Groundwater overdraft can also lead to a decline in the quality of the remaining water. In coastal areas, a primary threat is saltwater intrusion. Under natural conditions, pressure from freshwater in coastal aquifers prevents seawater from pushing inland. As the freshwater table lowers, this balance is disrupted, allowing saltwater to contaminate the supply and render it unusable for drinking or irrigation.
The problem is not limited to coastal regions. In any aquifer, a reduction in the total volume of water can lead to a higher concentration of existing contaminants. Pollutants from agricultural runoff or industrial discharge that were present in low concentrations can become more potent as the water diluting them decreases.
This degradation of water quality presents long-term management challenges. Once an aquifer is contaminated with saltwater or concentrated pollutants, remediation can be difficult and expensive, if not impossible. The issue is a global one, affecting coastal lowlands where a large portion of the world’s population resides.
Impacts on Surface Water and Ecosystems
The effects of groundwater overdraft extend beyond aquifers, impacting surface water bodies like rivers, lakes, and wetlands. Many of these surface features are interconnected with groundwater systems. For example, streams and rivers may be fed by groundwater seeping into the channel, which helps maintain their flow during dry periods.
When intensive pumping lowers the water table, this natural flow can be reversed, causing the stream to leak water into the depleted aquifer. This can lead to reduced water levels in rivers and lakes and, in severe cases, cause them to dry up completely. The loss of this water source harms the plants and animals that constitute riparian and aquatic ecosystems.
The reduction of streamflow affects fish populations, while the drying of wetlands harms birds and other wildlife that depend on these areas for breeding and feeding. In the southwestern United States, a region with naturally scarce water, overdraft has been documented to cause the elimination of vegetation and has taken a toll on dependent aquatic ecosystems.