Coastal communities worldwide rely heavily on underground water supplies, known as aquifers, for drinking, agriculture, and industry. This groundwater is a finite resource, and its excessive removal can lead to water scarcity, not just through shortage, but through contamination. This process, known as saltwater intrusion, transforms freshwater reserves into saline water, directly impacting the long-term viability of coastal settlements.
The Natural Balance of Coastal Aquifers
The freshwater found beneath coastal land exists in a state of natural equilibrium with the nearby ocean water. Because freshwater is less dense than saline ocean water, it forms a large, wedge-shaped lens that floats upon the underlying, denser saltwater. This freshwater lens extends inland from the coast, with a boundary where the two types of water meet, known as the interface.
This balance is maintained by the hydrostatic pressure created by the height of the water table above sea level. The depth of the freshwater below sea level is directly proportional to the height of the freshwater column above sea level. This relationship, formalized by the Ghyben-Herzberg principle, indicates that for every unit of freshwater head above sea level, the freshwater extends approximately 40 times that depth below sea level.
The pressure exerted by a small elevation of freshwater above the ocean surface holds the denser saltwater wedge in place. This mechanism prevents the ocean from naturally migrating inland into the aquifer. This pressure ensures that the freshwater flows slowly toward the sea, where it discharges naturally, completing the hydrologic cycle.
How Overpumping Triggers Saltwater Intrusion
The natural equilibrium of the freshwater lens is disturbed when human activity extracts groundwater at unsustainable rates. Pumping wells draw water from the aquifer, which causes the water table in the immediate vicinity of the well to drop, forming a localized depression. This localized drop is known as a cone of depression.
The excessive removal of freshwater reduces the hydraulic head, or the pressure exerted by the freshwater column. When this pressure drops, the hydrostatic force holding the saltwater interface down is significantly diminished. Because the saltwater is denser, it responds to the reduced pressure by moving inland and upward, displacing the freshwater.
The Ghyben-Herzberg relationship demonstrates why this effect is dramatic: a small drop in the freshwater table above sea level can result in a disproportionately large rise of the saltwater interface below. For instance, a one-foot drop in the water table may cause the interface to rise by approximately 40 feet. This rapid vertical movement, called upconing, can quickly reach the depth of a pumping well, contaminating the water supply.
When the saltwater interface rises to the level of the well screen, the extracted water becomes brackish, containing elevated levels of chloride and total dissolved solids. This contamination makes the water unsuitable for human consumption and is harmful to many crops if used for irrigation. This process causes water scarcity because a previously available source of potable water is effectively lost, often forcing communities to abandon their primary water source.
Strategies for Managing Groundwater Resources
Effective management of coastal aquifers focuses on two primary goals: limiting freshwater extraction and actively maintaining the freshwater pressure. Setting sustainable yield limits is a foundational strategy, which involves calculating the maximum amount of water that can be withdrawn from the aquifer without causing undesirable results. This often requires reducing the overall pumping rate or limiting the number of new wells installed near the coastline.
The location and depth of existing wells can also be adjusted to mitigate intrusion risk. Wells can be relocated farther inland, away from the immediate coastal zone, or their screens can be placed at shallower depths to avoid the rising saltwater interface. However, these adjustments are often temporary measures, as continued over-extraction will eventually affect the entire system.
Managed Aquifer Recharge (MAR) is a proactive strategy that involves intentionally adding water back into the aquifer to maintain the freshwater head. Techniques like injection wells are used to pump treated water, often reclaimed wastewater or surface water, directly into the ground near the coast. This injected water creates a hydraulic barrier, physically pushing the saltwater wedge back toward the ocean and preventing further intrusion.
Alternative water sources, such as desalination plants or increased use of surface water, can reduce the community’s reliance on the vulnerable coastal aquifer. Diversifying the water supply eases the pressure on the groundwater system, allowing the aquifer to recover its natural hydrostatic balance. These management techniques are essential for preserving the long-term availability of freshwater in coastal regions.