Saltwater intrusion is the movement of saline ocean water into freshwater aquifers. This phenomenon naturally occurs to some degree in nearly all coastal regions where groundwater systems connect with the sea. However, human activities and environmental changes have dramatically increased the rate and extent of this infiltration globally. This degradation of groundwater quality threatens drinking water supplies and ecosystems in coastal communities worldwide.
The Mechanism of Intrusion
The interaction between fresh and saltwater in a coastal aquifer is governed by the difference in their densities. Freshwater, which has a lower mineral content, is less dense than saline ocean water. This density difference causes the lighter freshwater to float on top of the heavier saltwater, creating a distinct interface deep within the aquifer.
Under natural conditions, the pressure from the inland freshwater flowing toward the sea maintains this interface in a stable position near the coastline. The denser saltwater extends inland beneath the freshwater in a formation known as a saltwater wedge. The position of this wedge is described by the Ghyben-Herzberg principle, which approximates that for every one unit of freshwater head elevation above sea level, the freshwater-saltwater interface extends about forty units below sea level.
Primary Drivers of Intrusion
The primary cause for accelerated intrusion is the excessive pumping of groundwater from coastal wells, which disrupts the natural balance. When water is extracted faster than it can be naturally recharged, the freshwater table drops. This reduces the pressure that holds the saltwater wedge back, allowing the denser saltwater to move inland and upward, contaminating the well.
Environmental changes exacerbate this problem, most notably sea level rise. A higher sea level increases the pressure exerted by the ocean on the coastal aquifer, pushing the saltwater interface further inland. Changes in precipitation patterns, often linked to climate change, can also reduce the natural recharge of the aquifer, lowering the freshwater head and accelerating the intrusion. Additionally, man-made structures like drainage canals can act as conduits, allowing tidal saltwater to flow inland and infiltrate the aquifer.
Consequences for Human and Natural Systems
Contamination of Water Supplies
The most immediate consequence of saltwater intrusion is the contamination of potable water sources. Even slight increases in salinity can render groundwater unusable for drinking, as chloride concentrations can quickly exceed the Environmental Protection Agency’s recommended limit of 250 mg/L. Communities are then forced to seek expensive alternative water supplies or invest in costly desalination and treatment infrastructure.
Agricultural Impacts
Agriculture is severely impacted as the use of brackish water for irrigation leads to soil salinization. Increased salt concentrations in the soil impair a plant’s ability to absorb water and nutrients. This results in stunted growth, poor germination rates, and reduced crop yields. In severe cases, this process transforms once-fertile farmlands into unproductive saline wastelands, threatening regional food security and farmer livelihoods.
Ecological Damage
Saltwater intrusion causes ecological damage to coastal ecosystems. Coastal wetlands and estuaries that rely on a specific fresh and saltwater balance are altered, leading to tree die-offs in coastal forests and the loss of freshwater-dependent species. The change in water chemistry can also mobilize nutrients in the soil, leading to increased nutrient runoff and excessive algae growth in nearby waterways. The corrosive nature of saline water accelerates the degradation of underground municipal infrastructure, such as pipes and wells.
Strategies for Management and Prevention
Managing saltwater intrusion requires strategies focused on reducing freshwater extraction and physically controlling the saltwater interface. The simplest measure is to reduce groundwater pumping rates and implement water conservation policies. This allows the freshwater head to recover and push the interface seaward. Furthermore, relocating existing pumping wells further inland or changing the timing of pumping can also help minimize the local drawdown that draws the saltwater cone upward.
More active management involves creating physical or hydraulic barriers to block the intrusion. One effective hydraulic method is the use of injection wells, which pump treated freshwater back into the aquifer parallel to the coastline. This artificial recharge increases the freshwater pressure, creating an underground barrier that holds the saline wedge back. Continuous monitoring of groundwater levels and salinity provides the necessary data to inform management decisions and adjust pumping or recharge rates before contamination occurs.