Saltwater intrusion is the movement of saline water into freshwater aquifers, degrading groundwater quality in coastal regions. It poses a significant environmental challenge, potentially contaminating drinking water sources and impacting agricultural land. Many coastal communities rely on these freshwater aquifers.
Understanding Groundwater Dynamics
Groundwater is water stored beneath the Earth’s surface within geological formations called aquifers. These aquifers consist of permeable rock or unconsolidated materials like sand and gravel that can store and transmit water. The upper surface of this saturated zone is known as the water table, which fluctuates with changes in water supply and withdrawal.
In coastal areas, freshwater in aquifers naturally meets saltwater from the ocean. Saltwater is denser and has higher pressure than freshwater due to differences in mineral content. This density difference causes saltwater to form a wedge shape beneath the lighter freshwater.
Fresh groundwater flows from inland areas towards the coast, where elevations and groundwater levels are lower. This natural flow maintains a balance, pushing back the denser saltwater and preventing its extensive inland movement. For every foot the freshwater table stands above sea level, the interface between freshwater and saltwater can extend about 40 feet below sea level.
Natural Pathways of Intrusion
Saltwater intrusion can occur through various natural processes. Geological characteristics of coastal areas, such as highly permeable sand and gravel aquifers, allow for easier movement of saltwater inland. Sea level changes can also cause the freshwater-saltwater interface to shift. For instance, a 40 cm rise in sea level can reduce the thickness of a freshwater lens by up to 50%.
Tidal influences and storm surges contribute to temporary intrusion. Storm surges can push saltwater further inland, allowing it to percolate into freshwater sources. Variations in rainfall, such as prolonged droughts, can reduce the amount of freshwater recharging aquifers. This decrease in freshwater availability thins the freshwater lens, making it more susceptible to saltwater intrusion.
Human Activities Accelerating Intrusion
Human activities significantly accelerate saltwater intrusion. Excessive pumping of freshwater from coastal aquifers is a leading cause. When water is extracted faster than it can be replenished, the water table drops, reducing the pressure exerted by the freshwater and allowing the denser saltwater to move further inland and upward into the aquifer. This imbalance disrupts the natural equilibrium that keeps saltwater at bay.
Coastal development and changes in land use also exacerbate the problem. Urbanization and agriculture increase the demand for water, leading to more groundwater pumping. The construction of canals, drainage ditches, and dredging activities create direct pathways for saltwater to enter freshwater systems. This direct connection bypasses natural barriers, allowing saltwater to intrude more easily into previously protected areas.
Human-induced climate change, particularly global sea level rise, directly pushes the saltwater wedge further inland and upward. Since 1880, the global average sea level has risen by 21–24 cm, with the rate of increase accelerating in the 21st century. This rise, caused by thermal expansion of warming oceans and melting ice, directly increases the volume of saltwater pressing against coastal aquifers. Rising sea levels, combined with land subsidence in some areas, intensify the intrusion by altering the hydraulic balance between fresh and saline water.