Coastal aquifers are underground layers of water-bearing rock or sediment. These formations represent a major source of freshwater for the dense populations that inhabit coastal zones. Growing demand from residential development, industry, and agriculture necessitates the withdrawal of large volumes of groundwater. When this water is pumped out at a rate exceeding the natural replenishment from rain and surface water, the delicate subsurface balance of the aquifer system is disturbed.
The Primary Concern: Saltwater Intrusion
The most widespread and significant problem resulting from heavy coastal pumping is saltwater intrusion. This process involves the movement of naturally occurring saline water from the ocean into the freshwater-bearing parts of the aquifer. Saltwater intrusion degrades the quality of the groundwater, contaminating what was once a reliable source of drinking water.
Freshwater is naturally less dense than seawater, causing it to float atop the denser, underlying saline water. Excessive pumping upsets this density-driven equilibrium by removing the lighter freshwater, allowing the heavier saltwater to move inland and upward. This contamination renders the water resource unusable for most human purposes, including both domestic supply and irrigation.
Understanding the Pumping Mechanism
In a stable coastal aquifer, the freshwater forms a buoyant lens that pushes down and holds back the wedge of underlying seawater. This hydrostatic balance is maintained by the height of the water table above sea level, which provides the necessary downward pressure. For every foot the freshwater table stands above sea level, the freshwater-saltwater interface is held approximately forty feet below sea level. This 40-to-1 ratio illustrates how a relatively small drop in the freshwater level can cause a dramatically larger rise in the saltwater interface below.
Heavy pumping causes the water table around the well to drop significantly, creating a depression in the water surface known as a “cone of depression.” This localized lowering of the freshwater pressure immediately reduces the force holding the saline water wedge at bay. As the freshwater is removed, the denser seawater responds by flowing into the area of lowered pressure. The saltwater wedge then migrates inland and upward, threatening the base of the well and the surrounding aquifer. The saltwater can rise vertically beneath the well, a phenomenon sometimes called “upconing,” which quickly contaminates the water being withdrawn.
Consequences for Water Supply and Land Use
The immediate practical effect of saltwater intrusion is the loss of potable water supply for coastal communities. Once a well begins to draw water with elevated salinity, it requires expensive and energy-intensive desalination treatment. This necessitates drilling deeper wells or importing water from distant, often non-coastal sources, further stressing regional water infrastructure.
Saltwater contamination also severely impacts agriculture, which relies heavily on groundwater in many coastal regions. Crops are highly sensitive to salt and experience stunted growth and reduced yields in salinized soil. Elevated salinity levels in the water and soil also mobilize important nutrients, such as nitrogen, which can then run off into surface waters. This nutrient loading contributes to harmful algal blooms and the subsequent depletion of oxygen in nearby estuaries, damaging local ecosystems and fisheries.
Beyond water quality, the presence of saltwater accelerates the deterioration of infrastructure. The increased salinity in the ground accelerates the corrosion of metal pipes, well casings, and pumping equipment. The premature decay of water distribution and sewage systems results in high repair and maintenance costs. In some areas, the removal of large volumes of groundwater can also cause the overlying land to compact and sink, a process called land subsidence, which further damages infrastructure and increases the risk of coastal flooding.
Strategies to Manage Coastal Aquifers
Mitigating saltwater intrusion requires a combination of reduced freshwater pumping and active management of the aquifer system. Reducing the rate of groundwater extraction allows the freshwater table to recover and push the saltwater back toward the sea. Water conservation measures and switching to alternative surface water sources can help reduce the overall demand on the coastal aquifer.
Active management techniques, known as Managed Aquifer Recharge (MAR), are also employed. This involves injecting treated wastewater or stormwater directly back into the aquifer through a series of injection wells. This artificial recharge creates a hydraulic barrier of freshwater that helps to maintain the pressure ridge against the intruding saltwater wedge.
Physical barriers, such as subsurface cut-off walls, can also be installed to block the movement of the saltwater. These impermeable barriers span the thickness of the aquifer. All management strategies depend on sophisticated monitoring systems, which use observation wells to track changes in water levels and salinity. This provides the necessary data to adjust pumping and recharge rates and ensure the long-term sustainability of the freshwater supply.