Under normal conditions, freshwater prevents salt water from encroaching on coastal aquifers. Along coastlines, freshwater and salt water are in equilibrium, such that less dense freshwater floats on more dense salt water. The interface between freshwater and salt water is maintained near the coast and below land surface. Saltwater intrusion happens when seawater flows into coastal freshwater as a result of changes in the elevation of groundwater near the coasts. Hydraulically connected groundwater and seawater may change their elevation gradient direction as a result of changes in water storage due to pumping and droughts or due to sea-level rise caused by melting ice. These changes create a diffusive zone, or a zone of dispersion--a brackish region of transition between freshwater and salt water.

A hydrostatic equilibrium is achieved by the weight of a unit column of freshwater extending from the water table to the saltwater-freshwater interface stabilized by a unit column of salt water extending from sea level to the same point on the interface, which is roughly 40 meters of freshwater below sea level for every meter of groundwater.

Freshwater resources in coastal regions are threatened by salinization. Salinization is the increase of chloride ion concentrations in freshwater aquifers along coastlines and in some instances inland. Saltwater intrusion, caused by both natural and human activities, occurs by lateral inflow from coastal waters and vertical influx to discharging wells. Groundwater pumping can reduce freshwater flow toward coastal discharge areas, causing salt water to be drawn toward the freshwater zones of the aquifer. Saltwater intrusion decreases freshwater storage in the aquifers, which can result in the abandonment of supply wells.

Once saltwater intrusion has occurred, it is almost impossible to reverse, making it a significant threat to freshwater resources. Many coastal areas are arid, increasing the magnitude of the threat. Indeed, in many such areas, including California, the Middle East, and the Mediterranean, the future sustainability of groundwater resources is at risk from overuse and from contamination, increasing the urgency of managing and protecting water resources in a sustainable manner.

A numerical ranking system known as the GALDIT index exists to assess the potential for saltwater intrusion. Developed by the EU-India INCODEV COASTIN project, this index rates coastal aquifers' vulnerability to saltwater intrusion based on a combination of factors, including aquifer hydraulic conductivity, depth to groundwater, level above the sea, distance from the shore, impact of existing seawater intrusions in the area, and thickness of the aquifer. The index also considers the stability of groundwater in the vicinity, categorizing it as unconfined, confined, or leaky.

Saltwater intrusion can be related to climate change and global warming. Increasing temperatures at the Earth's surface cause droughts, higher evaporation, increases in surface runoff, decreases in precipitation and infiltration, and ice melt. These changes affect groundwater storage and therefore groundwater elevation and gradient (Le Treut, 2007). Natural causes of saltwater intrusion include storm surges caused by hurricanes and other tropical systems and rises in sea level. The rise of sea levels pushes seawater further inland, threatening freshwater aquifers.

Anthropogenic saltwater intrusion can result from dredging canals in coastal zones that allow salt water to migrate farther inland. Louisiana, moreover, has experienced this type of saltwater intrusion as a result of overpumping the freshwater aquifer for potable water and irrigation needs, which reduces the head difference at the saltwater-freshwater interface and induces the flow of salt water into the freshwater system when that system is not capable of recharging completely, such as during drought conditions.

Freshwater systems benefit from mechanisms capable of restoring equilibrium after potentially damaging events. After Hurricanes Katrina and Rita, for example, salinity that had been introduced into the freshwater marshes was naturally removed by tidal flushing before substantial damage was done. Damage to freshwater systems was observed in areas not affected by tidal and precipitation flushing of salt water from the water table.

Coastal communities are taking into account the impact of rising sea level on water resources. The Florida Aquifer is endangered by saltwater intrusion, motivating Floridians to reduce groundwater pumping and find alternative water resources (Chachadi, 2001). This saltwater intrusion cannot be stopped, but the rate of its progression can be slowed down. In southwest Florida, the Southwest Florida Water Management District established the Southern Water Use Caution Area to mitigate saltwater intrusion, among other hazards due to overpumping the Florida Aquifer. Another form of mitigation used in California is to use treated wastewater to recharge the aquifer.

Since 1850, the Los Angeles area has used an injection-well barrier to push freshwater into the aquifer near the coast. The State of New Jersey constructed sea walls, bulkheads, and jetties to protect the freshwater aquifer from the rising sea level. Overall, in coastal communities exposed to storm surge, a sustainability plan for saltwater intrusion proposes to construct coastal barriers slowing storm surge to prevent the loss of wetland habitat. This project requires coordination by state and federal agencies, such as the United States Geological Survey, the Environmental Protection Agency, and the Army Corps of Engineers.

The sustainability of freshwater resources is associated with the sustainability of life in coastal communities. Sustainability plans to protect these resources are costly, but if such plans are not executed, freshwater may become unavailable in many heavily populated coastal areas. The threat of global and coastal climate change simply increases the vulnerability of these freshwater resources.

References

1. Chachadi, A. G., and J. P. Lobo-Ferreira. "Assessing Aquifer Vulnerability to Seawater Intrusion Using GALDIT Method: Part 1--Application to the Portuguese Aquifer of Monte Gordo." Coastal Policy Research Newsletter 4 (March, 2001): 7-9.

2. Le Treut, H., et al. "Historical Overview of Climate Change." In Climate Change, 2007--The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by Susan Solomon et al. New York: Cambridge University Press, Saltwater intrusion, 2007.

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