Saltwater Intrusion

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Saltwater intrusion has occurred to some degree in many of the coastal aquifers of the United States. Since saltwater cannot be used to irrigate crops or be consumed by people, saltwater intrusion can be very problematic to coastal communities that rely on fresh groundwater supplies for the livelihood. The USGS studies how excessive groundwater pumping, sea level rise, and other factors contribute to the encroachment of seawater into fresh groundwater supplies. This research aids those who manage the water supplies, allowing for better management strategies to protect people and their sources of water.

BACKGROUND

Under natural conditions, the seaward movement of freshwater prevents saltwater from encroaching on freshwater coastal aquifers. This interface between freshwater and saltwater is maintained near the coast or far below the land surface. The interface actually is a diffuse zone where freshwater and saltwater mix. This zone is referred to as the zone of dispersion or the zone of transition.

Groundwater pumping can reduce freshwater flow toward coastal areas and cause saltwater to be drawn toward the freshwater zones of the aquifer. Saltwater intrusion decreases freshwater storage in the aquifers, and, in extreme cases, can result in the abandonment of wells. Saltwater intrusion occurs by many ways, including lateral encroachment from coastal waters and vertical movement of saltwater near discharging wells. The intrusion of saltwater caused by withdrawals of freshwater from the groundwater system can make the resource unsuitable for use. Thus, groundwater management plans should take into account potential changes in water quality that might occur because of saltwater intrusion.

illustrations and photos of freshwater saltwater boundaries on Long Island

The Process of Saltwater Intrusion: The figure above illustrates how the process of saltwater intrusion into an aquifer system can occur. The boundary between fresh groundwater and saltwater is referred to as the freshwater/saltwater interface. Fresh groundwater discharging to the coast prevents the landward encroachment of saltwater. If too much freshwater is pumped from the aquifer system, then saltwater can migrate landward by a process referred to as “saltwater intrusion.” If a pumping well is close to the landward migrating freshwater/saltwater interface, saltwater could enter the well and contaminate the water supply, too.

WHERE HAVE WE SEEN IT HAPPEN?

One part of the U.S. that has dealt with saltwater intrusion is Florida. In Florida, saltwater has intruded into groundwater supplies through different compounding ways. For example, saltwater has encroached into aquifers because fresh groundwater levels have decreased relative to sea level, allowing higher gradient water to flow toward the freshwater. Also, leaking saltwater inland canals, leakage between aquifers, or even upwelling of saltwater from depth also have impacted freshwater aquifers. Water managers in Florida are using information from local, State, Tribal, and Federal saltwater-intrusion monitoring networks, such as from the USGS, to prevent and reverse saltwater intrusion.

 

RELATED USGS RESEARCH

The USGS studies and monitors freshwater/saltwater interfaces in coastal communities to help protect against saltwater intrusion. Here are a few links to demonstrate how the USGS does studies along each coast.

 

ADDITIONAL RESOURCES

U.S. Environmental Protection Agency (EPA)

National Ocean and Atmospheric Administration (NOAA)