Saltwater Intrusion Basics

Saltwater Intrusion Basics

Groundwater Monitoring, Management and Conservation Keep

Saltwater Intrusion Under Control

Almost two thirds of the world's population lives within 400 km of the ocean shoreline; just over half live within 200 km, an area only taking up 10% of the earth's surface (Hinrichsen, 2007).  Most of these coastal regions rely on groundwater as their main source of fresh water for domestic, industrial and agricultural purposes.  As the world's population continues to grow at an alarming rate, fresh water supplies are constantly being depleted, bringing with it issues such as saltwater intrusion and increasing the importance of groundwater monitoring, management, and conservation.

Freshwater-Saltwater Interactions

Saltwater intrusion is a major concern commonly found in coastal aquifers around the world.  Saltwater intrusion is the induced flow of seawater into freshwater aquifers primarily caused by groundwater development near the coast.  Where groundwater is being pumped from aquifers that are in hydraulic connection with the sea, induced gradients may cause the migration of salt water from the sea toward a well, making the freshwater well unusable.

Because fresh water is less dense than salt water it floats on top.  The boundary between salt water and fresh water is not distinct; the zone of dispersion, transition zone, or salt-water interface is brackish with salt water and fresh water mixing.

Under normal conditions fresh water flows from inland aquifers and recharge areas to coastal discharge areas to the sea.  In general, groundwater flows from areas with higher groundwater levels (hydraulic head) to areas with lower groundwater levels.  This natural movement of fresh water towards the sea prevents salt water from entering freshwater coastal aquifers (Barlow, 2003).

Groundwater pumping/development can decrease the amount of fresh water flowing towards the coastal discharge areas, allowing salt water to be drawn into the fresh water zones of coastal aquifers.  Therefore, the amount of fresh water stored in the aquifers is decreased (Barlow, 2003).

The Ghyben-Herzberg Relation assumes, under hydrostatic conditions, the weight of a unit column of freshwater extending from the water table to the salt-water interface is balanced by a unit column of salt water extending from sea level to that same point on the interface.  Also, for every unit of groundwater above sea level there are 40 units of fresh water below sea level.

X  Groundwater Level        Y  Sea Water Level

Salt-water interface in an unconfined coastal aquifer according to the Ghyben-Herzberg relation.

This analysis assumes hydrostatic conditions in a homogeneous, unconfined coastal aquifer.  According to this relation, if the water table in an unconfined coastal aquifer is lowered by 1 m, the salt-water interface will rise 40 m.

Generally, saltwater intrusion into coastal aquifers is caused by two mechanisms:

· Lateral encroachment from the ocean due to excessive water withdrawals from coastal aquifers, or

· Upward movement from deeper saline zones due to upconing near coastal discharge/pumping wells.

Saltwater intrusion into freshwater aquifers is also influenced by factors such as tidal fluctuations, long-term climate and sea level changes, fractures in coastal rock formations and seasonal changes in evaporation and recharge rates.  Recharge rates can also be lowered in areas with increased urbanization and thus impervious surfaces.  Intrusion has also occurred in areas because of water levels being lowered by the construction of drainage canals .

Most incidents of saltwater intrusion occur in coastal regions, as has been the focus of discussion thus far, but inland areas can also be affected.  Salinity issues in some regions surrounding the Rio Grande in New Mexico and Texas have been attributed to upwelling of deep-circulating groundwater, which is more saline due to natural underlying geologic formations (Doremus, 2008).  The more saline groundwater is brought to the surface through pumping for irrigation and other uses.  Similar occurrences have been noted in the Mississippi River Valley Alluvial Aquifer in Arkansas, where in response to pumping, there is also upward movement of saline water from deeper formations

Intrusion Occurrence

Incidents of saltwater intrusion have been detected as early as 1845 on Long Island, New York.  Intrusion occurs in coastal aquifers worldwide, and is a growing issue in areas including North Africa, the Middle East, the Mediterranean, China, Mexico, and most notably, the Atlantic and Gulf Coasts of the United States, and Southern California. The increased use of groundwater has caused the salt-water interface to move inland and closer to the ground surface along much of the U.S. Atlantic Coast, as well as Southern California.